Mid-infrared spectral observations Uranus acquired with the Infrared Spectrometer (IRS) on the Spitzer Space Telescope are used to determine the abundances of C2H2, C2H6, CH3C2H, C4H2, CO2, and tentatively CH3 on Uranus at the time of the 2007 equinox. For vertically uniform eddy diffusion coefficients in the range 2200-2600cm2s-1, photochemical models that reproduce the observed methane emission also predict C2H6 profiles that compare well with emission in the 11.6-12.5μm wavelength region, where the υ9 band of C2H6 is prominent. Our nominal model with a uniform eddy diffusion coefficient Kzz=2430cm2s-1 and a CH4 tropopause mole fraction of 1.6×10-5 provides a good fit to other hydrocarbon emission features, such as those of C2H2 and C4H2, but the model profile for CH3C2H must be scaled by a factor of 0.43, suggesting that improvements are needed in the chemical reaction mechanism for C3Hx species. The nominal model is consistent with a CH3D/CH4 ratio of 3.0±0.2×10-4. From the best-fit scaling of these photochemical-model profiles, we derive column abundances above the 10-mbar level of 4.5+01.1/-0.8×1019molecule-cm-2 for CH4, 6.2±1.0×1016molecule-cm-2 for C2H2 (with a value 24% higher from a different longitudinal sampling), 3.1±0.3×1016molecule-cm-2 for C2H6, 8.6±2.6×1013molecule-cm-2 for CH3C2H, 1.8±0.3×1013molecule-cm-2 for C4H2, and 1.7±0.4×1013molecule-cm-2 for CO2 on Uranus. A model with Kzz increasing with altitude fits the observed spectrum and requires CH4 and C2H6 column abundances that are 54% and 45% higher than their respective values in the nominal model, but the other hydrocarbons and CO2 are within 14% of their values in the nominal model. Systematic uncertainties arising from errors in the temperature profile are estimated very conservatively by assuming an unrealistic ""alternative"" temperature profile that is nonetheless consistent with the observations; for this profile the column abundance of CH4 is over four times higher than in the nominal model, but the column abundances of the hydrocarbons and CO2 differ from their value in the nominal model by less than 22%. The CH3D/CH4 ratio is the same in both the nominal model with its uniform Kzz as in the vertically variable Kzz model, and it is 10% lower with the ""alternative"" temperature profile than the nominal model. There is no compelling evidence for temporal variations in global-average hydrocarbon abundances over the decade between Infrared Space Observatory and Spitzer observations, but we cannot preclude a possible large increase in the C2H2 abundance since the Voyager era. Our results have implications with respect to the influx rate of exogenic oxygen species and the production rate of stratospheric hazes on Uranus, as well as the C4H2 vapor pressure over C4H2 ice at low temperatures.
We thank NASA’s Spitzer Space Telescope program for initial support of the data acquisition, reduction and its initial analysis, and we thank Tom Soifer for Director’s Discretionary Time on Spitzer (Program #467). This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work from the Spitzer program was provided by NASA through an award issued by JPL/Caltech. Another portion of our support was provided to JPL/Caltech, from NASA’s Planetary Atmospheres program. J. Moses acknowledges support from NASA Planetary Atmospheres Grant NNX13AH81G, as well as older grants from the NASA Planetary Atmospheres program. L. Fletcher acknowledges the Oak Ridge Association of Universities for its support during his tenure at the Jet Propulsion Laboratory in the NASA Postdoctoral Program (NPP), together with the Glasstone and Royal Society Research Fellowships during his current tenure at the University of Oxford. F.J. Martin-Torres was supported by the Spanish Economy and Competitivity Ministry (AYA2011-25720 and AYA2012-38707). During his contribution to this work, M. Line was supported by NASA’s Undergraduate Student Research Program.
The radiative-transfer calculations were primarily performed on JPL supercomputer facilities, which were provided by funding from the JPL Office of the Chief Information Officer. We thank Linda Brown, Helmut Feuchtgruber, Tristan Guillot, Mark Hofstadter, Kathy Rages, Larry Sromovsky, Larry Trafton, and J. Cernicharo for helpful and illuminating conversations, J. Schaefer for help in implementing dimer contributions into the H2 collision-induced opacity calculations, as well as Emmanuel Lellouch and an anonymous reviewer for insightful comments and suggestions.
- infrared obervations