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.