Completing a trio of posts on the state of giant planet science in 2013, based on notes, tweets and abstracts submitted to two large conferences, EPSC in London and the DPS in Denver. Late 2013 finds Saturn's northern springtime hemisphere (four years after the equinox, and four years until summer solstice) continuing to recover from the major 2010-11 storm, with the continued presence of the stratospheric 'beacon' (see 'Saturn's Stratospheric Vortex') and tropospheric anticyclone (see 'Saturn's Storm Vortex Survives!') spawned by the storm, and observations of the north polar hexagon and polar cyclone as they come into view from Earth (see 'Saturn's Hexagon Viewed from the Ground').
Cassini Returns to Saturn's Poles
|Cassini ISS stares down into the heart of Saturn's north polar |
My own research (presented at EPSC) concerned new observations of Saturn's poles by Cassini, now that the spacecraft has returned to an inclined orbit around the gas giant. I used thermal spectroscopy from Cassini/CIRS to show the continued presence of hot spots at both poles of Saturn, which correspond to cyclonic vortices at both poles irrespective of season. We also showed the cooling of the south polar 'autumn' stratosphere and hints of a warming north polar stratospheric vortex, as atmospheric temperatures, composition and clouds change in response to the increasing sunlight in the northern hemisphere. Saturn's south pole is now in the darkness of approaching winter, and Momary et al. (DPS) show that the hot cyclone still corresponds to a hurricane-like vortex with a cloudless 'eye' lying deeper than the surrounding cloud decks.
The north polar hexagon is now readily visible in spring sunlight, with new results from Cassini/VIMS (Momary et al., DPS), Cassini/ISS (Sayanagi et al., DPS) and ground-based observations (Sanchez-Lavega et al. EPSC) suggesting that the hexagon is slowly rotating westward in longitude, rather than being stationary as we previously thought. On the other hand, ground-based amateur observations presented by Delcroix et al., (EPSC) were able to resolve the hexagon vertices throughout 2013, but were not accurate enough to see any slow drifts. Sanchez-Lavega et al. suggest that the hexagon has the most stable rotational period of any feature on Saturn, making it an excellent candidate for constraining the deep internal rotation rate of the planet. Finally, Momary et al. showed discrete clouds racing around the edges of the hexagon, and a massive storm residing just poleward of the hexagon system, which seems to have become increasingly cloudy since 2008 and could be a 'shepherding storm' for the hexagon.
Scientists studying Saturn are interested in far more than the polar latitudes, and Cassini continues to provide a unique opportunity to study the seasonal evolution of a gas giant. Sinclair et al. (EPSC) and Li et al. (DPS) find intriguing differences in Saturn's temperatures, winds and composition between 2009/10 from Cassini and 1980/81 from Voyager, exactly one Saturnian year earlier. Edgington et al. (DPS) showed that Saturn's moving ring-shadow influences photochemistry and haze content during a year; Guerlet et al. (EPSC) and Sylvestre et al. (EPSC) used limb spectroscopy from Cassini to look at Saturn's evolving stratosphere, which will nicely feed into the new global climate model being developed by Spiga et al. (EPSC) for Saturn's middle atmosphere. Orton et al. (DPS) focussed on a type of wave activity, namely slowly moving thermal waves observed on Saturn by Cassini and ground-based infrared observations since 2003. These are very large-scale waves, and the strongest wave activity was discovered between 30-45S and 0-30N during southern autumn, with a strong correlation between tropospheric and stratospheric wave activity. They also found that that Saturn's waves occurred in trains over a limited longitude range but with little apparent correlation with known atmospheric storms (one possible cause of the waves), and with peak activity just before Saturn's northern spring equinox in 2009.
Over a duet of talks, Barth and Rages (DPS) describe Cassini observations of Saturn's limb at high phase angles and spatial resolutions of around 10 km, showing the presence and structure of particulates in Saturn's stratosphere. The hazes are likely a mix of material, including solid organics formed as a result of methane photolysis and electron deposition, as well as water condensation and hydrocarbon ices (e.g., butane, diacetylene). At even higher altitudes, Koskinen et al. (DPS) use Cassini/UVIS solar and stellar occultations to probe the temperatures of the thermosphere, finding exospheric temperatures (an exobase 2700-3000 km above Saturn's 1-bar level) ranging from 370-540 K and increasing from equator to pole by 100-150 K, consistent with auroral heating being redistributed to lower latitudes by some as-yet uncertain circulation.
Aftermath of the Great Storm
Finally, researchers are still using the wealth of Cassini and ground-based data from the springtime storm of 2010-11 to understand the processes governing Saturn's meteorology. Sromovsky et al. (DPS) summarise their recent Icarus paper on Cassini/VIMS reflected sunlight imaging of the storm system, finding an aerosol population of ammonia and water ice, with some ammonium hydrosulphide as a third component. This is the first identification of water ice on Saturn, required to improve the spectral fits around the 3-µm ammonia ice features, and supports the idea that the storm was powered by strong convection from the 10-20 bar depths of the water cloud. Fouchet et al. (EPSC, DPS) followed up on our Cassini study of Saturn's stratospheric vortex (the beacon), using the TEXES instrument on the IRTF in July 2011 to determine the vertical structure of temperatures near the vortex, probing higher altitudes than Cassini could sense. Finally, Li et al. (DPS) suggested that the 20-30 year periodicity of these enormous storms is caused by a prohibition of strong convection when the troposphere is warm, and the presence of water makes the column 'heavy'. As the troposphere cools below some critical point, convection can begin and produce a warm column that overshoots into the stratosphere. The resulting large-scale atmospheric adjustment causes ammonia vapour to condense and precipitate out as snow, causing the high brightnesses observed by the Cassini/RADAR teams at microwave wavelengths (indicating a depletion of ammonia). However, it remains unclear why these eruptions should occur only at certain latitudes, but it certainly sounds like progress.
Finally, my own use of the IRTF/TEXES instrument back in February 2013 yielded the first results on the origins of nitrogen on both Jupiter and Saturn, in my second presentation at EPSC. Although it's still a work in progress, the initial results seem to suggest that Saturn's primordial nitrogen came from the same place as Jupiter's..... so watch this space as I get the article written!
Key 2013 Saturn Papers
- Sromovsky et al., (2013), Saturn's Great Storm of 2010-2011: Evidence for ammonia and water ices from analysis of VIMS spectra, http://dx.doi.org/10.1016/j.icarus.2013.05.043
- Sinclair et al., (2013), Seasonal variations of temperature, acetylene and ethane in Saturn's atmosphere from 2005 to 2010, as observed by Cassini CIRS, http://dx.doi.org/10.1016/j.icarus.2013.03.011
- O'Donoghue et al., (2013), The domination of Saturn's low-latitude ionosphere by ring 'rain', http://dx.doi.org/10.1038/nature12049
- García-Melendo et al., (2013), Atmospheric dynamics of Saturn's 2010 giant storm, http://dx.doi.org/10.1038/ngeo1860
- Sayanagi et al., (2013), Dynamics of Saturn's great storm of 2010-2011 from Cassini ISS and RPWS, http://dx.doi.org/10.1016/j.icarus.2012.12.013
- Janssen et al., (2013), Saturn's thermal emission at 2.2-cm wavelength as imaged by the Cassini RADAR radiometer, http://dx.doi.org/10.1016/j.icarus.2013.06.008
- Abbas et al. (2013), Distribution of CO2 in Saturn's Atmosphere from Cassini/cirs Infrared Observations, http://dx.doi.org/10.1088/0004-637X/776/2/73
- Dyudina et al. (2013), Saturn's visible lightning, its radio emissions, and the structure of the 2009-2011 lightning storms, http://dx.doi.org/10.1016/j.icarus.2013.07.013
- Laraia et al., (2013), Analysis of Saturn's thermal emission at 2.2-cm wavelength: Spatial distribution of ammonia vapor, http://dx.doi.org/10.1016/j.icarus.2013.06.017
- Li et al., (2013), Strong Temporal Variation Over One Saturnian Year: From Voyager to Cassini, http://dx.doi.org/10.1038/srep02410
- Roman et al., (2013), Saturn's cloud structure inferred from Cassini ISS, http://dx.doi.org/10.1016/j.icarus.2013.03.015