Dynamics of submicrometric dust grains in Mercury’s exosphere

• We study the dynamics of the impact ejecta grains of the exosphere of Mercury inside its Hill sphere. • We focus on the submicrometric population as the fastest ejecta grains. • We estimate lower limits of size and speed for the detection of ejecta particles. The exosphere of Mercury is a collisionless-low-density layer of gravitationally bound atoms and molecules that extends from the surface up to several planetary radii. Most of these molecules follow ballistic trajectories after being released through different mechanisms like vaporisation from the hypervelocity impacts of micrometeoroids (with impacts speeds > 80 km / s ) mainly of cometary origin that also excavate considerable amounts of dust from which a dust exosphere is built. In the case of the Moon’s dust cloud, the exospheric dust population observed by Lunar Dust Experiment (LDEX) on board the Lunar Atmosphere and Dust Environment Explorer (LADEE) comprise grains with radii ≳ 0.3 μ m . It is expected that the impact dust grains of the clouds of Mercury and the Moon have similar sizes, however, given that Mercury is subject to much more intense weathering processes compared to the Moon a somewhat smaller ejecta dust grain population may be present at Mercury. The aim of this work is to study the dynamics of the impact ejecta grains of the exosphere of Mercury inside its Hill sphere, where the exosphere is confined. We focus on the submicrometric population and consider, not only grains with radii ∼ 0.1 μ m (as those observed at the Moon), but we will explore the possibility of grains as small as 10 nm , considering that the smallest (submicrometric) ejecta grains, are able to achieve the largest speeds. Although we are able to estimate some properties of the exosphere, one question that we try to answer is whether instruments like the Mercury Dust Monitor (MDM) on board the Mercury Magnetospheric Orbiter, also known as Mio, would be able to detect some of the grains of the hermean dust exosphere once in orbit around Mercury. Mio, from the Japan Aerospace Exploration Agency (JAXA) is part of the BepiColombo mission, which among its many objectives will study the hermean exosphere after November 2026. In particular, MDM is a piezoelectric (PZT) dust impact instrument that will be dedicated to study the distribution of interplanetary dust at the orbit of Mercury. Our estimations indicate that, it is likely, that MDM would be able to detect ejecta dust particles with radii as small as 50 nm as long as they have a relative speed of > 5 km / s with respect to the detector. On the other hand, it should be able to detect ejecta particles with radii ≳ 100 nm as long as they have impact speeds ⩾ 1 km / s .