Presentation #103.07 in the session “A Plethora of Icy Satellites”.
On the 18th May 2010 Cassini’s Composite Infrared Spectrometer (CIRS) observed Dione as its surface went into solar eclipse. Surface temperatures derived from each of CIRS’ ten detectors in focal plane 3 (FP3, 600 to 1100 cm-1, 9.1 to μm) show a rapid decrease in Dione’s surface temperature upon eclipse ingress. The bolometric Bond albedo derived from these detectors are consistent with one another (0.54±0.05 to 0.62±0.03), indicating Dione’s albedo is uniform across the observed region of its leading hemisphere. These albedos are also consistent with previous studies that used diurnal temperature variations to constrain albedo of Dione’s leading hemisphere (e.g. 0.49±0.11, Howett et al., 2014). The derived thermal inertias are also consistent across detectors: 9±4 MKS to 16±8 MKS. Furthermore the thermal inertias are also consistent with previous studies that constrain thermal inertia by investigating diurnal temperature variability (e.g. 8 to 12 MKS, Howett et al, 2014). The skindepth probed by thermal wave due to the eclipse is expected to be between 0.6 to 1 mm, which is much shallower than that probed by diurnal cycles (~50 mm). Thus the agreement in thermal inertia between the eclipse and diurnal studies indicates that the structure of Dione’s subsurface is uniform from its upper-surface (sub-mm) to sub-surface (sub-cm) scales. This is notably different to the trend observed in the Jovian system, where eclipse-derived thermal inertias are much lower than those derived from diurnal studies. The cause of this difference in uniformity is not known, but one possibility is the E-ring grains that bombard Dione’s leading hemisphere overturn it, causing it to become uniform to cm depths.