Modeling ion density in the exospheres of moons and other small bodies

Abstract:

The exospheres of small bodies in space can form when material leaves the surface through a variety of mechanisms such as heating and outgassing, micrometeorite impact, or ion bombardment. Mass spectrometers such as those carried on the LADEE, Rosetta, and Cassini spacecraft have shown that analysis of the gaseous material can provide a powerful means of determining information about the surface properties of the body being studied. However, most missions have focused on the detection of neutral gas and volatile species and were unable to provide details about the metal composition of the bodies studied. Ion mass spectrometry allows ejected ions of any type to be directly detected and has long been proposed as a means of determining the surface composition of bodies. Impacting solar wind or magnetospheric ions eject or sputter material from the surfaces, of which ~1% leaves the surface in an ionized state. Previous estimates for sputtered atomic densities have relied on popular ion implantation simulation software TRIM which has been shown to give incorrect results for light ions (H and He) at solar wind energies. Therefore, we have compared results from previous models to simulations carried out using the SDTrimSP code which was developed for low energy ion implantation. Sputtering yield estimates were then used to estimate the relative ion densities for a representative suite of meteorite compositions. Comparisons of model results with sputtering measurements from lunar soil are shown to match well, and a method of using ratios the ratios of detected elemental abundances as a means of identifying a object’s composition with a given meteorite classification will be presented.