Cometary impact effects at the Moon: Implications for lunar swirl formation

Relatively recent cometary impacts at the Moon could leave unique traces of their origins: high impact velocities and volatile abundances, combined with the presence of a dust- and ice-laden coma, may thermally and mechanically process the lunar surface in ways distinct from the impact of an asteroid. Here we analytically and numerically assess the consequences of a cometary impact at the Moon by considering the combined effects of a collision by the nucleus and inner coma. Our results show that cometary impacts entrain the finest fraction of lunar soil grains (<10 μm) over regional scales (∼100-1000 km), produce large masses of vaporized material, and likely generate transient magnetic fields that could exceed the Earth's surface field strength by a factor of 104. This combination of processes is consistent with a mechanism to generate lunar swirls: the diffuse, meandering disturbances in brightness and regolith texture that curl across much of the lunar far-side and are also commonly (but not exclusively) associated with magnetic anomalies. Previous observations of swirl features indicate that bright regions also possess a peculiar, altered regolith structure, which can be produced by the removal of fine soil grains. Regional scouring by an impacting comet explains both the structure and albedo variations: large dynamic pressures entrain the smallest grains within a near-surface flow of dusty plasma, disrupting the backscattering, “fairy-castle” structure of lunar soils in equilibrium with the airless environment. The resulting surface is brightened by compaction of the previously open, porous macrostructure. Darker lanes observed within swirl regions are interpreted as possible melt and/or vapor deposits. Finally, the intense magnetic fields generated during high-speed cometary impacts provide an explanation for correlations between swirl locations and magnetic anomalies.