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

Abstract

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) × 10 −5 and (4.1 ± 0.4) × 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) × 10 −5 for Uranus and between (5.1-7.7) × 10 −5 for Neptune. Conversely, adopting a cometary D/H for the protoplanetary ices between (15-30) × 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.

Isotopic compositions of asteroidal liquid water trapped in fluid inclusions of chondrites

“…12). The variation clearly shows that the fluid is under disequilibrium for oxygen isotopes (Geiss and Gloeckler, 2003;Linsky et al, 2006), Bulk Earth (Earth) (Lecuyer et al, 1998), Jupiter (J) (Lellouch et al, 2001), Saturn (S) (Lellouch et al, 2001), Uranus (U) (Feuchtgruber et al, 2013), Neptune (N) (Feuchtgruber et al, 2013), Enceladus (E) (Waite et al, 2009), Oort-cloud comets (OOC) (Balsiger et al, 1995;Biver et al, 2006;Bockelée-Morvan et al, 1998Hutsemékers et al, 2008;Meier et al, 1998;Villanueva et al, 2009), Jupiter-family comets (JFC) (Hartogh et al, 2011;Lis et al, 2013), chondrites (Robert, 2006), estimated compositions of water in carbonaceous (CC) and ordinary (OC) chondrites (Alexander et al, 2012) and fluid inclusion of chondrite (Fluid) (this study).…”

The comparative exploration of the ice giant planets with twin spacecraft: Unveiling the history of our Solar System

“…The Herschel observations of Uranus and Neptune (Feuchtgruber et al, 2013) respectively), suggesting a common source of icy planetesimals in the protoplanetary disk. Further insight on the conditions of the disk in its outer regions can be derived from the relative enrichment (with respect to the Solar values) of C, N, S and O, by determination of the abundances of the corresponding reduced forms.…”

“…Prevailing models of the shape of the heliosphere suggest a cometary-type interaction with a possible bow shock and/or heliopause, heliosheath, and termination shock (Axford, 1973;Fichtner et al, 2000). However, recent energetic neutral atom images obtained by the Ion and Neutral Camera (INCA) onboard the NASA spacecraft Cassini did not conform to these models .…”

A pebbles accretion model with chemistry and implications for the Solar system

“…This imply that this family of core accretion models are unable to conciliate the D/H ratio of Uranus & Neptune with their interior structures. This paradox was noted initially by Feuchtgruber et al (2013). Ali-Dib et al (2014b) proposed as a solution the formation of Uranus and Neptune on the CO iceline from predominantly CO ices.…”