The Phobos neutral and ionized torus

Abstract

Charged particle sputtering, micrometeoroid impact vaporization, and photon-stimulated desorption are fundamental processes operating at airless surfaces throughout the solar system. At larger bodies, such as Earth's Moon and several of the outer planet moons, these processes generate tenuous surface-bound exospheres that have been observed by a variety of methods. Phobos and Deimos, in contrast, are too gravitationally weak to keep ejected neutrals bound and, thus, are suspected to generate neutral tori in orbit around Mars. While these tori have not yet been detected, the distribution and density of both the neutral and ionized components are of fundamental interest. We combine a neutral Monte Carlo model and a hybrid plasma model to investigate both the neutral and ionized components of the Phobos torus. We show that the spatial distribution of the neutral torus is highly dependent on each individual species (due to ionization rates that span nearly 4 orders of magnitude) and on the location of Phobos with respect to Mars. Additionally, we present the flux distribution of torus pickup ions throughout the Martian system and estimate typical pickup ion fluxes. We find that the predicted pickup ion fluxes are too low to perturb the ambient plasma, consistent with previous null detections by spacecraft around Mars.

Phobos Surface Sputtering as Inferred From MAVEN Ion Observations

“…These enhancements and their durations can clearly be sufficient to significantly increase densities in the putative neutral torus of Phobos. Poppe et al () presented a numerical model of the Phobos neutral torus assuming a sputtered flux consistent with the long‐term value obtained in this study. A modeling effort could be conducted in the future to balance the increase of surface sputtered rate with the likely shortened neutral lifetime against ionization, to determine if and to what extent densities in the neutral torus of Phobos increased in March 2015.…”

“…In any case, both the increase of surface sputtered rate and ionization rate will result in an increase of the flux of ions created from the putative Phobos neutral torus. Poppe et al (2016) detail that these ions can be used to unambiguously detect the Phobos neutral torus, and section 5 of this study points out that periods of solar wind activity are conducive to this discovery.…”

Experimental Insights Into Space Weathering of Phobos: Laboratory Investigation of Sputtering by Atomic and Molecular Planetary Ions

“…Table 1 also shows a comparison to the Phobos surface composition used in the modeling by Poppe and Curry (2014) and Poppe et al. (2016), originally proposed by Vernazza et al. (2010) and Cipriani et al.…”

“…The values are averaged over the whole film, but very uniform film compositions were measured with ToF-ERDA. Table 1 also shows a comparison to the Phobos surface composition used in the modeling by Poppe and Curry (2014) and Poppe et al (2016), originally proposed by Vernazza et al (2010) and Cipriani et al (2011). It was derived from asteroid compositions, whose infrared spectra were found to agree well with Phobos.…”

“…Sputtering of Phobos has also been investigated as a particle source to form a neutral torus around Mars (Poppe et al., 2016). With a mean radius of 11 km, Phobos has a negligible gravitational pull and the resulting escape velocity is only 11.39 m/s (corresponding to about 7 × 10 −7 eV/amu).…”