LIFE AND DEATH ON MARS AND EARTH. K. Zahnle1 and N. H. Sleep2, 1Mail Stop 245-3, NASA Ames Research Center, Moffett Field CA 94035, USA ([email protected].nasa.gov), 2Department of Geo- physics, Stanford University, Stanford CA 94305, USA.

Failure to discover life on Mars has led a vival in deep subsurface environments appears great many experts to conclude that it must be more likely on Mars because (i) Mars's lower hiding. Where? The likeliest hiding places are background heat flow and lower gravity (the lat- deep beneath the surface, where geothermal heat ter permitting open porosity at greater depths) could permit liquid water. In this the search for allow deeper colonies, and (ii) the thermal heat life on Mars parallels the search for water on pulse from a major impact is briefer, and so Mars. Liquid water has been, at least on occa- penetrates less deeply. A third factor that favors sion, a geologically significant presence on the ultimate survival on Mars rather than Earth is surface. Channels were cut and plains dissected. that the bigger planet statistically experiences This water is now hidden, in all likelihood having more and bigger impacts. Impacts on the order of drained to the base of the porous regolith, where 1034 ergs are known to have occurred on the it fills possibly frozen aquifers. Presumably any Moon (S. Pole-Aitken) and Mars (Hellas), and surviving biota has followed the water from the 1035 erg impacts are likely on Earth. A 1035 erg surface to its hiding places in the deep. impact on Mars would sterilize only the upper Accordingly, we have extended our environ- few hundred meters of the subsurface, while a mental impact assessment of the environmental similar impact on Earth would boil the oceans hazards posed by large asteroid and comet im- and sterilize the subsurface to 1 km depth. pacts to Mars, and compare its case to Earth's. In An additional refuge that is poorly under- particular, we address the continuous habitability stood is ejection of weakly shocked material to of surface and subsurface environments. The space by impacts and its return to its home planet chief danger comes from globally distributed or transfer to a neighboring planet. It is clear that ejecta, which for very large impacts take the form such transfer is plausible from Mars to Earth of transient thick rock vapor atmospheres; both (Gladman et al. 1995), and it is not unlikely that planets suffered such impacts repeatedly. Ex- such transfer can go from Earth to Mars. The posed surfaces are sterilized. On Earth much of huge number of impact events on early Earth and the thermal radiation from the atmosphere is ab- Mars opens the possibility that we descend from sorbed by the ocean, the surface of which boils. an ancestral race of inadvertant spacefarers. This Shallow water quickly evaporates. Any surviving view of the solar system has gone from specula- life lives either in deep water or well below the tion to truism in less than five years (Davies, surface. On Earth, global thermal excursions are 1999). Overall, early Mars may have been safer buffered by the heat capacity of the oceans, espe- from impact sterilization than early Earth and cially the latent heat of condensation. But when probably was habitable before the Earth-Moon impacts are large enough to vaporize the oceans system formed. (>1035 J), thermal buffering serves only to pro- References: Sleep, N. H., & K. Zahnle long the disaster: very high surface temperatures (1998). "Refugia from asteroid impacts on early are maintained for the thousands of years it takes Mars and the early Earth." J. Geophys. Res. 103, to condense and rain out 270 atmospheres of 28529-28544. water vapor. Zahnle, K. & N. H. Sleep (1997). "Impacts Mars presents several differences. Without and the early evolution of life." In Comets and oceans thermal buffering does not occur; there- the Origin of Life, P. Thomas, C. Chyba, and C. fore relatively small impacts (1033 erg) fre- McKay, eds, Springer-Verlag, pp. 175-208. quently heat the surface everywhere to near the Sleep, N. H., K. Zahnle, J. F. Kasting & H. melting point. However, owing to the low mar- Morowitz (1989). "Annihilation of ecosystems tian escape velocity, the most energetic ejecta by large asteroid impacts on the early Earth." (including much of the rock vapor) more easily Nature 342, 139-142. escape to space, while massive quantities of less Davies, P. (1999). The Fifth Miracle: The energetic ejecta are globally distributed. Martian Search for the Origin and Meaning of Life. rock vapor atmospheres do form when ejecta Simon & Schuster. reenter the atmosphere, but the energy density is Gladman, B., Burns, J., Duncan, M., Lee, P., lower than for Earth. Obviously, survival in open & Levison, H. 1996. "The exchange of impact water is more likely on Earth, where the energy ejecta between terrestrial planets." Science 271, required to boil the oceans is enormous. But sur- 1387-1392.