Impacts into Titan's methane-clathrate crust as a source of atmospheric methane

Abstract

Titan is the only icy satellite in the solar system with a dense atmosphere. This atmosphere is composed primarily of nitrogen with a few percent methane, which supports an active, methane-based hydrological cycle on Titan. The presence of methane, however, is intriguing, as its lifetime is likely much shorter than the age of the solar system due to its irreversible destruction by UV photolysis. To explain Titan's current atmospheric methane abundance, it is hypothesised that a replenishment mechanism is needed. One such mechanism may be crater forming impacts; a methane-clathrate layer potentially covering the surface of Titan may act as a reservoir that releases methane when disrupted by impacts. Here, we perform impact simulations into methane-clathrate layers to investigate the amount of methane released via impacts. Our simulations show that the amount of methane released into the atmosphere depends on both the impactor size and the methane-clathrate layer thickness. A single 20-km-diameter impactor releases up to 1% of Titan's current atmospheric methane mass; the effect of impact obliquity and surface porosity may further increase the released mass by a factor of 2-3. The release rate from impacts is lower than the net loss rate by photolysis, but the released methane mass via impacts can enhance the lifetime of methane in Titan's atmosphere by up to 3%. Menrva-sized (> 400 km diameter) crater-forming impacts directly liberate of $\sim$15% Titan's current atmospheric methane. The direct heating of the atmosphere by the impactor might contribute to additional crustal heating and methane release.