The D/H ratio in the atmospheres of Uranus and Neptune from Herschel-PACS observations

Herschel-PACS measurements of the rotational R(0) and R(1) HD lines in the atmospheres of Uranus and Neptune are analyzed in order to derive a D/H ratio with improved precision for both planets. The derivation of the D/H ratio includes also previous measurements of the R(2) line by the Short Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The available spectroscopic line information of the three rotational transitions is discussed and applied in the radiative transfer calculations. The best simultaneous fit of all three lines requires only a minor departure from the Spitzer temperature profile of Uranus and a departure limited to 2K from the Voyager temperature profile of Neptune (both around the tropopause). The resulting and remarkably similar D/H ratios for Uranus and Neptune are found to be (4.4$\pm$0.4)$\times10^{-5}$ and (4.1$\pm$0.4)$\times10^{-5}$ respectively. Although the deuterium enrichment in both atmospheres compared to the protosolar value is confirmed, it is found to be lower compared to previous analysis. Using the interior models of Podolak et al. (1995), Helled et al. (2011) and Nettelmann et al. (2013), and assuming that complete mixing of the atmosphere and interior occured during the planets history, we derive a D/H in protoplanetary ices between (5.75--7.0)$\times10^{-5}$ for Uranus and between (5.1--7.7)$\times10^{-5}$ for Neptune. Conversely, adopting a cometary D/H for the protoplanetary ices between (15-30)$\times10^{-5}$, we constrain the interior models of both planets to have an ice mass fraction of 14-32%, i.e. that the two planets are rock-dominated.