Observations of the temporal evolution of Saturn's stratosphere following the Great Storm of 2010-2011. II. Latitudinal distribution of CO and stratospheric winds

Abstract

Saturn's Great Storm of 2010-2011 has produced two stratospheric hot spots, the "beacons," that eventually merged to produce a gigantic one in April and May 2011. This beacon perturbed stratospheric temperatures, hydrocarbon, and water abundances for several years. We aim to assess whether the beacon induced any perturbation in another oxygen species, namely CO. A second goal is to measure how the vortex perturbed the stratospheric wind regime. We conducted interferometric observations of Saturn in the submillimeter range with SMA and ALMA to spatially resolve the CO (J=3-2) and (J=2-1) emissions, respectively. We used a previously determined CO vertical profile as a template, to search for (i) the meridional distribution of CO and (ii) variations of the CO abundance associated with the storm. The high spatial and spectral resolutions of the ALMA observations enabled us to retrieve the winds from the Doppler shifts induced by the winds on the lines. Despite limitations resulting from the removal of baseline ripples, we find a relatively constant meridional distribution of CO. The average CO mole fraction implied by the adopted and rescaled 220-year-old-comet-impact vertical profile is (1.7$\pm$0.7)$\times10^{-7}$ at 0.3\,mbar, i.e., where the contribution functions peak. We also find that the CO abundance has not been noticeably altered in the beacon. The winds measured at 1\,mbar show striking differences with those measured in 2018, after the demise of the beacon. We find the signature of the vortex as an anticyclonic feature. The equatorial prograde jet is 100 to 200 m.s$^{-1}$ slower, and broader in latitude, than in quiescent conditions. We also detect several prograde jets in the southern hemisphere. Finally, we detect a retrograde jet at 74$^\circ$N which could be a polar jet caused by the interaction of the Saturn magnetosphere with its atmosphere.

Figures (15)

• Figure 1: Example of three raw spectra obtained with SMA at the limb of Saturn and at various latitudes. The standing wave removal stage comprises the subtraction of sine waves and/or polynomials from the raw spectra. An example of standing wave fit is provided for each raw spectrum, with the corresponding color.
• Figure 2: CO (J=3-2) line area map, as observed with SMA on March 13, 2010. The 1-bar level is shown with the black ellipse, the planet rotation axis is displayed with a dashed black line, and isolatitudes are indicated by gray contours. The expected position of the A and B rings is depicted by the gray filled area and the beam is illustrated with a white filled ellipse.
• Figure 3: Saturn continuum flux density images at 230 GHz. (Top) Saturn continuum image as observed with ALMA on January 14, 2012. (Center) Simulated continuum image in the conditions of the January 2012 observations. (Bottom) Residuals (observation - model) showing the overall good agreement between model and observations, especially at the limb. In all panels, the 1-bar level is shown with the black ellipse, the planet rotation axis is displayed with a dashed black line, and isolatitudes are indicated by gray contours. The expected position of the A and B rings is depicted by the gray filled area and the beam is illustrated with a white filled ellipse.
• Figure 4: CO (J=2-1) line area map from the combined ALMA observations of January 14 and 22, 2012. The 1-bar level is shown with the black ellipse, the planet rotation axis is displayed with a dashed black line, and isolatitudes are indicated by gray contours. The expected position of the A and B rings is depicted by the gray filled area and the beam is illustrated with a white filled ellipse.
• Figure 5: Zonal field of temperatures used for the analysis of the SMA data taken on March 13, 2010. The data are extracted up to 0.5 mbar from the Fletcher2018b dataset and expanded isothermally upward.