Evolution of Saturn’s north polar color and cloud structure between 2012 and 2017 inferred from Cassini VIMS and ISS observations

Abstract

Cassini/ISS imagery and Cassini/VIMS spectral imaging observations from 0.35 to 5.12 µm show that Saturn's north polar region (70 • -90 • N) evolved significantly between 2012 and 2017, with the region poleward of the hexagon changing from dark blue/green to a moderately brighter gold color, except for the inner eye region (88.2 • -90 • N), which remained relatively unchanged. These and even more dramatic near-IR changes can be reproduced by an aerosol model of four compact layers consisting of a stratospheric haze at an effective pressure near 50 mbar, a deeper haze of putative diphosphine particles typically near 300 mbar, an ammonia cloud layer with a base pressure between 0.4 bar and 1.3 bar, and a deeper cloud of a possible mix of NH 4 SH and water ice particles within the 2.7 to 4.5 bar region. Between the eye and the hexagon boundary near 75 • N were many small discrete bright cloud features that VIMS spectra indicate have increased opacity in the ammonia cloud layer. Our analysis of the background clouds between the discrete features shows that between 2013 and 2016 the effective pressures of most layers changed very little, except for the ammonia ice layer, which decreased from about 1 bar to 0.4 bar near the edge of the eye, but increased to 1 bar inside the eye. Inside the hexagon there were large increases in optical depth, by up to a factor of 10 near the eye for the putative diphosphine layer and by a factor of four over most of the hexagon interior. Inside the eye, aerosol optical depths were very low, suggesting downwelling motions. The high contrast between eye and surroundings in 2016 was due to substantial increases in optical depths outside the eye. The color change from blue/green to gold inside most of the hexagon region can be explained in our model almost entirely by changes in the stratospheric haze, which increased between 2013 and 2016 by a factor of four in optical depth and by almost a factor of three in the short-wavelength peak of its wavelength-dependent imaginary index. A plausible mechanism for increasing aerosol opacity with time is the action of photochemistry as the north polar region became increasingly exposed to solar UV radiation. For 2013 we found an ammonia mixing ratio of about 50×10 −6 in the depleted region between 4 bars and the NH 3 condensation level (∼ 1 bar), but the NH 3 results for 2016 are unclear due to very high retrieval uncertainties associated with increased aerosol opacity. We retrieved a deep abundance of about 5×10 −6 for PH 3 and a pressure breakpoint (where the PH 3 abundance begins to decline with altitude) that coincided with the main cloud top near 300 mbar, except when that cloud opacity was very low, at which point the PH 3 breakpoint pressure generally increased substantially, consistent with prior suggestions that the cloud layers shield PH 3 from destruction by UV radiation above the clouds. We found an average AsH 3 mixing ratio of 2×10 −9 with some evidence for a decline with altitude above the main cloud layer.

Saturn's Seasonal Atmosphere at Northern Summer Solstice

“…Early models constrained by the CCD spectral range were successful with broad vertical distributions. But multiple compact layers, even in the stratosphere and upper troposphere, can also match observations, as illustrated by Sromovsky et al (2020cSromovsky et al ( ,a, 2021. Evidence in flavour of compact layers, at least in the south polar region, are evident from the presence of antishadows (Sromovsky et al, 2020a) because physically thin layers are required to produce them.…”

“…Karkoschka and Tomasko (2005) found a large latitudinal variation in the magnitude of the imaginary index but not in its shape. Sromovsky et al (2021) also found little spectral shape variation in the north polar hexagon region but a significant increase in amplitude between 2013 and 2016. There is a surprisingly large unexplained variability among the many results dis-played in Fig.…”

“…These detections in storm regions were possible because vertical convection (strongly suggested by associated lightning, Dyudina et al, 2007;Dyudina et al, 2013) had apparently elevated deeper condensates to the visible cloud level, or very near to that level in the Storm Alley case. Additional examples were subsequently found in both polar regions (Baines et al, 2018;Sromovsky et al, 2020cSromovsky et al, , 2021 where the overlying putative diphosphine layer was found to be sufficiently transparent that the ammonia ice layer is not entirely obscured by it. In the undisturbed region ahead of the Great Storm of 2010-2011 Sromovsky et al (2013) found this layer to have an optical depth of about 7 at 2 µm, which is ten or more times what is found within the polar regions.…”

“…A layer of absorbing particulates in the 2-5 bar region is clearly required to modulate Saturn's 5-µm thermal emission. Plausible constituents are NH 4 SH and water ice according to ECCM predictions (Atreya and Wong, 2005), but neither alone provides an accurate spectral fit (Barstow et al, 2016;Sromovsky et al, 2020cSromovsky et al, , 2021. While NH 4 SH provides better model fits, a mixed composition seems likely.…”

“…While the seasonal range of HST and Cassini observations offers great potential to characterise and understand seasonal variabil- ity in Saturn's aerosols, a comprehensive analysis including the full range of the spectral constraints offered by Cassini remains to be accomplished. Most recent efforts have focused on storms (Oliva et al, 2016;Sromovsky et al, 2013Sromovsky et al, , 2016Sromovsky et al, , 2018 and polar regions (Sánchez-Lavega et al, 2006;Sanz-Requena et al, 2018;Sánchez-Lavega et al, 2020;Sromovsky et al, 2020aSromovsky et al, ,c, 2021, with limited latitudinal and temporal sampling, and only one Cassini study (Sromovsky et al, 2021) has combined both simultaneous coincident visual and near-IR constraints, using both reflected sunlight and thermal emission to constrain aerosol structure models in the north polar region.…”