Dione's Wispy Terrain: A Cryovolcanic Story?

Abstract

We examine the H₂O ice phase on the surface of Dione, one of Saturn's icy satellites, to investigate whether it might harbor cryovolcanic activity induced by a subcrustal body of water. Several studies have searched for such a signature, as summarized in Buratti et al.; however, none has yet produced sufficient evidence to dissipate doubts. In the radiation environment characteristic of Saturn's icy moons, the presence of crystalline H₂O ice has been used as a marker of a high-temperature region. Because ion bombardment will, over time, drive crystalline ice toward an increasingly amorphous state, the current phase of the H₂O ice can be used to gauge the temporal temperature evolution of the surface. We adopt a technique described by Dalle Ore et al. to map the fraction of amorphous to crystalline H₂O ice on Dione's surface, observed by the Cassini Visible and Infrared Mapping Spectrometer, and provide an ice exposure age. We focus on a region observed at high spatial resolution and centered on one of the faults of the Wispy Terrain, which is measured to be fully crystalline. By assuming an amorphous to crystalline ice fraction of 5% (i.e., 95% crystallinity), significantly higher than the actual measurement, we obtain an upper limit for the age of the fault of 152 Ma. This implies that the studied fault has been active in the last ~100 Ma, supporting the hypothesis that Dione might still be active or was active a very short time ago, and similarly to Enceladus, might still be harboring a body of liquid water under its crust.

Keywords: Saturnian satellites (1427); Surface ices (2117); Surface processes (2116).

Figure A1.

Examples of spectra returned by Cassini VIMS in the 0.9–5.1 μm range. (a) Enceladus’ spectrum with good signal-to-noise ratio (S/N). (b) Enceladus’ spectra showing saturation at short wavelengths. (c) Mimas’ spectrum with low S/N at longer wavelengths.

Figure A2.

Dione cluster map showing the location of pixels that belong to the clusters identified for this data set (see legend on the right for cluster IDs from 1 to 12, starting with 1 at the bottom) overlaid upon Dione’s base map (Schenk et al. 2011).

Figure A3.

Spectral averages for the pixels included in each of the clusters listed in the legend with their respective population.

Figure A4.

Calibration curves for the spectrally distinct regions of Dione’s surface.

Figure 1.

Dione global fraction of amorphous to crystalline H₂O ice map and high-resolution subset. Panel A shows both trailing (left side) and leading hemispheres (right side). The high-resolution region and Crater Creusa are respectively marked by solid and dashed red traces. Panels B and C show the maximum (red), average (green), and minimum (blue) amorphous H₂O ice fraction as a function of latitude and longitude respectively. Panels D and E are enlargements of the area of higher spatial resolution marked in red in panel A. The legend in panel E lists the percent spatial coverage and the corresponding amorphous fraction with its error. In panel A, the legend indicates the fraction of amorphous ice relative to crystalline.