A New In Situ Quasi-continuous Solar-wind Source of Molecular Water on Mercury

Abstract

Radar observations of Mercury and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging ( MESSENGER ) spacecraft data indicate the probable existence of water ice in the permanently shadowed polar regions. Generally, water is accepted to be of exogenous origin through delivery via comets and meteoritic impact. However, a continuous water formation process that involves thermal transformation of chemically stable mineral-bound hydroxyl groups produced by implanted solar-wind protons is readily available on the surface of Mercury. At typical temperatures prevailing on Mercury’s dayside surface, H 2 O can be produced from reactions involving OH groups on or within the H-saturated regolith grain interfaces. Similar reactions will also occur due to micrometeorite impact events on both the dayside and nightside. Once produced, H 2 O is released into the exosphere and then transported and processed via Jeans escape, photodissociation, dissociative adsorption, or condensation. Water reaching cold traps will be bound over geological periods. This simple water cycle will produce a highly chemically reduced surface and can deliver significant amounts of H 2 O to the permanently shadowed regions of Mercury over geological time periods. The overall process is an important but hitherto unnoticed source term that will contribute to the accumulation of water in the cold traps and polar regions of Mercury.