Uranus’s and Neptune’s Stratospheric Water Abundance and Vertical Profile from Herschel-HIFI* * Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Abstract

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 × 1014 cm−2 for Uranus and 1.9−0.3+0.2 ×1014 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 ∼0.03 mbar condensation level, which further limits water vapor to pressures ≤0.03 mbar. Neptune’s warmer stratosphere has a deeper ∼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 ∼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 4σ 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 × 104 cm−2 s−1 for Uranus and 12.7−2.0+1.3 ×104 cm−2 s−1 for Neptune. This suggests that Neptune’s ∼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).