Production of neutral gas by micrometeoroid impacts

We present the first direct laboratory measurement of vapor produced by simulated micrometeoroid bombardment. New in situ observations from the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft, and the anticipation of results from the Lunar Atmosphere and Dust Environment Explorer (LADEE), have highlighted the uncertainty surrounding the role of micrometeoroid impacts in sustaining planetary exospheres. In a recent series of experiments, the quantity of neutral molecules generated by impacts of simulated micrometeorids of 0.1–1 μm radius was measured using a fast ion gauge, over a speed range of 1–10 km/s. The quantity of neutrals released per unit projectile mass, N/m, is consistent with a power law N/m = vβ in the projectile speed v, with β ∼ 2.4. At the highest speeds tested, the number of neutrals liberated is equivalent to 5% of the atoms in the projectile; complete vaporization is projected at speeds exceeding 20 km/s.