history of life—for instance, the stellar flares and supernovas that would alter the cosmic ray flux into the atmosphere and onto the surface. Specifically, the biological effects of radiation and particle fluxes should be explored. Cosmic rays and high-energy photons may both produce ionization in cells, but their production, propagation, atmospheric penetration, and deposition are different. This report places some emphasis on particle and photon irradiation both because such radiation is a ubiquitous feature of the astronomical environment that is relevant to biology and because it is relatively understudied. Consideration of the astrophysical context of life elucidates the fact that all planets will exist in variable environments. While somewhat abstracted from a specific astronomical context, there are basic issues of how much variability is healthy, or even needed, for the robust evolution of complex life. These issues raise questions about molecular evolution in an astronomical context.
With finite personnel and finite time, the committee may not have addressed all the areas that could come within the purview of this report. Some representative areas that it believes are relatively understudied and especially amenable to focused effort in the near future are these:
The galactic environment,
Cosmic, solar, and terrestrial irradiation,
Interstellar and protostellar nebular chemistry,
Prebiotic chemistry and photosynthesis, and
Molecular evolution in a variable astronomical context.
The committee addresses these topics in some detail below. For each topic it attempts to outline relevant astronomical issues that are currently rather divorced from other disciplines of astrobiology but that are deemed to be life-oriented and hence solidly within the rubric “astrobiological research.” The committee also attempts to identify for each topic where interdisciplinary work could be needed.
One of the broadest questions one can ask is that about the role of the galactic environment in the origin, development, sustainability, and evolution of life. The galactic environment refers to the astronomical environment in which any life-hosting body resides. The galactic environment consists of the large-scale and highly irregular environment of the Galaxy, with its dense bulge and spiral arms filled with stars and gas of varying composition, analogous to the varied environments in the biosphere of Earth. The galactic environment is also defined by the particular type of star, its location in a dense or rarified portion of the interstellar medium, the star’s location with respect to the spiral arms or the central bulge, and the presence or absence of nearby perturbations caused by catastrophic or milder events that can affect life. As does life on an evolving Earth, the galactic ecological niche will inevitably vary with time as the star wanders through the Galaxy and encounters different conditions and as the star itself evolves. Stars like the Sun could very well have been born in rich clusters where neighboring young stars bathed the solar system with external radiation. In time, the clusters dissolved, leaving the Sun to wander its solitary way, as it does today. Some have referred to this overall galactic environment as a “disturbed galactic ecology.”
The question has been raised of whether there is a galactic habitable zone in the same sense as the habitable zones around the Sun and other stars, defined principally by the ability to sustain liquid water.