Vital Signs: Seismology of Icy Ocean Worlds

Abstract

Ice-covered ocean worlds possess diverse energy sources and associated mechanisms that are capable of driving significant seismic activity, but to date no measurements of their seismic activity have been obtained. Such investigations could reveal the transport properties and radial structures, with possibilities for locating and characterizing trapped liquids that may host life and yielding critical constraints on redox fluxes and thus on habitability. Modeling efforts have examined seismic sources from tectonic fracturing and impacts. Here, we describe other possible seismic sources, their associations with science questions constraining habitability, and the feasibility of implementing such investigations. We argue, by analogy with the Moon, that detectable seismic activity should occur frequently on tidally flexed ocean worlds. Their ices fracture more easily than rocks and dissipate more tidal energy than the <1 GW of the Moon and Mars. Icy ocean worlds also should create less thermal noise due to their greater distance and consequently smaller diurnal temperature variations. They also lack substantial atmospheres (except in the case of Titan) that would create additional noise. Thus, seismic experiments could be less complex and less susceptible to noise than prior or planned planetary seismology investigations of the Moon or Mars. Key Words: Seismology-Redox-Ocean worlds-Europa-Ice-Hydrothermal. Astrobiology 18, 37-53.

Holistic Approach for Studying Planetary Hydrospheres: Gibbs Representation of Ices Thermodynamics, Elasticity, and the Water Phase Diagram to 2,300 MPa

“…In order to better constrain the structure, evolution, and habitability of the interiors of water‐rich planetary bodies, a new generation of accurate measurements and internally consistent thermodynamic representations of aqueous solutions and ice polymorphs is required. Furthermore, since next‐generation planetary exploration missions are likely to investigate the seismic structure of icy worlds (Vance, Kedar, et al, ; Vance, Panning, et al, ; Panning et al, ; R. D. Lorenz et al, ; Stähler et al, ), data on seismic wave speeds in ices and aqueous solutions as a function of pressure and temperature are also needed.…”

Convection in Thin Shells of Icy Satellites: Effects of Latitudinal Surface Temperature Variations

“…This elongation indicates that a planar layer of contrasting velocities (low velocity interior to high velocity exterior) exhibiting high shear would exist through large portions of the middle of the ice shell (Figures and ; similar to shear in the Earth's mantle, e.g., Becker, ). This region would likely have a strong preferred crystallographic orientation and associated anisotropy that may be detectable with radar and seismic acquisitions of active ice shells such as expected for Europa (e.g., Barr & Stillman, ; Panning et al, ; Rudolph & Manga, ; Vance et al, ).…”

Seismic signal from waves on Titan's seas

“…Titan's silicate interior is ∼ 2110 km in radius (Vance et al, 2018a), covered by 460 to 550 km of water, resulting in a total average radius of 2575 km. This water is frozen at the uppermost 40 to 120 km (55 to 80 km, according to analysis of the Schumann resonance, see Béghin et al (2012)).…”

“…On top of the ice layer, lakes of liquid ethane and methane occupy much of Titan's northern hemisphere and some of its southern hemisphere. The seismic wave velocities β in Titan's ice have been modeled in Vance et al (2018a) and are on the order of 2000 m/s. The anelastic attenuation is strongly temperature dependent and therefore very low at the surface (94 K, Q ≈ 1000), but as the temperature increases rapidly within a few kilometers to 220 K, to stay at this value, Q might be as low as 70 for much of the crust.…”