The Search For Life in the Universe:
Mars, Moon, Europa, Titan, Io,
Encedalus, ExoPlanets, Nebular Clouds...
February - March 2010
Journal of Cosmology, 2010, Vol 5, 801-810.
Cosmology, November 25, 2009
Deputy Adviser, Ministry of Statistics & Programme Implementation, New Delhi, India
Key Words: Habitable zone, Exoplanets, Extremophiles, Panspermia, Earth, Mars, Europa
1. Introduction.
Many scientists and religious leaders believe life originated on Earth through processes that gave life to non-life. Many respond skeptically to the possibility there may be life on other planets. In contrast to the widely accepted beliefs in abiogenesis where the Earth is seen as the center of the biological universe, theories of panspermia view life as widespread throughout the cosmos. Although panspermia does not address the fundamental question as to when and where exactly life originated first in the universe, it nonetheless does rest upon the belief that life will be found on other planets (besides Earth) and satellites within the solar system and beyond (Lal 2008).
Central to theories of panspermia is the belief that the "seeds of life" are ubiquitous (Arrhenius 1908, 2009), and that these "seeds of life," i.e., living creatures, were embedded in meteors, asteroids, and comets, and deposited upon Earth as well as on other habitable bodies in the universe (Joseph 2009; Wickramasinghe et al., 2009). Considerable evidence has been presented in support of panspermia by Joseph (2000, 2009) and by Hoyle and Wickramasinghe (Hoyle and Wickramasinghe 1977, 2000; Wickramsinghe 1995). Wickramasinghe and colleagues have provided nearly convincing evidence that dust grains in interstellar clouds could contain spores, desiccated bacteria, and living microbes which survive in comets on cloud condensation to stars and planets.
The large-scale presence of organic molecules in the interstellar clouds, comets and asteroids, and evidence of amino acids in carbonaceous meteorites also support a cosmic perspective on the origin of life. One hundred and fifty different chemical compounds including several organic compounds and amino acids with C, H, O, and N as major constituents have been detected in the interstellar clouds, circumstellar envelopes, and comets since 1965 (Lammer et al. 2009). Glycine (CH2NH2COOH), the simplest amino acid has been identified in Sagittarius B2, a dense interstellar gas at the heart of Milky Way Galaxy (Kuan et al. 2003).
Mars, the Jovian Satellites – Europa, Ganymade, Io, and Callisto; and Saturn’s Enceladus and Titan are already hotspots for search for life within the solar system (Naganuma and Sekine 2010; Schulze-Makuch 2010). Search for life beyond the solar system is currently underway with the launching of NASA’s Kepler- and ESA’s COROT Missions. While the former has been designed to survey our region of the Milky Way Galaxy to discover hundreds of Earth-size and smaller planets in or near the habitable zone, the latter will be looking for rocky planets- several times larger than Earth, around nearby stars in our parent galaxy.
The first exoplanet (a gas giant) was discovered in 1995 around a nearby G-type star, 51 Pegasi. As of November 2009, 405 exoplanets of three distinct types – gas giants (hot Jupiter), hot-super-Earths in short period orbits, and ice giants have been discovered beyond our solar system. Astronomers have recently discovered the lightest Earth-like exoplanet with mass only thrice that of Earth with possibly abundant liquid water some 20.5 light years away in the constellation Libra (Mayor et al. 2009). The star, Gliese 581 falls into the category of low-mass spectral type-M red dwarf stars, around which low-mass planets in the habitable zone are most likely to be found. A number of planets have since been discovered orbiting red dwarf stars. Red dwarfs make up about 3/4 of the stars closest to our solar system. The majority of the discovered exoplanets to date, however, belong to the category of gas giants, where chances of finding life are almost ruled out. Guo et al. (2009) have estimated that the number of terrestrial planets in the habitable zones of host stars with masses < 4Msun is 45.5 billion. The majority of terrestrial planets in habitable zones in the Milky Way possibly orbit around K- type stars.
2. Circumstellar Habitable Zone.
Circumstellar Habitable Zone (CHZ), also known as the "Godilocks Zone," is a region of space within a solar system where an Earth-like planet capable of supporting life is most likely to be found (Fig. 1). Astronomers in the 1970's referred to Goldilocks Zone as a remarkable small region of space that is not too hot, not too cold, and is just right for sustaining life. However, in the 1970s, it was thought that life could not survive extreme conditions, i.e. no colder than Antarctica, no hotter than scalding waters, no higher than clouds, no lower than a few miles.
However, discoveries over the past four decades, calls for revision of the classical definition of Goldilocks Zone. These include the discovery of the whole ecosystems around deep-sea vents where sunlight never penetrates, and deep sea thermal vents where water is hot enough to melt lead, and the discovery of extremophiles with the extraordinary capability to survive and thrive in the harshest environments on Earth intolerable to virtually all other living creatures; scalding waters, subzero temperatures, extreme radiation, bone crushing pressures, corrosive acidic, and extreme salty and alkaline conditions.
Fig. 1 The HZ (upper panel) and the chemistry composition (lower panel) of an Earth-analog atmosphere as a function of distance from its host star. The dashed-dotted line represents the surface temperature of the planet and the dashed linescorrespond to the inner edge of the HZ where the greenhouse conditions vaporize the whole water reservoir (adapted from Kaltenegger and Selsis 2007).
Habitable zones are bounded by the range of distances from a star for which liquid water can exist on a planetary surface, depending on such additional factors as the nature and density of its atmosphere, and its surface gravity. In terms of orbital distance, the CHZ for our own solar system currently extends from at least 0.8AU to 1.6 AU (1 AU ~ 150 million Km). The limits of the Sun’s HZ are, however, not easy to fix. Discovery of primitive life, or fossils on Mars would extend the HZ of the Sun out to at least 225 million Km (~ 1.5 AU). Indeed, Mars itself is believed to have enjoyed warmer, wetter conditions in its early history, under which primitive organisms may have evolved.
Much farther from the Sun are the gas giants and their larger moons; at first sight inhospitable to life. The effect of tidal heating on those worlds orbiting the gas giants, e.g., Europa and Callisto, leading to existence of subsurface liquid water, makes them biologically interesting (Reynolds et al. 1987). If life is eventually discovered on on any of the Jupiter’s moons, the outer margin of the Sun’s habitable zone would be pushed out to almost 800 million km (~ 5.33 AU).
The hot inner edge of a CHZ is located at the orbital distance where a planet’s water is broken up by stellar radiation into oxygen and hydrogen. It is believed that massive disassociation of planetary water occurred on Venus (which has an average orbital distance of 0.7 AU) in the remote past of the planet’s history via runaway greenhouse effect. Conventionally, if a planet is located outside the habitable zone, it is believed to be incapable of sustaining life since it is impossible for liquid water to exist beyond the band. The size and location of CHZs of other stars depend on various factors such as size, brightness, and temperature of the star and other planetary factors.
Some astronomers are critical of the classical habitable zone theory as it encompasses only carbon – based life. Moreover, the classical HZ is defined for surface conditions only. Chemolithotrophic life, a metabolism that does not depend on the stellar light, can still exist outside the HZ thriving in the interiors of a planet where liquid water is available. Such metabolisms do not depend on photosynthesis for their energy requirement and growth. Many extremophiles have been discovered to be thriving deep within Earth’s crust, and in hydrothermal vents in deep ocean along the mid-oceanic ridges, such as East Pacific Ridge and Mid-Atlantic Ridge.
The range of star types that can support planets with Earth-like life may be limited to those lower mass stars that live long enough as stable luminous stars for planets to form and complex life to evolve. Jones et al. (2006) have estimated that it takes at least 1 Gyr to evolve life on a habitable planet.
Potentially habitable planets around lower mass stars such as M-type red dwarfs must orbit closer to these stars, perhaps one-fiftieth the distance of Earth to the Sun. This is because these stars are small and generate less heat than our Sun. Although all main sequence stars generate luminous energy by converting hydrogen into helium through thermonuclear fusion, stars more massive than 1.5 times that of the Sun (i.e stars of spectral types-O, B, or A dwarfs like Sirius) age too quickly to support the development of complex Earth-type life. Even the largest, possibly suitable stars, spectral type FO-4, may only be able to support Earth-type life for only around 2 billion years. Planets in favourable orbits around such stars thus may not have sufficient time to develop complex life such plants and animals (Kasting et al. 1993). Moreover, based on traditional views of star and planet formation, within a couple of billion years of a star’s birth, cometary and asteriodal bombardment may still be so intense that surviving on such planets would be nearly impossible.
On the opposite extreme, life may be unable to develop on planets orbiting stars with less than half of solar mass (e.g. smaller spectral type-M dwarfs like Porxima Centauri). These stars are likely to tidally lock planets into such proximal orbits that any surface liquid water will evaporate too quickly, before life can develop (Peale 1977). Tidal locking (synchronous rotation of the star and the planet) may eventually cause the destruction of any life including those which might rain down upon a planet due to panspermia. These planets would be unable to develop sustaining atmosphere through condensation on the cold, perpetually dark side of the planet. Most M-type red dwarfs stars regularly emit larger stellar flares which would tend to sterilize life on a close-orbiting Earth-type planet.
The large majority of stars close to our sun fall in the lower mass domain (M, K). A star with 25% the luminosity of the Sun will have a CHZ centred at about 0.50 AU. In contrast, a star twice the Sun’s luminosity will have a CHZ centred at about 1.4 AU. Low mass stars have closer orbital locations of their HZs (0.02AU-0.70 AU). They are longer active in X-rays and EUV radiation. Earth-like exoplanets may be exposed much stronger by coronal mass ejections (CMEs) and stellar winds than Sun-like stars within 1 AU.
NASA’s Kepler mission is currently searching for habitable planets around nearby main sequence stars that are less massive than spectral type A, but more massive than M- dwarf stars of types-F,G and K. Over 65 percent of the main sequence stars in our solar neighbourhood, that are possibly suitable (i.e, with a stellar mass between 0.5 and 1.5 times that of Sun) for hosting Earth–type planets, may be members of binary or multiple star systems (Duquennoy and Mayor 1991). In binary star systems, however, a planet must not be located too close to its 'home star' or its orbit will be unstable making it impossible for complex life to evolve.
In star systems with more than two stars, the limits on stable orbital distance are so stringent that the presence of Earth-like planets, where surface water would be liquid, is much less likely. In March 2007, astronomers using NASA’s infrared Spitzer Space Telescope, announced their findings that planetary systems and dusty disk of asteroids, comets, and perhaps planets may be at least as abundant in binary systems as they are around single stars.
2.1. Computation of Habitable Zone.
If the habitable zone is defined simply as the distance of a star where the effective temperature is in the range of 0°C to 100°C, it is then straightforward to calculate the radii of HZ’s inner and outer bounds. The relevant formula in this connection is :
L = 4π r2σ T4 .........................(i)
where, L = star’s luminosity,
r = the distance from the centre of star,
σ = Stefan-Boltzmann constant,
= 5.67 x 10-8 W m-2 K-1
and T = effective temperature (in Kelvin).
For the Sun, equation (i) yields a habitable range for the HZ of 0.7 to 1.5 AU. The HZ range for other stars can then be calculated from the formula given below:
L (star)/ L (sun) = r (star)2 / r (sun)2 .............(ii)
In the case of Vega, L (star)/ L (sun) = 53, which gives a range of HZ of 5.1 to 10.9 AU.
3. Galactic Habitable Zone (GHZ)
The Galactic Habitable Zone (GHZ) is a hypothesized spherical band that may be a likely place for terrestrial life to develop in a galaxy. It is essentially an extension of the circumstellar habitable zone to a galactic scale.
Lineweaver et al. (2004) have suggested four prerequisites for traces of life to be found on exoplanets within the Milky Way Galaxy: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution, and an environment free from life-extinguishing supernovae. They have defined the Galactic Habitable Zone as an annular region between 7 to 9 kiloparsees (23,000 to 29,000 light years) from the galactic centres, and composed of stars that formed between 8 to 4 billion years ago.
According to Guillermo Gonzalez, the width of GHZ is controlled by two factors. The inner (closest to the center of the galaxy) limit is set by threats to complex life: nearby transient sources of ionizing radiation and comet impacts. Such threats tend to increase close to the galactic center. The outer limit is imposed by galactic chemical evolution, specifically the abundance of heavier elements. Observation of stars in the Milky Way Galaxy suggests that the outer reaches of a spiral galaxy may be too poor in heavy elements to allow terrestrial complex life to exist.
Based on current theory and studies of extrasolar planets, metal-rich stars are believed to be more likely to have planets orbiting around them, as a certain minimum amount of metals is needed to form rocky bodies. A metallicity of at least half that of the Sun is required to build a habitable terrestrial planet. The mass of a terrestrial planet has important consequences for interior heat loss, volatile inventory, and loss of atmosphere. (Gonzalez et al. 2001). According to Valencia et al. (2006), terrestrial planets are rocky planets from one to ten Earth masses with the same chemical and mineral composition as the Earth.
The classic definition of GHZ excludes stars too close to the galactic centre, even if they are metal-rich star. This is exactly the case with our Sun which is conveniently distanced about 28,000 light years from the galactic centre.
Location on the outside of or outskirts of the Galaxy’s spiral arms and maintaining an orbital speed which prevents the star from being swept up and into the spiral arm is another requirement of the GHZ. Our Sun revolves at the same rate as the Galaxy’s spiral-arm rotation. This synchronization and the unusually circular orbit of our Sun around the galactic centre keeps it clear of the spiral arms where numerous stars and other life-neutralizing hazards abound.
Clearly, most stars even if orbited by Earth-like rocky planets, would be unable to sustain complex life. Only 10% of the Milky Way’s stars reside in galactic habitable zone with chemical and environmental conditions suitable for the development of complex Earth-type life.
4. Evolution of Probable Habitable Planets and Satellites:
Lammer et.al. (2009) have proposed a very useful classification for planets and the evolution of life, and has demarcated four habitat types:
Class I habitats represent planets or moons where stellar and geophysical conditions allow Earth-like life to take root and which would enable complex multi-cellular life forms to evolve.
Class II habitats include planets where life, similar to life on class I type planets may appear and flourish. However, due to different stellar and geophysical conditions, life will evolve differently than on Earth, or it may flourish briefly and then die out. The planets under this category gradually become more like Venus or Mars type worlds where complex life forms are unable to survive.
Class III habitats are planetary bodies with subsurface water layers or subsurface oceans which interact directly with a silicate-rich core (e.g. Europa). Presumably, microbes or simple multi-cellular eukaryotes may be able to survive in these environments.
Class IV habitats possibly have liquid water layers between two ice layers or liquid above ice (e.g. Ganymede, Callisto, Enceladus, and Titan -lakes). Life that evolves on Class IV planets or moons may include extremophiles, or forms which are not-carbon based or unlike those of Earth (Istock 2009; Naganuma and Sekine 2009; Schulze-Makuch 2009).
4.1 Class I Habitable Planets.
Class I habitable planets are likely to be found orbiting around G- type stars and K and F-types with masses close to G stars. These are the most likely candidates for harboring complex multi-cellular surface life forms. The stellar and planetary geophysical conditions are such that an evolving atmosphere and watery environment can be maintained; perfect for the evolution of complex life.
Earth is an ideal example of planetary habitat supporting life under this category. NASA's Kepler Mission defines Earth-type planets to be those that have mass between 0.5 and 2.0 times Earth's mass, or those having between 0.8 and 1.3 times Earth's radius.
Earth is in middle of our solar system’s habitable zone with Mars and Venus on either side. It is ideally positioned within the solar system for life to evolve and diversity. It has liquid water, a breathable atmosphere, and a suitable amount of sunshine. If Earth were a little closer to the Sun, it might be cloaked in a thick veil of greenhouse gases, as is the case with Venus; a little farther out and it would become like cold arid Mars.
4.2 Class II Habitable Planets.
Class II habitats include terrestrial planets which evolve differently from Earth. Class II planets are believed to orbit within Habitable Zones of low mass M and K-type stars. However, they are located either at the very edge of the habitable zone or very close to their suns so that their atmosphere-magnetosphere environments experience extreme exposure to stellar radiation and plasmas. Thus, they are likely to lose their atmospheres and oceans, which would make it difficult for life as we know it to evolve.
However it is possible that even under these conditions there may be niches where life may flourish, such as deep beneath the soil, particularly if early in its history the planet had oceans of water. Consider Venus or early Mars, both of which are believed to have had oceans of water early in their history. On the surface of Venus, liquid water is not supposed to exist because the surface temperature (480ºC) is above the critical temperature 374ºC for pure H2O. Exoplanets whose surface is below the critical temperature might host stable liquid water conditions under specific pressure conditions. Venus-like exoplanets with surface temperatures below 150ºC are are likely candidates for sustaining life (Cockell 1999).
Mars is another example of terrestrial planet under this category. Today, Mars is a dry, frozen desert that cannot sustain life on its surface. During 4-4.5 Gya, Mars may have had an atmosphere thick enough to maintain liquid water on the surface (Kulikov et al. 2007). Large impactors may have evaporated the Martian atmosphere early in the planet’s history, and the low gravity of the planet was not able to retain the gas in hot plumes created by these impactors. (Melosh and Vickery 1999). Yet another factor is the loss of its magnetic field, exposing the planet to the Sun's solar winds which may have blow off the atmosphere and oceans of Mars (Joseph 2009).
Liquid water cannot exist on the surface of Mars on account of its low atmospheric pressure of about 0.6 kPa compared to Earth’s 101.3 kPa (Chambers 1999). However, water, and life, could exist just inches below the surface. NASA’s Phoenix Mars Lander, which landed on the Mars Arctic Plain in May 2008, in fact confirmed presence of frozen water near the surface. In June 2000, evidence for subsurface water was discovered in the form of flood-like gullies. In March 2004, NASA announced that its rover, 'Opportunity' had gathered additional evidence that Mars, in the ancient past, was a wet planet. Mars Express orbiter had directly detected huge reserves of water ice at Mars' south pole in January 2004.
Trace amounts of methane were also discovered in the Martian atmosphere in 2003, and verified subsequently in 2004 (Krasnopolskya et al. 2004). Presence of methane on Mars is very intriguing, since it is an unstable gas. Its presence indicates that there must be an active source on the planet in order to maintain such levels in the atmosphere. Mars produces around 270 tonnes of methane annually (Vladimir 2005), but asteroid impacts account for only 0.8% of the total methane production. Lack of current volcanism and hydrothermal activity or hotspots are not favourable for geologic methane. The existence of life in forms of microorganisms such as 'methanogens' appear to be the most likely source. If microscopic Martian microbial life is producing methane, these microbes probably reside far below the surface, where it is still warm enough for liquid water to exist.
4.3 Class III Habitable Planets.
Class III habitats are planetary bodies with subsurface oceans in contact with silicate core. The Jovian moon Europa is an excellent candidate for Class III status. The astrobiological potential and possible habitability of class III habitats rest on the presence of liquid water, an adequate energy source to sustain the necessary metabolic reaction. Europa’s unlit interior is considered to be one of the most likely location for extant extraterrestrial life in our solar system (Schulze 2001).
The magnetic field data from the Galileo indicated that Europa has an induced magnetic field through interaction with Jupiter’s. This raises the possibility of a subsurface salty liquid water ocean, kept warm by tidally generated heat (Greenberg 2005). Life in a subsurface, Europa ocean could possibly be similar to microbial life in the deep oceans of Earth as can be be found in hydrothermic vents Anaerobic chemosynthetic bacteria and archaea that inhabit hydrothermic ecosystems, or those of Antarctic Lake Vostak, provide a possible model for life in Europa’s ocean.
4.4 Class IV Habitable Planets.
Under Lammer et al. (2009) proposed classification, Class IV habitats include planets and planetary satellites which have subsurface water oceans, water reservoirs or oceans on the surface which do not interact with a typical silicate bearing sea- floor like silicate -core. Subsurface oceans are speculated to exist between different ice-layers at Jupiter’s Ganymede and Callisto, and saturn’s Enceladus, and Titan (Naganuma and Sekine 2009; Schulze-Makuch 2009).
Findings of the Galileo space probes suggest that Ganymede, the largest satellite in the solar system is surprisingly Earth-like in some ways, including possessing a magnetic field and probably a molten core. It is composed primarily of silicate rock and water ice. Ganymede is likely to be fully differentiated with an iron sulphide and iron-rich liquid core. A saltwater ocean is believed to exist nearly 200 km below Ganymede’s surface (McCord et.al. 2001).
NASA’s Cassini spacecraft has shown that Saturn’s satellite Enceladus, possesses active plumes and jets whose source may include pockets of liquid water near the surface or a pressurized subsurface liquid water reservoir. The Ion Neutral Mass spectrometer (INMS) instrument onboard Cassini has found noncondensable volatile species (e.g. N2, CO2, CH4) in jet-like plumes over Enceladus' geologically active south polar terrain (Waite et.al. 2007). If N2 is present, it may reflect thermal decomposition of ammonia associated with a subsurface liquid reservoir, suggesting that water is coming into direct contact with very hot rocks; providing a source of heat as well as mineral sources for catalyzing reactions. If this scenario really exists on Encleadus, it is possible to speculate that the necessary ingredients may be present for the abiogenic creation of life through chemoautotrophic pathways. Ruling out for the moment, panspermia which is the most likely explanation for the presence of life on this body or other moons or planets (Joseph 2009; Wickramasinghe et al., 2009), it is reasonable to speculate that life on Enceladus and on Earth may have developed around deep-sea hydrothermal vents. In this model, life on Earth began in deep sea hot springs where 'arqueobacteria' extracted their energy from H2S and other Fe-compounds that billow out of the sea-floor.
Recent Cassini-Huygen discoveries are indicative of Titan’s potential for harbouring the ingredients necessary for sustaining life (Naganuma and Sekine 2009). These discoveries reveal that Titan is rich in organics, and contains a vast subsurface ocean with sufficient energy sources to drive chemical evolution (Coustenis and Taylor 2008).
Recent models of Titan’s interior, including thermal evolution simulation, suggest that the satellite may have an ice crust between 50 and 150 km thick, lying atop a liquid water ocean a couple of hundred kilometre deep, with some amount (~10%) of ammonia dissolved in it, acting as an antifreeze. Beneath this ocean lies a layer of high-pressure ice. Cassini’s measurement of a small asynchronicity in Titan’s rotation is probably explained by separation of the crust from the deeper interior by a liquid layer (Lorenz 2008).
Titan’s organic inventory is impressive and carbon-bearing compounds are widespread across the surface in the form of lakes, seas, dunes, and probably sedimentary deltas at the mouths of channels (Naganuma and Sekine 2009.). Methane on Titan seems to play the role of water on the Earth. Earth has a hydrological cycle based on water, and Titan has a cycle based on methane. Cassini Spacecraft’s close flybys have revealed presence of surface features on Titan similar to terrestrial lakes and seas. NASA scientists have identified a large lake near Titan’s south pole which is filled largely with ethane, and smattering of methane, nitrogen, and other hydrocarbons.
Analogies can also be made between the current organic chemistry on Titan and the chemistry which was active on the primitive Earth, and which many speculate gave rise to life. Titan is the only planetary body in our solar system other than Earth that is known to have liquid bodies on its surface. Several of the organic processes, which are occurring today on Titan, form some of the organic compounds which many believe served as key molecules in terrestrial prebiotic chemistry, such as hydrogen cyanide (HCN), cyanoacyetylene (HC3N), and cyanogen (C2N2). Presence of nitrogen (95%) methane, ethane, and other simpler hydrocarbons in Titan’s atmosphere makes it the most favourable atmosphere for pre-biotic synthesis (Naganuma and Sekine 2009). Ethane and other hydrocarbons are simply products from atmospheric chemistry caused by the breakdown of methane by sunlight.
5. Probability of Life on Habitable Moons of Exoplanets.
In our solar system, it is observed that the larger a gas giant, greater the total mass of its satellites. Going by this norm, extrasolar giants - more massive than Jupiter may have moons as large as Mars or even Earth (Le Page 2008). One important factor determining a moon's suitability for habitability of life is the stability of its orbit, which can be disrupted by the close proximity to its Sun. Computer simulations suggest that a moon with an orbital period < 45-60 days will remain safely bound to a massive gas giant or brown dwarf that orbits 1AU from a Sun-like star. The major moons of our own solar system's gas giants all have orbital periods well within this range, between 1.7 and 16 days. At the lower end of the range, the orbit will still stay outside the Roche limit where a moon would be sheared apart by tidal forces. Thus, giant planets and brown dwarfs in a star's habitable zone indeed seem likely to have large moons in stable orbits giving rise to possibility of traces of life being detected thereon.
Presence of atmosphere is another prerequisite for development of life-system on a moon. In order to have enough gravity to hold on to an atmosphere, a moon is required to be larger than Earth's own airless Moon, which has a mass 0.012 times that of Earth. For a body with Mars-like density and an Earth-like atmospheric temperature profile, calculations show that the mass of the habitable moon must be at least 7% of Earth's to facilitate retention of most of its atmosphere for 4.6 billion years (Earth's current age). One of the criteria for retention of atmosphere on any moon is the geologic activity needed to keep the carbonate-silicate weathering cycle going, which regulates the global atmospheric temperature on geologic time scales. Without this self-regulating cycle, an otherwise habitable world will fall into perpetual ice age, much as Mars is today.
Length of a moon's day is yet another significant factor responsible for habitability of life on a moon. Computer simulations have demonstrated that any large moon orbiting a giant planet or brown dwarf becomes locked into a synchronous rotation - one side of the moon always facing the planet – within a few hundred million years. This happened also with Earth's own Moon and many other large moons in the solar system. Assuming that large moons typically have orbital periods of 2-16 days, any potentially habitable moon would have a day; several times larger than Earth's. Simple calculations by Stephen Dole of the Rand Corporation in the 1960s showed that a body with an Earth-like atmosphere would become uninhabitable when the period of rotation exceeds 4 days, due to large swings in surface temperature (Le Page 2008).
Eccentricity is also an important parameter in deciding suitability of a moon for habitability of complex life. Though a number of recently discovered exoplanets have mean orbital distance that lies within the habitable zones of their stars, most of them have eccentric orbits that would not be conducive to habitability of complex eukaryotic life on their exoplanets and their moons on account of large variations in the amount of the varying amounts of sunlight reaching them. The mean insolation (measure of solar radiation energy in a given time) of the planet orbiting 16 Cygni B, for example, is about half that of the Earth - but in reality, ranges from 20% to 260% of the sunlight on our planet because of its eccentric orbit. Although the hardiest bacteria, archae and extremophiles might flourish under these conditions, multicellular organisms, if present on these planets and moons would have a difficult time surviving, except perhaps in sheltered isolated pockets, as they would be repeatedly frozen and fried.
6. Panspermia.
Most scientists subscribe to the view that life began through as yet unknown processes which yielded protocells possessing amino acids and nucleotides that were sparked with life (Menor-Salván 2009; Sidharth 2009). It is reasonable to assume that if life could arise on Earth through abiogenesis, then the same events can take place on other worlds if they posses the same chemistry. If so, life should be widespread, particularly microbial life. More complex life is another question.
However, if those advocating panspermia are correct, life only had to arise once, and could then be dispersed from planet to planet and solar system to solar system.
Joseph (2009) presents evidence indicating that periodic and powerful solar winds blow portions of the Earth's atmosphere into space, along with airborne microbes. Hoyle and Wickramasinghe (2000) maintain that life-carrying dust and debris from Earth is inevitably distributed throughout our solar system, and even on a galaxy-wide scale. Wickramasinghe and colleagues (2009) also theorize that comets hitting the Earth could eject material containing life into space only to land on other moons and planets.
In support of these intriguing possibilities is the accumulating evidence that microbes can survive the ejection from and the landing onto the surface of a planet, and a journey through space (see Joseph, 2009 and Wickramasinghe et al., 2009 for references). Microbes can also remain in a dormant state for hundreds of millions of years before returning to life (Vreeland et al. 2000 ; Gilichinsky 2002); findings which certainly support the advocates of panspermia.
If Wickramasinghe et al., (2009), Joseph (2009) and the "seeds of life" advocates are correct, life could have begun on Earth, and microbial life could have been and could continue to be deposited on every moon and planet in our solar system. Whether those life forms survive or evolve, would depend on if the planet or moon is a Class I, II, III, or IV. It is important to stress, however, that the advocates of panspermia do not believe life began on Earth but has a source much older than our planet (Joseph 2000, 2009; Sharov 2009; Wickramasinghe et al., 2009).
7. Concluding Remarks.
Discovery of over 400 exoplanets orbiting distant stars supports the likelihood that most Sun-like star may be ringed with orbiting planets. Even if we accept the lower estimate that 10 % of Sun-like stars have planets (Marcy et al. 2005), this would indicate hundreds of millions of stars in our galaxy and in billions of other galaxies, may provide an environment favorable to life.
Discovery of wide range of extremophiles flourishing in inhospitable environments on Earth has raised possibility that primitive microbial life may survive on several of the planets and moons in our solar system as well as on countless exoplanets in other solar systems.
Presently, we have very limited information about the location of either the constituents of building blocks of life or the building blocks themselves in our solar neighbourhood. It still remains far from clear as to what constitutes life or how life was first established. Amino acids, supposedly the building blocks of life, are definitely not alive. They are complex organic molecules present in all living organisms facilitating production of protein, which is so vital for genetic replication. Their existence barely demonstrates that a complex organic chemistry is at work. No laboratory experiment involving interactions among the known building blocks of life has produced any trace of life. The mere presence of organic molecules or basic building blocks of life, viz. amino acids, purines, and pyrimidines in interstellar clouds comets and exoplanets do not ensure the presence of life, though it certainly raises the possibility that they were biologically produced (Joseph, 2009, Wickramasinghe et al., 2000). If true, then life may be everywhere as claimed by the "seeds of life" advocates of panspermia.
At present, however, panspermia remains at best an intriguing possibility. Although there is no evidence to date that life can be created from non-life despite considerable effort in well-funded laboratories, the origin of life, and the ease at which is may take root on other worlds, remain unanswered questions. Until these issues are resolved satisfactorily and the nature of life is better understood, we can only say with certainty that Earth is the lone habitat in the universe where life has achieved an authentic foothold.
***
Journal of Cosmology, 2010, Vol 5, 811-817.
Cosmology, January 25, 2010
and the Search for ExtraTerrestrial LifeDuncan Forgan
SUPA, Institute for Astronomy, University of Edinburgh, Royal Observatory Edinburgh,
Blackford Hill, Edinburgh, EH9 3HJ, United Kingdom
Keywords: Astrobiology, Drake Equation, SETI, Fermi's Paradox, Panspermia
1. Introduction
Astrobiology is undergoing something of a phase transition. In the past two decades, it has progressed from broad discussions hamstrung by limited data, such as Fermi's first formulation of his classic Paradox (see Brin 1983 and Cirkovic 2009 for more detail), towards an era of burgeoning empiricism, driven largely by two separate observational astrophysical programs.
The first is the search for planets around other stars, begun in earnest with the first detection of a planet around the solar type star 51 Pegasi (Mayor et al., 1995). These discoveries have since led to a growing program devoted to exoplanet detection, with several methods of identifying planets populating diverse parameters. With 422 exoplanets so far detected, astronomers are receiving their first glimpse at data on the available niches for life in the Galaxy - while at the same jettisoning assumptions collected over the centuries when the only known planets to exist were those in our Solar System.
Secondly, missions in the Solar System itself have increased our knowledge of our neighbours. For example, Cassini has illustrated the commonalities between the Saturnian regular satellites and Earth - the presence of hydrocarbon lakes and watery matrices on Titan (e.g. Naganuma and Sekine, 2010; Stofan et al 2007), or geysers on Enceladus (Chela-Flores 2010; Parkinson et al 2007; Spencer and Grinspoon 2007). Discoveries such as these, coupled with our understanding of the adaptability of extremeophiles, erode the concept of the stellar habitable zone (Hart 1979, Kasting et al 1993) as the only locale in a star system where amenable conditions for life exist. Given the fact that microorganisms have been discovered even in the most life-neutralizing environments, from miles beneath the earth, at the bottom of the ocean, within radioactive waste, and in below zero temperatures within snow packs and ice, the notion of "habitability" as a discrete quantity for a celestial body is increasing difficult to apply even to the Earth (Spiegel et al 2008).
Prior to the discovery of the first extrasolar planets, and the subsequent identification of those within "habitable zones" most astrobiologists had to rely on simple algorithms such as Drakeʼs equation so as to compensate for the extreme paucity of data. Drake's elegant equation has led to some interesting statistical results (e.g. Maccone 2009). However, its simplicity prevents the useful incorporation of current observations.
Therefore, given the avalanche of recent discoveries, and as the old pillars of an isolated, unique, geocentric Earth at the center of the biological universe have now fallen, the onus is on theorists to fashion a modern coherent theory of Life in the Universe. Today's astrobiologists are at a distinct advantage compared to their predecessors, as numerical astrophysics and current astrophysical data allows the construction of more sophisticated models which are sufficiently constrained to be informative.
This paper will highlight some of the latest works in numerical astrophysics and will demonstrate how numerical simulations at many astrophysical size scales, have important implications for numerical astrobiology.
2. Drakeʼs Equation and Fermiʼs Paradox
Fermiʼs Paradox and Drakeʼs Equation have set the standards against which all numerical astrobiological analysis is judged. Fermiʼs Paradox is expertly reviewed in Brin (1983) and Cirkovic (2009), so only a brief summary appears here.
The paradox begins with the lack of evidence for extraterrestrial life/intelligence. If we estimate the expected timescale for crossing the Galaxy (as Fermi himself did, on the metaphorical back of an envelope), we come to a value of around 108 years. Given that the Galaxy has been in existence and developing life forms for over ten times that value, Fermi was led to the question: "where is everybody?".
However, as it has been correctly pointed out (Brin 1983; Cirkovic 2009) the calculation of this crossing timescale is somewhat perilous, containing as it does a plethora of hidden assumptions - what is the average crossing velocity? What course would extraterrestrial vehicles plot through the Galaxy? What motivation would a civilization have to carry out such a task? What if highly advanced extraterrestrials are not like humans? What if they are like ants, bees? Consider also that for much of human civilization communication did not involve radio transmission. Advanced civilizations have come and gone, building the great pyramids and the great wall of China, and yet they never sent a radio signal into space. If extraterrestrials had been listening then, they might conclude the Earth did not possess intelligent life. It would be a mistake to make the same assumption about life on other planets. Attempting to solve this paradox has provided the driving force for the field of astrobiology for the last fifty years, and a number of answers to the paradox have been formulated (Cirkovic 2009; Crater 2009).
While Fermiʼs Paradox requires quantitative analysis to be posed, arguably the foundation and cornerstone of numerical astrobiology is Drakeʼs Equation. The formulation is simple: begin by counting all the stars in the Galaxy to create a sample set. Then begin removing stars from the sample set if they do not meet certain criteria, e.g. if they do not host planets, if they do not host Earthlike planets, if they do not harbour inhabited planets etc. The final tally represents the total number of communicating civilizations in the Galaxy.
The mathematical result is elegantly simple:
3. Galactic Scale Simulations - Niches and Timescales
The computational cost of simulating Galaxies with resolutions down to planetary scales is prohibitive in most cases. The usual resort of numerical astrophysicists is to run one of two types of numerical simulation. "N Body" simulations follow the evolution of N particles (where N is typically of order 105 to 106) under the effect of forces such as gravity.
"Hydrodynamic" simulations are more sophisticated in that they model the effects of gravity, pressure and other forces on a fluid, by replacing the fluid by a set of grid cells or particles representing the fluid elements. These two techniques have been effective in simulating phenomena from the dark matter distribution of the Universe (the famous "Cosmic Web", e.g. Teyssier et al 2009), the birth of star clusters (e.g. Price and Bate 2009) to the evolution of stellar systems (e.g. Forgan and Rice 2009) and everything in between. The interested reader should consult Bodenheimer et al (2007) for an excellent overview of these numerical methods. However, these simulation techniques are limited by how many particles or grid cells are used, and cannot simulate an entire Galaxy down to the level of individual planets (at this time). Therefore, studies of interest for astrobiology and SETI focus on much simpler "static" simulations which discard much of the dynamics. This allows the modelling of billions of stars and planets without significantly increasing the computer runtime. Vukotic and Cirkovic (2007, 2008) used such models coupled with the concept of "global regulation mechanisms" (Annis 1999) to challenge Carterʼs classic argument against SETI. By allowing galactic gamma ray bursts (GRBs) to impede life across the entire Galaxy at the same instant in history, their numerical work demonstrates that the astrophysical timescale (e.g. the lifetime of a Main Sequence star such as the Sun) and the biological timescale required for the formation of intelligent life become correlated, undermining Carterʼs principal assumption that they are in fact uncorrelated (Cirkovic et al 2009). The age of the Galaxy is therefore the incorrect timescale to adopt for Fermiʼs Paradox, and should instead be the time from the last "resetting event". Being much shorter than the galaxy crossing timescale, the Paradox is then resolved.
Similar work has been carried out using current exoplanet data and theory in an attempt to compare hypotheses for life on a Galactic playing field (Forgan 2009, Forgan and Rice 2010). The method generates a Galaxy of planets, and allows life to form according to some hypothesis which establishes the criteria that must be met for life to form (planet surface temperature, stellar type, chemical composition etc). The simulation is then rerun several times, and the results averaged to identify statistical fluctuations. Unfortunately, statistical incompleteness in exoplanet data and uncertainty in the origin of life prevent models of this type from providing concrete predictions of the multiplicity of life in the Galaxy. What they do provide is twofold: firstly, they can compare two different hypotheses in terms of their relative trends; secondly, they provide an alternative to the Drake Equation, where each individual exoplanet discovery can be folded in intuitively to the input, and results can be presented as a function of space and time. This is particularly useful for testing models such as the "phase transition" model, where the number of civilizations in the galaxy increases from a low to a high value in a short time frame, another potential solution to Fermiʼs Paradox (Cirkovic and Vukotic 2008). As future studies of exoplanets grow, this method will be able to provide observational constraints on the current zoo of hypotheses for life, based on the available planetary architectures that exist.
The work of Cotta and Morales (2009) also deserves mentioning in this section. While their galactic model is perhaps the simplest of all, they pursue a devilishly complex problem, regarding the exploration of the Galaxy by remote probes. This has always been an important point in the discussion of Fermiʼs Paradox, as probes vastly decrease the timescale in which intelligent civilizations can make themselves known to us (Cirkovic 2009). However, the dynamics of how a fleet of probes explores a region of space is not obvious, being heavily dependent on the algorithm by which individual probes must select their course and survey pattern. Relying on previous study of an analogous problem, known as the "vehicle routing problem" (c.f. Toth and Vigo 2001), they use similar heuristic algorithms and extensive numerical simulation to simulate the routing procedure for a fleet of probes, giving them a star map and leaving them to construct an economical route to visit all star systems in the least time. They conclude that the non-detection of any such probes limits their potential number to less than 1000 in the Galaxy for any given epoch (and even less if they leave significant traces of their presence as they go). While the first steps with a simple model, it is an admirable attempt to address what is a demanding problem at the heart of current debate on extraterrestrial intelligence.
4. Solar System Creation and Life - Surviving Planet Formation and Asteroid Impact
Although lifeʼs origin is unknown, but assuming life is widespread, the survivability of complex life in a star system will be sensitive to the distribution of planetary and other astral bodies in it. In particular, the interactions between planets and asteroids, comets and other leftover debris from the planet formation phase will be crucial to the continuity of life. Positively, the transfer of organisms from one celestial body to the other - as current panspermia theory suggests - requires this interaction (Arrhenius 2009; Joseph 2009a), and may indicate that microbes participate in the generation of planetary atmospheres, climate, and environment, thus contributing to its habitability for more complex life forms (Joseph 2009b). Negatively, impacts from such objects onto an inhabited planet can decrease its habitability and its biodiversity significantly, potentially causing mass extinctions amongst metazoans (Barbee and Nuth 2009; Isvoranu and Badescu 2009; Napier 2009; Raup and Sepkoski 1982) or eradicating them entirely, with the only possible survivors being the extremophiles.
N Body simulations of planetary systems have a long and distinguished history (von Hoerner 1960,1963, Aarseth 2003), and are an important tool for planet formation theorists to determine the physical and dynamical processes that sculpt planetary systems. Key in much of these simulations is the presence of a disc of debris left over from the planet formation process, collected into dynamical groups, such as the asteroid belt which separates Mars and Jupiter. Astrobiologists have begun studying the effects of different planetary architectures on this leftover debris - particularly if they may come within close proximity of inhabited planets.
Broadly considered, some of this debris may be the remnants of planets expelled during the red giant phase of a solar system's death (Joseph 2009a). Current theory holds that our sun and solar system were created from a nebular cloud following the supernova of another star. Yet other supernova's may have perturbed that nebular cloud stimulating star formation. Although considered controversial, it is possible that as stars lose mass planets orbiting those stars may be ejected billions of years before supernova (Joseph 2009a). Yet other planets may be shattered. Atmosphere's and oceans ripped from these planets by powerful red giant solar winds may also be deposited in the nebular cloud (Joseph 2009a) thereby contributing to the creation of comets and the Oort cloud. According to current theories of panspermia, much of this debris, including comets, could contain microbial life. It is now well established that bacteria can survive journeys through space including the ejection from and reentry through the atmosphere to a planet's surface (Burchella et al., 2001; Burchell et al. 2004; Horneck et al. 1994, 1995, 2001; Mastrapaa et al. 2001; Nicholson et al. 2000). As this debris strikes Earth and other planets, some of these life forms may survive and may take up residence on these planets (Joseph 2009a). We know that Mars and our own planet experienced a period of heavy bombardment for over 700 billion years, therefore, according to theories of panspermia, life may have been deposited on these planets early in their respective histories (Joseph 2009a). If true, then the same mechanisms could be applied to other planets.
If asteroids and comets were to strike the modern Earth, the consequences could eradicate all but microbial life (Barbee and Nuth 2009; Isvoranu and Badescu 2009; Napier 2009). Therefore, instead of delivering life, these astral bodies could end life, leaving only microbes in their wake.
Horner, Jones and Chambers have studied the influence of Jupiter on three separate groups of objects in our Solar System - the asteroid belt between Mars and Jupiter (Horner and Jones 2008), the Centaur objects of the Kuiper Belt (Horner and Jones 2009) and the distant Oort Cloud beyond Pluto (Horner et al. 2010). Jupiter has long been thought of as a protector of Earth, capturing potential impactors in its gravitational well, effectively shielding Earth from increased bombardment and frequent mass extinctions and allowing intelligent life to successfully develop. This supposition was put to the test by running a series of around ten separate N Body simulations of the Solar System, containing around 105 particles, one particle for each of the four giant planets, and the rest for the asteroids/comets being studied. They varied Jupiterʼs mass in each simulation, from zero Jupiter masses (i.e. no Jupiter) to 2 Jupiter masses. They then measured the number of objects being deflected towards the inner solar system during the simulation (a duration of around ten million years). While Jupiterʼs presence can increase the rate of impacts from relatively nearby objects such as the asteroids in the belt between Mars and Jupiter and the Centaurs of the Kuiper Belt (compared to it not being present), it decreases the rate of impacts from the more distant Oort Cloud objects, showing that the survivability of complex or intelligent life on worlds like ours is sensitive to the planetary system it resides in, with a relationship that is not intuitively obvious.
5. Planetary Scale Simulations - Climate Models and Habitability
If life survives in other systems besides our own, then what will we see as observers? The ability of the transit planet detection method to probe the exoplanetʼs atmospheric spectrum (e.g. Tinetti et al 2007, Swain et al 2008) warrants the study of biomarkers - spectral features whose presence indicates chemical species which are most probably generated biotically, e.g. O2, O3, CH4+O2 or CH4+O3 (Lovelock 1975). While biomarkers limit the search to Earthlike organisms only, it is an important starting point in the current dearth of data regarding extraterrestrial life.
Several works have studied the detectability of Earthʼs biomarkers. Arnold et al (2009) simulate the Earthʼs surface and atmosphere using the Biome3.5 model at three epochs in its history - the present, the Last Glacial Maximum (a lower temperature regime), and the Holocene Optimum (a higher temperature regime). The model accounts for a series of components (oceans, ice, continents, grasslands, tundra, woodlands and other vegetation). They use these climate models (in conjunction with cloud cover models) to assess the strength of what is known as the vegetative red edge (VRE), a distinctive increase in spectral reflectance caused by green vegetation at λ ~ 700 nm. They show that the VRE is detectable in all three epochs, despite the extreme changes in climate (depending on the angle of observation - observing above the North Pole during the Last Glacial Maximum obviously produces a much reduced VRE).
Fujii et al (2009) take a slightly different approach, constructing light curves for extrasolar planets consisting of four main components - ocean, soil, vegetation and snow. This requires a scattered light model of the planet, taking into account its spin-orbit angles and observation angle, and scattered light contributions from the atmosphere, land and sea.
These curves are then fitted in an attempt to recover the fractional areas of each of the four components. While they admit there are several assumptions and simplifications that require improvement (the lack of clouds, for example), their work provides a means for observers to evaluate their data (alongside model-independent work, such as Cowan et al.'s (2009) measurements of Earth light curves). These techniques may also be extended to exomoons (Kalteneger 2009), potentially detectable using current instruments (Kipping et al 2009).
Perhaps most importantly, simulations of "pseudo-Earths" with differing rotation rates and land ocean ratios by Spiegel et al (2008) have shown that "conventional" habitability can be found in somewhat exotic circumstances. They show that habitability can only occur for a fraction of a terrestrial planetʼs orbit, or a fraction of its surface only (after all, the Earthʼs surface is not 100% habitable in its own right). This is reflected in Lammer et al. (2009)ʼs classification system for habitable planets, which shows that simply being an Earthlike planet is not sufficient for it to share the same level of habitability as the Earth itself (Lal 2010).
6. Discussion and Conclusions
The field of numerical astrobiology, while nascent, is growing apace from the contributions of numerical astrophysics as well as substantial contributions from other computational disciplines such as bio-informatics, evolutionary dynamics and network theory.
This paper highlights only a small fraction of recent advances in numerical astrophysics which have direct consequences for numerical astrobiology. We have moved from the simple analyses of Drake and Fermi into deeper understandings of how Life may be distributed in the Universe - glimpses of the complex relationships between species, biosphere, planet, star and galaxy. While we are not much further forward with concrete predictions for SETI, we are far better informed of our ignorance on the subject. Fifty years of subsequent human development have improved our perspective of technological civilization as a whole, while better studies of history are honing our understanding of how civilizations may develop sustainably in the future.
The first discovery of biomarkers will herald the new science of observational astrobiology, but we might also receive our first detections of life from unexpected sources - the most prominent being the future surveys of low frequency radio astronomy observatories such as the Low Frequency Array (LOFAR), and its successor, the larger scale Square Kilometre Array (SKA). While they are designed to detect hydrogen emission at cosmological distances - billions of light years away - they also cover the same frequency bands as those most commonly used by ourselves for telecommunication, radar and other functions. In fact, a human-esque civilizationʼs radio leakage could be detected by the SKA at distances of up to 300 light years (Loeb and Zaldarriaga 2006). The SKA will produce a phenomenal quantity of data, approximately 1 terabyte per minute. To inform any potential search of this data, theory must do all it can to guide observers to the most likely signal locations.
Studies like those referred to in this review are at the cutting edge of this effort, progressing at every astrophysical size scale, folding in new observational results as they arrive. There are also many opportunities for cross-disciplinary activity in all directions. It is the hope of the author that non-specialists reading this will be encouraged to make contacts across the traditional boundaries of science, and further the scientific endeavour of astrobiology.
***
Journal of Cosmology, 2010, Vol 5, 818-827.
Cosmology, January 25, 2010
Sulfur-Based, Silicon-Based, Ammonia-Based LifePabulo Henrique Rampelotto
Exobiology and Biosphere Laboratory - Southern Regional Space Research Center / National Institute for Space Research, P.O.Box 5021, 97105-900, Santa Maria, RS – Brazil.
Department of Biology, Federal University of Santa Maria P.O.Box 5096, 97105-900, Santa Maria, RS - Brazil
Keywords: Astrobiology, Extraterrestrial Life, Definition of Life, Anthropocentric Concept, Sulfur-Based Life, Silicon-Based Life, Ammonia-Based Life, Panspermia
1. The Search for Extraterrestrial Life
The ancient Greeks were among the first to explain astronomical phenomena in physical terms. It is known, for example, that Aristarco from Samos (320–250 BC) taught that the Earth was just one planet which, as others, moves around the sun and that stars were at great distances. Epicurus (341–270 BC) suggested that the universe is filled with other worlds where extraterrestrial life is possible. Since then, the idea of a universe consisting of many worlds, just like Earth and our solar system, has been raised many times in the course of human history (Crowe 1986).
In 1584, Giordano Bruno, a priest of the Dominican Order, published "Dell Infinito, universo e mondi" ("Of Infinity, the Universe, and the World"). Bruno wrote that the stars were just like our sun, that planets must orbit these suns and that sentient beings, just like the humans of Earth, lived on these planets: "There are innumerable suns and an infinite number of planets which circle around their suns as our seven planets circle around our Sun." However, according to Bruno, we are unable to see these planets and suns "because of their great distance or small mass."
Charles Darwin (1809–1882) also accepted pluralism of life in the cosmos, and in fact this cosmic perspective may have facilitated his attempt to explain by naturalistic means the origin of various terrestrial forms of life.
In 1908 Arrhenius (2009) proposed that the seeds of life flow throughout the cosmos. Arrhenius (2009) developed a cosmic model, known as panspermia, in which life may travel from world to world, seedings the stars with life. However, at that time, it was thought the Milky Way galaxy was the universe, and it was still unknown if planets circled any of the stars.
Until the middle 20th century, the possibility of habitable worlds other than our own remained merely speculative. It is only with recent advances in space exploration and discoveries in the fields of cosmology and astrobiology, and the identification of over 400 planets circling other stars, including super-Earths orbiting in habitable zones (Lal 2010), that the possibility of life on other planets has been given serious scientific consideration (Forgan 2010; Goertzel and Combs 2010; Istock 2010). These include recent discoveries in our own solar system, which has led to numerous scientists to raise the possibility of extant life on Mars (Houtkooper and Schulze-Makuch 2010; Leuko et al., 2010; Levin 2010), Europa (Chela-Flores 2010), Io (Schulze-Makuch 2010), Titan (Naganuma and Sekine 2010) and Encedalus (Chela-Flores 2010).
Moreover, research and discoveries related to planet formation in this and other solar systems has led many scientists to again embrace theories of panspermia (Burchell 2010; Joseph 2000, 2009a; Rampelotto 2009). A number of scientists now believe life began on other planets or other extraterrestrial environments billions of years before Earth was formed (Jose et al., 2010; Joseph 2010a,b; Naitoh 2010; Poccia et al., 2010; Sharov 2010).
In the coming decades there will be increased attention and scientific effort devoted to the search of extraterrestrial life not only in this solar system, but on extrasolar planets (Forgan 2010). These include the missions DARWIN by ESA and TPF (Terrestrial Planet Finder) by NASA, and direct imaging methods (Bounama et al 2007; Cockell et al 2009) to detect extrasolar Earth-like planets and find hints of life on them by examining the composition of their atmospheres. Undoubtedly, the search for extraterrestrial life is one of the most challenging and interesting scientific endeavors of the 21st century.
The design of life-detection experiments to be performed by telescopes and spacecraft landers depends on assumptions about what life is and what observations will be taken into account as evidence for its detection (Forgan 2010; Goertzel and Combs 2010).
Up to now, we only know life on Earth. Therefore, the search for extraterrestrial life has been the search for life as we know it, based on the nature of life on Earth. Certainly, it is highly likely that life forms just like those of Earth, dwell on other planets. Some extraterrestrial life forms may also share a common genetic origins as those of Earth (Jose et al., 2010; Joseph 2000, 2009b, 2010b; Sharov 2010); and those with common origins and which dwell on Earth-like planets may have even evolved in a similar manner resulting in species similar or even more advanced than humans (Joseph 2000, 2010a,b).
Exosolar planets that might sustain life are believed to orbit within a ‘habitable zone’, the region around a star in which water can remain liquid, and where atmospheres might contain carbon dioxide, water, and nitrogen (Lal 2010; Rekola 2009). Consequently, scientists have been looking for biomarkers produced by extraterrestrial life with metabolisms resembling the terrestrial ones, where water is used as a solvent and the building blocks of life are based on carbon and oxygen (Forgan 2010; Istock 2010; Lal 2010).
However, life-sustaining worlds need not orbit in a habitable zone. Nor do they need to closely resemble Earth (Chela-Flores 2010; Goertzel and Combs 2010; Istock 2010). We now know that microbes can thrive in almost every conceivable environment. Therefore, planets which are blazing hot, frigid, or whose atmospheres may contain high levels of sulphur, or which are radioactive and toxic, may be home to a variety of extremeophiles who thrive under these poisonous conditions (Houtkooper and Schulze-Makuch 2010; Leuko et al., 2010; Naganuma and Sekine 2010; Schulze-Makuch 2010), just as they do on Earth.
To search for life on planets other than the Earth we must be prepared to recognize life as we do not know it. We cannot rule out other planets just because they are not like our world. An infinite number of life forms may have been fashioned in alien environments with characteristics fundamentally different from those found on Earth. In this context, to recognize alien life, we must learn how to escape from our anthropocentric, Earth-centered way of thinking and abandon the pre-Copernican belief that our planet is the center of the biological universe and all life forms are just like us.
2. Understanding What Is Life
For millennia, philosophers, scientists, and theologians, have attempted define life (Popa 2004). And yet, there is no general accepted definition of life.
Nowadays scientists are content to define life using the "chemical Darwinian definition" that involves "self-sustaining chemical systems that undergo evolution at the molecular level" (Joyce et al 1994). It is a limited definition considering that life on Earth may have originated on other planets (Joseph 2009a; Rampelotto 2009). There are in fact a number of genetic-studies which purport to demonstrate that the common ancestors for Earthly life forms may have first began to form billions of year before the Earth was fashioned (Jose et al., 2010; Poccia et al., 2010; Sharov 2010). It has been speculated the first steps toward actual life may have begun with self-replicating riboorganisms (Jose et al., 2010) whose descendants fell to Earth and other planets through mechanisms of panspermia (Joseph 2009a) thereby triggering the RNA world and then life as we know it (Jose et al., 2010). However, this model of life is still based on life as we know it. In fact, the concept of a self-sustaining chemical process can be applied with some justification to other catalytic, self-sustaining physicochemical process, such as forest fires.
Life on some planets may be like life on Earth (Crater 2010). Life on other worlds may have a completely different chemistry, and may not even possess a genetic code. It would be extremely unfortunate to expend considerable resources in the search for alien life and not recognize it when we find it--or it finds us.
Life that may have been originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus could not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products. Life might be based on molecular structures substantially different from those we know.
Therefore, it may be a mistake to try to define life based on a single example – life on Earth. As pointed out by Cleland and Chyba (2002) definitions just tell us about the meanings of words in our language, as opposed to telling us about the nature of the world.
What we really need is a general theory of living systems, analogous to the theory of molecules that permits one to give an unambiguous answer to the question “what is water?” (Chyba and Hand 2005). Prior to molecular theory, the best a scientist could do in characterizing water would be to define it in terms of its sensible properties, such as being wet, transparent, odorless and tasteless. Once we had an understanding of the molecular nature of matter we could identify water in such a way that all ambiguity disappears: water is H2O. Thus, a precise answer to the question “what is water?” was possible only when situated within an appropriate scientific theory.
Again, however, this may trap us into an Earth-centered perspective. Life in the universe may not be like life as we know it. Therefore, the key to formulate a general theory of living systems is to explore alternative possibilities for life. In this context, first of all, we need to understand the fundamental features of life not just based on examples from Earth, but based on how life may form and then evolve on planets completely unlike Earth. By taking a broad view this will greatly improve the possibility of recognizing life if we come upon it elsewhere in the Universe.
3. Fundamental Features of Life
The search so far has focused on Earth-like life because that is all we know. Hence, most of the planning missions are focused on locations where liquid water is possible, emphasizing searches for structures that resemble cells of terran organisms, small molecules that might be the products of carbonyl metabolism and amino acids and nucleotides similar to those found in terrestrial proteins and DNA.
However, life that may have been originated elsewhere, even within our own solar system, could be unrecognizable compared with life here and thus could not be detectable by telescopes and spacecraft landers designed to detect terrestrial biomolecules or their products. We must recognize that our knowledge of the essential requirements for life and therefore our concept on it, is based on our understanding of the biosphere during the later stages of Earth history (Pilcher 2003). Since we only know one example of biomolecular structures for life and considering the difficulty of human mind to create different ideas from what it already knows, it is difficult for us to imagine how life might look in environments very different from what we find on Earth. In the last decades, however, experiments in the laboratory and theoretical works are suggesting that life might be based on molecular structures substantially different from those we know.
One of the fundamental features of life is its chemical complexity, which is based on polymeric molecules joined by covalent bonds. Carbon appears to be the only element capable of forming polymers that readily undergo chemical alterations under the physical conditions prevailing on Earth.
Organic molecules are now known to be common throughout the universe (Thaddeus 2006). Life, then, is assumed to be carbon-based.
However, our present knowledge of physics and chemistry suggests that an organism could have an entire non-carbon-based metabolism (Baross et al 2007). Silicon is a frequent mentioned option (Bains 2004). Like carbon, silicon can form four bonds. Silicon forms stable covalent bonds with itself and other elements. It can form stable tetra-, penta-, and hexa-coordinate compounds with N, C, and O bonds (Brook 2000).
Indeed, there is an immense diversity of structures that have been assembled from such chemistry (Bains 2004). Furthermore, the greater reactivity of silicon compared with carbon may be an advantage in cold environments. Thus, its chemical and structural flexibility in non-aqueous environments can provide analogues to most of the functions of terrestrial biochemistry.
Sulfur, nitrogen and phosphorus could also potentially form biochemical molecules (Schulze-Makuch and Irwin 2008). Sulfur is capably of forming long-chain molecules like carbon. Some terrestrial bacteria have already been discovered to survive on sulfur rather than oxygen, by reducing sulfur to hydrogen sulfide.
Phosphorus is similar to carbon in its ability to form form long chain molecules on its own, which would conceivably allow for formation of complex macromolecules. When combined with nitrogen, it can create quite a wide range of molecules, including rings.
Though a great variety of prebiotic molecules have been observed in the universe (Thaddeus 2006), they present different characteristics in comparison with specific classes of biomolecules. The same may be said when we analyze organic molecules in meteorites: they present structural diversity, a predominance of branched-chain isomers and its abundance decline with increasing chain length within homologous series (Pizzarello 2006).
The building blocks of life are based on polymers of various types (e.g. proteins, nucleic acids, and polysaccharides). Such polymers consist of a limited number of monomer units arranged in a linear array with some variation in the composition of the side chains. These various types of macromolecules to be functional in living systems are uniform with respect to chirality, i.e. they are homochiral polymers (Avetisov 2007; Tamura 2010). Though chiral enhancement can occur through abiotic processes (Pizzarello 2007), the presence of only one enantiomer, or at least a pronounced chiral excess, could be a sign of life.
To the chemical reactions required to support living processes occur, a liquid solvent is presumably necessary. This is because macromolecules need to be physically stable, yet capable of structural flexibility and chemical reactivity (Schulze-Makuch and Irwin 2008). On Earth, the only accessible common liquid is water and it is compatible with an enormous diversity of organic chemistry. Recent evidence indicates that other locations in the solar system may harbor liquid water (Chela-Flores 2010; Lewis 2004, Naganuma and Sekine 2010). Thus, water, as well as carbon, is assumed to be essential for life. Consequently, much of the astrobiological effort in recent years has been based on a ‘‘follow the water’’ principle.
However, novel studies have demonstrated that a variety of solvents may sustain life (Baross et al 2007). Among them, ammonia is the most cited. Ammonia, like water, dissolves many organic compounds. Many preparative organic reactions are done with this solvent in the laboratory. Although it is liquid at lower temperatures than water, the temperature range over which ammonia is liquid for relevant planetary surface pressures is greater than for water (Schulze-Makuch and Irwin 2006). The increased strength of the dominant base as well as the corresponding enhanced aggressivity of ammonia as a nucleophile imply that the biochemistry based on this solvent would have to be different in comparison with terrestrial life (Benner et al 2004). Considering that liquid ammonia may be abundant in the universe (Lewis 2004), ammonia may have also been employed in the development of alien life.
Ammonia is not the only polar solvent that might serve as an alternative to water. Sulfuric acid and formamide are reasonably good solvents that support chemical reactivity (Benner et al. 2004). Furthermore, there is no need to focus on polar solvents, like water, when considering possible habitats for life. Non-polar hydrocarbons such as methane and ethane are better than water for managing complex organic chemical reactivity since they do not destroy hydrolytically unstable organic species, as water does (Schulze-Makuch and Irwin 2006).
Many of the most important potential solvents found in the solar system exist only in their gaseous form on Earth. They become liquids at very low temperatures. Hence, they are known as cryosolvents. Low temperatures are, however, prominent throughout the cosmos, as are species that are liquid there. Therefore, cryosolvents cannot be dismissed as potential biosolvents (Baross et al 2007).
These studies suggest that if life originated independently in other worlds, which present different geochemical environment, it would likely have selected different solvents and biopolymers for metabolism. Thus, instead of searching for specific biosignatures that appeared later in the Earth's history, future missions should focus to search for the general characteristics of life, which means search for life’s material-independent signatures.
4. Searching for Life As We Don’t Know It
When we discuss about the search for extraterrestrial life, one of the most enticing questions that emerge in our mind is “how such exotic forms of life might look?” or “how similar or different from us will they be?”
Life is likely a result of physical and chemical contingencies presented in the world where it arises. Most of the geochemical and environmental processes of any world remain unclear. Even the conditions that were present in early Earth are not clearly understood, which makes the origin of terrestrial life a mystery far to be resolved (Rampelotto 2009). Furthermore, the history of life on Earth shows us that the evolutionary trajectory of a living system cannot be predicted. The diverse and unimaginable forms of life which arose during the Cambrian period are a good example of the variety of forms life may take. Therefore, the details of form and function that a different history of life elsewhere would take, cannot be known until we find it. However, despite the possibility of so much diversity, at the molecular level underlying mechanisms guide the development of any unimaginable living system. Thus, based on biochemical principles, it is possible to make predictions about the nature of exotic forms of life which may be found in our solar system.
5. Silicon-Based Life
Silicon can form long chains as silanes, silicones, and silicates. Among them, silanes have been considered the most proper compounds to sustain life because they present the closest analog to hydrocarbons, which are so important to terrestrial life processes. However, such silicon-based life would have to be different from life as we know on Earth.
Silanes burn spontaneously when in contact with oxygen to form silicate and molecular hydrogen. Hence, a biochemistry based on such compounds requires an ambient free of oxygen. The affinity of silicon to oxygen is so strong that whether silicon is placed in water, it will form a silica shell, stripping the oxygen from the water (LeGrand 1998). Thus, water is not a compatible solvent for silicon compounds. Methane, ethane or any compounds that contain methyl groups are more compatible solvents for a silicon-based system.
The strong Si-O bond can be avoided and the carbon scenario reproduced if oxygen is replaced by sulfur. Then the resulting ratio of bonding energies of Si-Si to Si-S is comparable to the ratio of the C-C to C-O bonding energies (Firsoff 1963). Also, silicon polymers have been obtained with nitrogen instead of oxygen, where nitrogen acts as an electron donor. In hydrogen poor environments, hydrogen is often replaced by a halogen such as chloride, and long linear chains of silicon and chloride are formed (Firsoff 1963). Large molecules based on Si-NH-Si backbone, with halogens as side-groups, could provide a basis for complex chemical systems. Silanes can form flexible, macromolecular assemblies in the form of sheets, strings, tubes, and other shapes, similar to those formed by lipid bilayers in carbon biochemistry (Unno et al 2000). Furthermore, oligosilanes having up to 26 consecutive Si–Si bonds can be chiral, support a variety of functionalized and non-functionalized side chains, have alkyl side chains that are generally soluble in nonpolar solvents and self-aggregate into amphiphilic structures in water, creating vesicles and micelles (Benner et al 2004).
Although the stability of silanes decreases with increasing chain length, if hydrogen is replaced by organic groups, stable compounds are obtained. For example, polysilanes with molecular weights of above 106 have been synthesized (Sharma et al 2002). Although polysilanes are not stable at the temperature and pressure conditions of Earth’s surface they are adequately stable at low temperatures, especially at higher pressures. These studies altogether suggest that whether silicon-based life exist, it may be restricted to an environment with minor amounts of oxygen, scarcity of water, a compatible solvent such as methane and low temperatures (at least below 0°C). Titan provides the best target in our solar system for investigating this possibility. It meets all the described criteria (Fulchignoni et al 2005; Naganuma and Sekine 2010). Although has been considered that the abundance of carbon compounds on Titan may compete with silicon as the building block of life, silicon may have advantage in such extreme cold environment due to its higher reactivity.
6. Ammonia-Based Life
Ammonia, as revealed by its physical properties, may be a good solvent for life. In fact, macromolecules such as proteins, amino acids, and nucleic acids contain both OH and NH2 functional groups in various combinations and proportions with which ammonia could easily interact. However, biochemistry based on this solvent would have to be different in comparison with terrestrial life. Since oxygen would oxidize and break down ammonia, ammonia-based life needs an environment without the presence of oxygen. In such environments, anaerobic metabolism is the alternative. Analogs of terrestrial biomolecules in which oxygen atoms are replaced by NH groups might yield an equally viable biochemistry (Raulin et al 1995). Synthesis reactions like the synthesis of proteins from amino acids through a peptide bond shows similarities in waterbased, ammonia-based and water-ammonia mixtures (Firsoff 1963).
Several lines of evidence suggest that the internal ocean of Titan contain NH3, mixed with water, in the form of a liquid layer below a rigid water-ice crust (Tobie 2005). Furthermore, its environment free of oxygen provides an excellent opportunity to find living systems which uses ammonia as a solvent. The presences of NH3 in the internal ocean of Enceladus (Waite et al 2009) also drew considerable attention to the presence of ammonia-based life on this satellite.
7. Sulfur-Based Life
Sulphuric acid has the reputation to be a strong corrosive agent. However, what is not realized is that the process, called hydrolysis, actually requires water. It is the water molecules that split proteins into small pieces; acid merely catalyses the process. Thus, due to its capacity to support chemical reactivity, sulphuric acid may be a reasonable solvent capable to sustain metabolism in non aqueous environments (Benner et al. 2004).
The Venusians atmosphere is the most proper ambient in the solar system where this exotic form of life may flourish. The clouds of Venus are composed mostly of aerosols of sulfuric acid and water is scarce (Markiewicz et al 2007). The layer of clouds 50 kilometers above the surface could provide a friendly environment, at similar pressures to those on Earth, and temperatures of 20 to 80°C (Svedhem et al 2007).
Life could have possibly originated in an early ocean on Venus when the planet's surface was younger and cooler; then retreated into the clouds when the planet heated. To protect them from the high amount of UV radiation received, such hypothetical living systems may use the compound cyclic-octa-sulfur (S8), which does not react with sulfuric acid. An analogous process is observed on Earth, where some purple sulfur bacteria, green sulfur bacteria and some cyanobacterial species deposit elemental sulfur granules outside of the cell (Tortora et al 2001). Such Venusians life forms may be phototrophic, using hydrogen sulfide, which is oxidized to produce granules of elemental sulfur (Schulze-Makuch et al 2004). Terrestrial purple sulfur bacteria use such anoxygenic process as source of energy (Herbert et al 2008).
8. Final Considerations
The discovery of exo-planets around stars other than the Sun continues to stimulate public and media interest. Undoubtedly, this attention has been driven by the prospects of finding evidence of alien life. At the moment, life on Earth is the only known life in the Universe, but there are compelling arguments to suggest we are not alone. As Carl Sagan said, the absence of evidence is not evidence of absence. This thought is well known in other fields of research. Astrophysicists, for example, spent decades studying and searching for black holes before accumulating today’s compelling evidence that they exist (Melia and Falcke 2001). The same can be said for the search for room-temperature superconductors, proton decay, violations of special relativity, or for that matter the Higgs boson. Indeed, much of the most important and exciting research in astronomy and physics is concerned exactly with the study of objects or phenomena whose existence has not been demonstrated.
With regards to our anthropocentric way of thinking, history tells us that it is prudent to be guided by the notion that terrestrial life is not special. This is generally known as the Copernican principle, which has altered our view of the Universe. Based on this principle, we have recognized that Earth is not in the center of the solar system, the solar system is not in the center of the Milky Way galaxy and the Milky Way galaxy is not in the center of the Universe.
Despite the considerable efforts on the search for extraterrestrial life, a manifest tendency exists today to judge alien life through a terracentric vision. However, the search for life should not and cannot be limited to the search for Earth-like features. This cosmic view of the diverse nature of extraterrestrial life, is a revolutionary perspective which has the potential to make a great impact on our way of thinking as profound as the Copernican revolution.
***
Journal of Cosmology, 2010, Vol 5, 828-832.
Cosmology, November 9, 2009
Centre for Astrophysics and Planetary Sciences, School of Physical Sciences, Ingram Building, University of Kent, Canterbury, Kent CT2 7NH, United Kingdom.
Key Words: Origin, life, Panspermia, abiogenesis, probability.
1. INTRODUCTION.
The idea that life can naturally migrate through space (Panspermia), annoys many people. The idea itself is an old one. In several European nations during the 19th century, leading scientists suggested that life may not have originated on the Earth. The logic they used had several key strands. It had long been "known" that life wasn’t spontaneously generated (e.g. the experiments of Spallanzani and Pasteur). Evolution was increasingly accepted, but the age of the Earth was still subject to debate, and some believed it was not old enough to have permitted the degree of evolution required to go from first generation of life to the profusion of life today. Others simply accepted the Copernican view that the Earth was neither unique, nor central to the Universe: Therefore to presuppose that life started here was just wrong.
By the early 20th century these ideas found an advocate in Svante Arrhenius, who discussed the idea that life could migrate through space and proposed that seeds came to the Earth from space (Arrhenius, 1908, or see Arrhenius, 2009). Throughout the 20th century the debate concerning Panspermia waxed and waned. The problems presented by the deleterious effects of radiation in space on living organisms were more fully understood. This was bad for Panspermia. But Martian meteorites were identified on Earth (Bogard and Johnson, 1983) and modelling showed that materials could move from planet to planet (e.g. Gladman et al., 1996 and Gladman, 1997).
Melosh (1988) made the conceptual leap and argued that this was a means for what he termed "rocky" or "litho-" Panspermia, i.e. life may move from planet to planet. To support this, Mileikowsky et al. (2000a;b) considered the various stresses any microorganisms might experience during such a transfer from Mars to Earth and concluded that this was not necessarily a sterilising event.
Various groups have also now undertaken extensive experimental work with relevance to Panspermia. Horneck et al. (2001a) (and similar work since) have shown that microbes can survive (short) expose in space, in real experiments in space! Survival under other stresses such as impact shock in the GPa range (Horneck et al., 2001b, Burchell et al., 2004, Stöffler et al., 2007) and in high speed impacts (> 1 km s-1) (Burchell et al., 2001, 2004) has also been demonstrated in the laboratory. That microbes in targets can be cultured from ejecta after high speed impact events has also been demonstrated (Burchell et al., 2003 and Fajardo-Cavazos et al., 2009).
Not all experiments are positive. Whilst rocks with microbial content have survived launch and recovery on the exterior of sounding rockets (Fajardo-Cavazos et al., 2005), experiments with sandstones placed in the heat shield of capsules returned from orbit suggest more significant heat processing which may be more hazardous to life (Parnell et al., 2008).
Nevertheless, slowly, step by step, results and measurements are being obtained which show that passage of life through space by natural means is not necessarily implausible. It is therefore no surprise that the idea of Panspermia keeps cropping up (see Burchell, 2004; Davies, 1988; Parsons, 1996).
By contrast, what can we say about the origin of life from non-life (abiogenesis)? This is what is required for a local origin of life. We have never observed it. Further we do not even understand what life is, as is illustrated by the difficulty of producing a definition of life (see Cleland and Chyba, 2002 for a discussion of this philosophical point). Those who favour abiogenesis argue that it obviously happened and is not as difficult as some make out (e.g. see Menor-Salván, 2009). Or they point out that even if life originated someplace else and came to the Earth, abiogenesis must have occurred somewhere (e.g. Sidharth, 2009), in which case why not here? By considering the molecular structure of life, some deductions can be made, for example the jump from chemistry to life may not have occurred all at once, but via an intermediate "RNA-world" (Orgel, 2004). But then how did the RNA-world evolve? And which steps led to life as we know it? There is much we do not understand.
The arguments in favour of abiogeneis are not yet conclusive and certainly not at the level that would permit an estimate of its degree of difficulty. Hoyle and his colleague Wickramasinghe argue that the degree of complexity involved in even the most basic living organism is so great, that the chance of random combinations producing it in random circumstances is vanishingly small (Hoyle and Wickramasinghe, 1999a). Therefore having it occur once is hard enough. This led them to consider Panspermia as a more likely source of the origin of life on Earth (Hoyle and Wickramasinghe, 1999b, also see papers in Hoyle and Wickramasinghe, 1999c).
However, few are convinced by the arguments suggested by Hoyle and Wickramasinghe. Attaching a mathematical significance to an event (abiogensis) about which we know so little is difficult. Some argue that chemistry constrains the form that complex molecules can take (e.g. Menor-Salván, 2009), so the resulting complexity, if it occurs, is already programmed to follow certain paths. Indeed, as noted by many (see Sidharth, 2009 for a recent discussion) chemistry is universal and thus complex organic molecules are widespread in space. However, this still ignores the assembly of such molecules into life forms (see discussion above), and it is this step that involves an enormous change in the degree of complexity.
2. LOGIC LEADING TO CHOOSING PANSPERMIA OR LOCAL ABIOGENESIS AS THE ORIGIN OF LIFE ON EARTH.
It would seem sensible to try to understand why some view Panspermia in a positive, or at least permissive, light whilst many others simply reject it. What seems to be occurring is an almost sub-conscious choice. Few argue from statistical considerations such as Hoyle and Wickramasinghe have done, and indeed some maintain that you cannot apply such an analysis to abiogenesis until we understand it. Instead many form their opinion based on an intuitive grasp of the issues. Sometimes they pick holes in individual arguments for and against the issues, but the motivation for doing so often seems determined by their prior belief. The author has never seen anyone in this debate change their mind after a discussion.
We can consider two factors which are key to the issue: The ease of Panspermia occurring and the ease of abiogenesis. Each of these can be considered on a scale ranging from "so easy it happens all the time" to "impossible". The two scales can then be plotted orthogonally to each other to form a scatter plot – this is shown in Fig. 1. Depending what value you assign to each of the two factors, you will inhabit a different region of Fig. 1.
Figure 1. Scatter plot of degree of difficulty for Panspermia vs. degree of difficulty for abiogenesis.
It may be that arguments and models can be made which rule out some types of Panspermia. Melosh (2003) for example, argues that whilst planetary ejecta may be leaving our Solar System (e.g. Martian ejecta whose heliocentric orbit is perturbed by passage close to Jupiter), the cross-section for it ever striking a planet in another Solar System is so vanishingly small that interstellar litho-Panspermia can be ruled out. But even then, advocates have proposed conceptual models that involve capture of such ejecta in comets and icy bodies around other stars, which then in turn liberate it during passage through the star forming region or the inner Solar System after planet formation (e.g., Napier, 2004, Wallis and Wickramasinghe, 2004). The debate continues.
3. CONCLUSIONS
It is not possible to currently calculate either of the probabilities required for Fig. 1. Each reader will have to make their own choices. These choices can be guided by models, experiments or just intuition. Interesting, like the Drake equation (see Burchell, 2006 for a recent review) choosing values seems to be more intuitive than calculated, and hence we find quite distinct ranges of outcomes. Where you place yourself on Fig. 1 seems to reflect something about how we view the Universe.
***
Journal of Cosmology, 2010, Vol 5, 1101-1120.
Cosmology, January 31, 2010
1University of California San Diego, La Jolla, CA 92093-0411, USA
2Cardiff Centre for Astrobiology, Cardiff University, 2 North Road, Cardiff CF10 3DY, UK
Keywords: Origin of life, comets, panspermia, evolution, horizontal gene exchange
1. Introduction
The contemporary scientific approach to the problem of the origin of life is being shaped mainly within the emergent discipline of astrobiology – a discipline that combines the sciences of astronomy, space science and biology. The fact that water and complex carbon-based organic molecules are ubiquitously present outside the Earth is leading some scientists towards a possibly erroneous point of view: that life is easily generated in situ from non-living matter – the ancient doctrine of spontaneous generation being essentially revived.
The astronomical origin of the "stuff" of life at the level of atoms is beyond dispute. The chemical elements C,N,O,P… and the metals that are present in all living systems were synthesised from the most common element hydrogen in nuclear reactions that take place in the deep interiors of stars (Burbidge, Burbidge, Fowler and Hoyle, 1957). The explosions of supernovae scatter these atoms into the frozen protoglobularstarcluster clumps (PGCs) of primordial earth-mass planets (PFPs) from which new stars, larger planets, moons and comets are produced by chains of PFP mergers (Gibson 2009ab, Schild and Gibson 2010, Gibson and Schild 2010; Joseph and Schild 2010). The dark matter of galaxies was independently discovered to be planets in clumps (Gibson 1996, Schild 1996) by fluid mechanical theory and by quasar microlensing, respectively. The combination of atoms into organic molecules can proceed in interstellar planets via well-attested chemical pathways, but only to a certain limiting level of complexity. The discovery of biochemical molecules in space, including comets and meteorites, crosses a limiting threshold, although the precise level of biochemical complexity that can be reached through chemistry alone is still in dispute (Joseph 2009a). New infrared space telescopes offer hope that the probability estimates for life on planets can be refined. All galaxies appear to support life (Joseph and Wickramasinghe 2010a), but not all PGCs.
The only secure empirical fact relating to the origin of life is encapsulated in a dictum eloquently enunciated by Louis Pasteur Omne vivum e vivo—all life from antecedent life (1857). If life is always derived from antecedent life in a causal chain that is clearly manifest in present day life and through the fossil record, the question naturally arises as to when and where this connection may have ceased. The continuation of the life-from-life chain to a time before the first life appears on our planet and before the Earth itself formed implies the operation of "panspermia".
The basic concept of panspermia has an ancient history going back centuries - to the time of classical Greece and even before – referring in general to the widespread dispersal of the "seeds of life" in the cosmos (Arrhenius, 1908; Hoyle and Wickramasinghe, 2000). Critics of panspermia often say that such theories are of limited value because they do not address the fundamental question of origins. Nevertheless the question of whether life originated in situon Earth, or was delivered here from the wider universe constitutes a scientifically valid line of inquiry that needs to be pursued.
Whilst the Francis Crick and Leslie Orgel’s idea of directed panspermia transfers the problem of origin to another site, possibly invoking intelligent intervention (Crick and Orgel, 1973), Fred Hoyle and one of the present authors (Wickramasinghe et al., 2009) have attempted to expand the domain in which cosmic abiogenesis may have occurred, focussing in particular on the totality of comets in our galaxy. Like Crick and Orgel (1973) Hoyle and Wickramasinghe were influenced by the estimated super-astronomical odds against the transition from organic molecules to even the most primitive living system (Hoyle and Wickramasinghe, 1982).
The currently fashionable view that all extraterrestrial organics arise abiotically – that is to say through non-biologic processes – has no secure empirical basis and is likely to be flawed (Joseph 2009a). On the Earth it is clear that life processes account for almost all the organic molecules on the planet. If biology can somehow be shown to be widespread on a cosmic scale, the detritus of living cells would also be expected to be widely distributed in the Cosmos. The bulk of the organic molecules in space would then be explained as break-up products of life-molecules. Inorganic processes can scarcely be expected to compete with biology in the ability to synthesise systems of biochemicals resembling the detritus of biology. So wherever complex organics are found in an astronomical setting, one might legitimately infer that biology has spread astrophysically (Wickramasinghe, 2010).
2. Abiogenesis
But what then of a first origin of life?
Charles Darwin, who laid the foundations of evolutionary biology, never once alluded to the origin of life in his 1859 book On the Origin of Species (Darwin, 1859). He had, however, thought about the problem and formulated his own tentative position in a letter to Joseph Hooker in 1871 thus:
3. Emergence of the idea of a cosmic imperative
Arguments relating to the improbability of life emerging from non-life has had a chequred history. The image of a "watchmaker" and of the complexity of a pocket watch was infamously introduced by William Paley, (1743–1805):
Nobel Laureate Jacques Monod, who made fundamental contributions to enzymology and gene expression, revived the same argument in a biochemical context and advanced the view that life on Earth arose by a freak combination of exceedingly improbable circumstances, so improbable indeed that it may have arisen only once in the entire history of the universe. He wrote in his book Chance and Necessity (Monod, 1971):
The improbability argument stated here has its critics of course. But the case for an infinitesimally small probability of abiogenesis cannot be peremptorily dismissed as wrong: at worst it might be considered indeterminate. Scientific arguments for abiogenesis remain ill-defined to the extent that no processes have been identified that can lead from prebiotic chemistry to life. The enormous probability hurdle needs somehow to be overcome.
The a priori probability for the emergence of crucial molecular arrangements needed for life (e.g. in the enzymes) have been variously estimated as being about 1 in 10x, where x is in the range ~ 100 to 40,000 (Orgel and Crick, 1975; Hoyle and Wickramasinghe, 1982). Even assuming the lower value in this range the probability of abiogenesis leading to life remains truly minuscule.
Since we know that life exists on the Earth two possibilities remain open. It arose on Earth against incredible odds, in which case the emergence of life on other planets would be multiply improbable, and an argument could then be made for life being unique to the Earth or to this solar system – a position that was passionately espoused by Jacques Monod. Such a pre-Copernican viewpoint is less appealing than the idea that life emerged on a cosmic scale that transcended the scale of the solar system, even the scale of the entire galaxy.
The resistance to all such ideas, in our view, is due to a fear that the argument would give succour to creationism. This is unfounded of course because abiogenesis must surely have happened according to our view – but only as an exceedingly rare, even unique event in the history of the cosmos. The spread and diffusion of life even from a single event of origination then turns out to be essentially unstoppable (Joseph and Wickramasinghe 2010a).
The probability of abiogenesis is greatly increased by recognizing that the interstellar medium is dominated by planets in clumps merging to make stars (Gibson 2009ab). Rather than the 1012 non-merging planets expected per galaxy in old cosmology, new cosmology requires more than 1019 and their moons and comets constantly recycling life giving stardust from the stars they produce and destroy.
4. Interstellar organics molecules and the origin of life on the Earth
In recent years Astrobiology has taken up the challenge of extending the Oparin- Haldane ideas of abiogenesis to a wider cosmic canvas. This has been prompted in large measure by the discovery of biochemically relevant molecules such as polycyclic aromatic hydrocarbons (PAHs: ie, oil-like) in interstellar space, an identification first reported in the journal Nature by Fred Hoyle and Chandra Wickramasinghe in 1977 (Hoyle and Wickramasinghe, 1977, 2000). Such molecules have now been inferred to exist in vast quantity not only within the Milky Way but in external galaxies as well (Wickramasinghe et al., 2004).
Figure 1 shows a Herschel infrared telescope image of the Aquila Nebula which contains clouds choc-a-bloc with organic molecules including PAHs and is an active site of star-births, the youngest stars being younger than a few million years. Such stellar and planetary nurseries are considered by many to be regions where a Urey- Miller-type chemistry is taking place on a grand scale (Joseph and Schild 2010). An unproved assertion is that life must be the inevitable end result of such chemistry- a cosmic imperative.
We argue that interstellar clouds represent not only a nursery for stars, but a graveyard of life – polycyclic aromatic hydrocarbons and other organic molecules present here arise from the destruction and degradation of life.
Support for the idea that life originated on Earth in a primordial soup is beginning to wear thin in the light of recent geological and astronomical evidence. It is becoming clear that life arose on Earth almost at the very first moment that it could have survived. During the period from about 4.3-3.8 Gyr ago (the Hadean Epoch) the Earth suffered an episode of heavy bombardment by comets and asteroids. Rocks dating back to the tail end of this epoch reveal evidence of an excess of the lighter isotope 12C compared with 13C pointing to the action of microorganisms that preferentially take up the lighter isotope from the environment (Mojzsis et al, 1996; Manning et al, 2006).
5. Origin of life in Comets
If we reject the option of an origin of life on the Earth against incredible odds as discussed in section 3, a very much bigger system and a longer timescale was involved in an initial origination event, after which life was transferred to Earth. The 4x1017 m PGC molecular clouds of planets and their atmospheres in the galaxy are of course much bigger in scale than anything on Earth, but in the constantly merging and recycling planetary interstellar medium one can achieve the production of organic molecules through a variety of liquid and gas-phase chemistry, particularly if supernovae in the planet clump have provided sufficient fertilizer and seeds. These organic molecules must enter a watery medium in suitably high concentrations to begin the presumptive prebiotic chemistry that eventually leads to life. In the formation of a solar system (and planetary nebulae systems approaching supernovae such as the Helix seen in Fig. 2), numerous solid objects form, such as larger planets, moons and comets. These originally icy objects would contain the molecules of the parent interstellar clouds of planets, and for a few million years after they condensed would have liquid water interiors due to the heating effect of radioactive decays (Wickramasinghe et al, 2009). If microbial life were already present in the parent interstellar medium, the newly formed comets could serve to amplify it.
Ignorance of frozen primordial planets in clumps as the dark matter of galaxies has led to a false indication of Λ dark energy by systematic SNIa dimming errors. Antigravity forces of the big bang are supplied by vortex dynamics of big bang turbulence, not a cosmological constant Λ. Λturb antigravity (turbulent dark energy) powers the initial expansion of the fireball of Planck particles and antiparticles by inertial vortex forces but frictionally dissipates soon after the big bang event. Prior to life being generated anywhere, primordial comets could provide trillions of "warm little ponds" replete with water, organics and nutrients, their huge numbers diminishing vastly the improbability hurdle for life to originate. Recent studies of comet Tempel 1 (Figure 3) have shown evidence of organic molecules, clay particles as well as liquid water, providing an ideal setting for the operation of the "clay theory" of the origin of life (Cairns-Smith 1966; Napier et al, 2007). It can be argued that a single primordial comet of this kind will be favoured over all the shallow ponds and edges of oceans on Earth by a factor 104, taking into account the total clay surface area for catalytic reactions as well as the timescale of persistence in each scenario. With 1011 comets, the factor favouring solar system comets over the totality of terrestrial "warm little ponds" weighs in at a figure of 1015, and with 1011 sun-like stars replete with comets in the entire galaxy we total up a factor of 1026 in favour of a cometary origin life.
6. Origin of Life in the Early Universe
Whilst the distribution of life via comets is unavoidable once an origin has been achieved somewhere, the events leading to the origin itself must still remain largely speculative. The existence of life on Earth logically demands an origin, albeit an exceedingly improbable origin, somewhere in the Universe. Although the set of comets in the galaxy discussed in section 4 was seen to confer a non-trivial advantage over sites of origin on Earth by an estimated factor 1024, there would be merit in seeking even wider horizons, going back further in time in the history of the Big-Bang Universe. The earliest opportunity for such an event arises after the recombination of ions and electrons – the plasma to gas transition that took place 300,000 years (1013 s) after the Big Bang. At that time the Universe scale of causal connection ct included a baryonic mass of about 1048 kg, or 106 protogalaxies, where c is the speed of light and t is the time since the big bang event. These starless H and 4He plasma clouds retained as fossils the density, strain rate and morphology of superclusters formed at 1012 s (30,000 years) and supervoids now expanded to 1025 m with 1024 m 1046 kg superclusters. Protogalaxies fragmented along weak turbulence vortex lines of the plasma epoch to form chain galaxy clusters observed by the Hubble Space Telescope ultra deep field (Gibson and Schild 2010, Fig. 1). The protogalaxy morphology is either linear or spiral at the base of the linear fragments according to Nomura direct numerical simulations of weak turbulence, giving the Nomura scale of 1020m from the fluid mechanics. The 1012 s density ρ ~ 10-17 kg m-3 is the density of PGCs.
Figure 4 illustrates how cosmology modified by modern fluid mechanics (New Cosmology, Gibson 2009ab) estimates the time life should appear and how it is likely to be dispersed to cosmological scales.
7. Astronomical Evidence
Identifying the composition of interstellar dust in clouds such as Fig. 1 has been a high priority for astronomical research since the early 1930’s (see Wickramasinghe, 1967). The dust absorbs and scatters starlight causing extinction of the light from stars, and re-emits the absorbed radiation in the infrared. An important clue relating to dust composition follows from studies of extinction of starlight. The total amount of the dust has to be as large as it can be if nearly all the available carbon and oxygen is condensed into grains. The paradigm in the 1960’s that the dust was largely comprised of water-ice was quickly overturned with the advent of infrared observations showing absorptions due to CH, OH, C-O-C linkages consistent with organic polymers. The best agreement for a range of astronomical spectra embracing a wide wavelength interval turned out to be material that is indistinguishable from freeze-dried bacteria and the best overall agreement over the entire profile of interstellar extinction was a mixture of desiccated bacteria, nanobacteria, including biologically derived aromatic (PAH) molecules as seen in Fig.5.
Whilst amplification of microorganisms within primordial comets could supply a steady source of primitive life (archeae and bacteria) to interstellar clouds and thence to planetary systems, the genetic products of evolved life could also be disseminated on a galaxy-wide scale. Such ideas were pioneered and developed by Joseph (2000, 2009b,c) and discussed also by others (Hoyle and Wickramasinghe, 1980, Napier, 2004, Wallis and Wickramasinghe, 2004; Wickramsinghe and Napier, 2008).
As first detailed and theorized by Joseph (2000), when prokaryotes, accompanied by viruses, were deposited onto planets already harboring life, genes were exchanged via horizontal gene transfer utilizing the same genetic mechanisms of exchange which are common among the microbes and viruses of Earth. These space-journeying microbes and viruses also exchanged and obtained genes from eukaryotes on innumerable planets. Microbes and viruses therefore began building up vast genetic libraries of genes coding for advanced and complex characteristics, and those shaped by natural selection. These genetic libraries maintained in the genomes of viruses and prokaryotes, made it possible to not only immediately adapt to every conceivable environment, but to biologically modify and terraform new planets and to promote the evolution and even the replication of species which had evolved on other worlds; a process Joseph (2000, 2009b,c) refers to as "evolutionary metamorphosis" and which he likens to embryogenesis. Hoyle and Wickramasinghe (2000), focusing on viruses, have referred to this as "evolution from space."
Hoyle and Wickramasinghe (2000) also pointed out, just as new computer programs can create errors in the computer's hardware, that in rare instances viruses also damage the genetic hardware of the host. However, for the most part, viruses deposited on Earth from passing comets, actually contain genes which promote evolution. When viruses from space insert their genes into the eukaryotic genome, the result is often of benefit to the host, just as most computer programs can enhance the functionality of the computer.
Joseph (2000, 2009b,c) sums it up this way: "Just as an apple seed contains the genetic instructions for the development of an apple tree, these genetic seeds of life contained the genetic instructions for the tree of life, and for every creature which has walked, crawled, swam, or slithered across the Earth...Genes act on the environment, and the biologically altered environment acts on gene selection, thereby expressing traits which had been encoded into genes acquired from life on other planets."
Therefore, according to Joseph, Hoyle and Wickramasinghe, evolution on Earth has been greatly influenced by genes acquired on other worlds. Because of the functionality of the "universal genetic code" and due to horizontal gene exchange, evolution has been coordinated on a galaxy-wide scale by microbes and viruses which have acquired and exchanged genes with microbes, viruses, and eukaryotes which evolved on innumerable planets and in every conceivable environment.
Intergalactic gene exchange is made possible by comets. Our present-day solar system appears to be surrounded by an extended halo of some 100 billion comets (the Oort Cloud) moves around the centre of the galaxy with a period of 240My. In fact, the term Oort cavity might be more appropriate, since an Oort cavity of size rO = (M/ρ)1/3 forms in a PGC when planets merge to form a protostar of mass M, where ρ is the PGC mass density.
Every 40 million years, on the average, the comet cloud becomes perturbed due to the close passage of a molecular cloud. Gravitational interaction then leads to hundreds of comets from the Oort Cloud being injected into the inner planetary system, some to collide with the Earth. Such collisions can not only cause extinctions of species (as one impact surely did 65 million years ago, resulting in the extinction of the dinosaurs), but they could also trigger the expulsion of surface material back into space. A fraction of the Earth-debris so expelled survives shock-heating and could be laden with viable microbial ecologies as well as genes of evolved life. Such life-bearing material could reach newly forming planetary systems in the passing molecular cloud within a few hundred million years of an ejection event. A new planetary system thus comes to be infected with terrestrial microbes terrestrial genes that can contribute, via horizontal gene transfer, to an ongoing process of local biological evolution.
Once life has got started and evolved on an alien planet or planets of the new system the same process can be repeated (via comet collisions) transferring genetic material carrying local evolutionary ‘experience’ to other molecular clouds and other nascent planetary systems. If every life-bearing planet transfers genes in this way to more than one other planetary system (say 1.1 on the average) with a characteristic time of 40My then the number of seeded planets after 9 billion years (lifetime of the galaxy) is (1.1)9000/40 ~ 2x109. Such a large number of ‘infected’ planets illustrates that evolution, involving horizontal gene transfer, must operate not only on the Earth or within the confines of the solar system but on a truly galactic scale (Joseph 2000; Joseph and Wickramasinghe 2010a,b). Life throughout the galaxy on this picture would constitute a single connected biosphere.
9. Life on Other Planets
Much astrobiological attention is being focussed nowadays on the planet Mars with attempts to find evidence of contemporary life, fossil life and potential life habitats. The Jovian moon Europa, the Venusian atmosphere, the outer planets and comets are also on the astrobiologist’s agenda but further down the time-line. The unambiguous discovery of life on any one of these solar system objects would be a major scientific breakthrough and would offer the first direct test of the concept of an interconnected biosphere.
The discovery of bacteria and archaea occupying the harshest environments on Earth continues to provide indirect support for panspermia. Viable transfers of microbial life from one cosmic habitat to another requires endurance of high and low temperatures as well as exposure to low fluxes of ionising radiation delivered over astronomical timescales, typically millions of years. The closest terrestrial analogue to this latter situation exists for microorganisms exposed to the natural radioactivity of the Earth, an average flux of about 1 rad per year. Quite remarkably microbial survival under such conditions is well documented. Dormant microorganisms in the guts of insects trapped in amber have been revived and cultured after 25-40 million years (Cano and Borucki, 1995); and a microbial population recovered from 8 My old ices has shown evidence of surviving DNA (Biddle et al, 2007). All this goes to show that arguments used in the past to ‘disprove’ panspermia on the grounds of survivability during interstellar transport are seriously flawed.
10. Microfossils in Meteorites
The topic of microfossils in carbonaceous chondrites has sparked bitter controversy ever since it was first suggested in the mid-1960’s (Claus, Nagy and Europa, 1963). Since carbonaceous chondrites are generally believed to be derived from comets, the discovery of fossilised life forms in comets would provide strong prima facie evidence in support of the theory of cometary panspermia. However, claims that all the micro structures (organised elements) discovered in meteorites were artifacts or contaminants led to a general rejection of the microfossil identifications. The situation remained uncertain until early in 1980 when H.D. Pflug found a similar profusion of "organised elements" in ultrathin sections prepared from the Murchison meteorite, a carbonaceous chondrite that fell in Australia on 28 September 1969 (Pflug, 1984). The method adopted by Pflug was to dissolve-out the bulk of minerals present in the thin meteorite section and examine the residue in an electron microscope. These studies made it very difficult to reject the fossil identification. More recent work by Richard Hoover (2005) and his team leaves little room for any other interpretation of these structures than that they are microbial fossils (Figs 7).
In conclusion we note that comets are beginning to acquire a prime importance and relevance to the problem of the origin of life. It would surely be prudent to study these celestial wanderers more carefully. From 1986 onwards infrared spectra of comets have shown consistency with the presence of biologically relevant material, perhaps even intact desiccated bacteria. With some 50-100 tonnes of cometary debris entering the Earth’s atmosphere on a daily basis the collection and testing of this material for signs of life should in principle at least be straightforward. Such a project was recently started in 2001 by the Indian Space Research Organisation, ISRO, in partnership with Cardiff University. Samples of stratospheric aerosols collected using balloon-borne cryosamplers were investigated independently in Cardiff, Sheffield and India and have revealed tantalising evidence of microbial life (Harris et al, 2002; Wainwright et al, 2003, 2004). A particularly interesting component of the collected samples was in the form of 10 micrometre clumps that have were identified by SEM and fluorescence tests as being viable but not culturable microorganisms (Figure 9).
***
No comments:
Post a Comment