HR: 1340h
AN: ED13D-0745    [Abstracts]
TI: Methane on Mars- Origin, Loss, Implications for Extinct or Extant Life
AU: * Atreya, S K
EM: atreya@umich.edu
AF: University of Michigan, Atmospheric, Oceanic and Space Sciences, 2455 Hayward St, Ann Arbor, MI 48109 United States
AU: Wong, A
EM: aswong@umich.edu
AF: University of Michigan, Atmospheric, Oceanic and Space Sciences, 2455 Hayward St, Ann Arbor, MI 48109 United States
AB: Detections of methane on Mars have been reported by two ground-based and one orbiter spacecraft. The global mixing ratio, 10 ppbv, is nearly a factor of 200 smaller than in the earth's atmosphere. On earth, the ultimate source of methane is predominantly biological. Biogenic source is a tantalizing possibility for the martian methane as well. Chemolithotrophic microbes, especially methanogens, that utilize CO and/or H$_2$ of the martian atmosphere or H$_2$ produced in sepentinization, i.e. hydration of ultramafic silicates, can produce CH$_4$ as a product of their metabolism. Cometary impacts are the most significant exogenous source of methane, but they fail to maintain a "steady state" mixing ratio CH$_4$ on Mars, considering an impact rate of 1 in 62 million years. On the other hand, an impact a few hundred to thousands of years in the past is not unlikely. We find that an impact of a ~160 m radius comet 100 years ago or a ~360 m radius comet 2000 years ago, for example, could supply sufficient CH$_4$ so that it would have steadily declined to the present global value of 10 ppbv. Hydrothermal processes now or in the past are also capable of producing methane on Mars. If aquifers exist or existed below the martian permafrost, numerical models show that up to 10$^{15}$ tons of CH$_4$ could have been produced in the process of alteration of the martian basalt at T$<$150 C. Water-rock reactions at higher temperature of 350-400 C produce methane, and are seen as black smoker vents several kilometers deep in the oceans on earth. Such methane producing processes could be occurring in the martian interior as well. It is important to note that despite the relatively short 400-1000 year lifetime of methane on Mars, its source need not be current, if a stable CH$_4$ -hydrate formed following abiotic (hydrothermal) or biogenic production (extinct microorganism metabolism) process. In that case methane could gradually leak out through hot spots. Any variation of CH$_4$ over Mars may be indicative of either currently active microbial colonies or localized subsurface hot spots but probably not a cometary source, unless a non uniform surface loss mechanism exists. The presence of oxidants in the surface, such as hydrogen peroxide produced due to triboelectric fields in dust devils and storms (Atreya et al., this meeting), may accelerate the removal of methane from Mars. EPO implications of such work will also be discussed.
DE: 6200 PLANETOLOGY: SOLAR SYSTEM OBJECTS (New field)
DE: 6207 Comparative planetology
DE: 6225 Mars
DE: 6605 Education
DE: 0614 Biological effects
SC: Education and Human Resourcese [ED]
MN: 2004 AGU Fall Meeting