RIME-REASON synergistic opportunities for detecting near-surface layering on icy moons

Knowledge of (near-)surface properties and their spatial heterogeneity will be critical to understanding the evolution of the top few-to-tens of meters of icy worlds, resulting in changes to its structure, density, and composition. On icy worlds, landform modification processes dominated by mass wasting and impact erosion (e.g., Ganymede) as well as potential plume fallout and refrozen brine infiltration (e.g., Europa), could leave behind layered deposits of varying density and thickness. Therefore, characterizing such heterogeneity (layering) can reveal much about the different processes acting on the near-surface environment. These can be studied with a multi-frequency/bandwidth approach applied to surface radar reflectometry measurements.

Radar reflectometry has been demonstrated to be a valuable technique for characterizing near-surface ice on Earth and Mars with mature plans for it to be applied to future observations of the Jovian icy moons, collected by the Europa Clipper and JUICE missions. Both missions host nadir-pointing ice-penetrating radar instruments: the Radar for Europa Assessment and Sounding: Ocean to Near- surface (REASON) on Europa Clipper operating at center frequencies of 60 MHz and 9 MHz, with bandwidths of 10 MHz and 1 MHz, respectively, and the Radar for Icy Moons Exploration (RIME) on JUICE at a single 9 MHz center frequency but bandwidths of 1 and 2.8 MHz.

Reflectometry is related to the measurement of the radar surface echo strength that is sensitive to surface and near-surface properties with a footprint-size spatial resolution of several kilometers (depending on the spacecraft altitude). The near-surface depth is bounded by the vertical resolution defined by the bandwidth of the transmitted signal. In void, [60/10 MHz], [9/2.8 MHz] and [9/1 MHz] radar systems have a vertical resolution of 15 m, 54 m and 150 m, respectively. The reflection strength of a homogeneous icy near-surface combined with a smooth surface is nearly independent of the radar system across the RIME-REASON frequency range. However, a sharp dielectric gradient from a horizontal discontinuity in the near-surface (e.g., layers) affects the effective surface reflection coefficient because of the phase shift between the surface and the near-surface reflections that are coherently integrated when measured at the receiver.

Previous applications of reflectometry rested on the assumptions implicit in the Radar Statistical Reconnaissance (RSR) technique, which has been regularly used to characterize bulk near-surface properties (e.g., porosity) and surface roughness, each predominantly dependent on the coherent and incoherent components of the total surface return, respectively. However, these previous applications of RSR utilized observations collected at near constant altitude. Europa Clipper and JUICE will both perform flybys of their targets of interest where altitude rapidly changes across the observation window. Thus, an understanding of how altitude (convolved with changes in the surface geology) can affect the balance between observed coherent and incoherent backscattered energy is necessary to confidently apply reflectometry to detect near-surface layering on Europa and Ganymede. 

Here, we simulate the radar surface echo from synthetic terrains, using a version of the multilayer Stratton-Chu coherent simulator with rough facets. We assess the coherent content of the total surface power to changes in altitude, by comparing the power derived from simulated surface echoes at the REASON/RIME shared center frequency (9 MHz) but different bandwidths (1 vs. 2.8 MHz). Coherent and incoherent geometric power falls off at different rates with altitude. Thus, the coherent content of the total return at a particular altitude over the target of interest could affect our ability to invert for the properties of near-surface layers. Note in particular that any landform modification of different terrain types (e.g., chaos terrain versus ridged plains on Europa, and dark versus bright terrains on Ganymede) may be better observed at different altitudes. In addition, our results provide valuable insight into targets and altitudes suitable for cross calibrating RIME and REASON [9/1 MHz] for comparative radar studies across the Jovian icy moons.