Radar Sounding of Europa’s Subsurface Properties and Processes:

Abstract

A primary objective of future Europa studies will be to characterize the distribution of shallow subsurface water as well as to identify any ice-ocean interface. Other objectives will be to understand the formation of surface and subsurface features associated with interchange processes between any ocean and the surface as well as regional and global heat flow variations. Orbital radar sounding, a now maturing technology, will be an essential tool for this work. We review the hypothesized processes that control the thermal, compositional, and structural properties, and therefore the dielectric character, of the subsurface of Europa's icy shell. We introduce fundamental concepts in radar sounding and then assess analog processes represented by, and sounded in, Earth's ice sheet. We use these Earth analog studies to define the radar imaging approach for Europa's subsurface that will be most useful for testing the hypotheses for the formation of major features.

Radar probing of Jovian icy moons: Understanding subsurface water and structure detectability in the JUICE and Europa missions

“…Frictional dissipation along faults or in local parts of the shell can cause concentration of heat, which might be expected to lead to a warmer and thinner crust locally, but it would also reduce the strain energy available elsewhere, allowing the shell to thicken there (Stevenson, 1996); moreover, for an ice shell >10 km in thickness, lateral pressure gradients are expected to mitigate any significant ice shell thickness variations (Nimmo, 2004). If the crust of Europa is only a few kilometers thick, this suggests that the liquid interior could rise up to the surface through open fractures, causing the formation of highly reflective areas for a radar instrument, while if the ice shell is thicker, convection can create regions of warm ice and localized intra-ice melting (Blankenship et al, 2009). Heating caused by geological processes, such as strike-slip motion between surface crustal blocks, may affect the shallow ice through sublimation by softening or melting surficial material, and by creating local melting (Nimmo and Gaidos, 2002).…”

“…Pappalardo et al (1998) uses geomorphological evidences from the Galileo images to support the hypothesis of a convective isothermal layer laying under a rigid conducting ice crust of a few kilometers. Convective diapirs might cause thermal perturbations in the shallow subsurface, potentially melting localized regions of the ice shell at depths ranging between 2 and 4 km (Blankenship et al, 2009). …”

“…Geophysical modeling by Chyba et al (1998) and Blankenship et al (2009) typically assume that the icy shell is composed of quasi-pure water ice mixed with dust and rocky inclusions. In contrast, the ice crust is likely to be heterogeneous in at least two other important ways.…”

Radar Signal Propagation and Detection Through Ice

“…Cold ice is more transparent than warm ice, so that it is expected that at the temperatures of the outer Solar System this technique could provide detection of much deeper subsurface structures (Blankenship et al 2010). …”

Seeking Europa's Ocean

“…Such an orbiting spacecraft would be able to characterize Europa's ocean, including its thickness and salinity, using the techniques of gravity and altimetry to measure Europa's tidal deformation, and magnetometry to much better characterize the satellite's induced magnetic field signature. Ice-penetrating radar could search for pockets of shallow water within the icy shell such as in chaos regions and along ridges, and could be correlated to geological indicators to understand surface-ice-ocean material exchange processes; a radar signal through sufficiently cold ice might even be able to directly detect the ocean (Blankenship et al, 2009). Compositional measurements, imaging, and charged particle readings could combine to indicate how Europa's surface features are created and evolve, and whether the surface chemistry is indicative of ocean chemistry, and whether that chemistry is conducive to life.…”