Uranus and Neptune in the Mid-Infrared: Recent Findings from VLT-VISIR and Future Opportunities with JWST-MIRI

Michael T Roman,Leigh N. Fletcher,Glenn S. Orton,Naomi Rowe-Gurney,Julianne Moses,Thomas K. Greathouse,Patrick G. J. Irwin,Yasumasa Kasaba,Takuya Fujiyoshi,Heidi B. Hammel,Imke de Pater,Arrate Antunano,James Sinclair,Henrik Melin, Deborah Bardet

crossref（2022）

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摘要

In this talk, we present highlights from our recent analyses of mid-infrared observations of Uranus and Neptune, and we look ahead to anticipated discoveries from the James Webb Space Telescope.   Drawing from a combination of archival and recent ground-based imaging and spectroscopy, we examine the spatial structure and trends of mid-infrared emission from the ice giant atmospheres.  We report on surprising temporal variability in the atmosphere of Neptune (see Figure 1) with an unexpected decline in stratospheric temperatures since at least 2003.  Recent VLT-VISIR imaging and spectroscopy are presented, revealing how this trend has progressed. In contrast, we show that no evidence yet exists of long-term thermal changes in Uranus’ stratosphere, but mid-IR observations of Uranus are still extremely limited. The observed spatial structure of Uranus’ mid-infrared emission is intriguing (Figure 2), as it is inconsistent with simple models of the atmospheric circulation and/or chemistry and its physical interpretation remains unclear. We share recent observations from VLT-VISIR and express the need for continued ground-based imaging for both Uranus and Neptune. Finally, we discuss how the James Webb Space Telescope MIRI observations will help greatly advance our understanding of the ice giants in the years ahead. In particular, given its supreme sensitivity compared to ground-based observations, JWST-MIRI observations will resolve the nature of Uranus stratospheric thermal/chemical structure and reveal how temperature and chemical abundances change with the seasons in the atmospheres of both ice giants.   <img src="" alt="" width="868" height="646" /> Figure 1: Observed changes in Neptune’s thermal-infrared brightness, a measure of temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness from Neptune’s stratosphere with time for all existing images taken by ground-based telescopes. Brighter images are interpreted as warmer. Corresponding thermal-infrared images (top) at wavelengths of ~12 µm show Neptune’s appearance in 2006, 2009, 2018 (observed by the European Southern Observatory’s Very Large Telescope VISIR instrument), and 2020 (observed by Subaru’s COMICS instrument). The south pole appears to have become dramatically warmer in just the past few years.    <img src="" alt="" /> Figure 2: Uranus' 13-micron images from VLT-VISIR in 2009 and 2018, with enhanced emission from high latitudes.  From current ground-based observations alone, it is unclear whether the enhanced radiances are due to greater temperatures or enhance acetylene abundances, but JWST will solve this mystery.

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