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

• Published in: The Astronomical journal Vol. 159; no. 2; pp. 45 - 60
• Main Authors: Roman, Michael T., Fletcher, Leigh N., Orton, Glenn S., Rowe-Gurney, Naomi, Irwin, Patrick G. J.
• Format: Journal Article
• Language: English
• Published: Madison The American Astronomical Society 01.02.2020; IOP Publishing
• Subjects: Acetylene; Astronomy; Atmospheric circulation; Atmospheric models; Atmospheric science; Cold traps; Emission analysis; Infrared imaging; North Pole; Outer planets; Photolysis; Planetary atmospheres; Planetary science; Seasonal variations; Stratosphere; Temperature structure; Thermal circulation; Thermal imaging; Troposphere; Tropospheric circulation; Upper troposphere; Upper tropospheric circulation; Uranus; Very Large Telescope; Wavelengths
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ab5dc7

We present results from mid-infrared imaging of Uranus at wavelengths of 13.0 and 18.7 m, sensing emission from the stratosphere and upper troposphere, acquired using the VISIR instrument at the Very Large Telescope, 2018 September 4-October 20. Using a combination of inverse and forward modeling, we analyze these northern midspring (Ls ∼ 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. well past equinox, with only a subtle increase in temperature at the emerging north pole. Additionally, spatial-temporal variations in 13 m stratospheric emission are investigated for the first time, revealing meridional variation and a hemispheric asymmetry not predicted by models. Finally, we investigate the nature of the stratospheric emission and demonstrate that the observed distribution appears related and potentially coupled to the underlying tropospheric emission six scale heights below. The observations are consistent with either midlatitude heating or an enhanced abundance of acetylene. Considering potential mechanisms and additional observations, we favor a model of acetylene enrichment at midlatitudes resulting from an extension of the upper tropospheric circulation, which appears capable of transporting methane from the troposphere, through the cold trap, and into the stratosphere for subsequent photolysis to acetylene.