The Influence and Detectability of Methane Clathrates on Seismic Velocity Structure: Applications to Titan and Icy Ocean Worlds

Abstract:

Icy ocean worlds in the outer solar system are of high interest to the planetary community. Saturn’s largest moon, Titan, is intriguing for several reasons, including its potentially seismically active surface, presence of liquid methane lakes, and the potential habitability of its subsurface ocean. The composition of the surface layer and ocean may control the thickness of the internal layers and whether there are high-pressure ices (e.g. Ice VI) between the ocean and silicate interior. Unlike other icy ocean worlds, such as Europa and Enceladus, Titan has a thick atmosphere containing methane. It has been postulated that methane clathrates would be stable throughout Titan’s hydrosphere and could contribute to Titan’s methane cycle and affect its potential habitability. In the next decade, NASA will launch the Dragonfly mission to Titan. Dragonfly’s payload includes a seismic experiment package, tasked with detecting and locating seismicity, and constraining the internal structure of Titan. Due to the predicted thickness of Titan’s ice shell (tens of kilometers), seismological investigations are key methods for constraining internal layers and possibly composition.

Here, we investigate the effects of composition on the seismic velocity structure of Titan. Specifically, we compare a methane clathrate rich layer to a pure water ice (Ih) layer to determine how properties such as density, seismic velocities, and bulk moduli are affected by composition. We use PlanetProfile to generate self-consistent one-dimensional interior structure models. These models are used as inputs for the Salvus, AxiSem, and InstaSeis programs to generate seismograms that could be detected by the Dragonfly mission. We generated distance-travel time curves to quantify the distances over which seismic waves could be detected, and how those curves are dependent on the ice shell’s composition. We further compare the amplitudes of key phases such as S, P, and reflections off the internal layers (e.g. ice-ocean) to determine the event magnitude and source-location distances over which Dragonfly could detect these key phases. These investigations will help determine if Dragonfly, or seismic instrumentation in general, can help constrain the composition of icy ocean world ice shells.