Saturn stratospheric composition and dynamics during its 2010-2013 Great Storm: I. Oxygen species

The observations of Saturn's clouds show that its atmosphere usually seems quite inactive compared to its tumultuous neighbor Jupiter. However, its apparent calm weather is disrupted every saturnian year by giant planetary storms, and six have already been reported (Sánchez-Lavega et al. 2018). The last storm to date started on December 5th, 2010, with an initial tropospheric perturbation in the northern hemisphere at 40°N (Fletcher et al. 2011, Fischer et al. 2011, Sánchez-Lavega et al. 2011). The tropospheric perturbation quickly spread out in latitude over 30° and totally encircled the planet within a few weeks.

The storm also affected the stratosphere where two stratospheric hot vortices appeared above the tropospheric perturbation (Fletcher et al. 2011). The two structures eventually merged in May 2010 and formed a giant hot vortex (Fletcher et al. 2012). These vortices were nicknamed "beacons" because of how bright they were in the infrared, and because of the planet's rapid rotation. A huge increase in temperature was reported in the newly merged beacon, up to 80 K compared to quiescent conditions in the mbar region. The beacon completely reshaped the atmospheric weather, circulation and composition, as notable changes in temperatures, winds and hydrocarbon abundances were reported (Fletcher et al. 2011, Fletcher et al. 2012, Moses et al. 2015).

Saturn's stratosphere harbors oxygen species, like H₂O and CO, which are provided by external sources (Cavalié et al. 2010, 2019). Under quiescent conditions, H₂O condenses in the stratospheric layers where the peak of temperature increase caused by the beacon was reported. The warmer temperatures may therefore have led to the sublimation of the H₂O stratospheric cloud and completely changed the local H₂O vertical distribution.

In this paper, we present H₂O and CO mapping observations carried out between 2010 and 2013 with the Submillimeter Array, the Atacama Large Millimeter/submillimeter Array and the Herschel Space Observatory. We will show the temporal evolution of their meridional and vertical distributions. We also retrieve some properties of the stratospheric H₂O cloud from the strong enhancement of H₂O emission in the beacon all along the storm as observed with Herschel.

References

Cavalié et al. (2010). A&A 510, A88.

Cavalié et al. (2019). A&A 630, A87.

Fischer et al. (2011). Nature 475, 75-77.

Fletcher et al. (2011). Science 332, 1413-1417.

Fletcher et al. (2012). Icarus 221, 560-586.

Moses et al. (2015). Icarus 261, 149-168.

Sánchez-Lavega et al. (2011). Nature 475, 71-74.

Sánchez-Lavega et al. (2018). Saturn in the 21st Century (CUP), 377-416.