Proposed plume source regions on Europa: No evidence for endogenic thermal emission

Abstract Recent observations from the Hubble Space Telescope (Roth et al., 2014a; Sparks et al., 2016, 2017) and reanalysis of Galileo data (Jia et al., 2018) are consistent with the presence of plumes erupting from Europa's surface. Because plumes are generally associated with surface hotspots, here we take a closer look at existing thermal data of the proposed source regions. We use temperatures measured with the Galileo Photopolarimeter-Radiometer (PPR) instrument plus one observation from ALMA, the Atacama Large Millimeter Array (Trumbo et al., 2017). By fitting a diurnal thermal model to the observed brightness temperatures, we find that they are consistent with passive re-radiation of sunlight and do not require endogenic emission. The surface at the Jia et al. (2018) location has similar thermal properties to nearby regions, with a thermal inertia of 87 mks (J m−2 K−1 s1/2) and a bolometric albedo of 0.38, assuming an emissivity of unity. The thermal properties at the Sparks et al. (2017) source location are substantially different from the surrounding terrain, with a thermal inertia of 105 mks, a bolometric albedo of 0.50, an emissivity of 0.9 at thermal infrared wavelengths, and an emissivity of 0.77 at mm wavelengths. Our fit includes mid-morning PPR observations that are not used by Trumbo et al. (2017) and result in lower values of the thermal inertia and albedo. We also examine the nighttime PPR observation that covers both of the proposed source locations and find that an Enceladus- type tiger stripe would have been easily observable in this image. Combining this with the results of Rathbun et al. (2010) who found that any 100 km2 hotspot with a temperature greater than ~200 K would also have been observable, we conclude that no endogenic feature brighter than either of these examples exist at the proposed source locations. So, any plumes originating in this region are either associated with a substantially smaller hot area, or, at the time of this PPR observation had time to cool to below detectable levels or didn't become active until after this observation was obtained.