NASA Astrobiology Institute Director’s Corner 5 January 2007

One of the most enjoyable responsibilities of being the NAI Director is staying on top of (or at least trying to!) all the great research done by NAI members and their colleagues. In just the last few months several major results were published by NAI members, in at least one case involving multiple NAI teams. Two of these studies were presented in Director’s seminars (high resolution podcasts available at http://nai.arc.nasa.gov/seminars/index.cfm) and a third will be in late January.

The first of the Director’s seminars was presented by NAI Postdoc Julie Huber. She reported on research by Mitch Sogin’s Marine Biological Laboratory NAI team that charted, for the first time, the underexplored “rare biosphere” of the deep oceans. It has been recognized for some time that the oceans harbor much more phylogenetic diversity than is present in the dominant microbial populations, and that this “genetic reservoir” may provide an enormous source of novel genetic information. Using a new high-throughput genetic sequencing technique, Mitch, Julie, and colleagues showed that there are thousands to tens of thousands of bacterial species present at abundances perhaps as low as a few per liter of seawater, in comparison to cumulative abundances of the dominant species of 108-109 per liter. The high degree of genetic diversity in this rare population indicates that it is old on evolutionary (and geologic) time scales.

One implication of these conclusions is that rare organisms could rapidly become dominant if a change in environmental conditions favored them over the then dominant population. Such changes in environmental conditions may have occurred globally in the geologic past, for example during “Snowball Earth” episodes when it appears that Earth was ice-covered to low latitudes, and can occur locally or regionally today in response to subsea volcanic eruptions, hurricanes, and other natural processes. The diversity and potential of this rare biosphere thus links the evolutionary timescales of interest to astrobiology with the much shorter timescales of interest to NASA’s Earth science program which focuses on understanding the Earth system’s response to natural or human-induced changes. Further, the techniques developed to characterize the rare biosphere are applicable to the detection of low abundance organisms on spacecraft we send to other planets to study habitable environments or search for evidence of life. This research thus illustrates how astrobiology can, and does, connect different aspects of NASA’s programs while advancing in fundamental ways our understanding of life on Earth.

A second discovery about Earth’s microbial ecosystems has direct implications for the potential of extraterrestrial environments to harbor life. The Indiana-Princeton-Tennessee NAI team (IPTAI) led by Lisa Pratt of Indiana University and T.C. Onstott of Princeton University has discovered an isolated microbial community nearly two miles underground that appears to derive all its energy from radioactive decay rather than directly or indirectly from sunlight. In this case radioactive decay leads to the decomposition of water into hydrogen and oxidants that convert naturally occurring sulfide minerals to sulfate. The dominant microbial species in this ecosystem then reduces the sulfate using the hydrogen as an electron donor, reforming water in the process. Isotopic noble gas analyses of the ecosystem’s bulk water indicate that it has been completely isolated from surface processes for millions to tens of millions of years. The ecosystem thus appears able to maintain itself completely independently of sunlight. If indeed strictly geological processes such as radioactive decay and geochemical oxidation can sustain microbial communities indefinitely, then subsurface environments on Mars and other rocky planets could potentially sustain life even though they lack a surface photosynthetic biosphere. Lisa and TC will present this research in the next Director’s seminar on January 29 (see the announcement elsewhere in this newsletter).

Finally, another NAI Postdoc and an NAI-supported graduate student working with a more senior team member have demonstrated that extrasolar planetary systems with “hot Jupiters,” i.e., Jupiter-size planets close to their parent stars, may also contain terrestrial planets in the habitable zone where liquid water can exist on a planet’s surface. This conclusion is somewhat surprising because hot Jupiters most likely formed much farther from their parent stars, in the cold outer regions of a proto-planetary disk, and then migrated inward through the habitable zone to the orbit in which we observe them. Some earlier studies had indicated that this migration would prevent the formation of terrestrial planets.

Postdoc Sean Raymond, graduate student Avi Mandell, and Steinn Sigurdsson modeled terrestrial planet growth during and after giant planet migration. They found that, in general, terrestrial planets can form in the “wake” of an inward migrating giant planet, and that terrestrial planets can form in the habitable zone when the final giant planet orbit is inside ~0.5 AU. Further, if there is not an additional giant planet that remains at several AU to gravitationally scatter icy planetesimals from the outer regions of the disk, then those icy bodies arrive at the habitable zone planet in great number, leading potentially to the formation of an “ocean world.” This work, which involved the NAI teams at the University of Colorado, Pennsylvania State University, the NASA Goddard Space Flight Center, and the Virtual Planetary Laboratory, indicates that a third of the roughly 200 known extrasolar planetary systems may contain a habitable zone terrestrial planet. This study and others to follow will help us prepare to interpret data from NASA’s Kepler mission, scheduled to launch in 2008, which will study planetary system architectures with the goal of identifying systems with habitable zone terrestrial planets.

These studies illustrate just a few of the ways that astrobiology provides an overarching framework for much of NASA science. Other connections are being made between field research, laboratory studies, and space missions. Check our website frequently for other research highlights (http://nai.arc.nasa.gov), continue reading our electronic newsletter for the latest results (available through the website or by sending a request to mboldt@arc.nasa.gov to be added to the distribution list), and participate in our electronic seminars or download the podcasts afterward. There’s lots more to come!