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The Astrophysical Context of Life Appendixes

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The Astrophysical Context of Life A Context and Statement of Task CONTEXT Policy This study is proposed to address issues raised in the recent assessment of astrobiology programs at NASA (Life in the Universe: An Assessment of U.S. and International Programs in Astrobiology, 2002). The committee that wrote the report found that research in certain key areas of astrophysics relevant to understanding the astronomical environment in which life arose on Earth (and, potentially, elsewhere in the universe) was not well represented within the broad range of issues being addressed by NASA’s Astrobiology program. This report is intended to highlight the potential contributions astronomers can make to the new field of astrobiology. Technical Life on Earth originated and has evolved over the last 3.5 billion plus years in a complex and highly variable astronomical environment. The Earth was assembled from interstellar gas already enriched in prebiotic molecules that were themselves the product of generations of stellar nucleosynthesis and chemical evolution in interstellar clouds. Asteroid and comet impacts have apparently altered the course of evolution, and interstellar dust continues to sift down onto the Earth. Long-lived radioactivities from stellar explosions heat Earth’s molten core, driving plate tectonics, and suffuse the mantle in the form of potassium-40. Life on or near to the surface of the Earth is strongly affected by the evolving radiation output from the Sun, interrupted by solar flares. Life is exposed to a continuous flux of cosmic rays that has probably varied significantly over geological times. Statistically, the Earth has been exposed to perhaps thousands of jolts of biologically significant radiation from supernovae and the possibility of exposure to an exotic event such as a gamma-ray burst has been considered.

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The Astrophysical Context of Life Qualitatively, the same history affects other solar system bodies and extrasolar planets that might harbor life, but the effects will be varied in import and detail. Thus, there are compelling reasons to argue that a full and complete picture of the origin and evolution of life on Earth and elsewhere must integrate the astrophysical context of life. One of the goals of the burgeoning intellectual field of astrobiology is to embed the core topics of biology, biochemistry, and paleogeology in the broadest appropriate context of astronomy. Relevant aspects of astronomy should inform the biology, chemistry, and geology, and vice versa, in order to facilitate intellectual exchange between those fields and to maximize the synergism within this innately multidisciplinary field. An example of the mutual interchange of all these fields comes in the attempt to define “habitable zones.” Classic habitable zones are those around host stars of different and evolving luminosity in the standard liquid-water paradigm, but the existence of extremophiles has led to other, more novel, paradigms. On a broader scale, the question has been raised as to whether there are habitable zones within galaxies. The latter concerns issues of the level of heavy elements required to support the growth of terrestrial planets and the degree to which galactic commotion is inimical to life. STATEMENT OF TASK The committee will study the means to augment and integrate the activity of astronomy and astrophysics in the intellectual enterprise of astrobiology, in NASA’s Astrobiology program, and relevant programs in other federal agencies. The goals of this study are as follows: Identify areas where there can be especially fruitful collaboration among astrophysicists, biologists, biochemists, chemists, and planetary geologists; Define areas where astrophysics, biology, chemistry, and geology are ripe for mutually beneficial interchanges and define areas that are likely to remain independent for the near future; and Suggest areas where current activities of NSF and other federal agencies might augment NASA programs. Examples of research questions that may be relevant in this study include the following: What is the role of galactic ecology in the development and sustainability of life? Is there a galactic habitable zone? What are the habitable zones around host stars in the liquid-water paradigm and plausible alternative paradigms with other solvents or biochemistry? Given a liquid water planet, how will the evolution of life depend on the mass of the parent star through its radiation, mass ejection, flares, and stellar wind? How does the “faint young sun” issue affect terrestrial life and life elsewhere? What sort of geochemical evidence can be summoned (e.g., He-3, Be-10, Fe-60) that constrains extraterrestrial influences? What are the links between interstellar and prebiotic chemistry? What is the role of gas-phase interstellar chemistry in producing basic bio compounds? How do protostellar disks provide the conditions for life-supporting planets? How were organic compounds distributed, processed, and differentiated on early Earth, on satellites, on extrasolar planets? What is the role of meteoric chemistry?

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The Astrophysical Context of Life What are the effects of bombardment? What role does bombardment play in the origin of life on Earth or elsewhere? Did life emerge during the heavy bombardment of the Hadean era? How do rocky planets get wet? What are the frequency and effect of subsequent bombardments? What is the biological role of radiation? What fraction of mutations are due to copy errors, to influences from within the biosphere, and to external astronomical sources? Is the level of mutations a selected biological phenotype? To what degree and under what conditions is chiral asymmetry, and possibly homochirality, induced by radiation? What is the evolutionary origin of radiation repair mechanisms and what commonality is there between these mechanisms, gene transfer, and meiois, all of which involve annealing strands of DNA? Is photosynthesis a requirement for a highly developed biosphere? What programmatic activities at NASA and other agencies can be developed to detect/confirm/ verify factors relevant to the topics outlined above? Are there in vitro or in silico experiments that can inform these issues outlined above? How do current and proposed NASA missions support prospects for the remote sensing of the geology, climate, weather, chemistry, and biology of planets around other stars? What additional missions can be developed to address these issues?