Performances of the Passive SAR Imaging of Jupiter’s Icy Moons

With the development of the JUpiter ICy moons Explorer (JUICE)/Radar for Icy Moons Exploration (RIME) [European Space Agency (ESA)] and Europa Clipper/Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) [National Aeronautics and Space Administration (NASA)] instruments, designed to study the subsurface of the Galilean moons, interest has been growing to study the performances of sounding radar orbiting these bodies. In the presence of strong Jupiter’s radio emissions, probing in a passive mode using these emissions as the emitter is considered. However, radar performances in this bistatic mode are dependent on the entire geometry of observation: the position of the source of emission, the spacecraft trajectory, and on the region probed. The 3-D Simulations are necessary to estimate the performances of these measurements. We analyze the influence of the geometry of observation by approximating Jovian radio bursts as radio impulses, simulating the signal scattered by a point target in a realistic 3-D geometry and computing the resolution. This allows a preliminary identification of scenarios of observation best suited for this radar mode. The influence of the correct localization of Jupiter’s emissions on the synthetic aperture radar (SAR) image is investigated as well, and interest in recovering the true position of Jupiter’s source is highlighted.

I. Introduction

Studying the presence of water and characterizing the tectonic structure in the first tens of kilometers of the crust of the Galilean icy moons are crucial to understand the formation and evolution of these bodies and could provide insight into habitable places within our solar system [1]. The most promising technique for directly detecting subsurface oceans is a penetrating radar. Two radar-sounding instruments, called Radar for Icy Moons Exploration (RIME) and Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) will be on board the missions JUpiter ICy moons Explorer (JUICE) [European Space Agency (ESA)] and Europa Clipper [National Aeronautics and Space Administration (NASA)] to provide information on the moons’ subsurface and characterize the depth of the ice up to 20 km with a 100-m resolution [2]. The use of low frequencies (< 30 MHz) is preferred to probe the moons’ subsurfaces, because the losses due to the surface roughness and absorption of the ice are reduced. However, Jupiter has an intense radio environment for frequencies < 40 MHz: the spectral flux density can reach at Europa, five orders of magnitude larger than the galactic background [3], thus probing at these frequencies would require a strong transmitter. This led to the addition of a very high frequencies (VHF) band at 60 MHz for REASON, and operation mainly on the anti-Jovian hemisphere, where the spacecraft is, in principle, shielded from the radio noise by the moon, for RIME. In addition, a passive mode, which would exploit Jupiter’s radio emissions in the 1–40-MHz band, could be used to operate the radar with low frequencies in the sub-Jovian hemisphere [4], [5]. This mode could be used as a complement to the active radar system using the same electronics and antenna and requiring low power, as a backup in case of system failure, or as the only solution to probe the sub-Jovian hemisphere of the moons. However, the passive radar operates in a complex bistatic geometry, where emissions and receptions are at different locations. While performances of active monostatic radars depend only on the radar’s trajectory and the position of surface probed, the performances of the passive radar will additionally depend on the 3-D position of Jupiter’s sources of emission. This complex geometry requires the use of 3-D simulations with realistic trajectories to identify the effects of the positions and orientations of Jupiter’s sources and the spacecraft’s on the radar’s return.