AAS 204th Meeting, June 2004
Session 21 Moon, Mercury, Vesta and Late Stellar Evolutio
Oral, Monday, May 31, 2004, 10:00-11:30am, 610/612

## [21.02] A Lyman Alpha Light Source and Diagnostic for Detecting Exposed Water Frost in Permanently Shadowed" Terrains of Mercury and the Moon.

S.A. Stern (SwRI)

At the poles of both the Moon and Mercury there exist cold trap regions that are believed to contain volatiles of various kinds, most notably including H2O ice. The stability of these volatiles relies on the fact that the cold traps are devoid of direct sunlight, which would otherwise warm the surface temperatures in these regions to levels which would sublimate away the volatiles on timescapes that are short compared to the age of the solar system. All observational evidence for the accumulation of volatiles in the lunar and Mercurian cold traps is indirect (e.g., in the form of ice-like radar backscatter signatures [and in the case of the Moon anomalously high concentrations of H in the uppermost regolith]).

As a result of the very darkness which allows the cold traps to retain volatiles, it is widely thought that there is no light in the cold traps-- thus making them impossible to observe without active (e.g., radar) illumination. This dilema has also prevented the direct, definitive spectrosopic identification of volatiles in the Mercurian and lunar polar cold traps.

Here I will quantitatively describe a new technique for directly observing the interior of these so-called, permanently shadowed terrains" by orbiting spacecraft around these two bodies, and determining if exposed H2O-ice exists there. The technique relies on two facts: (i) resonantly scattered Ly\alpha at 1216 Åfrom the interplanetary medium provides a clearly detectable (if monochromatic) lightsource that illuminates the cold traps and (ii) H2O-ice displays a characteristic and well-known absorption feature in the far UV.

Bulletin of the American Astronomical Society, 36 #2
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