Density Predictions of Mercury’s Exosphere Utilizing HEMO, the Hermean Exosphere Model of Oxygen, Including the Effects of Photodissociation

Abstract:

The work presented here builds on the results of Grotheer & Livi [2014], which found that the majority of the vapor produced due to meteoroid impacts on Mercury is caused by meteoroids with masses 4.2 x 10^-7 g ≤ m ≤ 8.3 x 10^-2 g. Meteoroids with a mass of 2.1 x 10^-4 g are the largest contributors to the vapor released by meteoroid impacts, thus here we focus on meteoroids with such masses as an input to a particle tracing simulation called the Hermean Exosphere Model of Oxygen (HEMO).

The HEMO simulations include 36 different particle species which can be released via meteoritic impact vaporization, based on the abundances determined by Berezhnoy & Klumov [2008]. After the initial simulation of the meteoroid impact, the released particles are affected by the gravitational pull of the planet Mercury, as well as the Sun’s radiation. Particles may be photoionized or in the case of molecules also photodissociated. Due to the effects of photodissociation, a total of 38 species are actually present in the simulation, since 2 species are not directly released by impact vaporization but may be created due to photodissociation.

These simulations record various pieces of information about each simulated particle, including position and velocity, for each time-step of the model. This information is then utilized to construct density profiles for each simulation run, as well as for aggregates of simulation runs with similar input parameters. The results are intended to aid the interpretation of results from the MESSENGER and BepiColombo missions to Mercury, with a particular focus on atomic and molecular oxygen.

References

Alexey A. Berezhnoy and Boris A. Klumov. Impacts as sources of the exosphere on Mercury. Icarus, 195(2): 511–522, 2008.

Emmanuel B. Grotheer and Stefano A. Livi. Small meteoroids' major contribution to Mercury's exosphere. Icarus, 227(1): 1-7, 2014.