Uranus's and Neptune's Stratospheric Water Abundance and Vertical Profile from Herschel-HIFI

Here we present new constraints on Uranus's and Neptune's externally sourced stratospheric water abundance using disk-averaged observations of the 557 GHz emission line from Herschel's Heterodyne Instrument for the Far-Infrared. Derived stratospheric column water abundances are 0.54(-0.06)(+0.26) 1.9(-0.3)(+0.2) 10(14) cm(-2) for Neptune, consistent with previous determinations using ISO-SWS and Herschel-PACS. For Uranus, excellent observational fits are obtained by scaling photochemical model profiles or with step-type profiles with water vapor limited to <= 0.6 mbar. However, Uranus's cold stratospheric temperatures imply a similar to 0.03 mbar condensation level, which further limits water vapor to pressures <= 0.03 mbar. Neptune's warmer stratosphere has a deeper similar to 1 mbar condensation level, so emission-line pressure broadening can be used to further constrain the water profile. For Neptune, excellent fits are obtained using step-type profiles with cutoffs of similar to 0.3-0.6 mbar or by scaling a photochemical model profile. Step-type profiles with cutoffs >= 1.0 mbar or <= 0.1 mbar can be rejected with 4 sigma significance. Rescaling photochemical model profiles from Moses & Poppe to match our observed column abundances implies similar external water fluxes for both planets: 8.3(-0.9)(+4.0) 12.7(-2.0)(+1.3) 10(4) cm(-2) s(-1) for Neptune. This suggests that Neptune's similar to 4 times greater observed water column abundance is primarily caused by its warmer stratosphere preventing loss by condensation, rather than by a significantly more intense external source. To reconcile these water fluxes with other stratospheric oxygen species (CO and CO2) requires either a significant CO component in interplanetary dust particles (Uranus) or contributions from cometary impacts (Uranus, Neptune).

Automated summary

New constraints on the externally sourced stratospheric water abundance of Uranus and Neptune are presented using observations from Herschel's Heterodyne Instrument for the Far-Infrared. The results show consistent water abundance for Neptune compared to previous studies, while for Uranus, the cold stratospheric temperatures and condensation level further limit the water vapor.