Christopher P. McKay and Wanda L. Davis, NASA Ames Research CenterMoffett Field, California
Life on Earth is widespread and appears to have been present on the planet since early in its history. Biochemically all life on Earth is similar and seems to share a common origin. Throughout geological history, life has significantly altered the environment of the Earth while at the same time adapting to this environment. It would not be possible to understand the Earth as a planet without the consideration of life. Thus life is a planetary phenomenon that is arguably the most interesting phenomenon observed on planetary surfaces.
Everything we know about life is based on the example of life on Earth. Generalization to other areas or alien forms of life must proceed with this caveat. Although we remain uncertain of the process or the time of its origin, the advent of life on Earth was established within one billion years after the formation of the planet. While life also requires energy and nutrients, liquid water is the single-most defining ecological requirement for life on Earth. Thus a liquid water environment is currently the best indicator of where to search for extraterrestrial life. We do not expect to discover liquid water environments on any of the recently discovered large extrasolar planets because they are too close to their stars. Looking out into the Solar System, however, we see evidence for liquid water. Europa appears to have a liquid water ocean underneath a global ice surface--the evidence is indirect but persuasive. Enceladus has geysers erupting from its South Polar area presumably powered by subsurface liquid water. There are several lines of evidence that suggest that liquid water existed on Mars in the past. Direct images from orbiting spacecraft show fluvial features on the surface of Mars. Orbital infrared spectrometers have found local regions that show minerals formed in liquid water environments. The Mars Exploration Rovers also have found evidence for past aqueous activity at their landing sites on Mars. Our understanding of life, albeit limited to one example and one planet, would suggest that life is possible on other planets whenever conditions allow for environments like those on Earth--energy, nutrients, and most critically liquid water. This suggests the possibility of early microbial life on Mars and forms the basis for a search for Earth-like planets orbiting other stars. Studies of a second example of life--a second genesis--to which we can compare and contrast terrestrial biochemistry will be the beginning of a more general understanding of life as a process in the universe. This implies a search for not just fossils but a search for the biochemical remains of organisms, dead or alive.
2. What is Life?
Our understanding of life as a phenomenon is currently based only on our study of life on Earth. One of the profound results of biology is the realization that all life forms on Earth share a common physical and genetic makeup. The impression of vast diversity that we experience in nature is a result of manifold variations on a single fundamental biochemistry. The biochemistry of life is based on 20 amino acids and 5 nucleotide bases. Added to this are the few sugars, from which are made the polysaccharides, and the simple alcohols and fatty acids that are the building blocks of lipids. This simple collection of primordial biomolecules (Fig. 1) represents the set from which the rest of biochemistry derives.
Except for glycine, the amino acids in Figure 1 can have either left handed (L-) or right handed (D-) symmetry.
FIGURE 1 The basic molecules of life.
FIGURE 2 The L and D form of the amino acid alanine.
Figure 2 shows the two versions, known as enantiomers (from the Greek enantios meaning opposite), for alanine. Life uses only the L-enantiomer to make proteins although there are some bacteria that use certain D-forms in their cell walls, and many others have enzymes that can convert the D-form to the L-form. In addition, L-amino acids other than the 20 listed in Figure 1 are occasionally used in proteins and are sometimes used directly, for example as toxins by fungi and plants. We do not yet understand how and why life acquired a preference for the L-amino acids over the D-amino acids; this is one of the key observations that theories for the origins of life seek to explain.
The genetic material of life--DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)--are both constructed from nucleotide bases that form the alphabet of life's genetic code. In DNA, these are adenine (A), thymine (T), cytosine (C), and guanine (G). In RNA, thymine is replaced by uracil (U). The nucleic acids each provide a four-letter alphabet in which the codes for the construction of proteins are based. This information recording system is found in all living systems.
The biochemical unity of life, in particular the genetic unity, strongly suggests that all living things on Earth descend from a common ancestor. This is the phylogenic unity of life as shown in Figure 3. These genetic trees are obtained by comparing the ribosomal RNA within each organism. Sections within the RNA are remarkably similar within all life forms. These conserved sections show only random point changes and not evolutionary trends. Thus the similarity between the genetic sequences of any two organisms is a measure of their evolutionary distance, or more precisely the time elapsed since they shared a common ancestor. When viewed in this way, life on Earth is divided into three main groups: the eucarya, the bacteria, and the archaea. The eucarya include the multicellular life forms encompassing all plants and animals. The bacteria are the familiar bacteria including intestinal bacteria, common
FIGURE 3 A phylogenetic tree showing the relatedness of all life on Earth. The last universal common ancestor is shown by the arrow. This is the root of the tree.
soil bacteria, and the pathogens. The archaea are a different class of microorganisms that are found in unusual and often harsh environments such as hypersaline ponds and H[sub.2]-rich anaerobic sediments. All methane-producing microbes are archaea. Archaea are also found in soils and grow on and in humans, producing methane in the gut. Archaea are not known to be human pathogens or to produce substances that are toxic to humans. Why some bacteria but no archaea are pathogenic is not yet understood.
2.1 The Ecology of Life: Liquid Water
In addition to describing the building blocks of life, it is instructive to consider what life does. In this regard it is possible to define a set of ecological or functional requirements for life. There are four fundamental requirements for life on Earth: energy, carbon, liquid water, and a few other elements. These are listed in Table 1 along with the occurrence of these environmental factors in the Solar System.
TABLE 1 Ecological Requirements for Life
TABLE 1 Ecological Requirements for Life
Occurrence in the Solar System
Photosynthesis at 100 AU light levels e.g.,
H[sub.2] + CO[sub.2] - CH[sub.4] + H[sub.2]O
Common as CO[sub.2] and CH[sub.4]
Rare, only on Earth for certain
N, P, S and other elements
Energy is required for life from basic thermodynamic considerations. Typically on the Earth this energy is provided by sunlight, which is a thermodynamically efficient (low entropy) energy source. Some limited systems on Earth are capable of deriving their energy from chemical reactions (e.g., methanogenesis, CO[sub.2] + 4H[sub.2] - CH[sub.4] + 2H[sub.2]O) and do not depend on photosynthesis. On Earth these systems are confined to locations where the more typical photosynthetic organisms are not able to grow, and it is not known if an ecosystem that was planetary in scale or survived over billions of years could be based solely on chemical energy. There are no known organisms on Earth that make use of temperature gradients to derive energy. These organisms would be analogous to a Carnot heat engine. Table 2 lists some of the most important metabolic reactions by which living systems generate energy. This list includes autotrophs (which derive energy from nonbiological sources) as well as heterotrophs (which derive energy by the consumption of organic material, usually other life forms).
Elemental material is required for life, and on Earth carbon has the dominant role as the backbone molecule of biochemistry. Life almost certainly requires other elements as well. Life on Earth utilizes a vast array of the elements available on the surface. However, this does not prove that these elements are absolute requirements for life. Other than H[sub.2]O and C, the elements N, S, and P are probably the leading candidates for the status of required elements. Table 3 lists the distribution of elements in the cosmos and on the Earth and compares these with the common elements in life.
As indicated in Table 1, sunlight and the elements required for life are common in the Solar System. What appears to be the ecologically limiting factor for life in the Solar System is the stability of liquid water. Liquid water is a necessary requirement for life on Earth. Liquid water is key
TABLE 2 Examples of Metabolic Pathways
to biochemistry because it acts as the solvent in which biochemical reactions take place and, furthermore, it interacts with many biochemicals in ways that influence their properties. For example, water forms hydrogen bonds with some parts of a large molecule, the hydrophilic groups, and repels other parts, the hydrophobic groups, thereby forcing these molecules to curl up with their hydrophobic groups in the interior and the hydrophilic groups on the exterior in contact with the water. Certain organisms, notably lichen and some algae, are able to use water in the vapor phase if the relative humidity is high enough. Many organisms can continue to metabolize at temperatures well below the freezing point of pure water because their intracellular material
TABLE 3 Elemental Abundances by Mass
contains salts and other solutes that lower the freezing point of the solution. No microorganism currently known is able to obtain water directly from ice. Many organisms, such as the snow algae Chlamydomonas nivalis thrive in liquid water associated with ice but in these circumstances the organisms are the beneficiaries of external processes that melt the ice. There is no known occurrence of an organism using metabolic methods to overcome the latent heat of fusion of ice thereby liquefying it.
TABLE 2 Examples of Metabolic Pathways
C[sub.6]H[sub.12]O[sub.6] - 2CO[sub.2] + 2C[sub.2]H[sub.5]OH
2. Anaerobic Respiration
C[sub.6]H[sub.12]O[sub.6] + 12NO[sub.3] [sup.[?]] - 6CO[sub.2] + 6H[sub.2]O + 12NO[sub.2] [sup.[?]]
3. Aerobic Respiration
C[sub.6]H[sub.12]O[sub.6] + 6O[sub.2] - 6CO[sub.2] + 6H[sub.2]O
1. Anoxic photosynthesis
12CO[sub.2] + 12H[sub.2]S + hn - 2C[sub.6]H[sub.12]O[sub.6] + 9S + 3SO[sub.4]
2. Oxygenic photosynthesis
6CO[sub.2] + 6H[sub.2]O + hn - C[sub.6]H[sub.12]O[sub.6] + 3O[sub.2]
CO[sub.2] + 4H[sub.2] - CH[sub.4] + 2H[sub.2]O
CO + 3H[sub.2] - CH[sub.4] + H[sub.2]O
4CO + 2H[sub.2]O - CH[sub.4] + 3CO[sub.2]
2CO[sub.2] + 4H[sub.2] - CH[sub.3]COOH + 2H[sub.2]O
3. Sulphate Reducers
H[sub.2]SO[sub.4] + 4H[sub.2] - H[sub.2]S + 4H[sub.2]O
4. Sulfur Reducers
S + H[sub.2] - H[sub.2]S
5. Thionic Denitrifiers
H[sub.2]S + 2NO[sub.3] [sup.[?]] - SO[sub.4] [sup.2[?]] + H[sub.2]O + N[sub.2]O
3S + 4NO[sub.3] [sup.[?]] + H[sub.2] - 3SO[sub.4] [sup.2[?]] + 2N[sub.2] + 2H[sup.+]
6. Iron Reducers
2Fe[sup.3+] + H[sub.2] - 2Fe[sup.2+] + 2H[sup.+]
1. Sulfide Oxidizers
2H[sub.2]S + 3O[sub.2] - 2SO[sub.4]S + 2H[sub.2]O
2. Iron Oxidizers
4FeO + O[sub.2] - 2Fe[sub.2]O[sub.3]
TABLE 3 Elemental Abundances by Mass
Because liquid water is universally required for known life and because it appears to be rare in the Solar System, the search for life beyond the Earth begins first with the search for liquid water.
2.2 Generalized Theories for Life
There have been many attempts at a definition of life, and perhaps such a definition would aid in our investigation for life on other planets and help unravel the origins of life on Earth. However, it is probable that there will never be a simple definition of life and it may not be necessary in a search for life on other worlds. Despite the fundamental unity of biochemistry and the universality of the genetic code, no single definition has proven adequate in describing the single example of life on Earth. Many of the attributes that we would associate with life--for example, self-replication, self-ordering, response to environmental stimuli, can be found in nonliving systems--fire, crystals, and bimetallic thermostats, respectively. Furthermore, various and peculiar life forms such as viruses and giant cell-less slime molds defy even a biological definition of life in terms of the cell or the separation of internal and external environments. In attempting a resolution of this problem, the most useful definition of life is a system that develops Darwinian evolution: reproduction, mutation, and selection (Table 4). This is an answer to the question what does life do?
We are able to answer the questions, what does life need? and what does life do?, even if we do not have a closed form compact definition of life. Thus, the requirements for life listed in Table 1 and the functions of life listed in Table 4 are very general; it is probably unwise to apply more restrictive criteria. For example, for evolution to occur some sort of information storage mechanism is required. However, it is not certain that this information mechanism needs to be a DNA/RNA-based system or even that it be expressed in structures dedicated solely for replication. While on the present Earth, all life uses dedicated DNA and RNA systems for genetic coding, there is evidence that at one time genetic and structural coding were combined into one molecule, RNA. In this so-called RNA world there would have been no distinction between genotype (genetic) and phenotype (structural) molecular replicating systems--both of these processes would have been performed by an RNA-replicating molecule. In present biology, the phenotype is composed of proteins for the most part. This example illustrates the difficulty in determining which aspects of biochemistry are fundamental and which are the result of the peculiarities of life's history on Earth.
In basing our consideration of life on the distribution we observe here on Earth as a general phenomenon, we suffer simultaneously from the problem that there is only one kind of life on this planet while the variety of that life is too complex to allow for precise definitions or characterizations. We can neither extrapolate nor be specific in our theories for life.
TABLE 4 Properties of Life
Some scientists have suggested that living systems elsewhere in the universe may exhibit vast differences from terrestrial biology and have proposed a variety of alternative life forms. One postulated alternative life form is based on the substitution of ammonia for water. Certainly ammonia is an excellent solvent--in some respects better than water. The range of temperatures over which ammonia is liquid is prevalent in the universe (melting point: [?]78deg, normal boiling point [?]33deg, liquid at room temperature when mixed with water) and the elements that compose it are abundant in the cosmos. Other scientists have suggested the possibility that silicon may be used as a substitute for carbon in alien life forms. However, silicon does not form polymeric chains either as readily or as long as carbon does and its bonds with oxygen (SiO[sub.2]) are much stronger than carbon bonds (CO[sub.2]) rendering its oxide essentially inert.
Although speculations of alien life capable of using silicon in place of carbon or ammonia in place of water are intriguing, no specific experiments directed toward alternate biochemistries have been designed. Thus we have no strategies for where or how to search for such alternate life or its fossils. More significantly, these speculations have not contributed to our understanding of life. One can only conclude that our unique understanding of terrestrial life is based on Earth systems, and wide-ranging speculations regarding alternate chemistries are currently too limited to be fruitful. Perhaps some day we will develop general theories for life or, more likely, have many sources of life to compare thereby allowing for complete theories. Basing our theories on Earth-like life should be considered a necessary first approach and not a fundamental limitation.
3. The History of Life on Earth
Several sources of information about the origin of life on Earth include the physical record, the genetic record, the metabolic record, and laboratory simulations. The physical record includes the collection of sedimentary and fossil evidence of life. This record is augmented by theoretical models of the Earth and the Solar System, all of which provide clues to conditions billions of years ago when the origin of life is thought to have occurred. There is also the record stored in the genomes of living systems that comprise the collective gene pool of our planet. Genetic information tells us the path of evolution as shaped by environmental pressures, biological constraints, and random events that connect the earliest genomic organism, the last universal common ancestor, and the present tree of life (see Fig. 3). There is also the record of metabolic pathways in the biochemistry of organisms that have evolved in response to changes in the environment while simultaneously causing changes to that environment. All of these records are palimpsests in that they have been overwritten--often repeatedly--over time. Laboratory simulations of prebiotic chemistry--the chemistry assumed present before life--can provide clues to the conditions and chemical solutions leading up to the origin of life. Experiments of DNA/RNA replication sequences can provide clues to the selection process that optimize mutations as well as provide a basic understanding of reproduction. Perhaps one day the process that initiates life will be studied in the laboratory or discovered on another planet.
The major events in the history of life are shown in Figure 4. As the Earth was forming about 4.5 Gyr ago, its surface would have been inhospitable to life. The gravitational energy released by the formation of the planet would have kept surface temperatures too high for liquid water to exist.
FIGURE 4 Major events in the history of the Earth and Mars. The period of moist surface conditions on Mars may have corresponded to the time during which life originated on Earth. The similarities between the two planets at this time raise the possibility of the origin of life on Mars.
Eventually, as the heat flow subsided, rain would have fallen for the first time and life could be sustained in liquid water. However, it is possible that subsequent impacts could have been large enough to sterilize the Earth by melting, excavating, and vaporizing the planetary surface, removing all liquid water. Thus, life may have been frustrated in its early starts. Following a sufficiently large impact, the entire upper crust of the Earth would be ejected into outer space and any remnant left as a magma ocean. Barring these catastrophic events, however, sterilizing the Earth is a difficult task because it is not sufficient to merely heat the surface to high temperatures. At present, microorganisms survive at the bottom of the ocean and even kilometers below the surface of the planet. An Earth-sterilizing impact must not only completely evaporate the oceans but must then heat the surface and subsurface of the Earth such that the temperature does not fall anywhere below about 200degC, which is the temperature required for heat sterilization of dry, dormant organisms. This is a difficult requirement because the time it takes heat to diffuse down a given distance scales as the square of the distance. Thus, heat must be applied a million times longer to sterilize to a depth of 1 km compared to a depth of 1 m.
It is not known when the last life-threatening impact occurred on Earth. As shown schematically in Figure 4, the rate of impact, extrapolated from the record on the Moon, rises steeply before 3.8 Gyr ago. It is therefore likely that the Earth was not continuously suitable for life much before 3.8 Gyr ago. There is persuasive evidence, including microbial fossils and stromatolites, that microbial life was present on the Earth as early as 3.4 Gyr ago. Stromatolites are large features--often many meters in size--that can be formed by the lithification of laminated microbial mats, (Fig. 5) although physical processes can result in similar forms. Phototactic microorganisms living on the bottom of a shallow lake or ocean shore may be periodically covered with sediment carried in by spring runoff, for example. To retain access to sunlight, the organism must move up through this sediment layer and establish a new microbial zone. After repeated cycles, a layered series of mats are formed by lamination of the sediments containing the organic material. One characteristic of these biogenic mats that distinguishes them from nonbiologically caused layering is that the response is phototactic, not gravitational, so that the layered structure is not usually flat but is more often dome-shaped because covered microorganisms in a lower layer on the periphery of the structure would move more toward the side to reach light. In this way, stromatolites can be distinguished sometimes from similar but nonbiological laminae. Often stromatolites contain microfossils--further testimony to their biological origin.
Microbial life, which is possibly capable of photosynthesis and mobility, appears to have originated early in the history of the Earth, possibly before the end of the late bombardment 3.8 Gyr ago and almost certainly not later than 3.4 Gyr ago. This suggests that the time required for the onset of life was brief. If the Greenland sediments are taken as evidence for life, it suggests that, within the resolution of the geological record, life arose on Earth as soon as a suitable habitat was provided. The microbial mats at 3.4 Gyr ago put an upper limit of 400 million years on the length of time it took for life to arise after clement conditions were present.
In principle, tt is possible to determine which organism on the Earth is the most similar to the last universal common ancestor. To do so, we must determine which organism has changed the least compared to all other organisms. For example, if some taxon of organism contains a certain mutation, but many do not, we can trace the mutation to an ancestor common to all organisms in that taxon. Within this related group of organisms, the most primitive
FIGURE 5 A stromatolite formed by cyanobacteria over 1 billion years ago from the Crystal Springs formation, Inyo County, California. Stromatolites are an important form of fossil evidence of life because they form macroscopic structures that could be found on Mars. It is therefore possible that a search for stromatolites near the shores of an ancient Martian lake or bay could be conducted in the near future. Expecting microbial communities to have formed stromatolites on Mars is not entirely misplaced geocentricism. The properties of a microbial mat community that results in stromatolite formation need only be those associated with photosynthetic uptake of CO[sub.2]. There are broad ecological properties that we expect to hold on Mars even if the details of the biochemistry and community structure of Martian microbial mats were quite alien compared to their terrestrial counterparts. Within stromatolites, trace microfossils can sometimes be found.
traits can be established based on how widespread they are. Traits that are found in all or most of the major groupings should be primitive, particularly if these traits are found in groups that diverged early. Traits found in only a few recently related groups are probably younger traits. This line of reasoning applied to the entire phylogenetic tree would indicate which organism extant today has the most primitive set of traits. This organism would therefore be most similar to the common ancestor. Studies of this type have indicated that the organisms alive today that are most similar, genetically and hence presumably ecologically, to the common ancestor are the thermophilic hydrogen metabolizing bacteria and perhaps the sulfur metabolizing bacteria. The arrow in Figure 3 represents the suggested position of the last common ancestor.
It is important to note here that the last universal common ancestor is not necessarily representative of the first organisms on Earth but was merely the last organism (or group of organisms) from which all life forms today are known to have descended. The common ancestor may have existed within a world of multiple lineages, none of which are in evidence today. If all life on Earth has indeed descended from a sulfur bacterium living in a hot springs environment, this could be the result of at least three possibilities. First it may be the case that hot sulfurous environments are important in the origin of life and the common ancestor may represent this primal cell. Second, the common ancestor may have been a survivor of a catastrophe that destroyed all other life forms. The survival of the common ancestor may have been the result of its ability to live deep within a hydrothermal system. Third, the nature of the common ancestor may be serendipitous with no implications as to origin or evolution of the biosphere.
For over 2 Gyr after the earliest evidence for life, life on the Earth was composed of only microorganisms. There were certainly bacteria and possibly one-celled eukaryotes as well. There seemed to be a major change in the environment of the Earth with the rise of photosynthetically produced oxygen beginning at about 2.5 Gyr ago, reaching significant levels about 1 Gyr ago and culminating about 600 Myr ago. (Figure 4 shows a timeline of Earth's history with these events.) Soon after the development of high levels of oxygen in the atmosphere, multicellular life forms appeared. These rapidly radiated into the major phylum known today (as well as many that have no known living representatives). In time, organisms adapted to land environments in addition to aqueous environments, and plants and animals appeared.
4. The Origin of Life
Numerous and diverse theories for the origin of life are currently under serious consideration within the scientific community. A diagram and classification of current theories for the origin of life on Earth is shown in Figure 6. At the most fundamental level, theories may be characterized within two broad categories: theories that suggest that life originated on Earth (Terrestrial in Fig. 6) and those that suggest that the origin took place elsewhere (Extraterrestrial in Fig. 6). The extraterrestrial or panspermia theories suggest that life existed in outer space and was transported by meteorites, asteroids, or comets to a receptive Earth. In this case, the origin of life is not related to environments possible on the early Earth. Along similar lines, life may have been ejected by impacts from another planet in the
FIGURE 6 Diagrammatic representation and classification of current theories for the origin of life.
Solar System and jettisoned to Earth, or visa versa. Furthermore, it has been suggested in the scientific literature that life may have been purposely directed to Earth (directed panspermia in Fig. 6) by an intelligent species from another planet.
The terrestrial theories are further subdivided into organic origins (carbon-based) and inorganic origins (mineral-based). Mineral-based theories suggest that life's first components were mineral substrates that organized and synthesized clay organisms. These organisms have evolved via natural selection into the organic based life forms visible on Earth today. The majority of theories that do not invoke an extraterrestrial origin require an organic origin for life on Earth. Theories postulating an organic origin suggest that the initial life forms were composed of the same basic building blocks present in biochemistry today, organic material. If life arose in organic form, then there must have been a prebiological source of organics. The Miller-Urey experiments and their successors have demonstrated how organic material may have been produced naturally in the primordial environment of Earth (endogenous production in Fig. 6). An alternative to the endogenous production of organics on early Earth is the importation of organic material by celestial impacts and debris--comets, meteorites, interstellar dust particles, and comet dust particles. A comparison of these sources is shown in Table 5. Table 6 lists the organics found in the Murchison meteorite and compares these with the organics produced in a Miller-Urey abiotic synthesis. Organic origins differ mainly in the type of primal energy sources: photosynthetic, chemosynthetic, or heterotrophic. The phototrophs and chemotrophs (collectively called autotrophs) use energy sources that are inorganic (sunlight and chemical energy respectively), whereas heterotrophs acquire their energy by consuming organics (see Table 2).
TABLE 5 Sources of Prebiotic Organics on Early Earth
TABLE 5 Sources of Prebiotic Organics on Early Earth
Energy Dissipalion (J yr[sup.[?]1])
Organic Production (in a reducing atmosphere) (kg yr[sup.[?]1])
1 x 10[sup.18]
3 x 10[sup.9]
5 x 10[sup.17]
2 x 10[sup.8]
Ultraviolet light (l < 270 nm)
1 x 10[sup.22]
2 x 10[sup.11]
Ultraviolet light (l < 200 nm)
6 x 10[sup.20]
3 x 10[sup.9]
Meteor entry shocks
1 x 10[sup.17]
1 x 10[sup.9]
Meteor post-impact plumes
1 x 10[sup.20]
2 x 10[sup.10]
6 x 10[sup.7]
Hydrothermal vent environments have been suggested for the subsurface origin of chemotrophic life. In the absence of sunlight, these organisms must utilize chemical energy (e.g., CO[sub.2] + 4H[sub.2] - CH[sub.4] + 2H[sub.2]O + energy). Alternatively, phototrophic life utilizes solar radiation from the surface for prebiotic synthesis. These organisms with the ability to chemosynthesize and photosynthesize can assimilate their own energy from materials in their environment. One feature that the various theories for the origin of life have in common is the requirement for liquid water because the chemistry of even the earliest life requires a liquid water medium. This is true if the primal organism appears fully developed (panspermia), if it engages in organic chemistry, and for the clay inorganic theories.
TABLE 6 Comparison of the Amino Acids in Murchison Meteorite and in an Electric Discharge Synthesis, Normalized to Glycine
TABLE 6 Comparison of the Amino Acids in Murchison Meteorite and in an Electric Discharge Synthesis, Normalized to Glycine
For many years the standard theory for the origin of life posited a terrestrial organic origin requiring endogenous production of organics leading to the development of heterotrophic organisms, generally known as the primordial "soup" theory. Recently there has been serious consideration for the chemotrophic origin of life, and at the present time the scientific community is split between these two views.
5. Limits to Life
In considering the existence of life beyond the Earth, it is useful to quantitatively determine the limits that life has
TABLE 7 Limits to Life
been able to reach on this planet with respect to environmental conditions. Life does not exist everywhere on Earth. There are environments on Earth in which life has not been able to effectively colonize even though these environments could be suitable for life. Perhaps the largest life-free zone on Earth is in the polar ice sheets, where there is abundant energy, carbon, and nutrients (from atmospheric deposition) to support life. However, water is available only in the solid form. No organism on Earth has adapted to using metabolic energy to liberate water from ice, even though the energy required per molecule is only [?]1% of the energy produced by photosynthesis per molecule. Table 7 lists the limits to life as we currently know them. The lower temperature limit clearly ties to the presence of liquid water, while the higher temperature limit seems to be determined by the stability of proteins, also in liquid water. Life can survive at extremely low light levels corresponding to 100 AU, roughly three times the distance between Pluto and the Sun. Salinity and pH also allow for a wide range. Water activity, effectively a measure of the relative humidity of a solution or vapor, can support life only for values above 0.6 for yeasts, lichens and molds. Bacteria require levels above 0.8. Radiation resistant organisms such as Deinococcus radiodurans can easily survive radiation doses of 1-2 Mrad and higher when in a dehydrated or frozen state.
TABLE 7 Limits to Life
Thermal denaturing of proteins
Algae under ice and deep sea
Depends on the salt
Yeasts and molds
May be higher for dry or frozen state
6. Life in the Solar System
Because our knowledge of life is restricted to the unique but varied case found here on Earth, the most practical approach to the search for life on the other planets has been to proceed by way of analogy with life on Earth. The argument for the origin of life on another world is then based on the similarity of other planetary environments with the postulated environments on early Earth. Whatever process led to the establishment of life in one of these environments on Earth could then be logically expected to have led to the origin of life on this comparable world. The more exact the comparison between the early Earth and another planet, the more compelling is the argument by analogy. This comparative process should be valid for all the theories for the origin of life, ranging from panspermia to the standard theory, listed in Figure 6.
Following this line of reasoning further, we can conclude that if similar environments existed on two worlds and life arose in both of them then these life forms should be comparable in their broad ecological characteristics. If sunlight was the available energy source, CO[sub.2] the available carbon source, and liquid water the solvent, then one could expect phototrophic autotrophs using sunlight to fix carbon dioxide with water as the medium for chemical reactions. Our knowledge of the Solar System suggests that such an environment could have existed on Mars early in its history as well as on Earth early in its history. While life forms independently originating on these two planets would have different biochemical details, they would be recognizably similar in many fundamental attributes. This approach by analogy to Earth life and the early Earth provides a specific search strategy for life elsewhere in the Solar System. The key element of that strategy is the search for liquid water habitats.
Spacecraft have now visited or flown past comets, asteroids, and most of the large worlds in the Solar System except Pluto; however, a spacecraft is en route to Pluto at the time of this writing. Observatory missions have studied all of the major objects in the Solar System as well. We can do a preliminary assessment of the occurrence of liquid water habitats, and indirectly life, in the Solar System.
6.1 Mercury and the Moon
Mercury and the Moon appear to have few prospects for liquid water, now or anytime in the past. These virtually airless worlds have negligible amounts of the volatiles (such as water and carbon dioxide) essential for life. There are no geomorphological features that indicate fluid flow. There is speculation that permanently shaded regions of the polar areas on Mercury can act as traps for water ice. Recent radar data support this hypothesis. However, there is no indication that the pressure and temperature were ever high enough for liquid water to exist at the surface. [See MERCURY.]
Venus currently has a surface that is clearly inhospitable to life. There is no liquid water on the surface, and the temperature is over 450degC at an atmospheric pressure of 92 times the Earth's. There is water on Venus but only in the form of vapor and clouds in the atmosphere. The most habitable zone on Venus is at the level in the atmosphere where the pressure is about half of the sea level on Earth. At that location, there are clouds composed of about 25% water and 75% sulfuric acid at a temperature of about 25degC; these might be reasonable conditions for life. It is possible therefore to speculate that life can be found, or survive if implanted, in the clouds of Venus. What argues against this possibility is the fact that clouds on Earth that are at similar pressures and temperatures do not harbor life. We do not know of any life forms that thrive in cloud environments. Perhaps the essential elements are there but a stable environment is required. [See VENUS: ATMOSPHERE]
Theoretical considerations suggest that Venus and Earth may have initially had comparable levels of water. In this case Venus may have had a liquid water surface early in its history when it was cooler 4 billion years ago because of the reduced brightness of the fainter early sun. Unfortunately, all record of this early epoch has been erased on Venus and the question of the origin of life during such a liquid water period remains untestable. [See VENUS: SURFACE AND INTERIOR]
Of all the extraterrestrial planets and smaller objects in the Solar System, Mars is the one that has held the most fascination in terms of the existence of life. Early telescopic observations revealed Earth-like seasonal patterns on Mars. Large white polar caps that grew in the winter and shrunk in the summer were clearly visible. Regions of the planet's surface near the polar caps appeared to darken beginning at the start of each polar cap's respective spring season and then spread toward the equator. It was natural that these changes, similar to patterns on the Earth, would be attributed to like causes. Hence, the polar caps were thought to be water ice and the wave of darkening was believed to have been caused by the growth of vegetation. The 19th century arguments for the existence of life, and even intelligent life, on Mars culminated in the book Mars as the Abode of Life by Percival Lowell in 1908 and in the investigations of the celebrated canals. The Mars revealed by spacecraft exploration is decidedly less alive than Lowell anticipated but its standing as the most interesting object for biology outside Earth still remains.
6.3.1 THE VIKING RESULTS
In 1976 the Viking landers successfully reached the Martian surface while the two orbiters circled the planet repeatedly
FIGURE 7 Schematic diagram of the Viking biology experiments.
photographing and monitoring the surface. The primary objective of the Viking mission was the search for microbial life. Previous reconnaissance of Mars by the Mariner flyby spacecraft and the photographs returned from the Mariner 9 orbiter had already indicated that Mars was a cold dry world with a thin atmosphere. There were intriguing features indicative of past fluvial erosion but there was no evidence for current liquid water. It was thought that any life to be found on Mars would be microbial. The Viking biology package consisted of three experiments shown schematically in Figure 7.
The Pyrolytic Release (PR) experiment searched for evidence of photosynthesis as a sign of life. The PR was designed to see if Martian microorganisms could incorporate CO[sub.2] under illumination. The experiment could be performed under dry conditions similar to those on the Martian surface or it could be run in a humidified mode. The CO[sub.2] in the chamber was labeled with radioactive carbon which could then be detected in any organic material synthesized during the experiment. The very first run of the pyrolytic release experiment produced a significant response. It was well below the typical response observed when biotic soils from Earth had been tested in the experiment, but it was much larger than the noise level. Subsequent trials did not reproduce this high result, and this initial response was attributed to a startup anomaly, possibly some small prelaunch contamination.
The Gas Exchange (GEx) experiment searched for heterotrophs, which are microorganisms capable of consuming organic material. The GEx was designed to detect any gases that the organisms released as a byproduct of their metabolism, bacterial flatulence. After a sample was placed in the chamber, the soil was first equilibrated with water vapor and then combined with a nutrient solution. At
TABLE 8 A Comparison of GEx O[sub.2] and LR [sup.14]C Results[sup.a ]
prescribed intervals, a sample of the gas above the sample was removed and analyzed by a gas chromatograph.
TABLE 8 A Comparison of GEx O[sub.2] and LR [sup.14]C Results[sup.a ]
GEx O2 (nmol cm[sup.[?]3])
Oxidant[sup.b ] (KO[sub.2] - O[sub.2])
LR CO[sub.2] (nmol cm[sup.[?]3])
Oxidant[sup.b ] (H[sub.2]O[sub.2] - O)
[sup.a ] After Klein 1979.
[sup.b ] Assuming a bulk soid density of 1.5 g cm[sup.[?]3].
Viking 1 (surface)
Viking 2 (surface)
Viking 2 (sub-rock)
The GEx results were startling. When the Martian soil was merely exposed to water vapor, it released oxygen gas at levels of 70-700 nanomoles per gram of soil, much larger than could be explained by the release of ambient atmospheric oxygen that had been absorbed onto the soil grains. The GEx results are summarized in Table 8. It was clear that some chemical or biological reaction was responsible for the oxygen release. A biological explanation was deemed unlikely since the reactivity of the soil persisted even after it had been heat sterilized to temperatures of over 160degC. Furthermore, adding the nutrient solution did not change the result that some chemical in the soil was highly reactive with water.
The Labeled Release (LR) experiment also searched for evidence of heterotrophic microorganisms. In the LR experiment, a solution of water containing seven organic compounds was added to the soil. The carbon atoms in each organic compound were radioactive. A radiation detector in the headspace detected the presence of radioactive CO[sub.2] released during the experiment. Any carbon metabolism in the soil would be detected as organisms consumed the organics and released radioactive CO[sub.2].
When the LR experiment was performed on Mars, there was a steady release of radioactive CO[sub.2] (Table 8). When the soil sample was heat sterilized before exposure to the nutrient solution, no radioactive CO[sub.2] was detected. The results of the LR experiment were precisely those expected if there were microorganisms in the soil sample. Taken alone, the LR results would have been a strong positive indication for life on Mars.
In addition to the three biology experiments, another instrument, a combination of a gas chromatograph and a mass spectrometer (GCMS), gave information pivotal to the interpretation of the biological results. This instrument received Martian soil samples from the same sampling arm that provided soil to the biology experiments. The sample was then heated to release any organics. The decomposed organics were carried through the gas chromatograph and identified by the mass spectrometer. The only signal was due to cleaning agents used on the spacecraft before launch. No Martian organics were detected. However, the samples were heated to only 500degC, and highly refractory organics would not have been volatilized at this temperature. In addition, it is now known that iron compounds in the soil could have interfered with the release of organics. The limit on the concentration of organics that would remain undetectable by the GCMS was one part per billion. A part per billion of organic material in a soil sample represents over a million individual bacterium, each the size of a typical Escherichia coli. This may not seem to rule out a biological explanation for the LR results. However, all life is composed of organic material and it is constantly exuded and processed in the biosphere. On Earth, it is difficult to imagine life without a concomitant matrix of organic material. This apparent absence of organic material is the main argument against a biological interpretation of the positive LR results.
The prevailing explanation for the reactivity of the Martian soil relies on the presence of reactive chemicals in the Martian atmosphere. In particular, hydrogen peroxide (H[sub.2]O[sub.2]) is assumed to be produced by ultraviolet light in the atmosphere and deposited onto the soil surface. Hydrogen peroxide itself could explain many of the LR results including the loss of reactivity with heating, but it cannot explain the thermally stable results of the GEX. However, peroxide, possibly abetted by ultraviolet radiation could somehow result in the production of the stable reactive chemicals responsible for the release of oxygen upon humidification and the breakdown of organics in the LR experiment. In addition, these reactive chemicals would have broken down any naturally occurring organic material or any material carried in by meteorites on the Martian surface. Table 8 also lists the concentration of oxidant necessary to explain the Viking results for typical models of the chemistry of the oxidants.
Amplifying the apparently negative results of the Viking biology experiments, the environment of Mars appears to be inhospitable to life. Although the atmosphere contains many of the elements necessary for life--it is composed of 95% CO[sub.2] with a few percent N[sub.2] and Argon and trace levels of water--the mean surface pressure is less than 1% of sea level pressure on the Earth, and the mean temperature is [?]60degC. The mean surface pressure is close to the triple point pressure of water, which is the minimum pressure at which a liquid state of water can exist. The low pressures and low temperatures make it unlikely that water will exist as a liquid on Mars. Because of seasonal transport, the available surface water on Mars is trapped as ice in the polar regions. In the locations at low elevation where the pressures and temperatures are sufficient to support liquid water, the surface is desiccated. Even saturated brine solutions cannot exist in equilibrium with the atmosphere near the equator. The absence of liquid water on the surface of Mars is probably the most serious argument against the presence of life anywhere on the surface of the planet. A second significant hazard to life on the Martian surface is the presence of solar ultraviolet light in the wavelengths between 190 and 300 nm. This radiation, which is largely shielded from Earth's surface by atmospheric oxygen and the ozone layer, is highly effective at destroying terrestrial organisms. Wavelengths below 190 nm are absorbed even by the present thin Martian CO[sub.2] atmosphere. Compounding the effects of UV irradiation, and perhaps caused by it, are possible chemical oxidants that are thought to exist in the Martian soil. Such strong oxidants have been suggested as the causative agent for the chemical reactivity observed at the Viking sites. [See MARS ATMOSPHERE: HISTORY AND SURFACE INTERACTION.]
6.3.2 EARLY MARS
There is considerable evidence that early in its history Mars did have liquid water on its surface. Images from the many orbiters show complex dendritic valley networks that are believed to have been carved by liquid water. These valleys are predominantly found in the heavily cratered, hence ancient, terrains in the southern hemisphere. This would suggest that the period of liquid water on Mars occurred contemporaneously with the end of the last stages of heavy cratering, about 3.8 Gyr ago, the same epoch at which life is thought to have originated on Earth (see Fig. 4). [See MARS, SURFACE AND INTERIOR.]
Figure 8 shows part of Nanedi Vallis on Mars. The canyon snakes back and forth, which is characteristic of liquid flow. On the floor of the canyon appears a small channel, which presumably was the flow of the river that carved the canyon. It would have taken considerable flow, although not necessarily continuous flow, for this river to have carved the much larger canyon. This image provides what is perhaps the best evidence from orbit that liquid water flowed on the surface of Mars in stable flow for long periods of time. Figure 9 shows evidence for liquid water form the surface rover missions. The "blueberries" seen at the Meridiani Site are interpreted as concretions formed in liquid water.
The presence of liquid water habitats on early Mars at approximately the time that life is first evident on Earth suggests that life may have originated on Mars during the same time period. Liquid water is the most critical environmental requirement for life on Earth and the general similarity between Earth and Mars leads us to assume that life on
FIGURE 8 Liquid water on another world. Mars Global Surveyor image showing Nanedi Vallis in the Xanthe Terra region of Mars. The image covers an area 9.8 km by 18.5 km; the canyon is about 2.5 km wide. This image is the best evidence we have of liquid water anywhere outside the Earth. (Photo from NASA/Malin Space Sciences).
Mars would be similar in this basic environmental requirement. More exotic approaches to life on Mars cannot be ruled out, nor are they supported by any available evidence.
It is interesting to consider how evolution may have progressed on Mars by comparison with the Earth. The history of Earth and Mars are compared in Figure 4, which shows that the period between 4.0 and 3.5 Gyr ago is the time when life is most likely to have evolved on both planets. On Earth, life persists and remains essentially unchanged
FIGURE 9 Blueberries. The triplet of connected spheres, dubbed blueberries, as seen in this MER Opportunity image, is a strong indication that they are concretions formed in the presence of water, not in volcanic eruptions or meteor impacts. Concretions are spherical mineral structures formed by groundwater percolating through porous rocks. On Earth, as concretions grow in close proximity to each other, their outer edges often intersect each other, producing connected spheres. (NASA/JPL)
for several billion years until the cumulative effects of O[sub.2] production induces profound changes on the atmosphere of that planet. On Mars, conditions become unsuitable for life (no liquid water) in a billion years or less. Thus, is it likely that if there were any life on early Mars it remained microbial.
The evidence of liquid water on early Mars, particularly that provided by the valley networks, suggests that the climate on early Mars may have been quite different than at present. It is generally thought that the surface temperature must have been close to freezing, much warmer than the present [?]60degC. These warmer temperatures are thought to have occurred as a result of a greatly enhanced greenhouse due to a thick ([?]1-5 atm) CO[sub.2] atmosphere. However, CO[sub.2] condensation may have limited the efficacy of the CO[sub.2] greenhouse but theoretical models indicate that CO[sub.2] clouds or CH[sub.4] could enhance the greenhouse and maintain warmer temperatures.
If Mars did have a thick CO[sub.2] atmosphere, this strengthens the comparison to the Earth, which is thought to have also had a thick CO[sub.2] atmosphere early in its history. The duration of a thick atmosphere on Mars and the concomitant warm, wet surface conditions are unknown but simple climate models suggest that significant liquid water habitats could have existed on Mars for [?]0.5 Gyr after the mean surface temperature reached freezing. This model is based on the presence of deep ice-covered lakes (over 30 m) such as those in the dry valleys of the Antarctic where mean annual temperatures are [?]20degC.
If we divide the possible scenario for the history of water on the surface of Mars into four epochs, the first epoch would have warm surface conditions and liquid water. As Mars gradually loses its thick CO[sub.2] atmosphere, the second and third epochs would be characterized by low temperatures but still relatively high atmospheric pressures. This is because the temperature would drop rapidly as the pressure decreased. During the second epoch, temperatures would rise above freezing during some of the year and liquid water habitats would require a perennial ice-cover. However, by epoch three the temperature would never rise above freezing and the only liquid water would be found in porous rocks with favorable exposures to sunlight. In epoch four the pressure would fall too low for the presence of liquid water.
A point worth emphasizing here is that the biological requirement is for liquid water per se. Current difficulties in understanding the composition and pressure of the atmosphere need not lessen the biological importance of the direct evidence for the presence of liquid water. In fact, as we observe in the Antarctic dry valleys, ecosystems can exist when the mean temperatures are well below freezing. Mars need not have ever been above freezing for life to persist.
The particular environment on the early Earth in which life originated is not known. However, this does not pose as serious a problem to the question of the origin of life on Mars as might be expected. The reason is that all of the environments found on the early Earth would be expected to be found on Mars, including hydrothermal sites, hot springs, lakes, oceans (that is planetary scale water reservoirs), volcanoes, tidal pools (solar tides only), marshes, salt flats, and others. Thus, whatever environment or combination of environments needed for life to get started on Earth should have been present on Mars as well, and at the same time.
Since the rationale for life on Mars early in its history is based on analogs with fossil evidence for life on the early Earth it is natural to look to the fossil record on Earth as a guide to how relics of early Martian life might be found. The most persuasive evidence for microbial life on the early Earth comes from stromatolites as discussed before. The resulting structures can be quite large: they are macroscopic fossils generated by microorganisms.
6.3.3 SUBSURFACE LIFE ON MARS
Although there is currently no direct evidence to support speculations about extant life on Mars, there are several interesting possibilities that cannot be ruled out at this time. Protected subsurface niches associated with hydrothermal activity could have continued to support life even after surface conditions became inhospitable. Liquid water could be provided by the heat of geothermal or volcanic activity melting permafrost or other subsurface water sources. Gases from volcanic activity deep in the planet could provide reducing power (as CH[sub.4], H[sub.2], or H[sub.2]S) percolating up from below and enabling the development of a microbial community based upon chemolithoautotrophy. An example is a methanogen (or acetogen) that uses H[sub.2] and CO[sub.2] in the production of CH[sub.4]. Such ecosystems have been found deep underground on the Earth consuming H[sub.2] produced by the reaction of water with basaltic rock, a plausible reaction for subsurface Mars. However, their existence is neither supported nor excluded by current observations of Mars. Tests for such a subsurface system involve locating active geothermal areas associated with ground ice or detecting trace quantities of reduced atmospheric gases that would leak from such a system. It is interesting to consider the recent reports of CH[sub.4] in the atmosphere of Mars at the tens of ppb level. If these reports are confirmed, it may be that this CH[sub.4] may be related to subsurface biological activity. However nonbiological sources of CH[sub.4] are also possible.
While it certainly seems clear that volcanic activity on Mars has diminished over geological time, intriguing evidence for recent (on the geological time scale) volcanic activity comes from the young crystallization ages (all less than 1 Gyr) of the Shergotty meteorite (and other similar meteorites thought to have come from Mars). Volcanic activity by itself does not provide a suitable habitat for life; liquid water that may be derived from the melting of ground ice is also required. Presumably, the volcanic source in the equatorial region would have depleted any initial reservoir of ground ice and there would be no mechanism for renewal, although there are indications of geologically recent volcano/ground ice interactions at equatorial regions. Closer to the poles, ground ice is stable. It is conceivable that a geothermal heat source could result in cycling of water through the frozen ice-rich surface layers. The heat source would melt and draw in water from any underlying reservoir of ground water or ice that might exist. [See METEORITES.]
Another line of reasoning also supports the possibility of subsurface liquid water. There are outflow channels on Mars that appear to be the result of the catastrophic discharge of subsurface aquifers of enormous sizes. There is evidence based on craters and stratigraphic relations that these have occurred throughout Martian history. If this is the case, then it is possible that intact aquifers remain. This would have profound implications for exobiology (as well as human exploration). Furthermore, it suggests that the debris field and out wash regions associated with the outflow channel may hold direct evidence that life existed within the subsurface aquifer just prior to its catastrophic release.
The collection of available water on Mars in the polar regions naturally suggests that summer warming at the edges of the permanent water ice cap may be a source of meltwater, even if short lived. In the polar regions of Earth, complex microbial ecosystems survive in transient summer meltwater. However, on Mars the temperature and pressures remain too low for liquid water to form. Any energy available is lost from the sublimation of the ice before any liquid is produced. It is unlikely that there are even seasonal habitats at the edge of the polar caps. This situation may be different over longer timescales. Changes in the obliquity axis of Mars can significantly increase the amount of insolation reaching the polar caps in summer. If the obliquity increases to over about 50deg, then the increased temperatures, atmospheric pressures, and polar insolation that result may cause summer liquid water meltstreams and ponds at the edge of the polar cap.
The polar regions may harbor remnants of life in another way. Tens of meters beneath the surface, the temperature is well below freezing (<[?]70degC) and does not change from summer to winter. These permafrost zones likely have remained frozen, particularly in the southern hemisphere, since the end of the intense crater formation period. In this case, there may be microorganisms frozen within the permafrost that date back to the time when liquid water was common on Mars, over 3.5 Gyr ago. On Earth permafrost of such age does not exist, but there are sediments in the polar regions that have been frozen for many millions of years. When these sediments are exhumed and samples extracted using sterile techniques, viable bacteria are recovered. The sediments on Mars have been frozen much longer (1000 times) but the temperatures are also much colder; it may be possible that intact microorganisms could be recovered from the Martian permafrost. Natural radiation from U, Th, and K in the soil would be expected to have killed any organisms but their biochemical remains would be available for study. The southern polar region seems like the best site for searching for evidence of ancient microorganisms since the terrain there can be dated to the earliest period of Martian history as determined by the number of observed craters.
6.3.4 METEORITES FROM MARS
Of the thousands of meteorites known, there are over 30 that are thought to have come from Mars. It is certain that these meteorites came from a single source because they all have similar ratios of the oxygen isotopes--values distinct from terrestrial, lunar, or asteroidal ratios. These meteorites can be grouped into four classes. Three of these classes contain all but one of the known Mars meteorites and are known by the name of the type specimen; the S (Shergotty), N (Nakhla), and C (Chassigny) class meteorites. The S, N, and C meteorites are relatively young, having crystallized from lava flows between 200 and 1300 million years ago (see Fig. 4). Gas inclusions in two of the S type meteorites contain gases similar to the present Martian atmosphere as measured by the Viking landers, proving that this meteorite, and by inference the others as well, came from Mars. The fourth class of Martian meteorite is represented by the single specimen known as ALH84001. Studies of this meteorite indicate that it formed on Mars about 4.5 Gyr ago in warm, reducing conditions. There are even indications that it contains Martian organic material and appears to have experienced aqueous alteration after formation. This rock formed during the time period when Mars is thought to have had a warm, wet climate capable of supporting life.
It has been suggested that ALH84001 contains evidence for life on Mars based on four observations. (1) Polycyclic aromatic hydrocarbons similar to molecules found in interstellar space are present inside ALH84001. (2) Carbonate globules are found in the meteorite that are enriched in [sup.12]C over [sup.13]C. The isotopic shift is within the range that on Earth, indicates organic matter derived from biogenic activity. (3) Magnetite and iron-sulfide particles are present that are similar to those produced by microbial activity. (4) Features are seen that could be fossils of microbial life, except that they are much smaller than any bacteria on Earth. As a result of more than a decade of study, most scientists currently prefer a nonbiological explanation for all of these results. Only the magnetite result is generally considered relevant, although not conclusive, evidence related to life.
ALH84001 does not provide convincing evidence of past life on Mars when compared to the multiple lines of evidence for life on Earth 3.4 Gyr ago including fossil evidence. However, the ALH84001 results do provide strong support to the suggestion that conditions suitable for life were present on Mars early in its history. When compared to the SNC meteorites, ALH84001 indicates that Mars experienced a transition from a warm reducing environment with organic material present to a cold oxidizing environment in which organic material was unstable.
6.4 The Giant Planets
The "habitable zone" in the inner Solar System provides the temperature conditions which can support liquid water on a planetary surface, but the outer Solar System is richer in the organic material from which life is made. This comparison is shown in Figure 10, which shows the ratio of carbon to heavy elements (all elements other than H and He) for various objects in the Solar System. Earth is in fact depleted in carbon with respect to the average Solar System value by a factor of about 10[sup.4]. It may be interesting then to consider life in the organic rich outer Solar System.
The giant planets Jupiter, Saturn, Uranus, and Neptune, do not have firm surfaces on which water could accumulate and form a reservoir for life. Here the only clement zone would be that region of the clouds in which temperatures were in the range suitable for life. Cloud droplets would provide the only source of liquid water. Such an environment might provide the key elements needed for life as well as an energy source in the form of sunlight. [See ATMOSPHERES OF THE GIANT PLANETS.]
FIGURE 10 Ratio of carbon atoms to total heavy atoms (heavier than He) for various Solar System objects illustrating the depletion of carbon in the inner Solar System. The x-axis is not a true distance scale but the objects are ordered by increasing distance from the sun. Mars is not shown since the size of its carbon reservoir is unknown.
There have been speculations that life, including advanced multicellular creatures, could exist in such an environment. However, such speculations are not supported by considerations of the biological state of clouds on Earth. There are no organisms that have adapted themselves to live exclusively in clouds on Earth even in locations where clouds are virtually always present. This niche remains unfilled on Earth and by analogy is probably unfilled elsewhere in the Solar System.
Following this line of thought leads us to search for environments suitable for life on planetary bodies with surfaces. In the outer Solar System, this focuses us on the moons of the giant planets. Of particular interest are Europa, Titan, and Enceladus.
Europa, one of the moons of Jupiter, appears to be an airless ice-shrouded world. However, theoretical calculations suggest that under the ice surface of Europa there may be a layer of liquid water sustained by tidal heating as Europa orbits Jupiter. The Galileo spacecraft imaging showed features in the ice consistent with a subsurface ocean and the magnetometer indicated the presence of a global layer of slightly salty liquid water. The surface of Europa is crisscrossed by streaks that are slightly darker than the rest of the icy surface. If there is an ocean beneath a relatively thin ice layer, these streaks may represent cracks where the water has come to the surface. [See PLANETARY SATELLITES.]
There are many ecosystems on Earth that thrive and grow in water that is continuously covered by ice; these are found in both the Arctic and Antarctic regions. In addition to the polar oceans where sea ice diatoms perform photosynthesis under the ice cover, there are perennially ice-covered lakes in the Antarctic continent in which microbial mats based on photosynthesis are found in the water beneath a 4-m ice cover. The light penetrating these thick ice covers is minimal, about 1% of the incident light. Using these Earth-based systems as a guide, it is possible that sunlight penetrating through the cracks (the observed streaks) in the ice of Europa could support a transient photosynthetic community. Alternatively, if there are hydrothermal sites on the bottom of the Europan ocean, it may be possible that chemosynthetic life could survive there--by analogy to life at hydrothermal vent sites at the bottom of the Earth's oceans. The biochemistry of hydrothermal sites on Earth does depend on O[sub.2] produced at the Earth's surface. On Europa, a chemical scheme like that suggested for subsurface life on Mars would be appropriate (H[sub.2] + CO[sub.2]).
The main problem with life on Europa is the question of its origin. Lacking a complete theory for the origin of life, and lacking any laboratory synthesis of life, we must base our understanding of the origin of life on other planets on analogy with the Earth. It has been suggested that hydrothermal vents may have been the site for the origin of life on Earth and if this is the case improves the prospects for life in a putative ocean on Europa. However, the early Earth contained many environments other than hydrothermal vents, such as surface hot springs, volcanoes, lake and ocean shores, tidal pools, and salt flats. If any of these environments were the locale for the origin of the first life on Earth, the case for an origin on Europa is weakened considerably.
Titan, the largest moon of the planet Saturn, has a substantial atmosphere composed primarily of N[sub.2] and CH[sub.4] with many other organic molecules present. The temperature at the surface is close to 94degK and the surface pressure is 1.5 times the pressure of Earth at sea level. The surface does not appear to have expansive oceans as once suggested but numerous small lakes have been discovered in the north polar region. However, the ground beneath the Huygens Probe was wet with liquid CH[sub.4], which was heated by the problem and formed vapor. [See TITAN.]
The spacecraft data from the Voyager and Cassini/Huygens missions, as well as ground-based studies, indicate that there is an optically thick haze in the upper atmosphere. The haze is composed of organic material, and the atmosphere contains many organic molecules heavier than CH[sub.4]. Photochemical models suggest that these organics are produced from CH[sub.4] and N[sub.2] through chemical reactions driven by solar photons and by magnetospheric electrons. The observed organic species and even heavier organic molecules are predicted to result from these chemical transformations. Laboratory simulations of organic reactions in Titan-like gas mixtures produce solid refractory organic substances (tholin) and similar processes are expected to occur in Titan's atmosphere.
Conditions on Titan are much too cold for liquid water to exist, although the pressure is in an acceptable regime. For this reason, it is unlikely that Earth-like life could originate or survive there. The organic material in Titan's atmosphere provides a potential source of energy and the liquid methane on the surface provides a possible liquid medium for life. Life in liquid methane could use active transport and large size to overcome the low solubility of organics in liquid methane and enzymes to catalyze reactions at the low temperatures. If carbon-based life in liquid methane existed on Titan, it could be widespread. With or without life, Titan remains interesting because it is a naturally occurring Miller-Urey experiment in which simple compounds are transformed into more complex organics. A detailed study of this process may yield valuable insight into how such a mechanism might have operated on the early Earth.
There is also some speculation that under unusual conditions Titan may have liquid water on or near the surface. This could have occurred early in its formation when the gravitational energy released by the formation of Titan would have heated it to high temperatures. More recently, impacts could conceivably melt local regions generating warm subsurface temperatures that could last for thousands of years. Whether such brief episodes of liquid water could have led to water-based life remains to be tested.
The Cassini mission has recently documented geysers erupting from the south polar region of Enceladus. [See PLANETARY SATELLITES.] Associated with this outflow of water, CH[sub.4] is present but no NH[sub.3]. The source of the water is considered to be a subsurface liquid water reservoir heated and pressurized by subsurface heat flow. Such a subsurface habitat could support the sort of anaerobic chemoautotrophic life that has been found on Earth. These systems are based on methanogens that consume H[sub.2] produced by geochemical reactions or by radioactive decay. The age or lifetime of any subsurface liquid water on Enceladus is not known, which adds uncertainly to speculations about the origin of life. The theories for the origin of life on Earth, shown in Figure 6, that would apply to Enceladus are panspermia and a chemosynthetic origin of life. The same that would apply to Europa.
If there is subsurface life in the liquid water reservoirs on Enceladus, then the geysers would be carrying these organisms out into space. Here they would quickly become dormant in the cold vacuum of space and would then be killed by solar ultraviolet radiation. But these dead, frozen microbes would still retain the biochemical and genetic molecules of the living forms. Thus a Stardust-like mission moving through the plume of Enceladus' geysers might collect lifeforms for return to Earth, which might provide the easiest way to get a sample of a second genesis of life.
Asteroids seem unlikely locations for life to have originated. Certainly they are too small to support an atmosphere sufficient to allow for the presence of liquid water at the present time. However, asteroids, particularly the so-called carbonaceous type, are thought to contain organic material, thereby playing a role in the delivery of organics to the prebiotic Earth. A more intriguing aspect of some asteroids is the presence of hydrothermally altered materials, which seems to indicate that the asteroids were once part of a larger parent body. Furthermore, conditions on this larger parent body were such that liquid water was present, at least in thin films. Containing both organic material and liquid water, the parent bodies of these asteroids are interesting targets in the search for extraterrestrial lifeforms. However, a thorough assessment of this possibility will require a more detailed study of carbonaceous asteroids in the asteroid belt. Meteorites found on the Earth provide only a glimpse of small fragments of these objects and no signs of extraterrestrial life have been found. But the samples are small and the potential for contamination by Earth life is great.
Comets are also known to be rich in organic material. However, unlike asteroids, comets also contain a large fraction of water. In their typical state this water is frozen as ice, which is unsuitable for life processes. As a comet approaches the sun, its surface is warmed considerably, but this leads only to the sublimation of the water ice. Liquid does not form because the pressure at the surface of the comet is much too low.
It has been suggested that soon after their formation the interior of large comets would have been heated by short-lived radioactive elements ([sup.26]Al) to such an extent that the core would have melted. In this case, there would have been a subsurface liquid water environment similar to that postulated for the present day Europa. Again the question of the origin of life in such an environment rests on the assumption that life can originate in an isolated deep dark underwater setting.
7. How to Search for Life on Mars, Europa, or Enceladus
If we were to find organic material in the subsurface of Mars, or in the ice of Europa, or entrained in the geysers of Enceladus, how could we determine if it was the product of a system of biology or merely abiotic organic material from meteorites or photochemistry? If that life is related to Earth life, it should be easy to detect. We now have very sensitive methods, such as the amplification of DNA and fluorescent antibody markers, for detecting life from Earth. The case of Earth-like life is the easiest but it is also the least interesting. If the life is not Earth-like, then the probes specific to our biology are unlikely to work. We need a general way to determine a biological origin. The question is open and possibly urgent. As we plan missions to Mars and Europa, we may have the opportunity to analyze the remains of alien biology.
One practical approach makes use of the distinction between biochemicals and organic matter that is not dependent on a particular organic molecule but results from considering the pattern of the organics in a sample. Abiotic processes will generate a smooth distribution in molecular types without sharp distinctions between similar molecules, isotopes, or chemical chirality. If we consider a generalized phase space of all possible organic molecules, then for an abiotic production mechanism the relative concentration of different types will be a smooth function. In contrast to abiotic mechanisms, biological production will not involve a wide range of possible types. Instead, biology will select a few types of molecules and build biochemistry up from this restricted set. Thus organic molecules that are chemically very similar may have widely different concentrations in a sample of biological organics. An example of this on Earth is the 20 amino acids used in proteins and the selection of life for the left-handed version of these amino acids. To maximize efficiency, life everywhere is likely to evolve this strategy of using a few molecules repeatedly. It may be that other life forms discover the same set of biomolecules that Earth life uses because these are absolutely the most efficient and effective set under any planetary conditions. But it may also be that life elsewhere uses a different set that is optimal given the specific history and conditions of that world. We can search for the repeated use of a set of molecules without knowing in advance what the members of that set will be.
We can apply this approach to the search for biochemistry in the Solar System. Samples of organic material collected from Mars and Europa can be tested for the prevalence of one chirality of amino acid over the other. More generally, a complete analysis of the relative concentration of different types of organic molecules might reveal a pattern that is biological even if that pattern does not involve any of the biomolecules familiar from Earth life. Interestingly, if a sample of organics from Mars or Europa shows a preponderance of D amino acids, this will suggest the presence of extant or extinct life and at the same time show that this life is distinct from Earth life. This same conclusion would apply to any clearly biological pattern that is distinct from the pattern of Earth life. The pattern of biological origin in organic material can potentially persist long after the organisms themselves are dead. Eventually this distinctive pattern will be destroyed as a result of thermal and radiation effects. Below the surface of Mars, both temperature and radiation are low, so this degradation should not be significant. On Europa the intense radiation may destroy the biological signature after several million years at depths to about 1 m below the surface ice.
8. Life about Other Stars
In the Solar System, only our own planet has clear signs of life. Mars, Europa, and Enceladus provide some hopes of finding past or present liquid water but nothing comparable to the richness of water and life on Earth. Our understanding of life as a planetary phenomenon would clearly benefit from finding another Earth-like planet, around another Sun-like star, that harbored life.
One way of formulating the probability of life, and intelligent life, elsewhere in the galaxy is the Drake equation, named after Frank Drake, a pioneer in the search for extraterrestrial intelligence. The equation and the terms used with it are listed in Table 9. The most accurately determined variable in the Drake equation at this time is R*, the number of stars forming in the galaxy each year. Since we know that there are about 10[sup.11] stars in our galaxy and that their average lifetime is about 10[sup.10] years, then R* [?]10 stars per year. All the other terms are uncertain and can be only estimated by extrapolating from what has occurred on Earth. Estimates by different authors for N, the number of civilizations in the galaxy capable of communicating by radios waves, range from 1 to millions. Perhaps the most uncertain term is L, the length of time that a technologically advanced civilization can survive.
The primary criterion for determining whether a planet can support life is the availability of water in the liquid state. This in turn depends on the surface temperature of the planet which is controlled primarily by the distance to a central star. Life appeared so rapidly on Earth after its formation that it is likely that other planets may only have had to sustain liquid water for a short period of time for life to originate. Planets orbiting a variety of star types could satisfy this criterion at some time in their evolution. The development of advanced life on Earth, and in particular intelligent life, took much longer, almost 4 billion years. Earth maintained habitable conditions for the entire period of time.
TABLE 9 The Probability of Life, and Intelligent Life, Elsewhere in Galaxy
Locations about stars in which temperatures are conducive to liquid water for such a long period of time have been called continuously habitable zones (CHZ). Calculations of the CHZ about main sequence stars indicate that the mass of the star must be less than 1.5 times the mass of our sun for the CHZ to persist for more than 2 billion years.
TABLE 9 The Probability of Life, and Intelligent Life, Elsewhere in Galaxy
The Drake Equation N = R* x f[sub.p] x n[sub.e] x f[sub.l] x f[sub.i] x f[sub.c] x L
The number of civilizations in the galaxy.
The number of stars forming each year in the galaxy.
The fraction of stars possessing planetary systems.
The average number of habitable planets in a planetary system.
The fraction of habitable planets on which life originates.
The fraction of life forms that develop intelligence.
The fraction of intelligent life forms that develop advanced technology.
The length of time, in years, that a civilization survives.
An interesting result of these calculations is that the current habitable zone for the sun has an inner limit at about 0.8 AU and extends out to between 1.3 and 1.6 AU, depending on the way clouds are modeled. Thus, while Venus is not in the habitable zone, Earth and Mars both are. This calculation would suggest that Mars is currently habitable. But we see no indication of life. This is owing to the fact that the distance from the sun is not the only determinant for the presence of liquid water on a planet's surface. The presence of a thick atmosphere and the resultant greenhouse effect is required as well. On Earth the natural greenhouse effect is responsible for warming the Earth by 30degC; without the greenhouse effect the temperature would average [?]15degC. Mars does not have an appreciable greenhouse effect, and hence its temperature averages [?]60degC. If Earth were at the same distance from the sun than Mars, it would probably be habitable because of the thermostatic effect of the long-term carbon cycle. This cycle is driven by the burial of carbon in seafloor sediments as organic material and carbonates. The formation of carbonates is due to chemical erosion of the surface rocks. Subduction carries this material to depths where the high temperatures release the sedimentary CO[sub.2] gas, and these gases escape to the surface in volcanoes that lie on the boundary arc of the subduction zones. The thermostatic action of this cycle results because the erosion rate is strongly dependent on temperature. If the temperature were to drop, erosion would slow down. Meanwhile the outgassing of CO[sub.2] would result in a buildup of this greenhouse gas and the temperature would rise. Conversely, higher temperatures would result in higher erosion rates and a lowering of CO[sub.2] again stabilizing the temperature.
Mars became uninhabitable because it lacks plate tectonics and hence has no means of recycling the carbon-containing sediments. As a result, the initial thick
FIGURE 11 Habitable zone (large rectangle) in terms of planetary temperatures and planetary mass. The objects of the Solar System (open squares) are shown as well as the newly discovered extrasolar planets (filled circles).
atmosphere that kept Mars warm has dissipated, presumably into carbonate rocks located on the floor of ancient lake and ocean basins on Mars. Mars lacks plate tectonics because it is too small, 10 times smaller than the Earth, to maintain the active heat flows that drive tectonic activity. The low gravity of Mars and the absence of a magnetic field also contributed to the loss of its atmosphere. Hence, planetary size and its effect on geological activity also play a role in determining the surface temperature and thereby the presence of liquid water and life. Figure 11 shows the habitable zone for a planet in terms of its surface temperature and mass. The planets of the Solar Systems and some of the extrasolar planets discovered as of 2006 are shown.
Life is a planetary phenomenon. We see its profound influences on the surface of one planet--the Earth. Its origin, history, present reach, and global scale interactions remain a mystery primarily because we have only one datum. Many questions about life await the discovery of another life form with which to compare. Mars in its early history is probably the best prospective target in the search for extraterrestrial lifeforms, although Europa and Enceladus are also promising candidates because of the likely presence of liquid water beneath a surface ice shell and the possibility of associated hydrothermal vent activity. In any case, it is likely that our true understanding of life is to be found in the exploration of other worlds--both those with and without life forms. We've only just begun to search.
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|Publication:||Encyclopedia of the Solar System, 2nd ed.|
|Article Type:||Topic overview|
|Date:||Jan 1, 2007|
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