Estimate of the neutral atoms' contribution to the Mercury exosphere caused by a new flux of micrometeoroids

¹ CISAS, University of Padova, via Venezia 15, 35131 Padova, Italy e-mail: patrizia.borin@unipd.it
² Department of Mineralogical and Petrological Science, via Valperga Caluso 35, 10125, Torino, Italy e-mail: marco.bruno@unito.it
³ INAF-Astronomical Observatory of Padova, Vicolo dell'Osservatorio 5, 35131 Padova, Italy e-mail: gabriele.cremonese@oapd.inaf.it
⁴ Department of Physics, via Marzolo 8, 35131 Padova, Italy e-mail: marzari@pd.infn.it

Received: 24 February 2010
Accepted: 7 May 2010

Abstract

Context. The planet Mercury has an extended and tenuous exosphere made up of atoms that are ejected from the surface by energetic processes, including hypervelocity micrometeoritic impacts, photon-stimulated desorption by UV radiation, and ion sputtering. The well known constituents of the Hermean exosphere are H, He, O, Na, K, and Ca but, from the new MESSENGER data from flybys, many others elements are expected, as for instance Mg.

Aims. Meteoroid impacts are an important source of neutral atoms in the exosphere of Mercury. We estimate the vapor and neutral atom production rates on Mercury caused by impacts of micrometeoroids of sizes between 5–100 m. The micrometeoritic flux is derived from a new statistical approach based on direct numerical integrations of dust particle trajectories under the action of the Poynting-Robertson drag and the gravitational attraction of all planets.

Methods. We included two different calibration sources for the meteoroid flux in our calculations of the vapor and neutral atoms and also considered both asteroidal and cometary sources for the dust. Three different surface compositions, which might be found on the planet, have been adopted, each with a different mass fraction of atoms in the regolith of the planet.

Results. We derive different values of neutral atom vapor production rates assuming different calibration sources for the meteoroid flux. The three simple mineralogical surface compositions show significant differences in the related production rates, and they are all greater than those reported in the previous papers assuming other dominant source mechanisms. Our neutral atom production rates are about one order of magnitude higher than the previous estimates. This implies that the impact vaporization has a much higher contribution than previously assumed.