Uranus in Northern Mid-Spring: Persistent Atmospheric Temperatures and Circulations Inferred from Thermal Imaging

TL;DR

This study uses mid-infrared imaging from VLT to analyze Uranus's atmospheric temperatures and circulation during northern mid-spring, revealing minimal temperature change, hemispheric asymmetry, and possible acetylene enrichment linked to circulation patterns.

Contribution

First detailed analysis of Uranus's stratospheric and tropospheric emissions during northern mid-spring, highlighting hemispheric asymmetry and coupling mechanisms not previously observed.

Findings

Upper tropospheric temperatures show little change since 1986.

Hemispheric asymmetry observed in stratospheric emission.

Evidence suggests acetylene enrichment due to atmospheric circulation.

Abstract

We present results from mid-infrared imaging of Uranus at wavelengths of 13.0 micron and 18.7 micron, sensing emission from the stratosphere and upper troposphere, acquired using the VISIR instrument at the Very Large Telescope (VLT), September 4-October 20, 2018. 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 micron 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 mid-latitude heating or an enhanced abundance of acetylene. Considering potential mechanisms and additional observations, we favor a model of acetylene enrichment at mid-latitudes 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.