Laboratory Measurements of the Microwave Absorption and Refraction of Ices Analogous to those on the Surfaces of Jovian Satellites

As part of its extended mission, NASA Mission Juno had close flyby encounters with Jupiter’s satellites Ganymede (June 2021) and Europa (September 2022). Among several instruments used in the flyby observations was the Juno Microwave Radiometer (MWR) which mapped the microwave emission from both bodies at 6 different wavelengths ranging from 1.3-50 cm. 

Based on existing models for the ices at Europa and Ganymede, the longest MWR wavelength (50 cm) may penetrate to depths as low as 24 km below the surface (Brown et al., JGR: Planets, May 2023)   Higher frequencies (such as channel 3, operating at 12 cm wavelength) combine emission and reflection from a higher, colder ice layer (~ 200m). Determination of the actual depth of penetration and the physical temperature of that layer depends on reliable understanding of the microwave dielectric properties (both refractive index and loss) of the ices at low temperatures (Zhang et al., GRL, June 2023).

In these laboratory measurements, ice samples are placed in a microwave resonator so as to determine the dielectric properties of laboratory analogs of the Europa/Ganymede ices. Previous studies by this group were conducted at both 2.5 GHz (using a partially filled cavity at JPL) and from 4.5-10.5 GHz (using a filled cavity at Georgia Tech), corresponding to Juno MWR channels 3, 4, and 5, respectively. 

Previous measurements at 2.5 GHz were conducted at temperatures from 93 K to 213 K, using pure water ices and ices with contaminants such as salts and sulfuric acid. Measurements made from 4.5 to 10.5 GHz were conducted at temperatures from 193 to 223 K, using pure water ice and ice contaminated with sulfuric acid (Steffes et al., AGU 2024).

New measurements with a wider range of contaminating constituent abundances, and a wider range of frequencies are now being conducted, and are reported here. The experiments are being conducted using ice which contains trace amounts of sulfuric acid, salts, and magnesium sulfide, which are thought to exist at Europa and Ganymede. It is noteworthy that the increase in the loss of the ice when contaminated with small amounts of contaminants is substantial. The new measurements presented here also include an analysis of accompanying uncertainties.

This work was supported by NASA Contract NNM06AA75C from the Marshall Space Flight Center supporting the Juno Mission Science team members at SwRI and Georgia Tech and under NASA contract NNN12AA01C supporting team members at the Jet Propulsion Laboratory, California Institute of Technology.