Planetary Biology, Evolution and Intelligence
The Astrobiology Roadmap asks three fundamental questions: (1) How does life begin and evolve; (2) Does life exist elsewhere in the universe? and (3) What is the future of life on Earth and beyond? The SETI Institute will conduct a set of coupled research projects in the co-evolution of life and its planetary environment, beginning with fundamental ancient transitions that ultimately made complex life possible on Earth. We conclude with a project that brings together many of these investigations into an examination of the suitability of planets orbiting M stars for either singlecelled or more complex life. Results will help the next generation scientific Search for Extraterrestrial Intelligence (SETI) choose the 105 to 106 target stars that it will survey for signs of technical civilizations using the new Allen Telescope Array (ATA) being built by the SETI Institute in partnership with the University of California, Berkeley.
SETI is a natural part of the continuum of research that comprises astrobiology. This is recognized explicitly in the Astrobiology Roadmap, which calls for a strategy “for recognizing novel biosignatures” that “ultimately should accommodate a diversity of habitable conditions, biota and technologies in the universe that probably exceeds the diversity observed on Earth.” From the life detection point of view, the Roadmap notes that “although technology is probably much more rare than life in the universe, its associated biosignatures perhaps enjoy a much higher 'signal-to-noise’ ratio. Accordingly, current methods should be further developed and novel methods should be identified for detecting electromagnetic radiation or other diagnostic artifacts that indicate remote technological civilizations.”
Overview of Proposed Research
The research in the Institute’s proposal intends to elucidate the co-evolution of life and its planetary environment, typically investigating globalscale processes that have shaped, and been shaped by, both. Throughout, we recognize the importance of pursuing the planetary evolution aspects of this research in the context of comparative planetology: since laboratory experiments are impossible over some of the time and spatial scales relevant to early Earth, we must supplement laboratory data with the insight as we can gain by exploring extraterrestrial environments that may provide partial analogs to the early Earth environment and its processes.
We begin by proposing two new investigations into the oxidation of early Earth’s environment. While the Earth’s “oxygen transition” has been recently discussed in terms of biological mechanims, two nonbiological mechanisms are being explored here (peroxy in rocks [Drs. Friedemann Freund and Lynn Rothschild] and aerosol formation in the early Earth’s atmosphere [Dr. Emma Bakes], in part in analogy to processes now occurring in the atmosphere of Saturn’s moon Titan). If such mechanisms were to be shown to be quantitatively significant, it would suggest that the oxygen transition on an Earth-like world could take place independently of the invention of any particular metabolic pathways (such as photosynthesis or methanogenesis) that have been proposed as driving this transition. Since Earth’s oxygen transition ultimately set the stage for the oxygenbased metabolism evidently essential for metazoa, understanding this transition is crucial to elucidating both Earth’s evolution and the evolution of complex (including intelligent) life. Our geological investigations are tightly coupled with microbiological experiments to understand the extent to which the proposed mechanism might have led to the evolutionary invention of oxidant protective strategies and even aerobic metabolism.
One of the major sinks for oxygen on early Earth would have been reduced iron. At the same time iron could have provided shielding against ultraviolet (UV) light that would have been reaching Earth’s surface in the absence of the ozone shield generated by atmospheric oxygen. Nanophase ferric oxide minerals in solution could provide a sunscreen against UV while allowing the transmission of visible light, in turn making the evolution of at least some photosynthesic organisms possible. We will test [Drs. Janice Bishop and Lynn Rothschild] this hypothesis through coupled mineralogical and microbiological work in both the lab and the field, and examine its implications not only for Earth but for Mars as well—with an emphasis on implications for upcoming spacecraft observations.
The survival of microorganisms in very high UV environments can also be tested empirically through the exploration of Earth’s highest altitude lakes and ponds, in Bolivia and Chile. We propose [Drs. Nathalie Cabrol and Edmond Grin] a series of investigations of these lakes to examine the strategies employed by these microorganisms.
Just as globalscale changes in oxygen (or iron) were critical for the early biosphere, so too would have been global processes involving other key “biogenic” elements such as carbon [Dr. Bakes] or nitrogen [Drs. Rocco Mancinelli, Amos Banin, David Summers, and Bishun Khare]. We propose coupled laboratory and field research to understand the partitioning of nitrogen on early Earth—and on Mars—between different possible reservoirs, and the abiotic to biotic transition in this cycling.
The work described so far examines the evolution of planetary surface habitability. With the recognition that a subsurface ocean likely exists on Jupiter’s moon Europa, we know that habitability in possibly entirely subsurface environments must also be explored. We propose spacecraft data analysis and modeling to examine the geology of Europa and its implications for the free energy sources that would be needed to power a europan biosphere [Drs. Cynthia Phillips and Christopher Chyba]. We will then couple these results with terrestrial analog work [Chyba] and direct lowtemperature laboratory experiments [Dr. Max Bernstein] to make predictions about the possible abundance and survivability of any oceanic biomarkers that might reach Europa’s surface through active geology. These results will have implications for astrobiological exploration of Europa from either an orbiter or a surface lander.
Finally, we suggest research (Drs. Peter Backus, Jill Tarter, and Chyba) to examine the prospects of planets orbiting dwarf M stars being habitable for either microscopic or complex life. The results of this work will directly influence the strategy employed in the next generation SETI search program to begin in 2005.