A.2 The Search for Origins: Findings of a Space Science Workshop, October 28–30, 1996
On September 25, 1996, NASA Administrator Daniel Goldin received a letter from Dr. John Gibbons, Assistant to the President for Science and Technology, requesting that NASA assemble a group of scientists to “offer an objective assessment of 1) major questions in space science; 2) how current U.S. programs respond to those questions; and 3) appropriate ‘next steps’ that might be taken.” The scope of the activity was given to include the universe, planets and planetary systems, and life and its development. Shortly after the dramatic announcement of possible evidence for fossil life on a martian meteorite found in Antarctica, President Clinton called for a space summit with congressional leaders to develop a consensus on NASA’s future. The purpose of the scientific gathering requested in Dr. Gibbons’ letter was to develop discussion papers on the above broad subjects for a symposium to be held for the benefit of Vice President Al Gore in advance of the summit.
On October 4, 1996, NASA Associate Administrator for Space Science Wesley T.Huntress wrote to Space Studies Board Chair Claude R.Canizares requesting assistance in identifying appropriate participants, organizing the workshop, and integrating participants’ contributions into a proceedings discussion paper for NASA’s use in briefing the Vice President. In response, a workshop was planned and carried out under Dr. Canizares, chair of the Board, and Dr. Anneila Sargent, chair of the Space Sciences Advisory Committee. The workshop was held on October 28–30 and resulted in a two-part proceedings consisting of a set of illustrated briefing charts and an accompanying narrative discussion paper. The package was delivered by NASA to the White House in late November, and a subset of the workshop participants (see Section 2 above, pp. 29–30) subsequently met with the Vice President on December 10.
The Search for Origins: Findings of a Space Science Workshop October 28–30, 1996
On October 28–30, 1996, a joint NRC-NASA workshop was held in Washington, D.C., resulting in a briefing package consisting of illustrated view charts and an accompanying narrative discussion paper. The package was delivered to the White House in late November and was presented for discussion with Vice President Al Gore on December 11.
This discussion paper reports the findings of three dozen biologists, planetary scientists, astronomers, and cosmologists assembled by NASA and the National Research Council at the request of the White House Office of Science and Technology Policy. They met in Washington, D.C. on October 28–30, 1996. In a workshop format, the group considered emerging directions in space science and identified ORIGINS as a unifying theme for future initiatives. Questions about our ORIGINS are as old as human thought; broadly interpreted, they comprise:
How did the Universe come to be what it is today?
What is the origin of life? What are the building blocks and conditions necessary for life, and how did these come about in the universe?
Is life unique to Earth? Can we find convincing evidence that life once existed or even now exists elsewhere in or beyond our solar system?
The study of ORIGINS follows the 15 billion year long chain of events from the birth of the universe at the Big Bang, through the formation of the chemical elements, of galaxies, stars and planets, through the mixing of chemicals and energy that cradled life on Earth, to the earliest self-replicating organisms and the profusion of life.
Recent discoveries from diverse disciplines attest that life is remarkably hardy and that each step in the chain of ORIGINS occurred surprisingly quickly. Discoveries in just the past few years provide the first scientific basis for believing that life may be widespread in the universe, in our solar system and beyond. We also have a new comprehension of the development of the universe, its constituent galaxies and stars, the number and variety of planetary systems, and the processes that shape them. For the first time in history, we have achieved the level of understanding and technical capability necessary to fill in “missing links” along the chain of ORIGINS by exploring on the Earth and outward in space, in the present and backward in time. To do so, we
need to understand more about the processes leading to the origin of life, about habitats suitable for life, and about the origins of the building blocks of the universe.
Answers to these questions are within reach. Major advances over the next 15 years can be realized by continuing and building upon the multidisciplinary programs that have brought us to this point. The current and planned space science programs of NASA begin the next steps in the quest for ORIGINS and pose the technology challenges needed for subsequent steps. Missions now under way and in planning, including upgraded instrumentation for the Hubble Space Telescope, the Advanced X-ray Astrophysics Facility, the Space Infrared Telescope Facility, the Stratospheric Observatory for Infrared Astronomy, the Mars Surveyor series, and other planetary and space astronomy and physics projects, will offer powerful tools for advancing the ORIGINS program. At the same time, while the ORIGINS challenge provides a unifying core for the space science program, neighboring disciplines address important problems of their own, and may unexpectedly contribute directly—as was the case for the recent analyses of martian meteorites. These related activities span the broad panoply of laboratory, field, and theoretical research conducted by NASA. Existing NASA planning processes, coordinated with NSF and other agencies and using peer review, are the best way to define the details and priorities of these programs.
Investment in a balanced and diversified ORIGINS program will yield a steady return of significant findings and, inevitably, major surprises. Over the next 15 years, scientists and the public could share the excitement of discoveries such as:
when and how primitive life emerged and flourished on Earth;
whether the martian meteorites found on Earth or rocks returned from Mars confirm that life existed on that planet;
the presence of a liquid water ocean on Jupiter’s moon Europa that could harbor primitive forms of life;
the detection of dozens of planetary systems, including some which may be conducive to life as we know it;
sharp pictures of planet-forming disks, infant stars and the growth of galaxies;
more detailed histories of the early stages of the universe, including maps of the dark matter seeds that grew to form galaxies.
The Hubble Space Telescope images of embryonic solar systems and the evidence for possible past life on Mars have aroused intense public interest in the ORIGINS of the universe and its contents. These breakthroughs are the astonishing returns from years of investment in many scientific disciplines. The ORIGINS quest informs, excites and inspires the public. Its outcome could well have as profound an effect on human thought as the Copernican and Darwinian revolutions.
Origins—Recent Advances and Next Steps
This is an extraordinary time for civilization and this nation. For the first time, we have achieved the level of understanding and the technical capability that allow us to grasp the full meaning of our origins, our history, and our context in the universe. For as long as humans have been able to think, we have been explorers, inventors, and dreamers, pondering how the universe came to be as it is, how the richness of life on this planet developed, and whether life on Earth is unique in the cosmos. In the past, these questions were answered by speculation and myth. Now they can be addressed with the scientific soundness of evidence and quantitative analysis.
In the past several years, indeed the past several months, scientists have reaped breathtaking returns from years of investment in basic science, from cosmology to microbiology, from astrophysics to analytic chemistry. Diverse disciplines are converging in a remarkable and unprecedented way to a focus on understanding our origins in the broadest and the deepest terms. For example, it is only recently that biologists have begun to understand the diversity and ubiquity of life forms in extreme environments on Earth, that astronomers have detected the first planets outside the solar system, and that cosmologists have measured the subtle ripples in the early universe which formed all present galaxies and stars. In these discoveries can now be perceived a chain of development that stretches from the instant that time began to our pondering these issues today.
The following narrative is intended to accompany and elaborate the briefing book presentation of the findings of the 1996 Space Science Workshop. The narrative is divided into four major sections: Life on Earth, Our Solar System as a Home for Life, Other Solar Systems, and The Universe. Each of these main sections provides additional information about recent advances and next steps in its area.
Life on Earth
The NASA exobiology program broadly addresses the origin and evolution of life in the universe, and has been instrumental in providing the basis of our new and exciting understanding that life may be widespread in the universe. The program begins with the cosmic origins and evolution of those elements necessary for life (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). It looks inward to the natures and limits of environments capable of supporting existing life on Earth
and the geological record of past life and its environments; it studies by means of new molecular biological tools the evolutionary relationships among existing organisms, thereby influencing our understanding of the nature of the common evolutionary ancestor of all known terrestrial life forms; and it investigates the characteristics that may have allowed the appearance of the first self-replicating “life-like” forms. Finally, the program again looks outward to identify and investigate extraterrestrial bodies on which environments potentially favorable to life exist or have existed in the past.
UBIQUITY OF LIFE ON EARTH WHEREVER LIQUID WATER AND USABLE ENERGY ARE PRESENT
Recent explorations of a number of extreme (extraordinary) environments on Earth reveal that diverse microbial life forms can be detected and even observed in surprisingly large numbers. These bacteria are able to thrive on unusual sources of energy and under conditions that do not support the growth of larger organisms. Nearly every moist environment that has been examined contains some form of life. At temperatures exceeding 100 C, deep-sea vents spew forth a diverse microbial biota, some of which can grow by metabolizing such unlikely sources of energy as hydrogen and sulfur. Deep subsurface terrestrial aquifers contain bacteria that consume energy sources released from basalt. Some hot springs contain a diverse and ancient microbiota yet to be cultivated. Some very acidic hot springs with temperatures near boiling are inhabited by bacteria that consume sulfur. In the extremely cold dry valleys of Antarctica microorganisms are found growing within rocks. Other bacteria found in abundance in evaporitic basins can grow only in the presence of saturated salt. The recent discoveries of life in such diverse environments as these on Earth greatly expand the range of habitats that can be considered suitable targets for life-detection missions on other planets.
IMPORTANT NEW INSIGHTS INTO RIBONUCLEIC ACID (RNA)-BASED CHEMICAL PROCESSES AS LIFE’S BEGINNINGS
An important new approach to the investigation of the origin of life is based on the proposal that the most important biopolymer in the first life forms was RNA. In this conjecture, RNA has two central functions: the storage of genetic information and the catalysis of chemical processes essential to maintaining life. This primitive life would have been much simpler than contemporary living organisms, which are based on proteins, deoxyribonucleic acid (DNA), lipids and carbohydrates. Recent laboratory studies have provided strong support for the idea that early life was RNA-based. It has been demonstrated that RNA can be formed from its monomers under prebiotic reaction conditions in a process catalyzed by clay minerals. The RNA prepared in this way has been shown to have the ability to store information. In a second set of studies it has been demonstrated that RNA with one type of catalytic activity can mutate into an RNA with a different structure and different catalytic function. These evolution studies, carried out in test tubes in the absence of life, suggest that life based on RNA was capable of evolving to contemporary life based on DNA and proteins.
DERIVATION OF EXISTING LIFE ON EARTH FROM MICROBES THAT LIVED AT HIGH TEMPERATURES IN THE ABSENCE OF OXYGEN
Over the past decade, biologists have enjoyed remarkable success in constructing the “Universal Tree of Life,” a family tree of all living organisms. The tree is based on the fundamental insight that all organisms contain subcellular components called ribosomes and that all ribosomes contain structurally similar components made of RNA. By comparing the sequence order of the building blocks in the DNA that provides the blueprint for these RNA structures, biologists now have a means of evaluating the evolutionary connectedness of organisms as dissimilar as baker’s yeast, blue whales and bacteria. The emerging family tree shows clearly that the plants and animals of our experience form only the tips of the Universal Tree. Organisms found on the tree’s innermost branches—the microbes—are those most likely to retain features that characterized life on the young Earth. Such organisms live at temperatures near the boiling point of water and do not use oxygen in cellular processes. In fact, many are killed by oxygen in even trace amounts. The Universal Tree thus suggests that our earliest known ancestors were microorganisms that lived in hot, oxygen-free environments such as exist today in the hot springs at Yellowstone National Park. Continuing study of environments as exotic as deep-sea hydrothermal systems or as familiar as farm ponds also indicates, however, that we still know very little about the biology and diversity of microbial life on our own planet.
EARTH LIFE ORIGINATED VERY EARLY AND EVOLVED VERY RAPIDLY
Within the past 5 years, NASA-supported research has resulted in discovery of the oldest firm evidence of life on Earth, a diverse assemblage of 11 species of microscopic organisms cellularly preserved in Western Australia’s sedimentary rocks nearly 3.5 billion years old. These fossil microbes appear to have been relatively advanced, perhaps including oxygen-producing photosynthetic cyanobacteria, and life therefore must have originated much earlier. In recent months, this supposition has been supported by other NASA-sponsored studies that resulted in detection in 3.85-billion-year-old rocks of southwestern Greenland
of graphitic carbonaceous matter that has an isotopic signature suggestive of a biological origin. If this carbon is a product of life, living systems must have originated quite rapidly, because Earth, like the Moon and Mars, was subjected to intense meteoritic bombardment from the time of its formation, 4.5 billion years ago, to about 3.9 billion years ago, including periodic impacts of bodies so large as to have vaporized the early ocean and sterilized the early planet. Life on our planet originated early, evidently easily, and evolved extraordinarily rapidly.
SURVEY THE EARTH’S BIOSPHERE AND MAP OUT THE “UNIVERSAL TREE OF LIFE”
Studies of diverse and exotic habitats for life on Earth have shown that living organisms are present wherever liquid water and a source of energy are available. Life thrives in environments from hot springs with temperatures above the boiling point of water, to permanently ice-covered lakes in Antarctica. A significant fraction of Earth’s living creatures live far beneath the surface in oceanic and continental sediments. In the latter environment, diverse anaerobic communities are supported in ecosystems driven by geochemically produced hydrogen. The possibility of such ecological niches on other planetary bodies is compatible with current knowledge. Exotic habitats must be explored more fully on Earth, so that the capability of living organisms to adapt to different environments and the ways in which life has adapted to changing environments can be better understood.
Recent studies using molecular approaches to establishing genealogical relationships have revolutionized the way we think about the natural history of life. Using these approaches, it has been possible to begin to map out the Universal Tree of Life, and this has led to the conclusion that the most likely common ancestors of existing life were heat-loving, hydrogen-metabolizing microorganisms. Molecular data gathered using these same approaches have yielded insights into the yet-undiscovered diversity of life on Earth. Ribosomal RNA sequences from many natural environments suggest that only a small fraction of Earth’s microbes have been cultured and identified with respect to their functional roles in nature. Estimates suggest that as much as 99% of the microbial populations on Earth have yet to be grown and studied in culture. It will be necessary to map out the ancestry of the entire range of microbes in order to fully understand the origin of life on Earth; this will in turn require field studies. Molecular sequencing will be needed to complete the Universal Tree for all newly discovered organisms.
ASSEMBLE AN INTEGRATED HISTORY OF LIFE AND THE ENVIRONMENTS FOR ITS DEVELOPMENT ON EARTH
While the Universal Tree is properly read as a history of life, it can also be understood as a chronicle of Earth’s environmental history, because the presence of microorganisms is closely tied to particular environmental conditions. Thus, the Universal Tree enables predictions about the history of both life and environments that can be tested against the direct history of life and environmental conditions encrypted in the geological record. Recent discoveries of fossils and new geochemical techniques for reconstructing past environments indicate that we are finally in a position to integrate information from comparative biology and the rock record in a way that will show how our planet and its biota have developed in concert through 4 billion years of Earth history. Remarkably, the first fossils date from a period very near to the origin of Earth and soon after a period of heavy cometary bombardment. These discoveries suggest that the origin of life was rapid and therefore easy once the terrestrial environment became relatively benign. The co-evolution of life and the environment will be a major theme wherever life may be found.
Interestingly, once life had originated on a planet it could have been transferred to other worlds by impact processes. Thus the origins of life may not have been isolated to individual planets. In recent years, many lunar and martian meteorites have been found on Earth. One rock from Mars probably falls on Earth each month. Interstellar and interplanetary dust grains also continuously transfer materials from deep space to the planets.
Our Solar System as a Home for Life
THE CHANGING SUN AND ITS EFFECTS ON THE PLANETS
From the moment fusion began in the Sun’s core, the center of the protoplanetary disk of gas and dust was bathed in light, heat, and energetic ionizing radiation. Evidence in meteorites suggests that the young Sun also rotated much more rapidly than it does today, generating a much stronger magnetic field that penetrated into the disk where boulders were forming from the dust. Observations of stars like the Sun show that a gaseous “wind” was also produced by the star’s interaction with the disk, in a manner still not understood. This wind may have cleared the inner part of the planet-forming disk of gas, resulting in the remarkable differences between the atmospheres of the giant and terrestrial planets.
Since that time, the Sun has gradually brightened by about 30% to its present luminosity, but has declined (by orders of magnitude) in its production of ionizing ultraviolet and extreme ultraviolet radiation. The Sun’s rotation has also slowed, and its wind has subsided to the contemporary solar wind. As the Sun’s evolution proceeded, the planets were affected by it. Each planetary body seems to record different responses in its atmosphere and surface. In particular, Venus and Mars show evidence of having lost large amounts of their original water, rendering these otherwise most Earth-like of the planets apparently inhospitable to life today. Observations of how Earth and the other planets currently interact with the various solar outputs, including light, ionizing radiations and solar wind, provide clues about the physical processes that the early planets and their atmospheres must have undergone in response to exposure to the more extreme young Sun.
METEORITE EVIDENCE THAT MICROBIAL LIFE ONCE MAY HAVE EXISTED ON MARS
For centuries Mars has been an object of fascination as a possible abode of life. The Mariner and Viking images of Mars, taken two decades ago, showed the planet to be cold and dry today, geologically active in the recent past, and possessing a much less extreme climate in more distant epochs. The Viking lander failed to find any evidence of organics, but rather documented a chemically reactive surface.
One of the most important recent developments in planetary science is strong evidence that laboratories and museums contain a number of rocks from Mars which arrived on Earth as meteorites. These rocks were blasted from Mars by large impacts and spent some time in space before they came within Earth’s gravitational attraction and fell to the ground as meteorites. Currently, 12 of the collected meteorites are believed to have come from Mars. Half of these fell in Antarctica, and most were collected in the annual expeditions of a joint NASA-NSF program. One was collected by a similar Japanese program. While all of these meteorites are igneous rock, most show evidence of low-temperature alteration by liquid water, and the evidence shows that some or most of this alteration occurred on Mars prior to the ejection of the rock into space. The oldest (4.5 billion years) of these meteorites (ALH84001) has recently been found to contain some features that could be interpreted as evidence of early microbial life on Mars. These features include the presence of organic chemicals, some of which may be from Mars, some very small iron oxide and iron sulfide grains that are very similar to those known to be made by microorganisms on Earth, and shapes and forms resembling fossil and living microorganisms on Earth, except that the forms found in the Mars meteorite are generally much smaller than known Earth microorganisms and microfossils. While none of these observations and interpretations is definitive for early microbial life on Mars, taken together they provide a strong impetus for more detailed study of the existing martian meteorites as well as for an intensive search for new ones in Antarctica.
PRESENCE OF THE NECESSITIES OF LIFE (WATER AND ENERGY SOURCES) IN VARIOUS SOLAR SYSTEM LOCATIONS, NOW OR IN THE PAST
To know when and where habitable zones may have existed in the past in the solar system, it is necessary to know when and where liquid water has been present. Over the past 4.5 billion years, the Sun’s luminosity has increased as it has matured. For the inner solar system, the habitable zone has moved as solar luminosity has increased and the composition of planetary atmospheres has evolved.
The story is likely to have been quite different for the outer solar system. Here, frictional heating of planetary interiors due to tidal flexing reduces the importance of solar luminosity as an energy source, and liquid water environments could have been stable for a much longer period of solar system history. Liquid water may be present on icy satellites of the jovian planets where tidal forces or the decay of radioactive elements in their interiors may heat them above the melting point of ice. The concept of tidal heating is best illustrated by Io, one of Jupiter’s four, so-called Galilean satellites. Io is, undoubtedly, the most active volcanic body in the solar system. The interior of Io constantly erupts molten sulfur as the gravitational attraction of Jupiter and its moons tugs on the small satellite, causing its outer shell to flex and crack.
While the environment of Io can hardly be considered a clement medium for life, this may not be true for its icy neighbors, especially Europa. The water-rich icy surface of Europa is quite smooth and free of mountains or craters, suggesting that it has a liquid interior. But the spectacular images obtained during a recent close flyby of the Galileo spacecraft revealed that Europa has a complex, fractured surface where plates of icy crust have pulled apart, allowing darker material to well up from below, filling the cracks between. The origin of the darker material is the subject of an ongoing debate. An exciting proposition is that this material is organic-rich ice that formed when liquid water from a subterranean ocean welled up between diverging plates of ice. This suggests the possibility of a subsurface organic-rich ocean, sustained by the gravitational heating of the interior. Where there is liquid water and the right mix of organic molecules, there is the possibility for life. Hydrothermal vents, similar to those discovered on the deep ocean floors on Earth, could exist on the floor of a Europan ocean. On Earth, such places teem with microbial life and, even in the absence of sunlight for photosynthesis, are able to sustain communities of many complex higher forms of life. To test such ideas, more information is needed about Europa’s surface composition and interior structure.
Life-supporting zones may also exist within the interiors of other jovian moons. Ganymede, one of Europa’s neighbors that is thought to be almost half water by weight, is also covered by an icy crust that has the appearance of a cracked egg shell. Rifted
mountain ranges encircle the surface of Ganymede for hundreds of kilometers, testifying to a past history of tectonic activity. Could Ganymede also harbor subsurface pools of liquid water where prebiotic chemical evolution may have led to some form of life? Similar questions can be asked of Enceladus, one of Saturn’s moons, where tidal heating may maintain warm interior zones of liquid water, and of Neptune’s moon, Triton. Subsurface hydrothermal processes may have operated even within the interiors of large asteroids.
Saturn’s giant moon Titan, larger than the planet Mercury, possesses a thick nitrogen atmosphere with abundant methane and other organic molecules. A rich chemistry powered by sunlight in the upper atmosphere produces hazes that hide the surface below, on which occasional pools of liquid water (created by impact or volcanism) undergo brief episodes of chemistry as yet unknown but perhaps on pathways toward life. Titan is the best extant planet-sized analog for Earth before it held life, and NASA’s upcoming Cassini-Huygens mission will reveal the nature of its surface and lower atmosphere.
PRIMITIVE REMNANTS OF THE FORMATION OF THE SOLAR SYSTEM
Materials that have remained relatively unmodified since the birth of the solar system provide the best opportunity to understand the past and unravel how the planets formed. Such materials must have been stored far from the sun’s warmth in bodies too small to be heated by internal geological processes. Comets, asteroids, and the newly discovered Kuiper Belt objects fulfill these requirements and enable us to explore the internal chemical beginnings of the solar system.
Comets are small dark bodies composed of a mixture of organic materials, refractory grains, and ices, predominantly water. Vast numbers reside at great distances from the Sun, but occasionally one is nudged into the inner solar system where it becomes more accessible to observation from Earth. Asteroids were born in the transition region between the rocky planets and the gaseous giants of the outer solar system. Because of their small sizes, asteroids and the meteorites that derive from them are relatively pristine and so may provide clues to the origin of the terrestrial planets.
Over the last few years, more than three dozen objects with radii of only 50 to 200 km have been detected in the Kuiper Belt at the outer edge of the observable solar system. Pluto itself might be considered the largest of these Kuiper Belt objects. They represent the most primitive remnants of planetary accretion, fragments that were not captured or scattered by the growing giant planets. As such, they may contain the best chemical records of the solar system’s original makeup, including delicate organic and volatile molecules from the molecular cloud out of which the solar system formed.
CHARACTERIZE THE SUN’S HISTORY AS IT INFLUENCES THE ORIGIN AND DEVELOPMENT OF LIFE; UNDERSTAND THE VARIABILITY OF THE SUN AS A STAR
The Sun has evolved over 4.5 billion years from its initial state as a rapidly rotating, strongly magnetic, violently active star with a massive solar wind. During this time, the Sun has contracted and grown hotter as hydrogen in the core fused into helium, and it is now about 30% brighter than in its youth. The problem today is to understand the convection and circulation within the Sun, because it is these fluid-like motions that generate and manipulate the magnetic fields that control the variable activity at the visible surface. This solar variability includes sunspots, flares, coronal mass ejections, x-ray emissions, and variations in total brightness. Precision observations of the activity and brightness of the Sun and monitoring of a large number of other solar-type stars are essential if we are to fill in the general picture of how stars like the Sun behave from birth to death. At the same time, the techniques of helioseismology are making the convection and circulation within the Sun available for direct study. Helioseismology, together with high-resolution studies of the motions and magnetic fields at the Sun’s surface and simultaneous optical, ultraviolet, and x-ray observations, will help to guide the theoretical studies and modeling needed to understand the past, present, and future of the Sun and other stars like it.
CHARACTERIZE THE CHEMICAL ENVIRONMENT, CLIMATIC EVOLUTION, AND GEOLOGIC HISTORY OF MARS
Mars holds a special place in both planetary science and exobiology. Since Mars is the planet most like ours, the synergistic study of the two worlds may help unlock some of Earth’s secrets. Even prior to the discovery of possible signs of past life in the meteorite ALH84001, exobiologists have found Mars most interesting as a place to trace the chemical evolutionary pathways that ultimately led to life on Earth. NASA’s new Mars Surveyor program will at its outset survey the global elemental, geochemical, and physical characteristics of Mars, with an emphasis on volatiles; this is useful both for comparative studies with Earth and as a basis for deciding the most likely abodes of life, if any, past and present. NASA also plans to place a series of landers, including rovers, on the surface; these will carry out simple scientific investigations and will provide a testbed for the technologies needed for sample return. Once promising sites for possible life have been identified, emphasis should shift to the
surface exploration of these favorable locations to seek more clear-cut evidence of past conditions, and to search for more direct confirmation of past life such as the presence of biogenic elements and compounds, and anomalous isotopic fractionations.
Fulfilling the primary objectives of the atmospheric sciences and of geophysics will require the deployment of a network of long-lived monitoring stations. Meteorological stations, when combined with simultaneous sounding from orbit, will provide essential data on the martian weather and will lead to much-improved general circulation models. These stations can also ascertain the seasonal cycles of carbon dioxide, water, and dust and thereby clarify the processes by which Mars’s layered sediments are deposited. If the polar laminae can be dated in situ, a chronology of martian climate change can be developed. In addition, accurate determinations of rare-gas and isotopic abundances, plus estimates of water trapped in the crust, may be useful in helping to characterize Mars’s ancient climate. These same stations could transmit seismic data to elucidate the Red Planet’s internal structure and could include sophisticated geochemical laboratories to assay local materials.
RETURN TO EARTH SOME WELL-CHOSEN MARS SAMPLES TO ELUCIDATE ITS PAST AND TO ENABLE A SEARCH FOR EVIDENCE OF BOTH FOSSIL AND EXTANT LIFE
While the recently announced findings from the Antarctic meteorite ALH84001 are not proof of life on Mars, they have yielded strong evidence that both hydrocarbons and minerals deposited by water are in ancient martian materials. It now appears that the critical ingredients required for the emergence of life—organics, water, and energy—have existed in the martian environment throughout its history. The expectation that life developed on Mars has therefore been significantly heightened.
Important immediate steps in pursuing the possibility of martian life are to expand the search for martian meteorites and to intensify the analysis of these meteorites. Beyond this, the crucial task is to collect well-chosen rock samples from Mars and bring them back to Earth for analysis. This task should be carried out in several steps. The first is to obtain a global-scale understanding of martian geologic and climatic history as it relates to life. This step has begun with the launch of the Mars Global Surveyor spacecraft. The next is to intensify the search for potential sites of past and present life. Mars Global Surveyor will begin this search, and future missions can extend it. Once good sites have been located, robotic vehicles can be sent to the martian surface to search for the most promising samples, collect them, and return them to Earth.
Mars could have both fossil evidence for life and extant life itself. But because the present martian surface environment is so harsh, evidence of fossil life will probably be much easier to find. Promising locations for fossil life can be identified by planned orbital missions, and promising samples can be collected by relatively simple surface vehicles. Extant life, however, may exist only deep below the surface, and hence may be difficult to locate and sample. The search for fossil life therefore can and should precede the search for extant life.
For any returned samples, it will be essential to have a facility where they can be received and processed, and state-of-the-art laboratories where they can be analyzed. Adequate receiving facilities do not exist now, and adequate laboratories are scarce; both need development before the first samples are returned.
EXPLORE POTENTIAL HABITATS ELSEWHERE IN THE SOLAR SYSTEM: EUROPA AND TITAN
Beyond Mars, most of the solar system is inhospitable to life because it is too hot or too cold, or too dry. A possible exception is Jupiter’s moon Europa, now being scrutinized by the Galileo spacecraft. Beneath its icy crust may lie a shallow ocean kept fluid by tidal heating, which may, as a result of ancient and continuing bombardment by organic-rich cometary materials, contain the ingredients for life. Saturn’s moon Titan is known, from Voyager and ground-based observations, to contain a wealth of complex organic molecules in its dense atmosphere and is suspected to have at least puddles of organic material on it surface. This satellite’s complement of prebiotic and proto-biologic molecules could provide a valuable benchmark in understanding the development of terrestrial life.
In its nominal mission, Galileo will provide high-resolution imaging of only a small fraction of Europa’s surface because of communications constraints imposed by the spacecraft’s unfurled antenna. Observations of the enigmatic satellite, at both visual and mineralogically diagnostic near-infrared wavelengths, will be substantially enhanced during a proposed extended Galileo mission, which would fly by Europa a dozen additional times at close range and various latitudes. These close encounters would significantly improve the characterization of the satellite’s surface geology and geophysics, but would not allow any complex chemistry to be discerned. A lander, carrying chemical laboratories, will ultimately be necessary to unravel the nature of the surface if the global orbital reconnaissance indicates some promise for biological activity; site selection for this lander would be based on the same principles as site selection at Mars.
Subsequent to the Cassini-Huygens mission, should the surface of Titan prove rich in complex organic molecules, follow-on missions to detect the extent and complexity of Titan’s organic chemistry could reveal preserves of molecules in the hazy boundary between the biological and nonbiological domains. The goal is to understand the level of chemical or possibly biochemical complexity achieved in this natural environment.
ANALYZE PRIMITIVE BODIES TO UNDERSTAND THE CHEMICAL INGREDIENTS, PROCESSES, AND PLANETARY CONDITIONS LEADING TO LIFE IN THE SOLAR SYSTEM
The chemistry that began with the Big Bang, continued in stars now dead, and still influences the properties of the clouds of gas and dust from which new stars are born is the starting point for the raw materials of life. How was this material transformed as the solar system began and planets formed? From where in the solar system did Earth receive its massive store of water and organic molecules during the first 10% of its history? Was it from nearby asteroid-like materials, or from early comets streaking inward from the realm of the giant planets?
Many of the clues may be locked in primitive bodies such as asteroids and comets, and in the vast reservoir of the Kuiper Belt. A small fraction of Kuiper Belt objects are perturbed, over time, into orbits that bring these bodies into the inner solar system as comets. Early in the history of the solar system similarly scattered fragments may have seeded Earth with water and organic molecules. A crucial next step to understanding whether Earth’s oceans and organic store came from this outer realm is reconnaissance of the Kuiper Belt using the most sophisticated chemical remote-sensing techniques available, first from large ground-based telescopes. These observations can limit the range of possible surface materials, but can yield only averages as results. Ultimately, flyby, rendezvous, or sample-return missions will be required. In situ measurements and complementary theoretical studies will enable us to determine the specific regions of the presolar nebula that are sampled by particular meteorites. This will enable us to construct a general picture of planetary accumulation processes.
A flyby of Pluto, which would complete the initial reconnaissance of the solar system, has been the subject of recent studies.
Other Solar Systems
OBSERVATIONAL EVIDENCE OF PLANETARY SYSTEMS AROUND OTHER STARS
Within the last year, the discovery that at least a dozen ordinary stars have planet-like companions has transformed the search for other planetary systems. The presence of these companions is implied by ground-based telescope observations that some stars “wobble,” presumably as a result of the gravitational pull of an orbiting object or objects. In addition, the inferred orbits and masses of the newly discovered companions are quite unlike those predicted by theoreticians. All have masses at least as large as that of Jupiter, for example. However, their detection suggests that we are on the threshold of being able to observe other planets.
DETERMINE HOW STELLAR AND PLANETARY SYSTEMS FORM AND EVOLVE
Understanding of the process of star and planetary system formation has improved significantly in recent decades, in large part as a result of NASA programs that encouraged the study of primitive solar system materials and supported observational and theoretical investigations of star-forming regions. Other agencies, such as NSF, have funded complementary, ground-based observations at near-infrared and millimeter wavelengths, leading to a perspective that intimately connects the formation of stars and potential planetary systems.
Stars are born in dense, dusty, and consequently dark clouds of molecular hydrogen. How do these clouds form and survive, and what controls their fragmentation into progressively smaller clumps in which aggregates of stars, or even individual stars, are formed? Clarifying this process would be a giant stride along the path to understanding the provenance of stars that, like the Sun, are capable of sustaining life.
There is ample observational evidence that most protostars are embedded in and developing from highly flattened, rotating disks of material. It now seems clear that these protostellar disks are the switching stations for material on its journey from interstellar clouds to stars and planetary systems. Disk properties, and their variation with age, strongly affect the nature of any planetary system that may subsequently develop. Overall disk properties, such as mass and angular momentum, are accessible to ground-based observational study, but very little information is currently in hand about their detailed structure and the way in which it changes as the disk and its associated star age. However, an understanding of these details is key to determining why some stars eventually emerge alone, why some have one or more stellar companions, why some may be accompanied by planets, and what kind of planets these will be. High-spatial- and high-spectral-resolution observations, making use of interferometric techniques in the infrared-to-millimeter wavelength regions of the spectrum, will be critical in probing the origins and evolution of these potentially planet-forming disks.
DISCOVER AND CHARACTERIZE PLANETS AROUND OTHER STARS
Determination of the frequency of occurrence and the overall structure of planetary systems around stars other than the Sun is now within reach. For the first time, it is possible to consider putting to the observational test theories of how the solar system formed. Detection of planetary systems is a demanding observational task, requiring both much more accurate instrumentation than was available a decade ago and new generations of ground- and space-based telescopes. While astronomers have recently used such systems to discover substellar-mass companions of several nearby Sun-like stars, the exact nature of these newly discovered companions remains unknown. To advance in this area, basic physical parameters such as mass and orbital character must be measured for a statistically significant sample. Current theories can then be improved or modified. Ground-based programs will be useful here, although only space-based instruments will be capable of detecting and studying planets that are similar to Earth in mass and surface conditions.
FIND AND STUDY POTENTIALLY LIFE-SUPPORTING PLANETS ORBITING OTHER STARS
There is general agreement that liquid water is essential to life as known on Earth. A search for life-supporting planets is therefore in reality an effort to detect planets that have suitably dense atmospheres and evidence of liquid water. These candidates would be expected to orbit their associated suns at distances where surface temperatures range between the freezing and boiling points of water. Further, their masses are unlikely to be very different from that of Earth.
The detection of such small and dim planets, located relatively close to their central star, is a daunting challenge that lies beyond the capability of existing telescopes. However, planets emit most of their energy at infrared wavelengths, and the energy radiated by a Sun-like star is less in the infrared than it is in the visible region of the spectrum. Studies suggest that it would be possible to search for, and subsequently study, life-supporting planets around other stars using a space-based, infrared observing system. Candidates for examination will be stars known from previous, less sensitive searches to have planetary companions. The goal will be to obtain a “family portrait” of another sun with its retinue of planetary companions. The long-term goal would be to detect and study Earth-sized planets with a view to determining atmospheric temperature and composition, and the possibility for life. Such studies will require innovative techniques, most likely interferometry carried out from platforms in space.
Understanding the origin of the universe, of galaxies, and of stars has long been one of the central themes of NASA’s space science program. Indeed, the 1990s have been the most productive period in the history of space astronomy; our theory of how the universe originated has been dramatically affirmed, and the long chain of events leading from the Big Bang to the present epoch is coming into sharper and deeper focus.
OBSERVATIONS OF INFANT STARS
It is now possible to fill in many chapters in the story of star formation. Stars are born in dense clouds of interstellar gas; the most massive burn their nuclear fuel, age, and ultimately die in enormous explosions called supernovae; these supernovae scatter ashes of carbon, nitrogen, oxygen, and heavier elements from the star’s nuclear cremation into interstellar space; over time, this debris cools and collapses to form new stars. By contrast, the lightest stars have not yet had time to go through this cycle even once. Observations of massive stars elucidate the production of different elements over the history of the galaxy. The existence of many slowly evolving, lower-mass stars gives an opportunity to study star formation over a range of protostellar ages and to characterize the birth of stars like the sun.
Understanding of the processes that take place inside stars is already quite well developed. There is, however, a paucity of information about the physical and chemical processes that take place in interstellar gas and dust. With time, the properties of these interstellar clouds change significantly, often by way of chemical reactions that are quite different from those known in terrestrial laboratories. In particular, a rich smorgasbord of often unfamiliar organic molecules, some of which may have a central role in prebiotic evolution, are actually observed in interstellar clouds. Detailed studies of interstellar chemistry are now under way to clarify how and when these molecules are formed.
Very recent Hubble Space Telescope optical images of the Eagle Nebula, a part of the Milky Way galaxy long known as a stellar nursery, illustrate the complex processes that transform cold interstellar gas clouds into new stars. The surface of the cloud is contorted into a series of columns that, themselves, display smaller protuberances. The complicated morphology of this surface, the boundary between a cold, dusty, and consequently opaque region and one that is hot and transparent to visible light, is due to photoevaporation by ultraviolet light from nearby hot stars that have already emerged from the clouds where they
formed. Infrared radiation can penetrate the dark clouds that obscure forming stars from optical view, and an infrared image of the same region displays for the first time a myriad of new stars in and around these clouds.
ORIGIN OF THE CHEMICAL ELEMENTS
The first stars, which emitted the first light, must have been made from gas that emerged as essentially pure hydrogen and helium from the Big Bang, uncontaminated with heavy elements. Theory indicates that it would be difficult for stars to form under these conditions. However, once the process has happened once, it takes only a few million years for nuclear reactions to produce carbon, nitrogen, oxygen, and other heavy elements, and for supernova explosions to disperse them, facilitating the birth of succeeding generations of stars. Recent measurements made with the Keck telescope in Hawaii have shown that creation of the elements began when the universe was one-tenth its present age, while some elements formed even earlier.
MANY MORE GALAXIES THAN EXPECTED
Looking out in space to the most distant observable luminous objects is looking back in time. Astronomers are now able to see galaxies as they were less than 2 billion years after the fiery birth of the universe, long before the formation of the solar system. In the past year, a very long observation with the Hubble Space Telescope of a tiny patch of the sky near the Big Dipper has indicated the existence of many more galaxies than previously anticipated. This patch was chosen because optical pictures made from the ground indicated that this region was as empty as possible of foreground objects. It offered an unobstructed view of the faintest and most distant objects that could be found.
Resembling a drop of water drawn from a tide pool and viewed under a powerful microscope, the little patch was discovered to be teeming with galaxies, self-organized islands that each contain billions of stars. It is estimated that a comparable survey of the whole sky would reveal about 100 billion galaxies, far more than previously expected, in a rich variety of shapes and colors, some surprisingly similar to our immediate neighbors, and some in forms that have never been seen before.
THE BEGINNING OF STRUCTURE IN THE UNIVERSE
To ground-based observers, the microwave radiation from the ancient universe looks the same in all directions; astronomers describe it as “smooth.” A major question related to origins is how the galaxies, stars, and planets of the present universe emerged. Where are the ripples that would presage the structure evident today?
Space observations made with the Cosmic Background Explorer have provided very precise maps of radiation emitted shortly after the Big Bang, when the universe was only some 300,000 years old, and much smaller. They show tiny ripples in the smoothness, the cosmic seeds from which present structure took shape. These galactic antecedents, the most primitive objects in nature that are likely to be observed directly, are immense clouds of hot gas. Most importantly, they are thought to contain a great amount of still-unobserved internal structure that might shed light on fundamental problems of cosmic evolution. In particular, they might lead to a better understanding of why the measured masses of galaxies, and even clusters of galaxies, disagree with the amount of mass that can be seen in the form of stars. In fact, most of the mass of the universe is in essentially invisible form, termed “dark matter.” Much contemporary research is devoted to locating, identifying, and characterizing dark matter.
UNDERSTAND THE FORMATION OF STARS AND PLANETS
The gravitational collapse of gas clouds is undoubtedly a key element in the formation of stars and planets, but the process is still shrouded in mystery. The division of labor between gravity and other forces, such as magnetic fields, stellar winds, and shock waves, is unclear. Both space- and ground-based observations during the past decade have opened up star and planet formation to detailed investigation. These deeply intertwined subjects together constitute one of the most important and active areas of contemporary astrophysics, linking astronomy to the planetary and geological sciences.
It is now possible to pose well-defined questions for space programs. Key areas for study include the detailed characterization of the disks that surround young stars and may go on to form planetary systems, the evolution of the dense molecular clouds that are apparently the source of all star formation in a spiral galaxy like ours, and the role of supernovae and other violent events in triggering star formation in molecular clouds. Space infrared observations will enable astronomers to draw aside the dusty cloaks surrounding local stellar nurseries like those in the Eagle Nebula. Measurements of the long-wavelength radiation from gas molecules in the clouds will help characterize such properties as density and temperature. Indirectly, this information could help in determining what environments are hospitable to prebiotic molecules and compounds. The Stratospheric Observatory for Infrared Astronomy and the Space Infrared Telescope Facility will permit long strides in these areas.