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 to derive a D/H ratio with improved precision for the two planets. The derivation of the D/H ratio also includes previous measurements of the R(2) line with 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 (each time around the tropopause). The resulting and remarkably similar D/H ratios for Uranus and Neptune are found to be (4.4 +/- 0.4) x 10(-5) and (4.1 +/- 0.4) x 10(-5), respectively. Although the deuterium enrichment in the two atmospheres compared to the protosolar value is confirmed, it is found to be lower compared to previous analyses. Using interior models from the literature 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) x 10(-5) for Uranus and between (5.1-7.7) x 10(-5) for Neptune. Conversely, adopting a cometary D/H for the protoplanetary ices between (15-30) x 10(-5), we constrain the interior models of the two planets to have an ice mass fraction of 14-32%, i.e., the two planets are rock-dominated.