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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 × 1014 cm−2 for Uranus and ×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: × 104 cm−2 s−1 for Uranus and ×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)

Abstract:

Numerous observational evidence has suggested the presence of active meteorology in the atmospheres of brown dwarfs. A near-infrared brightness variability has been observed. Clouds have a major role in shaping the thermal structure and spectral properties of these atmospheres. The mechanism of such variability is still unclear, and neither 1D nor global circulation models can fully study this topic due to resolution. In this study, a convective-resolving model is coupled to gray-band radiative transfer in order to study the coupling between the convective atmosphere and the variability of clouds over a large temperature range with a domain of several hundred kilometers. Six types of clouds are considered, with microphysics including settling. The clouds are radiatively active through the Rosseland mean coefficient. Radiative cloud feedback can drive spontaneous atmospheric variability in both temperature and cloud structure, as modeled for the first time in three dimensions. Silicate clouds have the most effect on the thermal structure with the generation of a secondary convective layer in some cases, depending on the assumed particle size. Iron and aluminum clouds also have a substantial impact on the atmosphere. Thermal spectra were computed, and we find the strongest effect of the clouds is the smoothing of spectral features at optical wavelengths. Compared to observed L and T dwarfs on the color–magnitude diagram, the simulated atmospheres are redder for most of the cases. Simulations with the presence of cloud holes are closer to observations.

Abstract:

Charon's exosphere may exhibit extreme seasonal dynamics, with centuries of quiescence punctuated by short lived (∼4 earth years) exospheric surges near the equinoxes, as spring sunrise bi-annually drives frozen methane off the polar night zones. Charon's pole-centric red spot has been proposed to be the product of Ly-α photolysis of frozen methane into refractory hydrocarbon “tholins”, but the role of exospheric dynamics in the red material's formation has not been investigated. We show with exospheric modeling that methane “polar-swap”, in which exospheric CH4 sublimated from the spring polar zone is rapidly re-frozen onto the autumn hemisphere, deposits ∼30 μm polar frosts too thick for Ly-α light to penetrate. Ethane, the primary methane photoproduct under these conditions, may unlike methane remain frozen decades after polar sunrise under solar wind exposure. Solar wind radiolysis of polar ethane frost synthesizes higher-order refractories that may contribute to the coloration of Charon's polar zones.