Abstract
Abstract Geophysics-focused missions and improved geophysical data sets are critical for the future exploration of icy ocean worlds. Of particular interest is the exploration of the Galilean moon, Europa, and the Saturnian moons, Titan and Enceladus. These bodies likely have geologically active surfaces and may harbor habitable subsurface environments. Placing any candidate signatures for life in context requires further knowledge of the interior of these worlds. While the surfaces of these bodies have been mapped, their interiors remain poorly understood. Geophysical approaches such as geodesy, seismology, ice-penetrating radar and altimetry measurements, and electromagnetometry would provide critical information regarding the interior of these icy ocean worlds. The approaches described below would answer key science questions regarding ice shell and ocean dynamics, internal structure and interior layer thicknesses, near-surface structure, and how material from the deep interior might be exchanged with the surface. Here we outline the geophysical environments of Enceladus, Europa, and Titan; some outstanding science questions that remain to be addressed; and examples of the geophysical approaches that can provide the context to better understand icy ocean worlds.
Highlights
Icy ocean worlds are of substantial interest to the planetary community for several reasons, including the potential habitability of their subsurface oceans and their geologically active surfaces
This is not an issue for missions like Cassini whose magnetometer was able to provide the first signs of plumes from the south pole of Enceladus (Dougherty et al 2006), or for the Europa Clipper whose goal is to verify the existence of a subsurface ocean, assess its conductivity, estimate its depth, and estimate the thickness of the overlying ice shell at the global scale (Howell & Pappalardo 2020)
For high-frequency (HF) radars, determination of the total electron content between the spacecraft and the surface through the dispersion of the radar signal can be used to probe the ionosphere (Safaeinili et al 2007; Scanlan et al 2019). This geophysical potential has been demonstrated by two radar sounders at Mars, the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on the Mars Express (Jordan et al 2009) and the Shallow Radar (SHARAD) on the Mars Reconnaissance Orbiter (MRO) (Seu et al 2004)
Summary
Icy ocean worlds are of substantial interest to the planetary community for several reasons, including the potential habitability of their subsurface oceans and their geologically active surfaces. Geophysical instrumentation can provide the data required to better understand internal layer interfaces and the dynamics within ice shells and any subsurface oceans. Seismology paired with gravity, electromagnetic (e.g., magnetotelluric), and radar studies can assess the depths of compositional and phase interfaces and infer the thermal state of the hydrosphere and possibly the deeper silicate interior These studies can characterize the current state of dynamics within the ice shell and fluid motions beneath the ice to infer how heat, redoxsensitive species, and components from the ocean might be transported between internal layers. A geophysical sensor network deployed as part of a landed mission could sense and process multiple geophysical signals to compute a subsurface image in situ and in real time
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