Passive probing of the Galilean Moons

Studying the presence of water and characterizing the tectonic structure in the firsts tens of kilometers of the crust of the Galilean icy moons is crucial to understand the formation and evolution of these bodies. The most promising technique for directly detecting subsurface oceans is a penetrating radar and the use of low frequencies (< 30 MHz) is preferred (Bruzzone et al., 2011). However, Jupiter has a loud radio environment for frequencies < 40 MHz (Cecconi et al., 2012), and the use of a passive mode, exploiting Jupiters decametric radio emissions, is considered to operate the radar with low frequencies in the sub-Jovian hemispheres. Yet, the passive radar operates in a complex bistatic 3D geometry where Doppler and delay information are not separable. This justifies the use of simulations with realistic orbitographies to identify the performances of the planned orbits and refine the scenarios of observation of the radar. In order to study the influence of geometry only, we do not take into consideration the stochastic character of Jupiters noise. The emission is then a simple impulsion located in the Jupiter auroral coronas: four sources are considered, at the Eastern and Western borders of Jupiters North and South auroral coronas. We compute the impact of the geometry on the final bistatic performances, using planned Juice orbits on a 3D geometry, computing the resolution arising from these different orbits. Since the Doppler and Delay information are not separated, the integration time influences the size of the resolution in the two dimensions on the 2D SAR image. Thus, while with an integration time of one minute, probing close to the center of the sub-Jovian hemisphere provides the best resolution, increasing the integration times to two minutes and higher leads to the best resolution along 45 of longitude. We then study the importance of locating correctly the position of the emitting source of Jupiters radio emissions. Considering the origin of emission at the center of Jupiter creates a shift and defocuses the point target pattern by creating an alias of the target. Implementation of methods such as autofocus are required to correct these effects. However, confusing the source with another source of emission of Jupiter seems to have little impact on the final quality.