While science often advances through the efforts of individual researchers, there are times when larger team efforts are needed. One of the purposes of the NASA Astrobiology Institute is to provide guidance and coordination for team efforts in astrobiology. Accordingly, on a beautiful autumn weekend more than 50 members of the NAI met at Jackson Hole Wyoming to consider the opportunities for this sort of “big science’. Unfortunately, the high peaks of Grand Teton National Park were invisible from our meeting location, and there was little time in any case to enjoy the surroundings. Rather, the attendees focused their attention on identifying scientific challenges and opportunities where the multidisciplinary and multi-institutional base of the NAI provides unique approaches to solving astrobiology problems.
Following a day of preliminary presentations, the attendees (three representatives from each of the NAI Teams plus staff from NAI Central and NASA Headquarters) divided into smaller groups. These groups discussed three areas for potential collaborative research: (1) studying the processes in protoplanetary or circumstellar disks (astronomy, cosmochemistry), (2) studying the early history of life on earth (paleobiology, geobiology), and (3) investigating the subsurface biosphere (microbial ecology, genomics). The recommendations from each of these three groups were presented on the final day of the retreat.
The first group called its proposed effort Disko, since it deals with protoplanetary disks and evolving planetary systems. These disks of hot gas and dust form as part of the process of star birth, and recently they have been studied by astronomical techniques ranging from visible light to microwaves. The primary objective of the Disko team is to integrate astronomical observations of these birthplaces of planets with dynamical and chemical theory and information derived from the chemistry of primitive bodies (comets and asteroids) in our own solar system. This work focuses on processing material through disks, beginning with the clouds of interstellar gas and dust and arriving finally at planets, some of which will be habitable.
One issue within the Disko area concerns disk time scales, which need to reconcile data from astronomical observations, dynamical theory, and the record of short-lived radioactive isotopes preserved in meteorites. Current opinion is that these lifetimes are very short and that planets may form in just a few million years, but there are discrepancies in this picture that need resolution. Since astrobiologists are especially interested in how water and other volatiles reached the planets, there should be a focus on the formation and dynamics of icy bodies and on the transport of volatiles to the Earth and other inner planets. More generally, there are interesting questions concerning the roles of chemistry in the interstellar clouds vs. chemistry in the disk itself, and how each contributes to the formation and delivery of prebiotic organic compounds.
The second group divided its theme of the early history of terrestrial life into three areas, arranged chronologically by eon of Earth history: the Hadean Eon, from the formation of the planet until about 4 billion years ago; the Archean Eon (4.0 - 2.5 billion years), which includes the oldest rocks and the oldest fossils; and the Proterozioc Eon (2.5 - 0.6 billion tears), which includes the rise of an oxygen atmosphere, first appearance of multicellular life, and (possibly) the episodes of snowball Earth.
In studying the Hadean World, we are limited by the almost complete absence of rocks from that period of time. In addition to continuing searches for terrestrial samples, the group also considered the potential of lunar science to provide evidence. One major question concerns the nature of the heavy bombardment of both Earth and Moon, which was critical in defining the environment in which life began -- was there a gradual decline or a late lunar cataclysm? Other questions deal with the responses of the biosphere to the bombardment, and the potential for cross-fertilization of Earth and Mars. A more thorough search of the lunar samples might illuminate these issues and could conceivably provide fragments of Hadean terrestrial rocks preserved on the Moon.
The major issues for the Achaean World include identification of the earliest fossilized life together with a better understanding of the composition of the early atmosphere and oceans. A search for additional rock samples should help determine the sources of organic carbon, the climate, biogeochemical cycles, tectonic regimes, and perhaps the time of origin of oxygen photosynthesis, which was so crucial to the future of the biosphere.
Study of the Proterozioc World naturally focuses on developments in the ocean, atmosphere, and biosphere that preceded the Cambrian (which is the time about 0.6 billion years before present when the rich fossil record begins). With new perspectives, this is the "not so boring billion years" that set the stage for our modern world -- what has been called the long, slow fuse to the Cambrian explosion of life. We want to understand when multicellular organisms appeared and to evaluate environmental constraints that may have delayed the evolution of complex life. Some of the suggested projects included study of the Black Sea as a possible analogue for the Proterozoic ocean and drilling the Indian Vindhyan succession, a well-preserved record from roughly 1.6 to 0.6 billion years before present. Using comparative methods to unravel the history of Proterozoic genomes may also provide key insights on when and how complex life began to evolve.
The third group proposed several new approaches to understanding the importance of subsurface life, a potentially very large and very old part of the terrestrial biosphere. In the past decade we have detected microbes living kilometers below both the land and the ocean floor, accessible only through drill holes and in deep mines. Most of these microbes were previously unknown, and indeed we do not yet have very much data on their nature. We need to determine the range of subsurface environments that can support life and to study the organisms that live there, including their genomic makeup.
Some of the broad themes that run through study of subsurface life are the sources and flow of energy, and the minimum levels of energy that are required to support life. We need to know the temperatures, salinity, and availability of water in these environments. Of particular interest is the question whether any subsurface environments are truly isolated from the surface. If we conclude that life can exist below the surface without drawing on an oxidizing surface and atmosphere, there are obvious implications for the search for life on Mars or Europa, where subsurface life has been hypothesized.
These are all fascinating projects, some of which can be pursued today, but many of which will require new resources. In future months the members of the NAI and other astrobiologists as well will be looking for opportunities to advance these lines of research.