In-situ production of organic molecules at the poles of the Moon

Details In-situ production of organic molecules at the poles of the Moon In-situ production of organic molecules at the poles of the Moon

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1730c&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P13D-1730

Biology

[5200] Planetary Sciences: Astrobiology, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

Samples returned by the Apollo missions showed no trace of organic materials. However, the poles of the Moon are utterly unlike the equatorial regions, and the LCROSS impactor detected a range of organic compounds including C2H4, CH3OH, and CH4 (Colaprete et al., 2010). These compounds may be of cometary origin, or they may have developed in situ. The lunar poles feature plausible conditions for production of organics from indigenous inorganic material and may provide an opportunity to test models of inorganic synthesis that can be applied to many surfaces in the solar system and interstellar clouds. Production of organics in situ requires the presence of the relevant elements (combinations of C, H, O, and N), sufficient mobility of elements to react with one another, and an energy source to drive reactions. Because of the low obliquity of the Moon, regions in topographic lows at the poles are permanently shaded from sunlight and measurements from the Diviner Lunar Radiometer have confirmed the extremely cold nature of some of these regions (Paige et al., 2010). The temperatures are low enough to trap even very volatile ices such as CO, but these low temperatures can also inhibit ion mobility. However, indirect illumination by light reflected off local topographic highs as well as the bombardment of the lunar surface by meteorites create temperature variation in the top 20 cm of regolith and expose icy material to a range of depths and temperatures. In addition to the presence of organic elements and temperature cycling, an energy source is needed to break bonds and enable reactions. Possible energy sources include scattered interstellar Lyman alpha UV radiation and galactic cosmic ray protons. However, Lyman alpha is confined to the optical surface and erodes surface ice (Morgan and Shemansky, 1991), so we investigate the deeper penetrating protons in the upper few centimeters where ices are better protected from loss. Laboratory measurements have demonstrated that energetic protons can stimulate organic synthesis in simple mixtures of C, H, O, and N-bearing ices. For example, a column density on the order of 10^17 molecules/cm^2 of CH3OH was produced from an H2O + CH4 ice mixture after a proton irradiation dose of 10 eV/molecule, and rose with increasing dose (Moore and Hudson, 1998). This establishes a rough order of magnitude for the dose required in the uppermost surface to produce organics from simple ices. We use the particle transport code MCNPX to calculate proton flux from cosmic rays at the poles to determine a dose rate and use lab measurements (Moore and Hudson, 1998) to estimate the production of organics from this process over time. Colaprete, A, and 16 co-authors (2010), Detection of water in the LCROSS ejecta plume, Science, 330, 463-467. Moore, MH and RL Hudson (1998), Infrared study of ion-irradiated water-ice mixtures with hydrocarbons relevant to comets, Icarus, 135(2), 518-527. Morgan, TH and DE Shemansky (1991), Limits to the lunar atmosphere. J. Geophys. Res. 96(A2), 1351-1367. Paige, DA, and 26 co-authors (2010), Diviner Lunar Radiometer observations of cold traps in the Moon's South Polar region, Science, 330, 479-482.

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