Hydrocarbon and oxygen photochemistry on Uranus as revealed from Spitzer/IRS observations
Abstract
Due to the very low atmospheric temperatures and hydrocarbon column abundances on Uranus, the planet appears very faint at mid-infrared wavelengths, making determinations of atmospheric composition difficult. The Spitzer Space Telescope Infrared Spectrometer (IRS) is two orders of magnitude more sensitive than previous space-based instruments, favoring the detection of faint molecular emission features from Uranus’ atmosphere (e.g., Burgdorf et al. 2006, Icarus 184, 634). Global-average Spitzer/IRS spectra acquired just days after the planet’s 2007 northern vernal equinox (Orton et al. 2013, manuscript submitted to Icarus) exhibit molecular emission features from CH4, C2H2, C2H6, C3H4, C4H2, and CO2 in Uranus’ stratosphere. We use these Spitzer/IRS observations to constrain new 1-D photochemical models for Uranus. Although the upper-stratospheric methane abundance is well determined from the observations, there is a range of model parameter space in terms of eddy diffusion coefficients Kzz and tropopause methane relative humidities that can reproduce the observed methane emission. However, all such models possess Kzz values that are considerably smaller than those on the other giant planets; the observations show no convincing evidence for significant changes in low-latitude Kzz values with time since the Voyager era. We highlight the differences in atmospheric chemistry and mixing between Uranus and the other giant planets (including the reasons behind the lower C2H6/C2H2 ratio on Uranus), discuss the implications of the observed C4H2 emission with respect to the vapor pressure of C4H2 over diacetylene ice at low temperatures, and summarize the implications with respect to the influx of external oxygen compounds and their corresponding upper-atmospheric haze components. This research was supported by the NASA Planetary Atmospheres program.
- Publication:
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AAS/Division for Planetary Sciences Meeting Abstracts #45
- Pub Date:
- October 2013
- Bibcode:
- 2013DPS....4531213M