PHYSICS AND CHEMISTRY OF THE SOLAR NEBULA

The solar system is thought to have begun in a flattened disk of gas and dust referred to traditionally as the solar nebula. Such a construct seems to be a natural product of the collapse of dense parts of giant molecular clouds, the vast star-forming regions that pepper the Milky Way and other galaxies. Gravitational, magnetic and thermal forces within the solar nebula forced a gradual evolution of mass toward the center (where the sun formed) and angular momentum (borne by a small fraction of the mass) toward the outer more distant regions of the disk. This evolution was accompanied by heating and a strong temperature contrast from the hot, inner regions to the cold, more remote parts of the disk. The resulting chemistry in the disk determined the initial distribution of organic matter in the planets; most of the reduced carbon species, in condensed form, were located beyond the asteroid belt (the outer solar system). The Earth could have received much of its inventory of pre-biological material from comets and other icy fragments of the process of planetary formation in the outer solar system.     

Comets as a possible source of prebiotic molecules

Prebiotic molecules derive from abiotic organic molecules, radicals, and ions that pervade the universe at temperatures as high as several 1000 K. Here we review the role of organic molecules that condensed at low temperatures before or during comet formation in the early history of the Solar System. Recent spacecraft encounters and ground-based observations of carbon-rich volatile and dust components of comet comae provide a broad database for the investigation of these organic molecules. New laboratory data for some potential cometary organics are presented. Probable icy organic constituents of the nucleus and CHON particles as likely candidates for the distributed sources of gas-phase organic species in the coma are discussed. There is broad agreement that many organic molecules observed in the coma originate from the dust that must have existed in the solar nebula at the time and place of comet formation. We conclude that complex organic molecules found in comets may be a source of prebiotic molecules that led to the origins of life.     

Molten earth and the origin of prebiological molecules.

Evidence for the molten Earth at its accretion time has been accumulated through the geochemical investigations and the observations of the surfaces of planets by space probes such as Venera 8, Mariner 9, Surveyor, Luna, and Apollo. The primitive terrestrial atmosphere might have been derived from the volcanic gases, as suggested by Rubey, but of a higher temperature than so far assumed. A thermochemical calculation of the composition of the volcanic gas suggests the following possibilities: (1) Large amounts of H2 and CO were present in the primitive atmosphere. This gives a theoretical basis for the HCN-production experiment by Abelson. (2) HCHO and NH3 existed in the primitive oceans, of the amount comparable with the weight of the present biosphere. (3) Plenty of NO3-, SO4, and PO4 were expected in the primitive oceans. The NO3- ions might have been useful for the nitrate respiration advocated by Egami. In an appendix, it is argued, on ;he basis of the observational evidence of the exospheric temperatures of planets by space probes, that a highly reducing atmosphere would (if it existed on the primitive Earth) have disappeared very quickly due to the thermal escape of hydrogen from its exosphere.     

The role of cometary particle Coalescence in chemical evolution

Important prebiotic organic compounds might have been transported to Earth in dust or produced in vapor clouds resulting from atmospheric explosions or impacts of comets. These compounds coalesced in the upper atmosphere with particles ejected from craters formed by impacts of large objects. Coalescence during exposure to UV radiation concentrated organic monomers and enhanced formation of oligomers. Continuing coalescence added material to the growing particles and shielded prebiotic compounds from prolonged UV radiation. These particles settled into the lower atmosphere where they were scavenged by rain. Aqueous chemistry and evaporation of raindrops containing nomomers in high temperature regions near the Earth's surface also promoted continued formation of oligomers. Finally, these oligomers were deposited in the oceans where continued prebiotic evolution led to the most primitive cell. Results of our studies suggest that prebiotic chemical evolution may be an inevitable consequence of impacting comets during the late accretion of planets anywhere in the universe if oceans remained on those planetary surfaces.     

Contributions of Icy Planetesimals to the Earth's Early Atmosphere

Laboratory experiments on the trapping of gases by ice forming at low temperatures implicate comets as major carriers of theheavy noble gases to the inner planets. These icy planetesimals may also have brought the nitrogen compounds that ultimately produced atmospheric N₂. However, if the sample of three comets analyzed so far is typical, the Earth's oceans cannot have been produced by comets alone, they require an additionalsource of water with low D/H. The highly fractionated neon inthe Earth's atmosphere may also indicate the importance of non-icy carriers of volatiles. The most important additional carrieris probably the rocky material comprising the bulk of the mass of these planets. Venus may require a contribution from icy planetesimals formed at the low temperatures characteristic of the Kuiper Belt.     

Extraterrestrial Organic Matter: A review

We review the nature of the widespread organic material present in the Milky Way Galaxy and in the Solar System. Attention is given to the links between these environments and between primitive Solar System objects and the early Earth, indicating the preservation of organic material as an interstellar cloud collapsed to form the Solar System and as the Earth accreted such material from asteroids, comets and interplanetary dust particles. In the interstellar medium of the Milky Way Galaxy more than 100 molecular species, the bulk of them organic, have been securely identified, primarily through spectroscopy at the highest radio frequencies. There is considerable evidence for significantly heavier organic molecules, particularly polycyclic aromatics, although precise identification of individual species has not yet been obtained. The so-called diffuse interstellar bands are probably important in this context. The low temperature kinetics in interstellar clouds leads to very large isotopic fractionation, particularly for hydrogen, and this signature is present in organic components preserved in carbonaceous chondritic meteorites. Outer belt asteroids are the probable parent bodies of the carbonaceous chondrites, which may contain as much as 5% organic material, including a rich variety of amino acids, purines, pyrimidines, and other species of potential prebiotic interest. Richer in volatiles and hence less thermally processed are the comets, whose organic matter is abundant and poorly characterized. Cometary volatiles, observed after sublimation into the coma, include many species also present in the interstellar medium. There is evidence that most of the Earth's volatiles may have been supplied by a late bombardment of comets and carbonaceous meteorites, scattered into the inner Solar System following the formation of the giant planets. How much in the way of intact organic molecules of potential prebiotic interest survived delivery to the Earth has become an increasingly debated topic over the last several years. The principal source for such intact organics was probably accretion of interplanetary dust particles of cometary origin.     

Cosmic Carbon Chemistry: From the Interstellar Medium to the Early Earth

Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowledge of abundant organic material in different space environments and investigate the connection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed.Carbon is a key element in the evolution of prebiotic material (Henning and Salama 1998), and becomes biologically interesting in compounds with nitrogen, oxygen and hydrogen. Our understanding of the evolution of organic molecules—including such compounds—and their voyage from molecular clouds to the early solar system and Earth provides important constraints on the emergence of life on Earth and possibly elsewhere (Ehrenfreund and Charnley 2000). Figure 1 shows the cycle of organic molecules in the universe. Gas and solid-state chemical reactions form a variety of organic molecules in circumstellar and interstellar environments. During the formation of the solar system, this interstellar organic material was chemically processed and later integrated in the presolar nebula from which planets and small solar system bodies formed. The remnant planetesimals in the form of comets and asteroids impacted the young planets in the early history of the solar system (Gomes et al. 2005). The large quantities of extraterrestrial material delivered to young planetary surfaces during the heavy bombardment phase may have played a key role in life''s origin (Chyba and Sagan 1992, Ehrenfreund et al. 2002). How elements are formed, how complex carbonaceous molecules in space are, what their abundance is and on what timescales they form are crucial questions within cosmochemistry.Open in a separate windowFigure 1.Carbon pathways between interstellar and circumstellar regions and the forming solar system.     

Prebiotic processes in planetary atmospheres

Within 40 years of experimental studies in prebiotic chemistry, most of the building blocks of the living systems have been synthesized in plausible conditions of the primitive Earth. The starting ingredients correspond to two complementary classes: volatile organics, and their non volatile oligomers. They may have been formed in the atmosphere on the primitive Earth and/or imported by extra-terrestrial sources. Organic chemistry is involved in meteorites, comets, in the giant planets and several of their satellites. Again this chemistry presents the two complementary aspects. In particular, with a dense reduced atmosphere rich in organic compounds in gas and aerosol phases, Titan appears as a natural laboratory for studying prebiotic chemistry at a planetary scale.     

Thermodynamic Constrains for Life Based on Non-Aqueous Polar Solvents on Free-Floating Planets

Free-floating planets (FFPs) might originate either around a star or in solitary fashion. These bodies can retain molecular gases atmospheres which, upon cooling, have basal pressures of tens of bars or more. Pressure-induced opacity of these gases prevents such a body from eliminating its internal radioactive heat and its surface temperature can exceed for a long term the melting temperature of a life-supporting solvent. In this paper two non-aqueous but still polar solvents are considered: hydrogen sulfide and ammonia. Thermodynamic requirements to be fulfilled by a hypothetic gas constituent of a life-supporting FFP’s atmosphere are studied. The three gases analyzed here (nitrogen, methane and ethane) are candidates. We show that bodies with ammonia oceans are possible in interstellar space. This may happen on FFPs of (significantly) smaller or larger mass than the Earth. Generally, in case of FFP smaller in size than the Earth, the atmosphere exhibits a convective layer near the surface and a radiative layer at higher altitudes while the atmosphere of FFPs larger in size than Earth does not exhibit a convective layer. The atmosphere mass of a life-hosting FFP of Earth size is two or three orders of magnitude larger than the mass of Earth atmosphere. For FFPs larger than the Earth and specific values of surface pressure and temperature, there are conditions for condensation (in the ethane atmosphere). Some arguments induce the conclusion than the associated surface pressures and temperatures should be treated with caution as appropriate life conditions.     

Comets and the formation of biochemical compounds on the primitive Earth--a review.

Thirty years ago it was suggested that comets impacting on the primitive Earth may have represented a significant source of terrestrial volatiles, including some important precursors for prebiotic synthesis (Oró, 1961, Nature 190: 389). This possibility is strongly supported not only by models of the collisional history of the early Earth, but also by astronomical evidence that suggests that frequent collisions of comet-like bodies from the circumstellar disk around the star beta Pictoris are taking place. Although a significant fraction of the complex organic compounds that appear to be present in cometary nuclei were probably destroyed during impact, it is argued that cometary collisions with the primitive Earth represented an important source of both free-energy and volatiles, and may have created transient, gaseous environments in which prebiotic synthesis may have taken place.     

Two Different Sources of Water for the Early Solar Nebula

Water is essential for life. This is a trivial fact but has profound implications since the forming of life on the early Earth required water. The sources of water and the related amount of delivery depend not only on the conditions on the early Earth itself but also on the evolutionary history of the solar system. Thus we ask where and when water formed in the solar nebula-the precursor of the solar system. In this paper we explore the chemical mechanics for water formation and its expected abundance. This is achieved by studying the parental cloud core of the solar nebula and its gravitational collapse. We have identified two different sources of water for the region of Earth's accretion. The first being the sublimation of the icy mantles of dust grains formed in the parental cloud. The second source is located in the inner region of the collapsing cloud core - the so-called hot corino with a temperature of several hundred Kelvin. There, water is produced efficiently in the gas phase by reactions between neutral molecules. Additionally, we analyse the dependence of the production of water on the initial abundance ratio between carbon and oxygen.     

The outer solar system: Perspectives for exobiology

The outer solar system contains many environments of interest for studies of the origin of life. Recent observations support the idea that Jupiter and Saturn have retained the mixture of elements originally present in the solar nebula. Subsequent low temperature chemistry has produced the expected array of simple molecules giving characteristic absorption bands in the spectra of these planets. Microwave and infrared observations show that the lower atmospheres are at temperatures above 300 K. Sources of energy for non-equilibrium chemistry seem available at least on Jupiter and the presence of an array of colored materials in the Jovian cloud belts has often been cited as evidence for the existence of complex abiogenic organic molecules. Further study of both planets in an exobiological context seems well worthwhile; potentially productive methods of investigation (including planned space missions) can be described and evaluated from this point of view. Uranus and Neptune are clearly deficient in light gases, but otherwise little is known with certainty about these distant planets. Again unusually high temperatures have been reported, but not above 273 K. Pluto and many of the outer planet satellites appear to represent a class of small bodies very unlike our neighbors in the inner solar system. Titan, Saturn's largest satellite, is especially interesting for our purposes because of its atmosphere. Methane and hydrogen are both present, and Titan's unusually reddish color again suggests the presence of organic compounds. The hydrogen-methane ratio is likely to be more similar to that of a primitive reducing terrestrial atmosphere than the ratios for Jupiter and Saturn, suggesting that in some respects this satellite may provide an even better model for early organic synthesis on the Earth. The problem of Titan's heat balance and atmospheric composition are currently under active investigation.     

Molten Earth and the origin of prebiological molecules

Evidence for the molten Earth at its accretion time has been accumulated through the geochemical investigations and the observations of the surfaces of planets by space probes such as Venera 8, Mariner 9, Surveyor, Luna, and Apollo. The primitive terrestrial atmosphere might have been derived from the volcanic gases, as suggested by Rubey, but of a higher temperature than so far assumed. A thermochemical calculation of the composition of the volcanic gas suggests the following possibilities:

1. Large amounts of H₂ and CO were present in the primitive atmosphere. This gives a theoretical basis for the HCN-production experiment by Abelson.
2. HCHO and NH₃ existed in the primitive oceans, of the amount comparable with the weight of the present biosphere.
3. Plenty of NO ₃ ⁻ , SO ₄ ⁻⁻ , and PO ₄ ⁻⁻⁻ were expected in the primitive oceans. The NO ₃ ⁻ ions might have been useful for the nitrate respiration advocated by Egami.

In an appendix, it is argued, on the basis of the observational evidence of the exospheric temperatures of planets by space probes, that a highly reducing atmosphere would (if it existed on the primitive Earth) have disappeared very quickly due to the thermal escape of hydrogen from its exosphere.     

Early Inner Solar System Impactors: Physical Properties of Comet Nuclei and Dust Particles Revisited

During the epoch of early bombardment, terrestrial planets have been heavily impacted by cometary nuclei and cometary dust particles progressively injected in the interplanetary medium. Stardust and Deep Impact missions confirm that the nuclei are porous, loosely consolidated objects, with densities below 1,000 kg m⁻³, and that they often release small fragments of ices and dust. Recent numerical simulations of the light scattering properties of cometary dust particles indicate that they are highly porous, most likely fractal, and rich in absorbing organics compounds (with a mixture ratio of e.g. 33 to 60% in mass for comet Hale–Bopp). Taking into account the fact that porous structures survive more easily than compact ones during atmospheric entry, such results reinforce the scenario of the early terrestrial planets enrichment – in organics needed for life to originate – by comets. *Title of paper or poster *Presented at: National Workshop on Astrobiology: Search for Life in the Solar System, Capri, Italy, 26 to 28 October, 2005.     

Comets and the formation of biochemical compounds on the primitive Earth – A review

Thirty years ago it was suggested that comets impacting on the primitive Earth may have represented a significant source of terrestrial volatiles, including some important precursors for prebiotic synthesis (Oró, 1961,Nature 190: 389). This possibility is strongly supported not only by models of the collisional history of the early Earth, but also by astronomical evidence that suggests that frequent collisions of comet-like bodies from the circumstellar disk around the star Pictoris are taking place. Although a significant fraction of the complex organic compounds that appear to be present in cometary nuclei were probably destroyed during impact, it is argued that cometary collisions with the primitive Earth represented an important source of both free-energy and volatiles, and may have created transient, gaseous environments in which prebiotic synthesis may have taken place.     

Dark matter in the solar system: Hydrogen cyanide polymers

In the presence of a base such as ammonia liquid HCN (bp 25 °C) polymerizes readily to a black solid from which a yellow-brown powder can be extracted by water and further hydrolyzed to yield-amino acids. These macromolecules could be major components of the dark matter observed on many bodies in the outer solar system. The non-volatile black crust of comet Halley, for example, may consist largely of such polymers, since the original presence on cometary nuclei of frozen volatiles such as methane, ammonia, and water makes them possible sites for the formation and condensed-phase polymerization of hydrogen cyanide. It seems likely, too, that HCN polymers are among the dark —CN bearing solids identified spectroscopically by Cruikshanket al. in the dust of some other comets, on the surfaces of several asteroids of spectral class D, within the rings of Uranus, and covering the dark hemisphere of Saturn's satellite Iapetus. HCN polymerization could account also for the yellow-orange-brown coloration of Jupiter and Saturn, as well as for the orange haze high in Titan's atmosphere. Implications for prebiotic chemistry are profound. Primitive Earth may have been covered by HCN polymers through cometary bombardment or terrestrial synthesis, producing a proteinaceous matrix that promoted the molecular interactions leading to the emergence of life.     

Aliens at home?

If we ponder how alien life might look like on other planets, we don''t have to go far, Simon Conway Morris argues, since life forms on Earth have already pushed life to the limits.When in 1609 Galileo first saw the moons of Jupiter, he must have been spellbound. I was certainly so enrapt when I saw Europa and her three companions strung like a line of jewels. Galileo may have appreciated the irony that my guide was a Jesuit priest, and the somewhat antiquated telescope we used was but a few yards from the Papal summer residence in Castel Gandolfo. Galileo prized open the door and before long, scientific imagination was fired by the prospect of innumerable inhabited worlds. As the centuries progressed, imagination raced ahead of facts, with the Moon optimistically colonized by Selenites, and Mars transformed by immense canals to supply the parched regions of a planet plunging into desertification. From this dying planet H.G. Wells propelled his aliens to terrorize southern England with immense tripods housing sinister octopoids.Now we might be closer to knowing if Wells was in any sense on the right track. The spectacular success in detecting extrasolar planets has produced a roster in excess of 450, and this technology potentially allows us to detect Earth-like planets. Even if many of the known planets are too large to be habitable and lie, for the most part, beyond the inferred ‘habitable zones'', before long we will get some clues as to how densely our galaxy is inhabited. The consensus points in two directions. First, life is a universal. Second, our biosphere will be of almost no use when it comes to comparisons. Let me draw your attention to a remarkably unappreciated fact: if you want to understand aliens, stay at home.Am I serious? After all it is already clear that extrasolar planetary systems are vastly different to our Solar System. Immense planets orbit their suns every few days, their surfaces far more torrid than that of Venus. Other planets most likely possess giant oceans, hundreds of kilometres deep. The diversity of moons and planets in our Solar System is a reminder of what may await us light years from Earth. Even among our neighbours, a case can be made for possible life in the clouds of Venus and Jupiter, the oceans of Europa and hydrocarbon lakes of Titan, and—with perennial optimism—in the permafrost of Mars. We might assume, therefore, that the range of environments available to life, its ‘habitation box'', is gigantic, and that Earth''s biosphere just nestles in one tiny corner. Oddly enough the evidence is exactly the opposite. Life on Earth has reached the limits of what is possible—anywhere.Temperature? The current limit on Earth is 122 °C. Plunging in the opposite direction the evidence is just as remarkable. At temperatures well below freezing, life carries on cheerfully. Even far beyond the eutectic, in which free water cannot form, organisms remain in a state of suspended animation with rates of damage and repair almost precisely matched. What of extreme desiccation? Evidently life has reached the limits of water activity. Entertainingly some of the hardiest forms are fungi that inhabit the weird alien world of Blue Stilton cheese. So, too, the bright colouration of salt pans is a familiar sight, and these osmotic extremes not only host rich microbial faunas but life that can flourish in the most bitter of brines. What of the extremes of pH—bleach versus battery acid? Once again, alkaliphiles and acidophiles disport themselves in ponds and streams that would have the Health and Safety officers in a state of panic. Pressure, either crushingly high or extremely attenuated? Life, of course, exists in the deepest oceanic trenches, but how much deeper might be viable? The weakest link seems to be the pressure sensitivity of the phospholipid membranes, suggesting that even on planets with titanic oceans life won''t survive much deeper than in the Mariana Trench. The same argument applies to the deep crust: at about 5 km the crushingly high pressures also coincide with the thermal limits imposed by the geothermal gradient. Shall we look to the skies? Clouds carry bacteria, but even at quite modest heights it seems to be accidental freight rather than a nebulous ecosystem.Terrestrial life has conquered nearly all of the ‘habitation box'' and its evolution begs so many questions. Are some forms, such as the hyperthermophiles, survivors from the Earth''s apocalyptic beginnings? Maybe, but most have clearly been reinvented several times. Getting to the limits of life isn''t that difficult, but how do extremophiles not only survive but flourish in these environments? Often the adaptations seem minor, which merely means they are more subtle than we might realize. What of the future? So far as the Earth is concerned it must cope with ever increasing solar luminosity: the last men will long predecease the last microbe. Possibly long before, we will engage in the first great galactic diaspora; but wherever our biologists journey they may find that life ‘out there'' got no further than the blue jewel that is Earth.     

Cometary supply of terrestrial organics: Lessons from the K/T and the present epoch

The time-scales of import with respect to the physical survival of planet-crossing bodies (asteroids, comets, meteoroids, dust) in the inner solar system are considered, and characteristic times for different masses reviewed. These physical lifetimes range from 10³–10⁵ yr for dust (masses 10^–12–10^–6 g), and 10⁵–10⁶ yr for small meteoroids (masses 10^–6–1 g), to 10⁷–10⁸ yr for larger bodies; bodies with aphelion distance 4 AU may have dynamical lifetimes lower than these figures due to ejection from the solar system by Jupiter. Values of terrestrial impact velocities and probabilities are given for characteristic orbits of long- and short-period comets, Earth-crossing asteroids, and near-Earth dust. Zodiacal dust particles, which are predominantly in near-circular, low-inclination orbits, have sufficiently low arrival velocities ( 20 km sec^–1) at the Earth to make organic survival plausible. Alternatively larger objects with short periods, perihelia near 1 AU, and i20°, will also impact at < 20 km sec^–1, but their impact probabilities are smaller. This argues for organic delivery predominantly from dust rather than directly through meteoroids, asteroids or comets. Such dust may have delivered the amino acids deposited over at most 10⁵ yr at the K/T boundary: this is also the appropriate time-scale for the hierarchical disintegration of a giant comet and its daughter products, and the accumulation by the Earth of the dust produced, but is too short for deposition by discrete large bodies produced in the disintegration of a large comet. This supports the conjecture that some organics arrived in the first 10⁹ yr of the planet's history as constituents of cometary dust gently decelerated in the atmosphere, allowing survival to the surface.     

Dust in the universe: Implications for terrestrial prebiotic chemistry

In the present review we analyze the available literature on the distribution of dust in the Universe, methods of its observation and determination of the chemical composition, and the roles for terrestrial prebiotic chemistry. The most plausible natural sources of dust on the Earth in the prebiotic era are sedimentation of interplanetary dust, meteoritic and cometary impacts, volcanic eruptions, and soil microparticulates; the interplanetary medium being among the most powerful supplier of the dust matter. Two fundamental roles of dust particles for the origins of life are considered: (1) catalytic formation of prebiotic compounds; and (2) delivery of organic matter to the Earth by space dust particles. Due to the fact that there is only approximate information on the chemical composition and properties of interstellar, circumstellar, and major part of interplanetary dust, even the simulating experiments are difficult to perform. Until these gaps are filled, it seems reasonable to focus efforts of the scientists dealing with dust-driven catalytic formation of prebiotically important compounds on the volcanic and meteoritic/cometary impact environments.