Review of laboratory and modeling efforts to understand and predict the evolution of water and hydroxyl on the Moon

Details Review of laboratory and modeling efforts to understand and predict the evolution of water and hydroxyl on the Moon Review of laboratory and modeling efforts to understand and predict the evolution of water and hydroxyl on the Moon

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13h..02h&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P13H-02

Mathematics

Logic

[6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

As part of the Lunar Science Institute node on polar environment, experimental and theoretical investigations into the interaction of water with lunar analogs under lunar surface conditions have been conducted with results used to simulate the evolution of water and hydroxyl over the lunar surface. To determine the thermal stability of water and hydroxyl on the different terrains of the Moon, temperature programmed desorption experiments were conducted on lunar analogs (albite, JSC1A, basaltic glass) and activation energies for their desorption were derived (1,2,3). Results demonstrate a compositional and crystallinity dependency on the adsorption of water and hydroxyl. Minerals with highly electronegative charge-compensating cations, such as feldspars, adsorb more water, more strongly than do simpler silicate minerals or glasses. Activation energies range from weakly bound like water-ice on glass or stronger for chemisorbed water, which has an activation of over 1 eV (3,4). These results are consistent with spacecraft observations of greater water and hydroxyl abundances associated with anorthositic terrain. As a partial model of the hydrologic cycle in the upper meter of the Moon, a Monte Carlo simulation was developed using input parameters including a range of desorption energies for water and hydroxyl, solar wind flux, diurnal surface and subsurface temperatures, surface composition, and which includes processes for surface saturation and reduction by solar wind, as well as rejuvenation by regolith turn-over [5]. Implantation of keV solar wind protons will produce hydroxyls in the upper 10 - 20 nanometers by scavenging oxygen from the silicate minerals and forming Si-OH. Over a lunation, hydroxyl will recombinatively desorb as water and possibly H2 at illuminated non-polar latitudes. Desorbed H2O will re-accumulate at cold traps, both locally and at the poles. The model predicts an accumulation of molecular water at the poles, the formation of hydroxyl in mornings and afternoon, and their thermal loss during mid-day, with a steady-state being reached in ~30 years. These model results are in general agreement with the observed distribution of water and hydroxyl on the Moon as observed by Epoxi and the Chandrayaan-1 M3 [6,7]. [1] Dyar, M. D. et al., (2010), Icarus, 208, 425. [2] Hibbitts, C. A. et al. (2011), Icarus, 213(1), 64. [3] Poston et al., (2011), JGR, submitted. [4] Poston et al., (2011), this conference. [5] Grieves et al.(2010), 41st LPSC, # 1533, 2552. [6] Sunshine, J. M. et al. (2009), Science, 326(5952), 565. [7] Pieters, C. M. et al. (2009), Science, 326(5952), 568.

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