The Evolution of Saturn's Northern Storm of 2010-2011 and Environs as Viewed by Cassini/VIMS

Physics

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[0343] Atmospheric Composition And Structure / Planetary Atmospheres, [5704] Planetary Sciences: Fluid Planets / Atmospheres, [5739] Planetary Sciences: Fluid Planets / Meteorology

Scientific paper

Images and spectra acquired by the Visual Infrared Mapping Spectrometer on board the Cassini Orbiter reveal that the current northern storm on Saturn is remarkable for (1) its location - the first significant storm seen in northern mid-latitudes since 1906, (2) its duration - presently approaching 8 months, and (3) its power, as indicated by the relatively massive ammonia-laden clouds it produces that reveal significant, persistent transport of materials over at least one bar of depth (> 30 km of altitude). Situated near 35 degrees north latitude (planetocentric) near the maximum of a westward jet, the storm head moves westward at ~ 2.7 degrees per day, or ~ 27 m/s. Multi-spectral images of the feature and its environs in 352 colors spanning nearly all longitudes were acquired by VIMS on February 24, May 11 and July 12, 2011. In all imagery, the head of the storm appears atypically dark in ammonia-ice sensitive wavelengths 2.73-3.1 micron, indicating significant amounts of ammonia ice. Simultaneously, the feature appears bright at pseudo-continuum near-infrared wavelengths, particularly at 4.08 micron, indicating an atypically massive cloud of large particles. Some 3-5 degree of latitude to the north and south of the cloudhead, streamers of such large-particle ammonia clouds extend more than 150 degrees of longitude to the east. While these streamers appear nearly equivalent in brightness in diagnostic wavelengths in the February 24, 2011 observations, the northern streamer clearly dominates in the May 11, 2011 map. As well, a new dark spot, the first observed associated with this storm, appears more than 250 degrees of longitude downstream of the cloudhead in the May 11, 2011 images and persists through the July 12, 2011 observations. Its appearance may be associated with the dissipation of overlying cloud features as the dark spot wandered eastward, Similar in size (> 3000 km) and spectral appearance to the dark spots associated with the 2008 southern storm (Baines et al, Planetary and Space Sci., 57, 1650-1658,2009), this feature suggests upwelling of materials from the ~10-bar level, as previously proposed for the southern storm. New imagery expected in August 2011 as well as additional analysis of the July, 2011 images will be discussed and dynamical implications presented, including possible relationships of the northern storm with the String of Pearls feature observed for 5 years in the same latitude and longitude range but which has not been observed since the advent of the storm in December 2010.

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