Serendipitous infrared observations of Europa by Juno/JIRAM

Abstract

We performed spectral analyses on four serendipitous observations of Europa acquired by the JIRAM instrument (Adriani et al., 2017) onboard the Juno mission. Due to the fixed orientation of the spacecraft spin axis and orbital plane placed along terminator, Europa is observed at approximately 90° phase angle from distances >3.35E5 km resulting in a best spatial resolution of 80 km/px. The one thousand spectra dataset returned to the ground offers a limited coverage in longitude but an almost complete excursion in latitude from poles to equator. With a 2 to 5 μm spectral range and a sampling of about 10 nm/band, JIRAM reflectance spectra allow extraction of different spectral indicators suitable to characterize surface composition, grain size distribution and temperature: the 2 μm water ice band center appears on average centered at 2.038 μm as a result of the presence of both amorphous and crystalline ice phases and non-ice materials. The reflectance level on the 2.4 μm inflection shoulder is comparable to the maximum absorption on the 2 μm band pointing to the presence of non-ice materials, like magnesium chlorinated or hydrated magnesium salts, mixed with water ice. Assuming water ice as the principal endmember, weestimate the surface regolith grain size by comparing the 2 μm band depth and the ratio between I/F at 3.63 and 2.27 μm (Filacchione et al., 2012). On average, we measure values of 0.6 for the first and 0.1 for the latter: such values are compatible with grain sizes in the range of tens to hundreds of microns. Despite the low signal-to-noise beyond 2.7 μm, due to the intrinsic low-reflectance of Europa's surface and very high phase angle values occurring during the observations, JIRAM is able to measure the wavelength of the reflectance peak around 3.6 μm, which is a proxy of water ice temperature (Filacchione et al., 2016a). In the dataset JIRAM has obtained the peak's maximum wavelength occurs at 3.64 μm corresponding to a maximum diurnal temperature of 132 K.