A hexagon in Saturn’s northern stratosphere surrounding the emerging summertime polar vortex

Fletcher, Leigh N.; Orton, Glenn S.; Sinclair, James; Guerlet, Sandrine; Read, P. L.; Antuñano, Arrate; Achterberg, R. K.; Flasar, F. M.; Irwin, Patrick; Bjoraker, G. L.; Hurley, J.; Hesman, B. E.; Segura, M.; Gorius, N.; Mamoutkine, A. A.; Calcutt, S. B.

doi:10.1038/s41467-018-06017-3

Cited by 44 publications

(87 citation statements)

References 69 publications

(176 reference statements)

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“…This choice is made because of the unknown optical properties of the aerosols in the 600‐ to 800‐mbar range. The sources of spectral line data are those listed in supporting information Table S1 of Fletcher, Orton, et al (2018).…”

Section: Tropospheric Temperatures and Aerosol Opacitymentioning

confidence: 99%

Characterizing Temperature and Aerosol Variability During Jupiter's 2006–2007 Equatorial Zone Disturbance

et al. 2020

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We use ground-based mid-infrared (8-20 μm) data acquired by three different instruments between 2005 and 2008 to characterize the variability of tropospheric temperature and aerosol opacity during the 2006-2007 Equatorial Zone disturbance. This disturbance is part of a repeating pattern of cloud-clearing events at Jupiter's equator, observed as a significant brightening at 5 μm (sensing the 2-to 7-bar region) and darkening at visible wavelengths (sensing the ∼0.7-bar pressure level). The data reveal a brightness temperature increase of ∼3.1 K between 2005 and February 2007 at 8.6-μm sensing tropospheric aerosol opacity and temperature near 0.6-0.8 bar. At wavelengths sensing tropospheric ammonia and temperatures between 150 and 600 mbar, the brightness temperature remains largely invariant between 2005 and 2008. The tropospheric vertical temperature profile and the tropospheric aerosol opacity were derived from images captured in different filters on four different dates, one for each year. The retrieved aerosol opacity at ∼0.6-0.8 bar shows a decrease at 2-5 • S of ∼45% in 2006 and ∼65% in 2007, with respect to 2008. This is consistent with cloud clearing/thinning during the coloration of the Equatorial Zone at visible wavelengths. This brightening at 8.6 μm started in 2005 and preceded the brightening at 5 μm, which started in April 2006. The results also suggest that cloud clearing during the Equatorial Zone disturbances is not simply the result of tropospheric warming, at least at p < 0.7 bar. We propose that cloud clearing occurs due to a decrease in the ammonia gas upwelling at the equator. Plain Language Summary Jupiter's equatorial latitudes between ∼7 • , known as the Equatorial Zone (EZ), undergo dramatic planetary-scale disturbances that completely alter its appearance at different altitudes of the troposphere between 0.7 and 4 bar. Here we characterize the last EZ disturbance, observed in 2006-2007, to investigate what atmospheric conditions vary during these disturbances. Retrieved aerosol opacity at ∼0.6 bar shows a decrease at 2-5 • S of ∼45% in 2006 and ∼65% in 2007, with respect to 2008, consistent with cloud clearing/thinning during these events. This removal of aerosol opacity is observed to start in 2005, a year before the deeper clouds are cleared. These results indicate that the EZ disturbance involves the clearing of both the ammonia ice cloud near 0.7 bar and the deeper NH 4 SH clouds with each cloud deck responding at different times. Results also suggest that cloud clearing during the EZ disturbances is not simply the result of tropospheric warming in the middle-to-high troposphere. We propose that cloud clearing occurs due to a decrease in the ammonia gas upwelling from deeper levels.

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Section: Tropospheric Temperatures and Aerosol Opacitymentioning

confidence: 99%

Characterizing Temperature and Aerosol Variability During Jupiter's 2006–2007 Equatorial Zone Disturbance

et al. 2020

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“…At 2.5 cm −1 , temperature retrievals are constrained between approximately 20 and 0.5 mbar. In Jupiter's auroral regions, there is sensitivity to temperatures at pressures as high as the 10-µbar level due to the presence of upper-stratospheric heating at this altitude (Sinclair et al, 2017a(Sinclair et al, ,b, 2018. Spectral emission lines of C 2 H 4 at approximately 950 cm −1 predominantly sound the 10-to 0.1-mbar pressure range with a secondary peak in sensitivity at the 5-µbar level.…”

Section: Vertical Sensitivitymentioning

confidence: 99%

Jupiter's auroral-related stratospheric heating and chemistry III: Abundances of C2H4, CH3C2H, C4H2 and C6H6 from Voyager-IRIS and Cassini-CIRS

Sinclair

Moses

Hue

et al. 2019

Icarus

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We present an analysis of Voyager-1-IRIS and Cassini-CIRS spectra of Jupiter's high latitudes acquired during the spacecrafts' respective flybys in November 1979 and January 2001. We performed a forward-model analysis in order to derive the abundances of ethylene (C 2 H 4 ), methylacetylene (CH 3 C 2 H), diacetylene (C 4 H 2 ) and benzene (C 6 H 6 ) in Jupiter's northern and southern auroral regions. We also compared these abundances to: 1) lower-latitude abundances predicted by the 'Model A' photochemical model, henceforth 'Moses 2005A', and 2) abundances derived at non-auroral longitudes in the same latitude band. This paper serves as an extension of Sinclair et al. (2017a), where we retrieved the vertical profiles of temperature, C 2 H 2 and C 2 H 6 from similar datasets. We find that an enrichment of C 2 H 4 , CH 3 C 2 H and C 6 H 6 with respect to lower-latitude abundances is required to fit the spectra of Jupiter's northern and southern auroral regions. For example, for CIRS 0.5 cm −1 spectra of Jupiter's southern auroral region, scale factor enrichments of 6.40 +1.30 −1.15 and 9.60 +3.98 −3.67 are required with respect to the Moses 2005A vertical profiles of C 2 H 4 and C 6 H 6 , respectively, in order to fit the spectral emission features of these species at ∼950 and ∼674 cm −1 . Similarly, in order to fit the CIRS 2.5 cm −1 spectra of Jupiter's northern auroral region, scale factor enrichments of 1.60 +0.37 −0.21 , 3.40 +1.89 −1.69 and 15.00 +4.01 −4.02 with respect to the Moses 2005A vertical profiles of C 2 H 4 , CH 3 C 2 H and C 6 H 6 were required, respectively.. Outside of Jupiter's auroral region in the same latitude bands, only upper-limit abundances of C 2 H 4 , CH 3 C 2 H and C 6 H 6 could be determined due to the limited sensitivity of the measurements, the weaker emission features combined with cooler stratospheric temperatures (and therefore decreased thermal emission) of these regions. Nevertheless, for a subset of the observations, derived abundances of C 2 H 4 and C 6 H 6 in Jupiter's auroral regions were higher (by greater than 1-σ) with respect to upper-limit abundances derived outside the auroral region in the same latitude band. This is suggestive that the influx of energetic ions and electrons from the jovian magnetosphere and external solar-wind environment into the neutral atmosphere in Jupiter's auroral regions drives enhanced ion-related chemistry, as has also been inferred from Cassini observations of Saturn's high latitudes (Guerlet et al., 2015;Koskinen et al., 2016;. We were not able to constrain the abundance of C 4 H 2 in either Jupiter's auroral regions or non-auroral regions due to its lower (predicted) abundance and weaker emission feature. Thus, only upper-limit abundances were derived in both locations. From CIRS 2.5 cm −1 spectra, the upper limit abundance of C 4 H 2 corresponds to a scale factor enhancement of 45.6 and 23.8 with respect to the Moses 2005A vertical profile in Jupiter's non-auroral and auroral regions.

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“…The hexagon was also visible in Composite Infrared Spectrometer retrieved temperature maps at 100‐mbar pressure, but was not seen in earlier maps probing higher levels (1–6 mbar; Fletcher et al, ). However, recent (2016–2017) Composite Infrared Spectrometer temperature observations (Fletcher et al, ) do see the polar hexagon near 5 mbar, along with enhanced acetylene and ethane poleward of the hexagon. Fletcher et al () argue that stratospheric aerosols may be localized poleward of the vortex and contribute to the sharp rise in stratospheric heating.…”

Section: Discussionmentioning

confidence: 97%

“…However, recent (2016–2017) Composite Infrared Spectrometer temperature observations (Fletcher et al, ) do see the polar hexagon near 5 mbar, along with enhanced acetylene and ethane poleward of the hexagon. Fletcher et al () argue that stratospheric aerosols may be localized poleward of the vortex and contribute to the sharp rise in stratospheric heating. Radiative transfer modeling of Cassini ISS images by Sanz‐Requena et al () confirms the presence of stratospheric aerosols in the 1–100‐mbar range with an optical depth at the hexagon latitude of 0.06 ± 0.01 at 0.4 μm.…”

Section: Discussionmentioning

confidence: 97%

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

et al. 2019

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Cassini's final orbits in 2016 and 2017 provided unprecedented spatial resolution of Saturn's polar regions from near‐polar spacecraft viewing geometries. Long‐wavelength channels of Cassini's Ultraviolet Imaging Spectrograph instrument detected Saturn's UV‐dark north polar hexagon near 180 nm at planetocentric latitudes near 75°N. The dark polar hexagon is surrounded by a larger, less UV‐dark collar poleward of planetocentric latitude 65°N associated with the dark north polar region seen in ground‐based images. The hexagon is closely surrounded by the main arc of Saturn's UV aurora. The UV‐dark material was locally darkest on one occasion (23 January 2017) at the boundary of the hexagon; in most Ultraviolet Imaging Spectrograph images the dark material more uniformly fills the hexagon. The observed UV‐dark stratospheric material may be a hydrocarbon haze produced by auroral ion‐neutral chemistry at submicrobar pressure levels. Ultraviolet Imaging Spectrograph polar observations are sensitive to UV‐absorbing haze particles at pressures lower than about 10–20 mbar.

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A hexagon in Saturn’s northern stratosphere surrounding the emerging summertime polar vortex

Cited by 44 publications

References 69 publications

Characterizing Temperature and Aerosol Variability During Jupiter's 2006–2007 Equatorial Zone Disturbance

Characterizing Temperature and Aerosol Variability During Jupiter's 2006–2007 Equatorial Zone Disturbance

Jupiter's auroral-related stratospheric heating and chemistry III: Abundances of C2H4, CH3C2H, C4H2 and C6H6 from Voyager-IRIS and Cassini-CIRS

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

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