Thermal imaging of Uranus: Upper-tropospheric temperatures one season after Voyager

Abstract

We report on 18--25 µm thermal imaging of Uranus that took place between 2003 and 2011, a time span roughly one season after the thermal maps made by the Voyager--2 IRIS experiment in 1986. We re--derived meridional variations of temperature and para--H2 fraction from the Voyager experiment and compared these with the thermal images, which are sensitive to temperatures in the upper troposphere of Uranus around the 70--400 mbar atmospheric pressure range. The thermal images display a maximum of 3 K of equivalent temperature changes across the disk, and they are consistent with the temperature distribution measured by the Voyager IRIS experiment. This implies that there has been no detectable change of the meridional distribution of upper--tropospheric / lower--stratospheric temperatures over a season. This is inconsistent with seasonally dependent radiative--convective--dynamical models and full global climate models that predict some variability with season if the effective temperature is meridionally constant. We posit that the effective temperature of Uranus could be meridionally variable, with the additional possibility that even the small temperature variations predicted by the GCMs are overestimated. IntroductionWith its 98° rotational axis tilt from its orbital plane, the atmosphere of Uranus is subject to extremes of seasonal variability in 84--year cycles of daylight and darkness. The thermal structure and response of the atmosphere to these extremes were determined only once in 1986 with close--up thermal spectroscopy of Uranus by the Voyager--2 IRIS experiment (e.g. Flasar et al. 1987, Conrath et al. 1990). It measured the north pole near winter solstice (planetocentric solar longitude Ls=273 o ) in the midst of its long winter darkness, yielding upper tropospheric (80--mbar) temperatures that were 1--2 K warmer than at the equator (Conrath et al. 1998). In order to determine the effect of extremes of variable sunlight on different latitudes in Uranus, we needed to determine temperatures near the same upper--troposphere levels as measured by the IRIS experiment. This is not possible from Earth using the same 30--50 µm wavelength range as the Voyager IRIS experiment because of strong absorption by water vapor in our own atmosphere. However, we can measure temperatures in a portion of the 75--650 mbar pressure range covered by the Voyager IRIS experiment from the 18--25 µm spectral range that is accessible from ground--based observatories. The IRIS experiment was also sensitive to the relative abundances of para--H2 vs. ortho--H2 ratio, parameterized by the para--H2 fraction, whose departures from local equilibrium can serve as a tracer of vertical motion. Ground--based observations that extend to 25 µm are also sensitive to the para--H2 fraction at one level, around 200 mbar of atmospheric pressure. We report here Earth--based spatially resolved thermal images of Uranus between 18 and 25 µm taken between 2003 and 2011 (Ls=354--14 o ). Following the Voyager IRIS measurements by 17--25 years, they...

Uranus in Northern Midspring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging

“…Using a combination of inverse and forward modeling, we analyze these northern mid-spring (L s ∼46 • ) images and compare them to archival data to assess seasonal changes since the 1986 southern solstice and subsequent equinox. We find the data are consistent with little change (< 0.3 K) in the upper tropospheric temperature structure, extending the previous conclusions of Orton et al (2015) well past equinox, with only a subtle increase in temperature at the emerging north pole. Additionally, spatial-temporal variations in 13-µm stratospheric emission are investigated for the first time, revealing meridional variation and a hemispheric asymmetry not predicted by models.…”

“…To evaluate temporal changes in the 18.7-µm images, we compared our new data to Voyager data (the oldest spatially resolved thermal data of Uranus available) following the techniques of Orton et al (2015), as described in Section 3. In the case of the 13.0 µm stratospheric emission, the Voy- (Orton et al 2014b).…”

“…First, we forward-modelled emission from the temperatures inferred from the Voyager measurements to produce a synthetic image of Voyager-era emission with 2018 geometry-effectively what we would have observed at 18.7 µm in 2018 if the upper tropospheric temperatures near 200 mbar remained unchanged since 1986. These temperatures were taken directly from Orton et al (2015), which reanalyzed Voyager 2-IRIS spectra acquired over the four days surrounding the closest approach to Uranus on January 24, 1986. These data covered both the southern and northern hemispheres as the spacecraft passed Uranus at distances ranging from 4.2 to 112 Uranus radii.…”

“…Spectra were inverted to yield temperatures as function of latitude and pressure from 70-400 mbar and then extended to lower pressures based on Spitzer observations (Orton et al 2014b). We refer the reader to Orton et al (2015) for further details on how the Voyager temperatures were retrieved.…”

“…Since Orton et al (2015) found no significant changes in the upper tropospheric temperatures between the 1986 solstice and the 2007 equinox (using a wider range of VISIR data and the same radiative transfer code applied in the present study), we chose to only assess changes between the 1986 Voyager data and our 2018 18.7-µm data.…”