* Astronomy

Saturn's weird hexagon changes colour

The mysterious hexagon at Saturn's northern pole has changed colour from blue to gold, scientists have said.
The hexagon is essentially a rotating cloud pattern; its shape may originate as the result of large differences in the speeds of Saturn's winds.
The observation was made by Nasa's Cassini spacecraft, which has been exploring the sixth planet since 2004. The change in colour is thought to be the result of seasonal changes on the planet.
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Title: Seasonal Variability of Saturn's Tropospheric Temperatures, Winds and Para-H2 from Cassini Far-IR Spectroscopy
Author: Leigh N. Fletcher, Patrick G.J. Irwin, Richard K. Achterberg, Glenn S. Orton, F. Michael Flasar

Far-IR 16-1000 µm spectra of Saturn's hydrogen-helium continuum measured by Cassini's Composite Infrared Spectrometer (CIRS) are inverted to construct a near-continuous record of upper tropospheric (70-700 mbar) temperatures and para-H2 fraction as a function of latitude, pressure and time for a third of a Saturnian year (2004-2014, from northern winter to northern spring). The thermal field reveals evidence of reversing summertime asymmetries superimposed onto the belt/zone structure. The temperature structure that is almost symmetric about the equator by 2014, with seasonal lag times that increase with depth and are qualitatively consistent with radiative climate models. Localised heating of the tropospheric hazes (100-250 mbar) create a distinct perturbation to the temperature profile that shifts in magnitude and location, declining in the autumn hemisphere and growing in the spring. Changes in the para-H2 (fp) distribution are subtle, with a 0.02-0.03 rise over the spring hemisphere (200-500 mbar) perturbed by (i) low-fp air advected by both the springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of high-fp air at northern high latitudes, responsible for a developing north-south asymmetry in fp. Conversely, the shifting asymmetry in the para-H2 disequilibrium primarily reflects the changing temperature structure (and the equilibrium distribution of fp), rather than actual changes in fp induced by chemical conversion or transport. CIRS results interpolated to the same point in the seasonal cycle as re-analysed Voyager-1 observations show qualitative consistency, with the exception of the tropical tropopause near the equatorial zones and belts, where downward propagation of a cool temperature anomaly associated with Saturn's stratospheric oscillation could potentially perturb tropopause temperatures, para-H2 and winds.

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Title: Saturn's aurora observed by the Cassini camera at visible wavelengths
Author: Ulyana A. Dyudina, Andrew P. Ingersoll, Shawn P. Ewald, Danika Wellington

The first observations of Saturn's visible-wavelength aurora were made by the Cassini camera. The aurora was observed between 2006 and 2013 in the northern and southern hemispheres. The color of the aurora changes from pink at a few hundred km above the horizon to purple at 1000-1500 km above the horizon. The spectrum observed in 9 filters spanning wavelengths from 250 nm to 1000 nm has a prominent H-alpha line and roughly agrees with laboratory simulated auroras. Auroras in both hemispheres vary dramatically with longitude. Auroras form bright arcs between 70 and 80 degree latitude north and between 65 and 80 degree latitude south, which sometimes spiral around the pole, and sometimes form double arcs. A large 10,000-km-scale longitudinal brightness structure persists for more than 100 hours. This structure rotates approximately together with Saturn. On top of the large steady structure, the auroras brighten suddenly on the timescales of a few minutes. These brightenings repeat with a period of about 1 hour. Smaller, 1000-km-scale structures may move faster or lag behind Saturn's rotation on timescales of tens of minutes. The persistence of nearly-corotating large bright longitudinal structure in the auroral oval seen in two movies spanning 8 and 11 rotations gives an estimate on the period of 10.65 ±0.15 h for 2009 in the northern oval and 10.8± 0.1 h for 2012 in the southern oval. The 2009 north aurora period is close to the north branch of Saturn Kilometric Radiation (SKR) detected at that time.

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Title: Cloud Features and Zonal Wind Measurements of Saturn's Atmosphere as Observed by Cassini/VIMS
Authors: David S. Choi, Adam P. Showman, Robert H. Brown

We present an analysis of data about Saturn's atmosphere from Cassini's Visual and Infrared Mapping Spectrometer (VIMS), focusing on the meteorology of the features seen in the 5-micron spectral window. We present VIMS mosaics and discuss the morphology and general characteristics of the features backlit by Saturn's thermal emission. We have also constructed a zonal wind profile from VIMS feature tracking observation sequences using an automated cloud feature tracker. Comparison with previously constructed profiles from Voyager and Cassini imaging data reveals broad similarities, suggesting minimal vertical shear of the zonal wind. However, areas of apparent wind shear are present in the VIMS zonal wind profile at jet stream cores. In particular, our analysis shows that the equatorial jet reaches speeds exceeding 450 m/s, similar to speeds obtained during the Voyager era. This suggests that recent inferences of relatively slower jet speeds of ~275-375 m/s are confined to the upper troposphere and that the deep (>1 bar) jet has not experienced a significant slowdown. Our measurements of the numerous dark, spotted features seen in the VIMS mosaics reveals that most of these features have diameters less than 1000 km and reside in confined zonal bands between jet stream cores. We propose that these spot features are vortices and that VIMS and ISS are sensing the same vortices at two different pressure levels. The local structure at the zonal jet streams remains complex, as VIMS may be sensing cloud features that are deeper than the NH3 cloud deck.

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