Thermal structure of Uranus' atmosphere

Area of Science:

• Planetary Science
• Atmospheric Physics
• Radiative Transfer

Background:

• Uranus' atmospheric thermal structure is complex, with discrepancies between models and observations, particularly in the stratosphere.
• Previous studies suggest a role for atmospheric hazes and internal heat flux in Uranus' energy budget, but definitive explanations remain elusive.

Purpose of the Study:

• To investigate the role of stratospheric hazes and internal energy flux in Uranus' atmospheric thermal structure using a radiative-convective equilibrium model.
• To reconcile discrepancies between model predictions and observational data for Uranus' temperature profile.

Main Methods:

• Applied a radiative-convective equilibrium model constrained by Voyager and post-Voyager atmospheric data.
• Analyzed the impact of stratospheric aerosols, methane abundance, H2 opacity, and photochemical hazes on the thermal structure.
• Investigated radiative transfer in the 7.8-micrometer band and its effect on the upper stratosphere.

Main Results:

• The baseline model reproduced observed tropospheric and stratospheric temperatures but found the stratosphere (10(-3) to 10(-1) bar) to be too cold.
• Observed stratospheric hazes did not significantly warm this region, and postulated hazes were inconsistent with constraints.
• A small abundance of hot methane in the lower thermosphere, absorbing radiation in the 7.8-micrometer band, could warm the upper stratosphere.
• Internal heat fluxes below approximately 60 erg cm-2 sec-1 were found inconsistent with the tropospheric temperature profile.

Conclusions:

• Stratospheric hazes play a limited role in warming Uranus' mid-stratosphere.
• Hot methane radiation is a potential mechanism for warming Uranus' upper stratosphere.
• A lower limit on Uranus' internal heat flux is established, approximately 60 erg cm-2 sec-1.