Evolution of Impact Melt Pools on Titan

Abstract

Titan is an ocean world with a dense atmosphere, where photochemistry produces complex organic molecules that fall to the surface. An important astrobiological question is whether this material can mix with water and form molecules of biological interest. Large impacts heat the moon's subsurface and create liquid water melt pools. A recent study investigated impacts into Titan's clathrate-covered ice shell. Methane clathrates are stable at Titan's surface conditions and have low thermal conductivity, making them efficient insulators that can lead to steep thermal gradients and a thin stagnant lid. The authors showed that the clathrate layer thickness primarily influences the melt distribution, while its volume is governed by the impactor size. Here, we investigate the fate of melt formed during an impact into a clathrate-covered ice shell. Our results show two different behaviors: in cases when less melt is produced, the subsurface melt pool remains close to the surface and freezes on timescales less than or similar to 25 kyr; in cases when larger volumes of melt are produced, a downward-oriented transport of the molten material occurs. As it descends, part of the melt freezes but some may reach the ocean within a few kyr under certain conditions; vertical impacts, high surface porosity, low viscosity, and tidal heating all favor this surface-to-ocean exchange. While providing insights on parameters that allow a subsurface melt pool to remain liquid beneath a Selk-sized crater for a few kyr, this study suggests that Dragonfly may be able to sample melt deposits where organics reacted with liquid water to produce biomolecules. Titan, Saturn's largest moon, harbors a subsurface ocean beneath its ice shell. The moon also has an atmosphere, which is rich in large organic molecules that settle onto its surface. When atmospheric methane reacts with surface water ice, it forms methane clathrate. A clathrate layer atop Titan's ice shell affects the formation of Titan's impact craters, as it is both stronger than ice and a better insulator. Here, we study the effect of this clathrate layer on the formation and subsequent fate of melt pools produced by impacts on Titan. We investigate whether the melt descends through the ice shell to reach the ocean or remains near the surface and freezes. Our results show only a limited range of scenarios where impact melt reaches the ocean. In the majority of models, impact melt freezes near the surface within short timescales, ranging from a few thousands to tens of thousands of years. This implies that surface organic molecules may have interacted with subsurface melt pools. These results provide a positive outlook for NASA's Dragonfly mission, which will explore Titan's Selk crater in search of organic materials that have reacted with water to potentially form molecules of biological interest. We studied the evolution of impact melt pools in Titan's ice shell using numerical simulations of two-phase thermal convection While most melt pools never reach the ocean, we observe surface-to-ocean exchange in a small part of the investigated parameter space Our results suggest that Dragonfly may be able to sample melt deposits where organics reacted with liquid water to produce biomolecules