Evolution of Stratospheric Chemistry in the Saturn Storm Beacon Region.

The giant northern-hemisphere storm that erupted on Saturn in December 2010 triggered significant changes in stratospheric temperatures and species abundances that persisted for more than a year after the original outburst. The stratospheric regions affected by the storm have been nicknamed "beacons" due to their prominent infrared-emission signatures (Fletcher, L.N. et al. [2011]. Science 332, 1413). The two beacon regions that were present initially merged in April 2011 to form a single, large, anticyclonic vortex (Fletcher, L.N. et al. [2012]. Icarus 221, 560). We model the expected photochemical evolution of the stratospheric constituents in the beacons from the initial storm onset through the merger and on out to March 2012. The results are compared with longitudinally resolved Cassini/CIRS spectra from May 2011. If we ignore potential changes due to vertical winds within the beacon, we find that C2H2, C2H6, and C3H8 remain unaffected by the increased stratospheric temperatures in the beacon, the abundance of the shorter-lived CH3C2H decreases, and the abundance of C2H4 increases significantly due to the elevated temperatures, the latter most notably in a secondary mixing-ratio peak located near mbar pressures. The C4H2 abundance in the model decreases by a factor of a few in the 0.01-10 mbar region but has a significant increase in the 10-30 mbar region due to evaporation of the previously condensed phase. The column abundances of C6H6 and H2O above ~30 mbar also increase due to aerosol evaporation. Model-data comparisons show that models that consider temperature changes alone underpredict the abundance of C2H x species by a factor of 2-7 in the beacon core in May 2011, suggesting that other processes not considered by the models, such as downwelling winds in the vortex, are affecting the species profiles. Additional calculations indicate that downwelling winds of order -10 cm s -1 near ~0.1 mbar need to be included in the photochemical models in order to explain the inferred C2H x abundances in the beacon core, indicating that both strong subsiding winds and chemistry at elevated temperatures are affecting the vertical profiles of atmospheric constituents in the beacon. We (i) discuss the general chemical behavior of stratospheric species in the beacon region, (ii) demonstrate how the evolving beacon environment affects the species vertical profiles and emission characteristics (both with and without the presence of vertical winds), (iii) make predictions with respect to compositional changes that can be tested against Cassini and Herschel data, and higher-spectral-resolution ground-based observations of the beacon region, and (iv) discuss future measurements and modeling that could further our understanding of the dynamical origin, evolution, and chemical processing within these unexpected stratospheric vortices that were generated after the 2010 convective event.

Thermal structure and dynamics of Saturn's northern springtime disturbance.

Saturn's slow seasonal evolution was disrupted in 2010-2011 by the eruption of a bright storm in its northern spring hemisphere. Thermal infrared spectroscopy showed that within a month, the resulting planetary-scale disturbance had generated intense perturbations of atmospheric temperatures, winds, and composition between 20° and 50°N over an entire hemisphere (140,000 kilometers). The tropospheric storm cell produced effects that penetrated hundreds of kilometers into Saturn's stratosphere (to the 1-millibar region). Stratospheric subsidence at the edges of the disturbance produced "beacons" of infrared emission and longitudinal temperature contrasts of 16 kelvin. The disturbance substantially altered atmospheric circulation, transporting material vertically over great distances, modifying stratospheric zonal jets, exciting wave activity and turbulence, and generating a new cold anticyclonic oval in the center of the disturbance at 41°N.

12C/13C ratio in ethane on titan and implications for methane's replenishment.

The (12)C/(13)C abundance ratio in ethane in the atmosphere of Titan has been measured at 822 cm(-1) from high spectral resolution ground-based observations. The value, 89(8), coincides with the telluric standard and also agrees with the ratio seen in the outer planets. It is almost identical to the result for ethane on Titan found by the composite infrared spectrometer (CIRS) on Cassini. The (12)C/(13)C ratio for ethane is higher than the ratio measured in atmospheric methane by Cassini/Huygens GCMS, 82.3(1), representing an enrichment of (12)C in the ethane that might be explained by a kinetic isotope effect of approximately 1.1 in the formation of methyl radicals. If methane is being continuously resupplied to balance photochemical destruction, then we expect the isotopic composition in the ethane product to equilibrate at close to the same (12)C/(13)C ratio as that in the supply. The telluric value of the ratio in ethane then implies that the methane reservoir is primordial.