The properties of the Lunar dust exosphere

Abstract:

During close flybys of the Galileo spacecraft on the Jovian moon Ganymede, the onboard dust detector discovered that the moon is wrapped in a faint dust cloud. The generation of these dust clouds is a general phenomenon - all airless bodies in the solar system are expected to maintain a dusty, surface bound exosphere due to the continual bombardment by micrometeoroids of interplanetary or even interstellar origin. The Galilean moons Europa and Callisto were found to have dust atmospheres as well. The Cassini dust detector CDA provided some evidence for dust exospheres around Saturn’s ice moons Enceladus and Rhea, and even Pluto and its moon Charon were proposed to have a dust exosphere.

Impacts of fast interplanetary meteoroids with the satellites’ surfaces produce ejecta particles populate tenuous clouds around the moons. This process is very efficient: a typical interplanetary 10−8 kg micrometeoroid impacting the Earth’ Moon produces a large number of dust particles, whose total mass is about 650 times that of the impactor. The ejecta particles move on bound trajectories, most of which have lower initial speeds than the moon’s escape velocity and re-collide with the surface. Particles ejected fast enough to escape from the moon’s gravity may form tenuous dust rings such as Jupiter’s gossamer rings.

The Lunar Dust EXperiment (LDEX) on the Lunar Atmosphere and Dust Environment Explorer (LADEE) is the first instrument flown in the vicinity of the Moon, which is sufficiently sensitive to observe the lunar dust exosphere. The spacecraft was launched in September 2013 and operated about the Moon on a low altitude orbit between October 2013 and April 2014. The collected data set is larger than any other existing observation of a dust exospheres by orders of magnitudes and deepened our insight into the physics of this important phenomenon.

This talk will report about first insights into the dynamical properties of the Lunar dust exosphere based on a in-depth analysis of the LDEX data. We will present for the first time size and speed distributions of the Lunar ejecta as function of their altitude and their sites of origin on the Lunar surface. We will discuss the implications of our findings for the mean field models currently favored to describe the density profile of bound dust exospheres.