Cautionary analysis of spectral radiance from Io’s active volcanoes derived from Galileo Near Infrared Mapping Spectrometer (NIMS) data

Between 1996 and 2001, the Galileo Near Infrared Mapping Spectrometer (NIMS) obtained one hundred and ninety observations of the volcanic jovian moon Io. These are the best observations to date of thermal emission from Io’s diverse, powerful, and multitudinous volcanoes. Rathbun et al. (2018) [Astron. J., 156, 207] published a list of 287 measurements of 3.5 µm spectral radiance from some of Io’s volcanoes, derived from NIMS data. However, when compared with previous published estimates, these spectral radiances greatly underestimate thermal emission from Io’s most powerful volcanoes, including Loki Patera, Pele, Amirani and Pillan, all important contributors to Io’s heat flow. The underestimations are in large part due to the methodology used by Rathbun et al., which incorporated null values into the quantification of spectral radiance. Previously published analyses (in particular, Davies et al., 2012, GRL, 39, L01201) utilize unadulterated NIMS radiance products and techniques that yield accurate spectral radiances. These radiances compare well with contemporaneous ground-based observations obtained by the NASA Infrared Telescope Facility (IRTF). Spectral radiance null values in the 3- to 4.3-µm range used by Rathbun et al. were caused by detector saturation. Rathbun et al. further suggest that the number of hot spots in a given area is a suitable proxy for total heat flow. This hypothesis is tested and is found to be false.