MIRI is a mapping spectrometer that acquires 3D ‘tiles’, where two dimensions are spatial and the third is spectral, covering the wavelengths between 4.9 µm and 27.9 µm at a spectral resolution R ≈ 3,500. The figure pictured here shows a composition of four tiles obtained on 13 and 14 November 2022. Three of them cover a vertical strip across Saturn’s disk, going from the north pole down to the equator. The fourth one images a corner of the extensive ring system. The MIRI tiles are superimposed on a recent image of Saturn obtained by Hubble and use an appropriately chosen RGB composite to highlight the atmospheric structures (R = 10.3 μm, G = 10.1 μm, B = 11.6 μm — note that the tile on the rings uses a different composite: R = 15.5 μm, G = 14.6 μm, B = 13.5 μm). The banded structure of Saturn’s atmosphere is clearly visible and various gaseous species both at tropospheric and stratospheric level, including a first detection of propane absorption, as well as two aerosol layers at different depths, can be identified. Saturn was in the middle of its northern summer at the time of the observations, and the authors find signatures of seasonal evolution such as the reversal of the stratospheric circulation between the two hemispheres compared to winter, which had already started when Cassini ended its mission in 2017 (just at the beginning of the northern summer).
This figure is remarkable for various reasons. In a sense, it is a continuation of the monitoring performed by Cassini, which had a mapping spectrometer in its payload (the Visible and Infrared Mapping Spectrometer, VIMS). MIRI of course cannot beat the spatial resolution that only being in situ can provide, but it has ten times VIMS’s spectral resolution, allowing it to separate the contribution of individual molecules in the absorption bands. The spectral range of MIRI and VIMS is also complementary, as they overlap only in a small window around 5 µm (where VIMS was quite noisy). As previous observations of Saturn in the mid-infrared were only performed by the Composite Infrared Spectrometer (CIRS), a more standard spectrometer onboard Cassini, or by full-disk images from ground-based telescopes, MIRI is looking at a hitherto unmapped wavelength range that includes the transition region between reflected sunlight and thermal emission (5.1–6.9 µm). Future observations with more refined calibration and simultaneous campaigns with NIRCam and JWST’s near-infrared spectrometer NIRSpec will be able to perform in-depth studies and continue the seasonal monitoring of Saturn’s atmosphere.
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