Deuterated Water in Comets: Importance, Current Status, and Future Directions*

Comets are comprised of icy planetesimals left over from planet formation, and contain the least processed remnant material from solar system formation. This makes them ideal for constraining the origin and thermal evolution of water in the solar nebula. D/H ratios in water may depend on distance of formation from the proto-sun, and on when the icy material incorporated into comets was last subjected to the gas-phase D/H exchange driven by vertical mixing between the planetary disk midplane and photosphere. 

Reports of cometary HDO/H₂O ratios to date suggest an intrinsic dispersion spanning approximately 1 to 3 times that of Earth’s ocean water (Vienna Standard Mean Ocean Water, or VSMOW). However, this is based on a relatively small sample of measured comets (fewer than 20). Therefore, the majority of reported HDO measurements are susceptible to systematic biases and uncertainties inherent to the diverse observational and analysis techniques used. Furthermore, with the exception of space-based measurements of two comets, HDO and H₂O have yet to be sampled simultaneously, which makes them vulnerable to potential time-varying water production for individual comets. 

In this talk, the evolution of deuterium in water will be traced from the cold dense pre-solar molecular cloud core, through the shielded proto-solar disk midplane to existing measurements of cometary HDO. Prospects for future measurement of multiple HDO lines simultaneously with other water isotopes (H₂¹⁶O, as well as H₂¹⁷O and H₂¹⁸O) will also be discussed.