Radar attenuation in Europa's ice shell: Obstacles and opportunities for constraining the shell thickness and its thermal structure

Abstract

Young surface and possible recent endogenic activity make Europa one of the most exciting solar system bodies and a primary target for spacecraft exploration. Future Europa missions are expected to carry ice‐penetrating radar instruments designed to investigate its subsurface thermophysical structure. Several authors have addressed the radar sounders' performance at icy moons, often ignoring the complex structure of a realistic ice shell. Here we explore the variation in two‐way radar attenuation for a variety of potential thermal structures of Europa's shell (determined by reference viscosity, activation energy, tidal heating, surface temperature, and shell thickness) as well as for low and high loss temperature‐dependent attenuation model. We found that (i) for all investigated ice shell thicknesses (5–30 km), the radar sounder will penetrate between 15% and 100% of the total thickness, (ii) the maximum penetration depth varies laterally, with deepest penetration possible through cold downwellings, (iii) direct ocean detection might be possible for shells of up to 15 km thick if the signal travels through cold downwelling ice or the shell is conductive, (iv) even if the ice/ocean interface is not directly detected, penetration through most of the shell could constrain the deep shell structure through returns from deep non‐ocean interfaces or the loss of signal itself, and (v) for all plausible ice shells, the two‐way attenuation to the eutectic point is ≲30 dB which shows a robust potential for longitudinal investigation of the ice shell's shallow thermophysical structure.

Five decades of radioglaciology

“…Two instruments have been selected for upcoming missions to Ganymede and Europa: the 9 MHz frequency Radar for Icy Moons Exploration (RIME) instrument on board the European Space Agency's Jupiter Icy Moons Explorer (JUICE) and the 9 and 60 MHz frequency Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument on board NASA's Europa Clipper (Bruzzone and others, 2013; Pappalardo and others, 2015; Lorente and others, 2017; Blankenship and others, 2018). These sounders are designed to probe the moons' interiors and have penetration depths which are functions of surface roughness, volume scattering, ice-shell thermal structure, chemistry and the character of the ice/water interface (Moore, 2000; McKinnon, 2005; Blankenship and others, 2009; Bruzzone and others, 2011; Schmidt and others, 2011; Berquin and others, 2013; Grima and others, 2014b; Pettinelli and others, 2015; Di Paolo and others, 2016; Grima and others, 2016; Aglyamov and others, 2017; Heggy and others, 2017; Kalousová and others, 2017; Campbell and others, 2018; Gerekos and others, 2018; Michaelides and Schroeder, 2019; Culha and others, 2020). The addition of a dual-channel VHF band on REASON also allows for characterization of the European ionosphere, altimetric investigation of Europa's shell and tides, and dual-frequency or interferometric clutter discrimination (Grima and others, 2015; Carrer and Bruzzone, 2017; Castelletti and others, 2017; Haynes and others, 2018a; Steinbrügge and others, 2018; Scanlan and others, 2019).…”

Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

“…The ability to predict compositional variations around putative hydrological features in Europa's shell will both constrain how these features form and inform spacecraft observations (Blankenship et al, 2009; Kalousova et al, 2017). The high salt contents associated with both fractures and lenses make them prime candidates for detection by ice penetrating radar instruments (e.g., RIME onboard JUICE, REASON onboard Europa Clipper; Plaut, 2019), as salt substantially alters the dielectric properties of ice (e.g., Blankenship et al, 2009; Schroeder et al, 2016).…”

“…Thus, beyond the ice shell's chemistry, the dynamics of impurity entrainment will affect the potentially appreciable, and ongoing, hydrological activity within Europa's ice shell in the form of perched water lenses, fractures, dikes, and sills (e.g., Manga & Michaut, 2017; Michaut & Manga, 2014; Schmidt et al, 2011; Walker & Schmidt, 2015). Moreover, interpretation of measurements taken by Europa Clipper's ice penetrating radar, REASON, depends critically on ice composition and dielectric properties (Blankenship et al, 2009; Kalousova et al, 2017; Weeks & Ackley, 1986). If the ice shell is impurity‐rich, it has the potential to reflect and attenuate radar signals, which can be used to investigate the ice shell's interior structure but may also prevent observation of the ice‐ocean interface (Kalousova et al, 2017).…”

“…Moreover, interpretation of measurements taken by Europa Clipper's ice penetrating radar, REASON, depends critically on ice composition and dielectric properties (Blankenship et al, 2009; Kalousova et al, 2017; Weeks & Ackley, 1986). If the ice shell is impurity‐rich, it has the potential to reflect and attenuate radar signals, which can be used to investigate the ice shell's interior structure but may also prevent observation of the ice‐ocean interface (Kalousova et al, 2017).…”

Ocean Dynamics of Outer Solar System Satellites

“…Looking specifically to the Jovian system, the Europa Clipper and JUICE missions will determine the ocean thickness and salinity and may be able to place constraints on spatial variations of ice shell thickness (e.g., Grasset et al, 2013;Phillips & Pappalardo, 2014). Ice penetrating radar will provide information on ice shell thermophysical structure and constrain ice-ocean exchange processes (e.g., Kalousová et al, 2017), while magnetometer measurements may allow probing of ocean currents through their induction of magnetic fields (e.g., Tyler, 2011). I thank Jonathan Aurnou, Baptiste Journaux, and Steve Vance for their helpful comments as well as Gabriel Tobie and Christophe Sotin for their constructive reviews.…”