37th DPS Meeting, 4-9 September 2005
Session 20 Outer Planets I
Oral, Tuesday, September 6, 2005, 9:00-10:30am, Law LG19

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[20.07] The Deep Winds of Saturn: First Measurements of the Zonal Windfield Near the Two-Bar Level

K. H. Baines, T. W, Momary (JPL/CalTech), M. Roos-Serote (Observatorio Astronomico de Lisboa)

We present the first measurements of the two-bar level zonal windfield of Saturn, as revealed using 5.1-micron images obtained by the Visual Infrared Mapping Spectrometer onboard the Cassini/Huygens orbiter. At this wavelength, the indigenous thermal radiation upwelling from depth dominates reflected sunlight. Overlying clouds attenuate this radiation, allowing the clouds to be mapped on both the day and nightside. Tracking these backlit clouds enables the determination of winds at a relatively deep level in the atmosphere near the two-bar level near where relatively large particles (> ~5 micron radius) reside within a major cloud layer, possibly composed of ammonium hydrosulfide. Comparison of these winds with those determined by tracking sunlit cloudtops enables the estimation of vertical windshears. Typically, we track features over multiple planetary rotations during a four-day period, from distances of ~ 5 million km corresponding to a VIMS IFOV of ~2500 km, thus yielding uncertainties in the derived winds of ~ 10 m/sec. Thus far, we find zonal winds from 63 degrees north to 53 degrees south latitude are close to those measured two decades ago by Voyager, in both high-speed jets and slow-speed regions. However, direct comparison with contemporary cloudtop wind measurements by Cassini/ISS (Porco et al., 2005, Science 307, 1243-1247) indicates a strong wind shear exists at depth in the Equatorial Region, with winds increasing with depth from the stratosphere above the 100-mbar level downward to at least the 2-bar level. At the equator, we find a tropospheric windshear of ~ 1.8 m/(s-km) in the 1-2-bar region, assuming the Porco et al. (2005) 0.75-micron filter results pertain to the 1-bar level. This is comparable to the to the upper-level shear value of ~ 2.2 m/(s-km) assuming a 100-mbar level for the 0.727 micron methane band measurements of Porco et al. (2005), and close to the 1.5 m/(s-km) value observed in the 1-2 bar region of Jupiter by the Galileo probe (Atkinson et al., 1998, JGR 103, 22,911-22,928). Outside the Equatorial Region, shears are significantly less.

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