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tokens: molecules released from the nucleus and an additional source of D-poor water molecules from sublimating icy grains (see dash-dotted line in Fig. 2 ). Laboratory experiments on samples of pure ice show small deuterium fractionation effects ( Lécuyer et al. 2017 ). In experiments with water ice mixed with dust, the released water vapor is depleted in deuterium, explained by preferential adsorption of HDO on dust grains ( Moores et al. 2012 ). This effect goes in the opposite direction to the observed trend, while conversely the VSMOW D/H value from subliming icy grains is likely representative of buried nucleus water ice. Alternatively, a non-steady-state regime of water ice sublimation could explain the factor 2 − 3 deuterium enrichment in water vapor released from the nucleus with respect to VSMOW ( Podolak et al. 2002 ). However, this would occur in episodic time intervals along the comet orbit ( Podolak et al. 2002 ), whereas the HDO/H 2 O values measured for comet 67P at different periods are very uniform ( Altwegg et al. 2017 ). 5. Conclusion Enlarging the number of accurate D/H measurements in both Jupiter-family and Oort cloud comets is required to better constrain the observed correlation. Taking these measurements from the ground is challenging. Nearly simultaneous spectroscopic observations of low-energy rotational lines of and HDO in a matching field of view encompassing a large fraction of the coma using the GREAT spectrometer aboard SOFIA can play a key role in this endeavor. In this context, the next close apparition of comet 67P/Churyumov-Gerasimenko in 2021 November will offer an excellent opportunity to re-measure the D/H ratio in this comet using spectroscopic techniques. The understanding of the observed correlation calls for detailed investigations of the mechanisms leading to dust and chunk ejection, and cometary hyperactivity. Further experimental and modeling work on evaporative fractionation is also clearly needed, and may ultimately establish that all comets share the same Earth-like water D/H ratio, with profound implications on the early solar system and the origin of the Earth’s oceans. Appendix A: Observations and modeling A.1. Observations The observations of comet 46P/Wirtanen reported here were carried out using the GREAT heterodyne spectrometer ( Heyminck et al. 2012 ) aboard SOFIA during five flights between 2018 December 14 and 20 UT, out of Palmdale, CA, USA. The instrument was operated in its upGREAT HFA/4GREAT (HFA/4G) configuration ( Risacher et al. 2018 ; Durán et al. 2017 ), which allows simultaneous observations at five different frequencies. The lowest frequency band, 4G-1, was used for the observations reported here. The tuning setup and the basic instrument characteristics are summarized in Table A.1 . Although several other transitions of interest were covered in the higher-frequency channels, only OH was detected at a low signal-to-noise ratio, and the other upper limits are not constraining