Microwave Measurements of the Galilean Satellites

In this proposed work we will interpret the Microwave Radiometer (MWR) data of the Juno flybys of Ganymede, Europa, and Io. Microwave radiometry is widely used for atmospheric temperature, density and composition of atmospheres, such as Juno’s MWR was designed to do. However, such data are also profoundly important in understanding solid surface temperatures and structure. Solid surfaces can be thought of as a very dense atmosphere, with MWR soundings reaching 10’s of meters into the interiors of Europa and the other Galilean satellites, rather than the 100’s of kms in Jupiter’s atmosphere. Specifically, microwave brightness temperatures are controlled by: 1) the wavelength of the observation (with longer wavelengths seeing deeper), 2) the physical temperature of the subsurface (which depends on illumination, thermal properties and geothermal heat), and 3) the dielectric properties of the subsurface (which depend on material properties such as density and minerology of the material).

In a series of NASA-funded studies, the PI has used data from the Earth’s Moon from the Chang’E orbiters (which carried a 3-37 GHz radiometer, similar to MWR) and observations at ~1.4 GHz and 420 MHz from the VLA to reveal the subsurface temperature, dielectric, and density structure of much of the lunar surface. One of the most intriguing outcomes of this research has been the proof-of-concept of using microwave radiometry to back out the subsurface thermal gradient and therefore geothermal heat flux of the Moon. This has led to a NASA DALI-funded microwave spectrometer instrument based on the Juno MWR instrument specifically designed to constrain the geothermal gradient and heat flux of the Moon or other bodies without requiring drilling.

Here we describe our strategy to used radiative transfer models of the MWR data of the Galilean satellites to constrain information about their the near surface structure, composition, and temperatures. Short wavelength data (1.37, 3, 5.75 cm) is dominated by near-surface density, dielectric properties, and diurnal temperatures. These data will allow the PI to constrain the surface density on Ganymede and Europa (which is important for potential landing site conditions) and potentially detect anomalous dielectric materials on the surface of Io. Long-wavelength (11.55, 25 and 50 cm) data will likely see well below the diurnally varying surface, and allow for constraint of the geothermal gradient, which is especially interesting on ocean worlds such as Ganymede and Europa, where the geothermal gradient would provide a strong constraint on the thickness and rheology of the ice shell. We will model shell thickness and both near and deep subsurface temperatures using JunoCam albedo.

We understand data will be limited. The 1000 km Ganymede flyby will result in ~200-350 km surface resolution depending on frequency (or about 15-25 data points across the body at each frequency); the 320 km flyby of Europa about 67-112 km resolution (27-46 data points); the 1500 km Io flyby will result in only 315-530 km data (7-12 data points). However, these are also limited by the speed of the flyby as MRW is fixed to the rotating spacecraft, resulting in gaps between this full coverage. Assuming a 5 rotation per minute rate we expect Ganymede data will be limited to about 24 points per frequency, Europa only about 6-7 points at each frequency, and Io as few as 2 resolved points. However limited, we argue the uniqueness of this data provides a compelling case for detailed analysis.

This proposed work directly addresses the objectives of the Juno Participating Scientist Program as it uses MWR data (aided by JunoCam) to look at the Extended Mission flybys of the Galilean satellites by Juno. This work will also feed-forward into currently funded NASA efforts, such as the Europa Clipper mission, potential Europa surface lander, potential missions to Io, and our DALI microwave radiometer.