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The evolution of deep clouds on Saturn using 5-micron observations

Presentation #201.03 in the session “Giant Planets 3: Saturn”.

Published onOct 26, 2020
The evolution of deep clouds on Saturn using 5-micron observations

The Great Storm of 2010-11 erupted from Saturn’s deep atmosphere at 40°N. In the aftermath of this planet-encircling storm, this latitude became exceptionally clear of cloud opacity down to at least 5 bars (Sromovsky et al. 2016). By 2013 only a clear (bright at 5 μm) and relatively uniform band remained (see Fig. 1). In addition to imaging, we used high resolution (R=20,00-35,000) spectra from Keck/NIRSPEC and IRTF/iSHELL at 5 μm to study Saturn’s deep cloud structure. NIRSPEC spectra from 2013 revealed a region at 40°N devoid of deep clouds. Our iSHELL data from 2019 indicate that this region has completely clouded over. Meanwhile, a new storm at high northern latitudes (65°N to 70°N) was reported in 2018 (Sanchez-Lavega et al. 2020). Our images in 2019 and 2020 (Fig. 1) show cloud clearing at 65°N. Spectra at 5 μm can reveal whether the observed changes in cloud opacity are due an NH3 cloud or due to deeper clouds, such as NH4SH. In clear regions, absorption lines of PH3 at 5.07 μm and CH3D lines at 4.66 μm are very broad due to pressure and opacity broadening. In cloudy regions, these lines are much weaker. The dramatic differences in PH3 and CH3D line shapes between bright and dark regions provides evidence for thicker clouds in regions that are dark at 5 μm. We modeled CH3D and PH3 line shapes in our data from 2019 and 2020 to constrain the optical thickness and pressure level of the clouds at 65°N and at the Equator.

  1. Bjoraker, G. L. et al. 2018. The Gas Composition and Deep Cloud Structure of Jupiter’s Great Red Spot. Astron. J. 156, 101-115.

  2. Sanchez-Lavega, A. et al. 2020. A complex storm system in Saturn’s north polar atmosphere in 2018. Nature Astronomy 4, 180-187.

  3. Sromovsky, L. A. et al. 2016. Cloud clearing in the wake of Saturn’s Great Storm of 2010-2011 and suggested new constraints on Saturn’s He/H2 ratio. Icarus 276, 141-162.

We observed Saturn at 5 µm to track the evolution of storms and associated cloud-clearing. Left: IRTF image of Saturn at 5.1 µm in May 2013 shows cloud-clearing at the latitude (40ºN) of the Great Storm of 2010-11. Center: The region at 40ºN has clouded over by 2019 but there is a new clearing near 65ºN that coincides with the latitude of a storm observed in 2018. Right: The clearing near 65ºN has persisted into 2020.

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