Yung-Ching Wang

Modeling of the O+ pickup ion sputtering efficiency dependence on solar wind conditions for the Martian atmosphere

Sputtering of the Martian atmosphere by O(+)pickup ions has been proposed as a potentially important process in the early evolution of the Martian atmosphere. In preparation for the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we performed a study using a Monte Carlo model coupled to a molecular dynamic calculation to investigate the cascade sputtering effects in the region of the Martian exobase. Pickup ion fluxes based on test particle simulations in an MHD model for three different solar wind conditions are used to examine the local and global sputtering efficiencies. The resultant sputtering escape rate is 2x10(24)s(-1) at nominal solar wind condition and can be enhanced about 50 times when both the interplanetary magnetic field (IMF) strength and the solar wind pressure increase. It is found that when the IMF strength becomes stronger, both the pickup ion precipitation energies and the resultant sputtering efficiencies increase. The related escape flux, hot component, and atmospheric energy deposition deduced from the MAVEN measurements may reveal clues about the prominent enhanced sputtering effects. Significant hemispheric asymmetries can be observed related to the solar wind electric fields. The shielding by the crustal fields and the recycling onto the nightside due to different magnetic field draping features can also lead to regional variations of sputtering efficiencies. The results suggest that disturbed or enhanced solar wind conditions provide the best prospects for detecting sputtering effects for MAVEN mission.

Statistical studies on Mars atmospheric sputtering by precipitating pickup O+: Preparation for the MAVEN mission

With the upcoming MAVEN mission, the role of escape in the evolution of the Martian atmosphere is investigated in more detail. This work builds on our previous modeling of the atmospheric impact of the the pickup O+ sputtering effects for varioussolar wind parameters, solar EUV intensities, and the surface crustal field distributions. Relationships between the incident ions properties and the ejected hot neutral components, often referred to as atmospheric sputtering, are derived for application to proposed MAVEN ion spectrometer measurements of precipitating O+. We show how our simulation results can be used to constrain the sputtering effects under present conditions and to interpolate toward estimates of sputtering efficiencies occurring in earlier epochs. Present-day sputtering under typical circumstance is estimated to be weak, but possibly detectable as an exospheric enhancement. The ultimate goal of estimating the importance of atmospheric sputtering effects on the evolution ofthe Martian atmosphere can be better deduced by the combining MAVEN measurements with models, and the sputtering response relations derived here.