A Distributed Radar Sounder Concept for Subsurface Exploration of Saturn's Moon Enceladus: Feasibility and Potential

Since the very first observations of the Moon, radars have been more and more employed as payloads of planetary exploration missions, in various operational modes like altimeters, SARs or radar sounders. Especially radar sounders provide unique measurement capabilities for the subsurface exploration of planetary bodies, as demonstrated by the MARSIS and SHARAD instruments and planned for the REASON [1] and RIME [2] instruments of the Europa Clipper and Juice missions, aimed on the exploration of Jupiter’s icy moons. Radar sounders are nadir-looking sensors that transmit pulsed electromagnetic radiation that propagates through the subsurface due to its relatively low frequency. Each dielectric discontinuity in the ground material reflects part of the signal towards the radar. The analysis of the recorded echoes provides crucial information on the subsurface structure and composition. Despite the capability of achieving good performances, the abovementioned instruments are limited by the almost omnidirectional antenna characteristic of dipole antennas that are commonly used because of the large antenna size at low frequencies. Due to the omnidirectional characteristic, surface clutter, i.e., spurious signals from off-nadir directions, is collected that potentially masks the signal of interest coming from subsurface layers in nadir direction, thus hindering the subsurface data interpretation.

To overcome those limitations, we investigate the feasibility and potential of a distributed radar sounder satellite configuration for an Enceladus mission scenario, in the frame of an ESA study. Distributed radar sounding configurations have been already proposed for Earth Observation of icy regions (e.g., the STRATUS concept [3]). Such a formation flying satellite configuration allows for synthesizing a large antenna array that potentially provides the following advantages with respect to a traditional radar sounding configuration: 1) suppress the surface clutter through beamforming techniques, 2) increase the signal to noise ratio, 3) possibility of exploiting interferometric techniques for subsurface DEM generation and clutter interpretation, and 4) possibility of performing 3D tomographic imaging of the subsurface.

We present an analysis of a distributed HF-band radar sounder for the subsurface exploration of Enceladus including 1) a science case derivation, 2) orbit and formation implications, 3) radar operational concepts, 4) instrument and satellite system architecture implications, and 5) performance assessment. The formation consists of up to 7 satellites, one complex mother satellite (~1.5 tons) implementing the radar signal transmission and other power and mass demanding functionalities (e.g., communication, down-link, data storage, on-board processing), and the other satellites (~200 kg) implementing transponder functionalities that receive the radar echoes and forward it to the mother satellite in a MirrorSAR [4] configuration. A main criticality is the strongly perturbed gravitational environment at Enceladus [5] posing challenges on the orbits and the formation flying capabilities. Potential orbit and formation concepts are presented as well as a performance assessment for the subsurface sounder exploration of Enceladus based on the envisioned satellite formation, attenuation and backscatter models, different operational concepts, and different beamforming approaches.

References:

[1] Blankenship et al., 2009. [2] Bruzzone et al., 2013. [3] Bruzzone et al., 2021. [4] Krieger et al., 2017. [5] Benedikter et al., 2022.