Lunar Surface Hydration Constrained by LADEE's Observations of Exospheric Water
Abstract
It has long been thought that the Moon was dry upon its formation and therefore devoid of native water. Recent observations suggest that water from external sources may be extensive on the lunar surface. Here, we report the first detection of signatures of near-surface water released into the lunar exosphere using observations collected by the Neutral Mass Spectrometer onboard the Lunar Atmosphere and Dust Environment Explorer spacecraft during its 8 month-long mission. The water signatures were measured as 736 distinct episodic, short-lived signal increases above the instrument background level of 5 cm^{-3}. The most intense event was recorded with a magnitude of 600 cm^{-3}. An analytical description of the behavior of the instrument when subjected to exospheric water reveals that these fluctuations in instrument background capture variations of the underlying source of water released into the exosphere. Our study shows that the intensity and frequency of these water events are correlated with the Moon's encounter with known intense meteoroid streams. These fluctuations carry information on both the nature of the meteoroids that triggered the water release into the exosphere and the nature of the reservoir that was sequestering the water beforehand. In order to shed light on the nature of the water reservoir, we use a Monte Carlo model to simulate the evolution of released water produced by the impact of meteoroids on the surface of the Moon. The model shows that the signatures detected by the NMS instrument are commensurate in size and distribution with vapors released from shallow depths in the lunar soil by impacts that occurred at various distances from the spacecraft. Using this model, information about water concentrations in the lunar soil and its variation with depth were inferred by comparing the distribution of the intensity of the observed water events against the distribution that would result from a given impactor flux distribution.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.277B