Cassini UVIS Detection of Saturn's North Polar Hexagon in the Grand Finale Orbits

Pryor, W. R.; Esposito, L. W.; Jouchoux, A.; West, Robert A.; Grodent, Denis; Gérard, Jean‐Claude; Radioti, Aikaterini; Lamy, Laurent; Koskinen, Tommi

doi:10.1029/2019je005922

Cited by 7 publications

(7 citation statements)

References 57 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…That analysis found extremely low shortwave absorption in the upper troposphere, and stronger absorption in the stratosphere, but with a much flatter spectral variation than found by Karkoschka and Tomasko (2005). Stratospheric absorption in the north polar stratosphere was also confirmed by the detection of a UV dark polar hexagon at 180 nm by the Cassini Ultraviolet Imaging Spectrometer (Pryor et al 2019).…”

Section: Parameterization Of the Chromophore Absorptionmentioning

confidence: 71%

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

Sromovsky

Baines

Fry

2021

Icarus

View full text Add to dashboard Cite

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.

show abstract

Section: Parameterization Of the Chromophore Absorptionmentioning

confidence: 71%

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

Sromovsky

Baines

Fry

2021

Icarus

View full text Add to dashboard Cite

show abstract

“…11.11. The weight of evidence seems to favour shortwavelength absorption at least being present in the stratosphere, as that has been detected in short-wavelength observations by Cassini UVIS imaging near 180 nm (Pryor et al, 2019), as well as in UV HST observations (Karkoschka and Tomasko, 2005). There is also quite a variety in derived magnitudes of absorption, as illustrated in Fig.…”

Section: Saturn's Chromophorementioning

confidence: 76%

Saturn's Seasonal Atmosphere at Northern Summer Solstice

Fletcher¹,

Sromovsky²,

Hue³

et al. 2020

Preprint

Self Cite

View full text Add to dashboard Cite

The incredible longevity of Cassini's orbital mission at Saturn has provided the most comprehensive exploration of a seasonal giant planet to date. This review explores Saturn's changing global temperatures, composition, and aerosol properties between northern spring and summer solstice (2015-2017), extending our previous review of Cassini's remote sensing investigations (2004( -2014( , Fletcher et al., 2018c to the grand finale. The result is an unprecedented record of Saturn's climate that spans almost half a Saturnian year, which can be used to test the seasonal predictions of radiative climate models, neutral and ion photochemistry models, and atmospheric circulation models. Hemi-

show abstract

“…The hexagon was discovered in Voyager images taken in 1980 and was discovered again by Cassini VIMS, before the spring equinox when the North Pole was still in darkness ( Baines et al 2009b ). Its effects extend into the stratosphere to altitudes of 0.5 mbar ( Adriani et al 2015 ; Fletcher et al 2018b ; Pryor et al 2019 ; Sanz-Requena et al 2018 ; Sanchez-Lavega et al 2020a , 2020b ). Based on ISS and VIMS imaging, the hexagon is a meandering jet whose 6-lobed pattern moves slowly or not at all relative to the nominal rotation rate for Saturn, which was defined following the Voyager encounter ( Desch and Kaiser 1981 ).…”

Section: Vortices Mean Flow and Eddiesmentioning

confidence: 99%

Cassini Exploration of the Planet Saturn: A Comprehensive Review

Ingersoll

2020

Space Sci Rev

View full text Add to dashboard Cite

Before Cassini, scientists viewed Saturn’s unique features only from Earth and from three spacecraft flying by. During more than a decade orbiting the gas giant, Cassini studied the planet from its interior to the top of the atmosphere. It observed the changing seasons, provided up-close observations of Saturn’s exotic storms and jet streams, and heard Saturn’s lightning, which cannot be detected from Earth. During the Grand Finale orbits, it dove through the gap between the planet and its rings and gathered valuable data on Saturn’s interior structure and rotation. Key discoveries and events include: watching the eruption of a planet-encircling storm, which is a 20- or 30-year event, detection of gravity perturbations from winds 9000 km below the tops of the clouds, demonstration that eddies are supplying energy to the zonal jets, which are remarkably steady over the 25-year interval since the Voyager encounters, re-discovery of the north polar hexagon after 25 years, determination of elemental abundance ratios He/H, C/H, N/H, P/H, and As/H, which are clues to planet formation and evolution, characterization of the semiannual oscillation of the equatorial stratosphere, documentation of the mysteriously high temperatures of the thermosphere outside the auroral zone, and seeing the strange intermittency of lightning, which typically ceases to exist on the planet between outbursts every 1–2 years. These results and results from the Jupiter flyby are all discussed in this review.

show abstract

Cassini UVIS Detection of Saturn's North Polar Hexagon in the Grand Finale Orbits

Cited by 7 publications

References 57 publications

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

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

Saturn's Seasonal Atmosphere at Northern Summer Solstice

Cassini Exploration of the Planet Saturn: A Comprehensive Review

Contact Info

Product

Resources

About