Deuterated water ice on the satellites of Saturn

Abstract

The deuterium to hydrogen ratio in water ice in a planetary body carries important information on the history of water processing and delivery in the protostellar nebula. For a giant planet satellite, the D/H ratio is also affected by the processes and temperatures of the circumplanetary or circumstellar environment in which the satellites formed. Here we present robust JWST spectroscopic detections of the 4.14 $\mu$m O-D stretch absorption line (analogous to the 3 $\mu$m water O-H stretch) on the mid-sized Saturnian satellites and use these detections to infer a D/H ratio on each satellite. Within the limitations of the technique, we find that all of the satellites are consistent with having a D/H ratio of about $1.5\times$ Vienna Standard Mean Ocean Water (VSMOW), which is about an order of magnitude higher than the value of the atmosphere of Saturn. A much higher previously reported D/H ratio for Phoebe is ruled out at the 10$\sigma$ level, and a 3$\sigma$ upper limit of 2.3 $\times$ VSMOW is obtained. The elevated D/H ratios demonstrate that the solid planetesimals and pebbles that built the satellites never sublimed and re-equilibrated with the gaseous circumplanetary disk. The similarity of the D/H measurements across all satellites suggest that the D/H ratio of water ice in the vicinity of Saturn at the time of satellite formation was also approximately 1.5 $\times$ VSMOW.

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