ИСТИНА

Jovian icy moons are in a certain sense unique objects of the Solar System. Their wave scattering characteristics, e.g. albedo and polarization curves in optical and radar wave bands, notably distinguish them from other types of celestial bodies, i.e. terrestrial and giant planets, comets and asteroids. At least several of them have global liquid water layers (oceans) beneath many kilometers thick icy crusts [1]. Despite of many observational evidences of that, ice thickness and ocean depth are at present rather uncertain. Immediate instrumental assessment of the ice thickness would help to address some key question of icy moons’ geology, to constrain parameters of present geological models and derive some implications for chemical composition and internal heat balance and the thermal regime of icy moons. Electromagnetic probing is the only way to explore the internal structure remotely, e.g. with a ground penetrating radar. Instruments of this type proved to be very useful for investigations of Lunar and Martian interiors. In the vicinity of Jupiter, strong electromagnetic noise generated in the Jovian ionosphere and magnetosphere can mask weak echo signals coming from beneath the icy shell. Transmitted power of the instrument is strictly limited due to the energy budget of the spacecraft and requirements of electromagnetic compatibility with other instruments and devices on board. However, that noise can be effectively used as a signal source for the so-called passive radar. The passive radar in fact is a correlation reflectometer, probing the object with radio waves coming from an external source rather than from its own local transmitter. Both types of radars have their specific advantages and disadvantages. The active radar is somewhat more convenient in use, since it provides wider freedom of choice in selection of operating frequencies, signal waveforms and other technical parameters, and observational strategies. The passive radar, instead, does not have a transmitter, and can have extremely low weight, size and power consumption. As an option, it can be combined with an active radar instrument and share with it some common units, e.g. the antenna system. In this talk, we discuss and compare the capabilities of instruments of both types to probe thick and rough icy crusts of the Jovian satellites. We present results of the numerical simulations of performance of active [2] and passive [3] radars on Ganymede for typical geological units of its icy crust. We also built a working prototype of the passive radar instrument [3] and tested it in the laboratory for detection of ionospheric echoes of noise-like radio signals. The study is partially supported by the Max-Planck-Institut für Sonnensystemforschung. One of the authors (Y.I) is grateful to the German academic exchange service (DAAD) for the scholarship 50015739 supporting this research. Support from Russian Fundamental Research Fund with the grant 15-02-05476 "Development of new techniques and means of meteorological radar sounding of atmospheric precipitation in the millimeter wave band" and Russian National Fund with the grant 17-77-20087 is also kindly acknowledged. Y.I. thanks the administration of the Scientific Research Computing Center of the Moscow State University for granting the access to the computational resources of the parallel computing systems SKIF-GRID "Tchshebyshev" and “Lomonosov”. References: [1] Vance, S., Bouffard, M., Choukroun, M., Sotin,C., Ganymede's internal structure including thermodynamics of magnesium sulfate oceans in contact with ice. // Planet.Space Sci. 2014. V.96. P. 62–70. [2] Ilyushin Ya.A. Subsurface radar location of the putative ocean on Ganymede: Numerical simulation of the surface terrain impact. // Planetary and Space Science 2014. V.92. P.121-126. [3] Hartogh P., Ilyushin Ya. A. A passive low frequency instrument for radio wave sounding the subsurface oceans of the Jovian icy moons: An instrument concept. // Planetary and Space Science. 2016. V.130. P.30-39.