Simulations of the Na+/O+ ratio observed by MESSENGER and implications for oxygen-bearing volatiles in Mercury's exosphere

Heavy ions have been observed near Mercury by the Fast Imaging Plasma Spectrometer (FIPS) during every orbit of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The majority of these heavy ions fall into two groups, consisting of mass per charge ranges of 21-30 amu/e and 16-20 amu/e. These groups are most likely dominated by Na+ and O+ ions, respectively, that originated from Mercury's exosphere and surface. We present the first comparative models of these ion species using magnetospheric fields derived from a magnetohydrodynamic (MHD) simulation of Mercury's magnetosphere. The expected ion production rates were initialized using exospheric models and experimental values of ionic yields measured with simulants of Mercury soil. Three magnetospheric states were simulated, two in which the interplanetary magnetic field is purely northward and southward, respectively, and one representing conditions encountered by MESSENGER during its first Mercury flyby. We find that we can account approximately for the distribution and flux levels of Na+ observed by FIPS with purely a photoion source for this species. In contrast, the observed Na+/O+ ratio cannot be reproduced with only an atomic oxygen exosphere. This result suggests either that O+ is released by precipitating electrons onto the surface with yields of ~0.001 O+ per incident electron, or that additional oxygen-bearing gases must be present in the exosphere. From our models and FIPS observations, we derive stringent limits for exospheric water, OH, CO, and CO2 that are two to three orders of magnitude less than previous limits from Mariner 10 observations.