A consensus theory of planetary formation is generally in hand: gradual accumulation of solids within a primarily gaseous, flattened circumstellar accretion disk, which itself is a byproduct of the formation of its parent star from a dense, rotating interstellar cloud of gas and dust. However, this theory has been studied in only a very narrow range of initial conditions, possibly important physics has been neglected, and it has little or no predictive capability. For example, recent discoveries of giant planets in circular orbits very close to solar-type stars were unexpected and are still not completely understood. There are, as of this writing, eight new giant planets known to orbit solar-like stars; at least one of these orbits within the "habitable zone" of its parent star. These new data provide not only a challenge to the current theoretical paradigms, but clear direction as to parts of parameter space in which both theoretical models and observations of extrasolar systems need more exercise. Furthermore, given the wide range of conceivable environments, we might ask "what makes a planet habitable?" (an associated question is "habitable for what kind of organism?").
Advances in technology are enabling not only new observations of these mature (if unanticipated) extrasolar planetary systems, but also of "protoplanetary nebulae" within which the planetary formation process is still ongoing. These observations are capable of telling us the extent, mass, gas and solid content, and thermal structure of the material from which planets form. In order to comprehend the new, surprising diversity of planetary systems, we must continue to study the early stages of planetary formation under a range of conditions, as well as to establish the full range of ultimate outcomes of the process.
In addition to observations of remote extrasolar planetary systems from ground and space, we are fortunate to have in hand, or accessible by spacecraft, actual material which survives from the days of the early accumulation of our own planets. So-called "primitive material" preserves clues as to the materials from which, and the processes by which, planets formed. To be found in these primitive materials are presolar grains which carry clues as to the variety and number of stellar precursors of our own system, complex organic material which might preserve the signature of interstellar chemistry, once-molten silicate "chondrules" with composition, size, and mineralogy diagnostic of the pre-accretionary environment, and, in one recent case, suggestive evidence for past life on another planet.
There is compelling evidence that cellular life existed on Earth 3.56 billion years ago. Recently, a persuasive argument was made that terrestrial life was already present toward the end of the period of heavy bombardment of the early Earth by asteroids and comets from 4.0 to 3.9 billion years ago. This implies that ancestors of contemporary life emerged rather quickly, on a geological time scale, and perhaps also survived the effects of large impacts. Such catastrophic events would have strongly favored survival of thermophilic organisms which thrive at high temperatures. This scenario is consistent with the phylogenetic record, which indicates that the last common ancestor was thermophilic. This record also supports the view that life might have arisen first near marine hydrothermal vents. The possibility remains, however, that the first common ancestor lived at moderate temperatures and only later adapted to thermophilic conditions, in which case ocean surfaces and near-shore shallow environments might have spawned life.
All present-day forms of life are cellular, with lipid bilayer membranes forming the primary barrier that separates the interior of a cell from the external environment. It has been proposed that similar, encapsulating structures (vesicles) made of simple membrane-forming material could have self-assembled in the protobiological environment. The presence of such membrane-forming material in carbonaceous meteorites is consistent with this idea. Furthermore, recent experiments showed that vesicular lipid bilayer structures can grow by spontaneous addition of membrane-forming material from the surrounding medium, and can encapsulate both ions and macromolecules. Besides separating intracellular components from the diluting effect of the environment, cell membranes also provide a barrier for separating charges, a fundamental process in bioenergetics. From phylogenetic data we infer that the earliest cells probably used chemical rather than photochemical energy sources. It has also been proposed that membranes helped stabilize the secondary structure of peptides (protein precursors) having appropriate sequences of polar and nonpolar amino acids. Some of these peptides may have been capable of performing basic protocellular functions, such as catalysis, signaling, and energy transduction, without requiring the existence of separate molecules capable of storing and transmitting genetic information (i.e., nucleic acids).
Alternatively, it has been postulated that there was a time in protobiological evolution when RNA played a dual role as both genetic material and a catalytic molecule ("the RNA world"). However, this appealing concept encounters significant difficulties. RNA is chemically fragile and difficult to synthesize abiotically. The known range of its catalytic activities is rather narrow, and the origin of an RNA synthetic apparatus is unclear. Therefore, it may be more likely that RNA and proteins co-evolved in protocells, rather than evolving independently. The co-evolutionary process leading to division of cellular functions between these molecules, however, is not at all clear. Understanding the emergence of life requires studies that extend beyond the origin of biopolymers and cellular structures. All these components necessarily assembled into auto-catalytic, self-reproducing systems capable of evolution and selection. Based on theoretical arguments, it has been suggested that sets of mutually catalytic molecules can reproduce and evolve without templating, resulting in a primitive metabolism without a genome. However, only a limited number of experimental studies have been performed in this area.
The recent discovery of organic, possibly even biogenic, material in a martian meteorite (ALH84001) opens the exciting possibility of extending the search for the origin of life to places beyond the Earth. Although current findings on ALH84001 are inconclusive regarding possible life on Mars, future exploration might lead to fundamentally new insights into prebiotic chemistry and protobiological evolution, the record of which is lost on the Earth.
The processes which modified the environment vary widely both in their magnitude and time scales. For example, the increase in solar luminosity, the declining rates of comet and meteorite impacts, the exchange of volatile materials between Earth's mantle and crustal reservoirs, and the stabilization of continents have all exerted dominant controls on the surface environment. However, because these processes themselves evolved very slowly, they required 108 to 109 year time scales to cause global changes. The effects of plate tectonics, erosion, sedimentation, and glaciation acted more quickly, causing changes over 104 to 108 year time scales. Faster still have been the effects of ocean and climate dynamics and ocean-atmosphere-biosphere interactions, which can vary on 1 to 104 year time scales. Already, human activity has dramatically altered patterns of erosion, sedimentation, climate patterns, species biodiversity, primary productivity and ocean-atmosphere-biosphere exchange. These changes are happening over a few decades. In the earlier "natural" world, such changes would have required typically thousands to millions of years to occur. How will plants, animals and the microbial world respond to such rapid change?
Microorganisms are supremely adapted for coping with change. Should global conditions deteriorate, the small size of microbes allows them to "hide" in niches. Small cell size imparts a high surface/volume ratio, which allows rapid rates of chemical exchange with the cell's surroundings. Thus microbes can rapidly exploit favorable conditions. The diverse biochemistry of microbes permits them not only to survive, but even to prosper under environmental extremes. Already by 3.5 billion years ago, widespread microbial communities accommodated large meteorite impacts, UV irradiation, desiccation, wide excursions in temperature and salinity, and a long menu of chemical substrates as sources of energy and organic matter. For example, our early biosphere adapted to major changes in volcanism, coastal environments, atmospheric composition, and the oxidation state of the oceans and atmosphere. On the other hand, microorganisms can themselves contribute to environmental change by, for example, affecting rates of erosion and sedimentation or by influencing the atmosphere's inventory of reactive gases. Microbes responsible for infectious diseases evolve to circumvent medical treatments, thereby continually challenging human populations.
In contrast with the bacteria, plants and animals are much larger, more complex and highly specialized. They typically depend upon a more limited suite of nutrients and a relatively narrow range of conditions for their survival. Accordingly, environmental change, human-induced or otherwise, can more easily trigger catastrophe within ecosystems which sustain these complex eukaryotic organisms. Modern challenges to the biosphere include rising atmospheric levels of CO2, SO2, CH4, CO, and N2O due to fossil fuel burning and agriculture (causing greenhouse climate effects as well as direct biospheric effects), declining ozone levels (leading to increased ultraviolet radiation), invasions of foreign species, and land use changes whose effects include the following: soil salinization, overgrazing, increased soil erosion, altered energy balance, loss of biodiversity, species extinctions, declines in food and fisheries, and chemical pollution.
While large meteorite impacts, such as the one which marks the Cretaceous/Tertiary boundary, were perhaps more severe than modern human-induced changes, impacts still serve as useful models for the effects of catastrophic change on the biosphere. For example, the severe "winter" which had been predicted to follow a large impact alerted us to the "nuclear winter" which might follow thermonuclear war. Also, impacts remind us that catastrophism probably does play at least a limited, but still important, role in the long-term evolution of our biosphere. The role of impacts in evolution was perhaps most pronounced during the earliest stages of Earth's history, when impact rates were much higher.
Electromagnetic radiation and gravity are two fundamental environmental variables that dramatically affect biological systems. On Earth, gravity is effectively constant in magnitude and direction, and the natural radiation environment has modest variability. These physical variables are difficult to control in space, and consequently can severely limit our ability to sustain life beyond the surface of the Earth.
How the radiation environment beyond the Earth affects biological systems is only partially understood. In space, galactic cosmic rays and particles from solar events can be lethal to terrestrial life forms. We have a very limited ability to predict solar events, and our understanding of shielding techniques to manage radiation risks is poor. Further, our ability to characterize the radio-biological effectiveness of various ionized and non-ionized particles, is limited. Space travelers beyond low Earth orbit must, therefore, monitor the Sun for solar storms as a matter of life or death.
Clearly, the effects of various forms of radiation on RNA and DNA are issues of major concern. Currently we are ignorant of the relationships among chromosomal damage, chromosomal aberrations, and carcinogenesis. The direct effects of high energy particles on the nervous system are also poorly understood, as are biological mechanisms for the repair of radiation damage.
Gravity profoundly affects many biological systems, both directly and indirectly. The cardiovascular, musculoskeletal, and neurovestibular systems all undergo dramatic changes in space, where organisms are deprived of terrestrial gravity. For example, fluids shift from the lower limbs and lower torso to the upper torso and the head; blood volume is reduced; anti-gravity muscles in the lower limbs and torso tend to atrophy; bones that formerly supported the organism against gravity become less dense and more fragile; vestibular-ocular reflexes are altered, and the nervous system re-calibrates itself to function in the absence of gravity. Although these changes are generally benign for functioning in space, they can seriously compromise an organism's ability to function in a new gravitational environment and upon return to the Earth.
Humans currently use multiple countermeasures to minimize the effects of non-terrestrial environments on physiological systems for periods of more than one year. These countermeasures, which include training procedures, protective garments, physical exercise, conditioning devices, and various pharmacological agents, may be of only limited value to sustain life beyond the Earth's biosphere for prolonged periods of time that ultimately will include multiple generations. Artificial gravity, provided by continuous or intermittent centrifugation, lower-body negative pressure exercise chambers, or other techniques, may be necessary. Our experience with artificial gravity for humans in space is limited to a single, brief, Gemini flight experiment, and our current knowledge base is inadequate to assess the need for artificial gravity to sustain life beyond the Earth's biosphere.
Critical psychological variables in small group interactions during prolonged isolation in a perpetually hostile environment away from the home society are not well understood. The interactions of gravity, radiation, and isolation in non-terrestrial environments have never been studied systematically. Thus, many fundamental questions in the life sciences will need to be answered before we can assure that terrestrial life forms can be sustained beyond the Earth's biosphere for prolonged periods.
With current technology, we are able to maintain terrestrial life beyond the Earth for periods in excess of one year. To sustain terrestrial life beyond the Earth for longer periods, it is necessary to create a micro-environment that is similar to that on Earth, at least initially. This environment must provide an atmosphere with a ppropriate partial pressures of O2 and allow for gas exchanges to support metabolism; it must provide adequate liquid water, appropriate microorganisms, adequate gravity, food, thermal protection, and radiation protection; it must allow for the partial recycling of nutrients and waste-products; finally, it must be stable and reliably sustainable for an indefinite period of time.
A program to extend human presence to Mars will inevitably have both exploration and what we may term habitability goals. If evidence that life once evolved on Mars is discovered, human explorers will provide much of the scientific capability needed (beyond robotic capabilities projected for the next several decades) to investigate how the pre-biotic seeds of microbial life evolved and subsequently prospered or perished.
Theory, laboratory experimentation, subterranean terrestrial sampling and meteoritic evidence suggest that microbial life could have evolved on early Mars. Our present lack of direct knowledge about subterranean martian environments should make us cautious, therefore, about concluding (as seems common) that any such early life would inevitably have become extinct on a planet where present surface conditions are indeed extremely hostile. To answer questions about possible extant life we need to explore the subsurface below the cryosphere, which extends to kilometer depths, and into the warmer martian hydrosphere. Although a thorough exploration of the martian subsurface by robots alone is feasible in principle, the combined effects of great communication distances and intrinsically limited machine intelligence might well require postponement of such exploration for many generations. Therefore, some astrobiologists are considering whether human exploration of Mars may be legitimately identified as a real scientific priority as the only efficient and timely way in which we will be able to study, at first hand, a second sample of life (all terrestrial life being linked to a common ancestor).
The consequences of the discovery of life, past or present, on Mars in the coming decades will have profound implications beyond just the intense interest of molecular biologists. (Likewise, although it will be much harder to disprove the case, the determination that Mars never evolved life would also have profound implications.) Scientists and non-scientists alike will immediately appreciate the improbability that humans are "alone" in our galaxy. The discovery of life on Mars will surely add priority to the search for life elsewhere in our solar system (e.g. in the subterranean oceans of Europa), to the search for Earth-like planets orbiting other stars in our galaxy, and to the search for extraterrestrial intelligence. More generally, the stimulation of such a discovery of martian life is also likely to lead us to a recognition that, having the technological means at hand, we can be on the verge of becoming a multi-planet civilization, with Mars as our second abode.
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Last updated Feb-11-1997
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