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

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 μm. 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.

Escape rates and variability constraints for high‐energy sodium sources at Mercury

“…Our derived Na production rates that match the observed tail brightness are 3.6 × 10 6 and 1.8 × 10 6 atoms cm −2 s −1 for a 3000 and 5000 K vaporized population, respectively, if this was the only source of escaping sodium. This is approximately the minimum meteoritic source rate determined by Borin et al [2010] accounting for asteroid dust migration into the inner solar system. Even at 5000 K, however, the width of the tail is significantly narrower than our observations, and analysis of the LCROSS impact plume suggests a temperature of only 1000 K, which may be more realistic [ Killen et al , 2010].…”

“…[47] Burger et al [2010] estimated an impact vaporization source rate of 3.5 Â 10 5 atoms cm À2 s À1 , about half that assumed by . The recent study by Borin et al [2010] suggests it may be substantially higher still due to migration of dust particles by the Poynting-Robertson effect. Our derived Na production rates that match the observed tail brightness are 3.6 Â 10 6 and 1.8 Â 10 6 atoms cm À2 s À1 for a 3000 and 5000 K vaporized population, respectively, if this was the only source of escaping sodium.…”

“…Simulations of these more energetic mechanisms reproduce the gravitational escape rates, while staying well below the expected dayside column abundance. The source rate of vaporized sodium caused by micrometeoroids was not constrained, as the impacting flux at Mercury is highly model dependent [e.g., Borin et al, 2010]. [32] Table 1 quantifies the escape rates requisite for the observed brightness during the 18 May 2008 tail observation.…”

Monte Carlo modeling of north‐south asymmetries in Mercury's sodium exosphere

“…The dashed line in Figure d shows the result of a 15 times increase in micrometeor vapor. Such intense meteoritic flux is consistent with recent estimates [e.g., Borin et al ., ], and the width of the tail at distance is better reproduced compared to localized PSD enhancements at the cusp footprints. Yet such high impact production rates are not consistent with this data set, since adding them to nominal values for midlatitude sputtering and PSD yields a negligible north‐south asymmetry, even with a κ of 9.…”

Small meteoroids’ major contribution to Mercury’s exosphere

“…In the vicinity of comet Halley, dust in the mass range of 10 −15 –10 −6 g was calculated to have a density of 3.5 g cm −3 , while in the 10 −2 –10 5 g range the density dropped to 0.3 g cm −3 (Hughes, 1979). In the course of this analysis we assume that the meteoroids have a mass density of 2.5 g cm −3 , which is the most recent and more often cited value (Borin et al, 2010; Bruno et al, 2007; Cremonese et al, 2005; Grün et al, 1985; Love and Brownlee, 1993; Mann et al, 2004). …”