Other
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p12c..01l&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P12C-01
Other
[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [5422] Planetary Sciences: Solid Surface Planets / Ices, [5462] Planetary Sciences: Solid Surface Planets / Polar Regions, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
It was the great geochemist Harold Urey who first called attention to peculiar conditions at the poles of the Moon where the very small inclination of the lunar spin axis with respect to the sun causes craters and other depressions to be permanently shaded from sunlight allowing very low temperatures. Urey suggested that the expected low temperature surfaces could cold trap and collect any vapors that might transiently pass through the lunar environment. Urey's notion has led to studies of the poles as a new research area in lunar science. The conditions and science of the poles are utterly unlike those of the familiar Moon of Neil Armstrong, and the study of the poles is similar to our understanding of the Moon itself at the dawn of the space age, with possibilities outweighing current understanding. Broadly, we can treat the poles as a dynamic system of input, transport, trapping, and loss. Volatile sources range from continuous, including solar wind, the Earth's polar fountain and micrometeorites, to episodic, including comets and wet asteroids, to nearly unique events including late lunar outgassing and passage through giant molecular clouds. The lunar exosphere transports volatiles to the poles, complicated by major perturbances to the atmosphere by volatile-rich sources. Trapping includes cold trapping, but also in situ creation of more refractory species such as organics, clathrates and water-bearing minerals, as well as sequester by regolith overturn or burial by larger impacts. Finally, volatiles are lost to space by ionization and sweeping. Spacecraft results have greatly added to the understanding of the polar system. Temperatures have been precisely measured by LRO, and thermal models now allow determination of temperature over the long evolution of the lunar orbit, and show very significant changes in temperature and temperature distribution with time and depth. Polar topography is revealed in detail by Selene and LRO laser altimeters while direct imaging of interiors of polar shadowed craters has been accomplished by many instruments from the ultraviolet to the radar. Imaging radars on Chandrayaan-1 and LRO have identified anomalous craters that may contain rich water ice deposits. Neutron spectrometers on Lunar Prospector and LRO directly detected hydrogen enhancements at both poles. Spectacularly, the LCROSS impact experiment detected a wide range of volatile elements and species at Cabeus crater in the lunar south polar region. While these measurements have catapulted polar science forward, much remains to be understood about the polar system, both from analysis of the current data, and new missions planned and in development. The general state of the lunar atmosphere is planned to be addressed by the UV and neutral mass spectrometers carried by the planned NASA LADEE (Lunar Atmosphere And Dust Environment Explorer) spacecraft creating an important baseline. But more data is necessary, from an in situ direct assay of polar volatiles to measurements of species and fluxes into and out of the cold traps over lengthy timescales.
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