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Imaging-Mode Stellar Occultations by Saturn from Cassini VIMS

Presentation #508.02 in the session Origin and Evolution of Giant Planet Systems II (Oral Presentation)

Published onOct 23, 2023
Imaging-Mode Stellar Occultations by Saturn from Cassini VIMS

During Cassini’s 13 years in orbit around Saturn, the Visual and Infrared Mapping Spectrometer (VIMS) performed over 100 stellar occultation experiments on the planet, covering a wide range of latitudes and seasons. The principal goals of observing Saturn stellar occultations with VIMS were to obtain temperature-pressure profiles of the planet’s stratosphere through inversion of light curves, and to obtain vertical profiles of hydrocarbon abundance in the upper stratosphere with which to test photochemical and eddy-diffusion models such as those of Moses et al. (2000 Icarus) and Moses and Greathouse (2005 JGR). VIMS performed most of its experiments in occultation mode, but a small subset (9) were done in imaging mode. This work uses the imaging-mode occultations to study the anatomy of an occultation so that we can calibrate a model for the occultation-mode data. These imaging-mode data probe deeper into the atmosphere (~50 mbar vs ~10 mbar) and provide a greater field of view on the occultation event, but sacrifice temporal resolution. With a greater field of view, we can directly measure the bending angle of the ray of starlight through Saturn’s atmosphere as the star’s image is refracted by Saturn’s atmosphere across the detector. From this, we relate the predicted refractive attenuation of the starlight, assuming no other sources of opacity (e.g. aerosol hazes), to the observed attenuation. Doing so, we test this assumption in the region of Saturn’s stratosphere that the occultation-mode dataset probes (~1-10 mbar) in advance of that future analysis. We can also measure when the refractive bending exceeds the single pixel of the occultation-mode data so that we will be better able to disentangle the effects of refraction from the effects of atmospheric attenuation in the occultation-mode dataset.

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