Comment on “Zambon, F. et al. Io Hot Spot Distribution Detected by Juno/JIRAM. Geophysical Research Letters, 50, e2022GL100597 (2023)”

The quantification and distribution of thermal emission from Io’s active volcanoes reflects volcanic advection within Io and provides some insight as to patterns of internal heating (Davies et al., 2015; Hay et al., 2020; Matsuyama et al., 2022; Ross et al., 1990; Veeder et al., 2012; Veeder et al., 2015). Davies et al. (2024) analysed all image data of Io obtained by the Jovian Infrared Auroral Mapper (JIRAM) (Adriani et al., 2017) on the Juno spacecraft from Juno orbits PJ5 (March 2017) to PJ43 (July 2022). Davies et al. processed images collected at 3.5 and 4.8 μm and identified 266 hot spots. They compared their hot spot detections with those of Zambon et al. (2023), who published a list of 242 hot spots detected in the same JIRAM data from orbits PJ10 (December 2017) through PJ33 (April 2021). However, Davies et al. found only 156 of the Zambon et al. (2023) hot spots, as well as others that Zambon et al. did not report. Both Davies et al. (2023) and Zambon et al. (2003) stacked JIRAM images obtained during a given flyby in order to increase signal to noise, enhancing thermal emission from faint hot spots. These methodologies used NAIF SPICE kernels to identify Io in the image frame, and allowed the position and viewing geometry for every on-planet pixel to be determined. Where sufficient observations existed, and to allow for the spacecraft geometry changing over a number of acquisitions, Zambon et al. grouped images into a series of superposition products (“super images”), and identified and determined the position of hot spots in each product. Davies et al. (2024) used the same approach, co-registering and stacking images. However, Davies et al. (2024) found that use of SPICE kernels alone was insufficient to accurately determine the position of hot spots on Io’s surface. Without further adjustment, uncertainty in position could be up to four pixels. At resolutions of 40 to >100 km/pixel, this is a significant distance, especially when processing data in Io’s polar regions where such uncertainty would translate into substantial movement in estimated hot spot longitude. To minimise positional error, for each orbit Davies et al. (2023) reprojected a selection of JIRAM frames to point perspective and adjusted and aligned each individual frame using the position of Io’s limb and any surface features that were visible. This accurate superpositioning and stacking improved contrast and revealed many faint hot spots.