Deuterated Water in the Outer Solar System

Deuterated water is a powerful tracer of the inheritance of unprocessed interstellar ice in planetary systems, giving a window into how interstellar ices, organics, and dust are directly incorporated into the outer regions of disks. In our own protoplanetary nebula, unprocessed dust grains delivered from cold molecular clouds could have carried water ice with a D/H ratio enriched by orders of magnitude above the bulk solar system. Direct incorporation of this ice into growing bodies in the outer solar system would lead to the unmistakable isotopic signature of the presence of interstellar ices in our planetary system

Unfortunately, attempts to accurately measure the D/H in water ice from the icy outer solar system beyond the giant planets have yielded inconsistent results. D/H can be measured in multiple species in cometary coma as the comets arrive into the inner solar system, but the comets measured have been found to have D/H ranging from the terrestrial value to enrichments by about a factor of 4, sometimes within the same comet. The sublimation and jetting processes active on these rapidly heating comets could lead to strong fractionation effects that could easily change D/H measured in the gaseous coma, making any interpretation of the values difficult.

Measuring D/H of water ice in the solid phase on the surface of cold distant inactive objects should give results significantly less affected by fractionation effects and could finally reveal the true D/H on objects in the outer solar system. JWST observations have now demonstrated that deuterated water can be directly detected via the 4.13 µm O-D stretch (analogous to the 3 μm O-H stretch), leading to the promise of detections of deuterated water through the outer solar system.

We examine all JWST data in which the 4.13 µm feature of deuterated water might be present, including the satellites and rings of Saturn, the satellites of Uranus, and Centaurs and Kuiper belt objects. We present detections or upper limits to the detections of these lines on objects throughout the solar system.

Converting the detections of the 4.13 μm absorption to a D/H ratio requires significant laboratory calibration and validation. We describe our current progress on this task.