The Young Age of the LAMP-observed Frost in Lunar Polar Cold Traps

Abstract

The Lunar Reconnaissance Orbiter/Lyman Alpha Mapping Project (LAMP) ultraviolet instrument detected a 0.5-2% icy regolith mix on the floor of some of the southern pole permanently shadowed craters of the Moon. We present calculations indicating that most or aU of this icy regolith detected by LAMP (sensed to a depth of <l μm) has to be relatively youngless than 2,000 years old-due to the surface erosional loss by plasma sputtering (external ionized gas-surface interactions), meteoric impact vaporization, and meteoric impact ejection. These processes, especially meteoric impact ejection, will disperse water along the crater floor, even onto warm regions where it will then undergo desorption. We have determined that there should be a water exosphere over polar craters (e.g., like Haworth crater) and calculated that a model 40-km-diameter crater should emit -10 H20 per second into the exosphere in the form of free molecules and ice-embedded particulates. Plain Language Summary Spacecraft orbiting the Moon have detect a low density water frost on the floor of some of the south polar craters-regions that are known to be very cold and can trap water and other volatiles. The floor of these craters is also exposed to the space environment including incoming meteors and ionized gases from the sun (that migrate into the shadowed craters via plasma expansion processes). We show that the flux of these external environmental processes can erode the frost-with a layer of 1/2 of a micron of frosty soil being eroded on time scales of less than 2,000 years. Further, the products of this erosion are ejected into space above the crater and thus could be detected by properly instrumented polar orbiting spacecraft.