A high-amplitude thermal inertia anomaly of probable magnetospheric origin on Saturn’s moon Mimas

► A thermal anomaly is discovered on Mimas’ surface using Cassini/CIRS data. ► High thermal inertias are shown to produce the anomalous temperatures. ► Anomaly coincides with high near-UV reflectance and high-energy electron bombardment. ► Magnetospheric bombardment causes the physical change to Mimas’ surface structure. Spectral maps of Mimas’ daytime thermal emission show a previously unobserved thermal anomaly on Mimas’ surface. A sharp V-shaped boundary, centered at 0°N and 180°W, separates relatively warm daytime temperatures from a cooler anomalous region occupying low- to mid-latitudes on the leading hemisphere. Subsequent observations show the anomalous region is also warmer than its surroundings at night, indicating high thermal inertia. Thermal inertia in the anomalous region is 66 ± 23 J m - 2 K - 1 s 1 2 , compared to < 16 J m - 2 K - 1 s 1 2 outside the anomaly. Bolometric Bond albedos are similar between the two regions, in the range 0.49–0.70. The mapped portion of the thermally anomalous region coincides in shape and location to a region of high-energy electron deposition from Saturn’s magnetosphere, which also has unusually high near-UV reflectance. It is therefore likely that high-energy electrons, which penetrate Mimas’ surface to the centimeter depths probed by diurnal temperature variations, also alter the surface texture, dramatically increasing its thermal inertia.