Determining the Sputtered Secondary Ion Densities at Phobos and Deimos: A combined computational and experimental study

Introduction: Space weathering by ion irradiation is ubiquitous on the surfaces of airless bodies in the Solar System. Sputtering occurs when solar wind (SW) or magnetosphere ions (MI) impact the suraces of bodies in space. Asteroids and moons are too small to maintain a significant atmosphere, and therefore they are directly exposed to ionizing radiation from the solar wind and magnetospheric plasmas. Incident ions can transfer sufficient energy to surface species to cause them to desorb and potentially escape to space. A small fraction of the sputtered species can escape as ions, called sputtered secondary ions (SSI). Mass, charge, and energy analysis of the sputtered ions using secondary ion mass spectrometry is highly diagnostic of the irradiated surface composition. The upcoming JAXA MMX mission will carry a Mass Spectral Analyzer (MSA) instrument will be capable of making measurements of SSI around its target bodies Phobos and Deimos (P&D). However, there is currently limited estimates of SSI yields from relevant surface compositions under relevant irradiation conditions, and the expected SSI fluxes around P&D are not well constrained.

Background: Although P&D are exposed to both the SW and MI and SSI are expected to be present throughout their orbits. However, several challenges arise when attempting to derive a precise surface composition from a measured SIMS spectra, or when estimating the expected count rates and elemental ratios that will be observed by MSA for a given composition: (i) the relative abundances measured by SIMS are not directly correlated with the actual surface composition, and (ii) the relative and absolute SSI yields (# of SSI ejected per incident ion) likely depend on the surface chemistry and exposure history, and on the incident ion type and energy.

Results: A combined computational and experimental approach has been used in order to better constrain the solar wind sputtering rates of small, rocky bodies. First, a series of SIMS measurements in the laboratory were carried out to determine the relative ion sputtering ratios from several lunar samples of known composition. Then, using Monte Carlo simulations of sputtering due to both solar wind and magnetosphere ions and the measured SSI energy distributions to determine the total sputtering yields, the total abundance and relative composition of sputtered ions can be determined for an arbitrary small body. This work will (1) estimate the the SSI yields from analog Mars and Carbonaceous Chondrite analog materials and correlate the expected yields with the surface composition, and (2) provide estimates the SSI fluxes and densities during their orbits around Mars. Further, this work will demonstrate how measurement of the elemental ratios of SSI can be used to estimate the potential origins scenarios for small bodies.