Deuterated water ice on the satellites of Saturn

TL;DR

This study leverages JWST to detect the 4.14 μm O-D stretch in water ice on Saturn's mid-sized satellites, enabling D/H measurements that probe the history of water delivery and processing in the Saturnian system. Using a radiative-transfer–based ratio of the 4.14 μm O-D feature to the 2 μm H2O band, the authors derive D/H values that cluster around $1.5 \times$ VSMOW across most satellites, with Saturn's atmosphere remaining much more depleted. The findings imply that the ices constituting these satellites did not equilibrate with the gaseous circumplanetary disk, pointing to formation from elevated-D/H solids, and suggesting a shared reservoir at Saturn’s distance. Phoebe is constrained to an upper limit of $2.8 \times$ VSMOW, and the results collectively inform satellite- and planet-formation scenarios, highlighting the need for further modeling and laboratory work to refine the O-D spectral interpretation.

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 $μ$m O-D stretch absorption line (analogous to the 3 $μ$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$σ$ level, and a 3$σ$ 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.

Figures (4)

• Figure 1: JWST spectrum of the leading hemisphere of Dione with an inset highlighting the 4.14 $\mu$m O-D stretch absorption and the 4.26 $\mu$m CO$_2$ absorption.
• Figure 2: The continuum-divided spectra of the Saturnian satellites, in the region of the O-D absorption. The red line shows a gaussian fit to the data using a fixed width. Most satellites have separate measurements for the leading and trailing hemispheres, but Hyperion and Phoebe, which are not synchronously rotating, only have single measurements. The O-D absorption is robustly detected at nearly every satellite.
• Figure 3: The ratio of the depth of the 4.14 $\mu$m absorption to 2 $\mu$m absorption for each of the satellites. The leading hemispheres are shown as black points while the trailing hemispheres are red. Hyperion and Phoebe, which are non-synchronously rotating, are shown in blue. The D/H value derived using the C19 calibration is shown on the right. The value measured for Enceladus is shown as the bold horizontal line, while the 1$\sigma$ upper and lower limits are shown as thinner dashed lines. The D/H value shown for Phoebe is derived from calibration to a radiative transfer model, rather than from the 4.14 to 2$\mu$m depth ratio.
• Figure 4: A comparison of the continuum-divided JWST spectrum of Phoebe and the VIMS spectrum of Phoebe from C19. Also shown is the average spectrum of all of the Saturnian satellites with robust 4.14 $\mu$m absorption detections.The JWST data rule out an absorption with the position and depth of that seen by VIMS at the 10 $\sigma$ level.