Simon, and three co-authors, reported March, 2021 that in using the Hubble Space Telescope, Saturn was observed in 2018, 2019, and 2020, just after the northern hemisphere summer solstice. Analysis of multispectral imaging data reveals three years of cloud changes associated with a 70° N storm that began in 2018. Additionally, there is an increase in equatorial brightness and perhaps haze optical depth at 0° to 7° N.
There are small midsummer changes at the north pole, with a thin blue feature near the polar hexagon's outer edge disappearing between 2019 and 2020 and increasingly reddish polar haze. Zonal winds at most latitudes remain close to values obtained by the Cassini mission with a slight increase of winds in the equatorial zone. Yearly cloud changes, while noticeable, are small compared with the changes observed between the Voyager (northern spring) and Cassini (southern summer to northern spring) eras, but further observations will provide a longer baseline for comparison.
The authors concluded that in analyzing Hubble imaging data of Saturn from 2018 to 2020 at wavelengths from 225 to 889 nm by using latitudinal I/F profiles at each wavelength, principal component analyses, and filter ratio indices, the authors have monitored short-term changes in reflectance and color after the northern summer solstice. The largest apparent changes occurred at equatorial and high northern latitudes.
The equator became brighter at wavelengths from ~400 to 760 nm, which are most sensitive to tropospheric cloud opacity. Using filter ratios, the cloud color is very slightly bluer in 2020; however, there are no changes, within the uncertainties, in cloud height or opacity. However, the principal component analysis does indicate a possible slight increase in tropospheric and stratospheric haze optical depths has occurred. As the two methods are potentially sensitive to different altitudes or particle sizes, further evolution of this region with season should reveal the cause for equatorial brightening.
At high northern latitudes, the 60°–75° N region was perturbed by storms in 2018 and 2020. These storms caused apparent cloud color changes, as well as increased stratospheric haze opacity and altered aerosol particle sizes. Further north, the polar region has progressed to redder colors, with possible changes in cloud opacity. This region should continue to evolve as solar insolation rapidly changes over time.
Zonal wind profiles were also computed, and changes were compared with solar insolation variation. The zonal winds from HST images are similar to the results from Cassini at most latitudes, despite the large difference in solar insolation. Differences in polar latitude winds are possibly artifacts from the lack of spatial resolution and polar geometry, while differences in the equatorial zone are possibly real and may merit further study in the future.
When connected with Voyager-era observations, these spacecraft data sets span approximately 1.5 Saturn years, albeit with gaps in coverage. Continued Hubble observations from the OPAL program will be crucial for understanding Saturn's seasonal evolution in the post-Cassini era.
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