There are several well-posed problems that are ready for accelerated study by astronomical observations. Recent indirect radial velocity observations of extrasolar planets, and millimeter-wave, infrared, and HST observations of circumstellar gas and particle disks, are outstanding examples of the tip of this iceberg. Certainly, observations of mature planetary systems around a large variety of stellar types of various ages will be needed to determine what kinds of planets form around what kinds of stars. Direct or indirect detection of planets in "mature" systems is the focus of NASA's planning efforts to date. However, there remain critical gaps in our understanding of the earlier "protoplanetary" stage; these include the actual absolute (not assumed relative) abundances of gas and solids, the nebula radial extent under different initial conditions, its radial and vertical temperature structure, the particle-to-planet accumulation time scale, the role and distribution of angular momentum of in-falling material, the properties of stellar winds, and the role of magnetic fields. We are completely ignorant of whether any of these properties vary with stellar type and/or with star formation environment (i.e., solitary or densely clustered), and there are hints in current data that protoplanetary systems in the two different star forming regions in Taurus and Ophiuchus have rather different properties.
The observations need to be conducted over a wide range of wavelengths between the short microwave (millimeter) and near-infrared spectral regions to penetrate the thick nebular dust envelopes and sample the mid-plane where planet formation is occurring. Infrared (Keck and follow-on) and millimeter-wave interferometers are needed to resolve protoplanetary disk structure at 1 AU resolution. NASA's planned "Origins" program proposes a series of space-based infrared telescopes and interferometers; other approaches were mentioned which are less connected technologically but are also worthy of consideration (photometric detection, balloon missions, HST upgrades, far-IR [100 micron] interferometer, microlensing, etc.). Several key theoretical questions are ripe for interdisciplinary attack. These include the properties of a densely clustered protostellar environment, the role of ionization, grain charging, and electromagnetic forces, the role of global wave modes and/or energetic infall itself in nebula evolution, the presence, extent, duration, and energetics of nebula turbulence, the possibly wide-ranging migration of protoplanets (and solid material in general) within the nebula and even relative to the nebula gas. It was noted that the Sun-Earth Connections theme of the Space Science Enterprise might be tapped to a larger extent for its expertise in electrodynamics and stability or instability of weakly ionized media, properties of stellar and solar winds, and of dusty plasmas.
It was generally felt that augmentation of the numbers of primitive meteorites returned, catalogued, and analyzed from the Antarctic would yield commensurate rewards. Much of the nation's analytic resources are Apollo-era and worthy of considerable upgrading. Of particular interest might be the sort of ultra-high resolution analytic equipment capable of studying the internal structure of putative nano-fossils (such as found in ALH84001) or of obtaining accurate age dates and/or isotope data on small mineral samples. Exploration of this sort of "inner space" is probably just as demanding of technology and expertise, and as rewarding in terms of understanding of the planetary formation process, as comparable efforts devoted to exploring "outer space."
Concerning the issue of habitability, there are many unknowns (even in the case of our own planet). For instance, what stellar and/or planetary conditions are most important for the origin of life, or for its evolution and increasing complexity? Are stellar photons of extreme energies and charged particle fluxes positive or negative factors? What is the role of internal planetary activity (tectonics) in truncating or prolonging the habitable era? Water is generally agreed to be the sine qua non of life, but how is water distributed across growing planets or reintroduced on mature planets that lost it or never had it at all? Does chaotic planetesimal dynamics spray icy objects from the outer solar system onto mostly formed inner planets? What is the composition of the "primitive" objects that even today impact the terrestrial planets? What is the mass and extent of the Kuiper belt of primitive planetesimals? Are terrestrial-sized satellites of close-in giant planets orbiting M dwarf stars habitable? Are they dynamically stable? Studies of the "habitable zone" using planetary scale climate evolution models might profit from more association with the sophisticated modeling supported in the Earth science community (such as to treat cloud feedback effects).
Since liquid water is required to support life, other, possibly transient, sub-surface micro-environments in the outer solar system and large asteroids could have been conducive to the origins of life, a prime example being Europa. These environments should be considered in future missions and their prebiotic and biological potential should be assessed by model studies.
Studies of ancient ecosystems could explore the relationship between the microenvironment and the diversity of microbiota and how these changed over time. Comparative studies of modern and ancient ecosystems could identify those aspects of the microenvironment which are crucial to microbial diversity and evolution and how they changed over geologic time. Microbial communities in hydrothermal systems (including hot spring deposits) and in groundwater are especially important analogs both for understanding the very early fossil record on Earth and for guiding the search for evidence of past life on Mars. Also, the changes in morphology and chemistry which accompany fossilization should be examined.
Regarding Earth's "macroenvironment," we should identify those mechanisms which directed the long-term increase in atmospheric O2 and the decline in atmospheric CO2 levels. To accommodate these changes, microbes modified their pathways of CO2 uptake, invented protocols for detoxifying oxidants, devised new O2-requiring biosynthetic pathways, and so forth. What were the nature and timing of these innovations? What were the composition and abundance of trace biogenic gases in the ancient atmosphere, particularly before significant levels of O2 were attained? What were/are the significant feedback effects involving biota, trace gases and climate? How did oxygen-utilizing eukaryotes evolve in response to these changes? Given anticipated land-use changes today, what role will trace gases play in future climate change?
How does an entire ecosystem, including its microbes, respond to abrupt environmental perturbations? The natural microbial world is a rich source of information about the mechanisms which could permanently change those ecosystems which sustain plants and animals. Can these microbial effects be detected before permanent change occurs? Ecosystem-level studies could monitor the effects of change. For example, such studies could explore the following: (a) the ecology of microbial communities which are still relatively unaltered by human activity, (b) the sensitivity of ecosystems to changes in specific parameters, singly or in combination (e.g., CO2 levels, UV irradiation, soil acidity, reductions in biodiversity, etc.), and (c) the relationships between the biota, climate, geography and hydrology.
Studies also could be pursued for plants and animals, specifically in the following areas: a) the influence of extraterrestrial phenomena such as impacts upon evolution, b) the physical and biological drivers of mass extinctions, and (c) ecosystem, hydrologic and climate changes which impact the natural ecosystem and also public health.
Extrasolar planets will eventually be examined to search for other biospheres. Life should ultimately be detectable through spectroscopic analyses of a planet's atmospheric composition. Under what conditions does the presence of abundant atmospheric O2 definitely indicate life? Aside from abundant O2 levels, what other atmospheric compositions are definitive indicators of a biosphere? Is the early history of our own atmosphere actually representative of other evolving, habitable planets?
Obviously an effective research and exploration program requires that new cross-disciplinary technologies be developed to exploit novel approaches for getting answers. These involve, for example, the development of new microsensors for probing the dynamics of microbial ecosystems, field sensors to monitor gas exchange between ecosystems and the atmosphere, new approaches in remote sensing and so forth. An effective technology program is one which is closely integrated with the research program and responds effectively to new needs as they arise.
Integrated quantitative models should be constructed which help to develop a deeper understanding between physiology, ecology, and the environment. Such models could account for changes over various time scales, and ultimately they should be able to predict community responses to perturbations. We also should model other planets which might still be habitable but which are different from Earth. How would different planetary sizes, solar insolation or volatile inventories affect the evolution of the planet and its biosphere? Such insights would greatly enrich our understanding of Mars as well as those extrasolar rocky planets which we are destined to discover.
To sustain terrestrial life beyond the Earth's biosphere for prolonged periods of time will require new fundamental knowledge and an integration of that knowledge in many disciplines. Further, we need a more profound understanding of closed or semi-closed ecological systems. Interdisciplinary studies involving radiation physics, gravitational biology, genetics, neurobiology, and developmental biology are required to provide the critical understanding.
The International Space Station is an essential evolutionary test bed for research on the effects of the space environment in biological development and evolution, as well as the only place where the effects of gravity on living systems can be investigated systematically. For example, we do not fully understand the role of gravity in development. Are there thresholds and critical periods for the effects of gravity; how are phenotypes and genotypes affected by gravity at the cellular, system, and organism level?
Improved models for definition and prediction of solar events, the development of advanced radiation shielding techniques, and enhanced understanding of genetic biological radiation repair mechanisms are of particular importance. Other interdisciplinary studies involving gravitational physics and life sciences are also needed. Just as importantly, we need a new interdisciplinary perspective to integrate the information that will assure the long term survival of terrestrial species beyond the Earth's biosphere.
A regenerative life support system (i.e. one which can be fully restored/replenished), will ultimately be needed to sustain terrestrial life beyond the Earth. Thus, a biodome will be required. Unfortunately, we cannot fully specify all the necessary characteristics of the required micro-environment at this time because we do not understand all the control mechanisms that function to maintain closed or nearly-closed ecological systems. A research biodome is an important tool that will be needed to help us examine these relationships. The transition from constant re-supplying to the use of in situ resources will eventually be necessary to sustain terrestrial life beyond the Earth's biosphere.
Cross-disciplinary studies continue to provide insights into the Earth's complex ecosystem, and these can ultimately be applied to developing artificial life support systems. This remains a promising area for future cross-disciplinary efforts, for the better we can characterize those events that alter the Earth's biosphere, the more adequately we will be able to specify what is needed to provide stable and sustainable life support systems in space.
If Mars does have an extant subterranean biosphere, then our exploration of that biosphere raises the serious environmental and ethical questions that we face on our own world where species are endangered and lost sometimes even before we have discovered and characterized them. The need to avoid contaminating and changing a presently unknown biologic environment was raised at the workshop and acknowledged to be not only a potential scientific catastrophe but also a real policy issue, as yet without an assigned advocate.
Further, even if it should turn out that Mars is now a sterile planet, environmental issues will confront us -- issues relating to the control of the pollution and waste associated with an expanding human base. In the much longer term our technology may provide us with the ability to seriously consider "terraforming" regions of Mars (or even the entire planet). Today terraforming another planet amounts to little more than a thought experiment, but human history demonstrates that such conjectures can indeed become reality, usually with severe unintended consequences.
In the absence of any other organization likely to grapple with the ethical dilemmas involved in the future expansion of humans beyond the Earth, the astrobiology component of NASA's space research program appears to be the natural home for analysis of exploration ethics.
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Last updated Feb-11-1997
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