Interpretation of shadows and antishadows on Saturn and the evidence against south polar eyewalls

TL;DR

This paper challenges the interpretation of Saturn's south polar shadows as hurricane-like eyewalls, proposing instead that they are caused by translucent aerosol layers with optical depth steps, supported by radiative transfer modeling.

Contribution

It provides a new explanation for Saturn's south polar shadows, arguing against the hurricane eyewall analogy and supporting a model involving aerosol layer edges.

Findings

Shadows are caused by aerosol layer edges, not eyewalls.

Optically thick eyewalls would produce different brightness patterns.

Shadow features are consistent with optical depth steps in aerosol layers.

Abstract

Cassini spacecraft observations of Saturn in 2006 revealed south polar cloud shadows, the common interpretation of which was initiated by Dyudina et al. (2008, Science 319, 1801) who suggested they were being cast by concentric cloud walls, analogous to the physically and optically thick eyewalls of a hurricane. Here we use radiative transfer results of Sromovsky et al. (2019, Icarus, doi.org/10.1016/j.icarus.2019.113398), in conjunction with Monte Carlo calculations and physical models, to show that this interpretation is almost certainly wrong because (1) optically thick eyewalls should produce very bright features in the poleward direction that are not seen, while the moderately brighter features that are seen appear in the opposite direction, (2) eyewall shadows should be very dark, but the observed shadows create only 5-10\% I/F variations, (3) radiation transfer modeling of clouds in this region have detected no optically thick wall clouds and no significant variation in pressures of the model cloud layers, and (4) there is an alternative explanation that is much more consistent with observations. The most plausible scenario is that the shadows near 87.9 deg S and 88.9 deg S are both cast by overlying translucent aerosol layers from edges created by step decreases in their optical depths, the first in the stratospheric layer at the 50 mbar level and the second in a putative diphosphine layer near 350 mbar, with optical depths reduced at the poleward side of each step by 0.15 and 0.12 respectively at 752 nm. These steps are sufficient to create shadows of roughly the correct size and shape, falling mainly on the underlying ammonia ice layer near 900 mbar, and to create the bright features we call antishadows.