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Sub-surface Observations of Ganymede’s Ice Shell from the Juno Microwave Radiometer

Presentation #509.06 in the session Icy Moons Interior and Ice Shell (Oral Presentation)

Published onOct 23, 2023
Sub-surface Observations of Ganymede’s Ice Shell from the Juno Microwave Radiometer

We report on observations of Ganymede’s ice shell from the Microwave Radiometer on the Juno Mission. On 7 June 2021, Juno flew within 1000 km of the surface. During the flyby, Juno’s Microwave Radiometer (MWR) observed Ganymede obtaining several swaths across Ganymede using Juno’s spin to partially map Ganymede’s ice shell in six channels ranging from 600 MHz to 22 GHz. The radiance at these frequencies originates from successively deeper layers of the sub-surface from the highest to lowest frequency. The MWR observations cover a latitude range from 20S to 60N and an east longitude range from -120 to 60 degrees, roughly centered on the Perrine region. The local solar time varies from around noon to mid-night over the longitude range. We present resolved brightness temperature maps and associated spectra of Ganymede with a spatial resolution of up to ~140 km (approximately 1/40th of Ganymede’s diameter). The maps and spectra are sensitive to prominent localized thermal features in addition to the various types of terrain seen in visible and infrared images of Ganymede. Comparing the microwave spectra with maps of Ganymede reveal spectral differences corresponding to different types of terrain visible on Ganymede including bright and dark geological features. Juno’s wide range of wavelengths probe various depths providing information on porosity, water ice purity, thermal inertial and dielectric constants of the various ice regions as well as linear features thought to be associated with tectonic activity. Initial analysis of Juno’s microwave radiometer reported an ice shell conducting layer depth of <150 km, a large degree of reflection and that the bright regions appear significantly more reflective in the microwave than the dark regions (Brown et al 2023, Zhang et al 2023). Here we report on new analyses that demonstrate significant subsurface differences exist in the ice shell as a function of terrain type and suggest a significantly thinner average conductive ice shell thickness. These observations provide new constraints on the subsurface properties and complement future radar sounding observations from the JUICE and Clipper missions.

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