Estimation of the dust flux near Mercury

Abstract

In this paper, we consider the dust flux in the vicinity of Mercury due to interplanetary particles using Divine's five population model (J. Geophys. Res. 98(E9) (1993) 17029). The average accretion rate is 16 metric tonnes per day. The (mass weighted) mean impact speed is 26 km s −1 . Due to Mercury's non-negligible eccentricity, the accretion rate is found to vary by a factor of two along the orbit. However, detection rates of interplanetary particles on an impact ionisation detector mounted on a Mercury orbiting spacecraft can vary by more than a factor of ten while Mercury travels around the Sun. The variations in detection rates will reveal the dependence of the interplanetary dust cloud on heliocentric distance and ecliptic latitude (in the range covered by Mercury's orbit). An impact ionisation detector, which is sensitive to particles in the mass range m=10 −16– 10 −9 g and has a target area of 100 cm 2 would detect of order 10 particles per day. A momentum sensor, which is sensitive to particles m>10 −12 g would detect one particle every 2–3 days. We estimate that the number of impact light flashes which could be detected by the high-resolution camera on the Planetary Orbiter, planned within the ESA's BepiColombo mission, is between 0.4 and 660 light flashes during the entire mission. This result is dependent on the fraction of energy which is converted to light during an impact on the surface of Mercury. Since Mercury's thin exosphere cannot protect the planet from hypervelocity impacts of interplanetary meteoroids, Mercury will be surrounded by the ejecta created during these impacts. For the impact generated dust cloud we adopt the model described by Krivov and Hamilton (Icarus 128 (1997) 335). Although the uncertainties of this model are large, we conclude that the ejecta should be detectable at least statistically, if the dust sensor is mounted on a spinning spacecraft which is in an elliptical orbit around Mercury.