The Na exosphere reservoir for Mercury and the Moon: Models constrained by MESSENGER and LADEE data

Abstract:

The Na exosphere of Mercury is flux-limited, and a careful accounting of the surface reservoir is necessary in order to understand the relative importance of proposed source mechanisms for this exosphere. At the Moon, a similar analysis has not yet been performed, but recent data acquired by the Kaguya spacecraft suggest an analogous depletion of the dayside reservoir for exospheric Na. New measurements of the lunar exosphere obtained by the Lunar Atmosphere and Dust Environment Explorer (LADEE) and of Mercury's exosphere by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provide the opportunity to constrain the main parameters determining the reservoirs for these exospheres. We present a model of the exosphere-surface system that attempts to unify these two datasets. We have developed a simulator that accepts as input partially constrained microphysical parameters of the gas-surface interaction (e.g., source rates/cross sections for different source processes, degree of thermal accommodation) to make testable predictions regarding the exosphere and uppermost surface (top 10^-10 m). These flux-balance simulations demonstrate that the lunar exosphere is limited by the recycling rate of Na atoms mobilized by micrometeoroid vaporization. An important consequence is that, to obtain consistency with ground-based observations of the Na exosphere, the inferred impact vapor at the Moon must peak near the equator and decrease towards the poles because of migration of surface particles toward the poles through exospheric transport. Micrometeoroid streams can have a long-term effect on the lunar exosphere because, as shown in our model, particles introduced by such streams survive in the soil and exosphere for at least two lunations. Important but secondary effects must be provided by the solar wind in order to account for the variations observed within one lunation by LADEE. In its application to Mercury, the code uses new estimates of the micrometeoroid impact vaporization rate and solar wind influx onto Mercury's high-latitude regions. We demonstrate that a model similar to that for the Moon can approximately reproduce the near-surface densities and the dawn-dusk asymmetry in the Na exosphere observed by MESSENGER during part of a Mercury year.