Saturn's zonal wind profile in 2004-2009 from Cassini ISS images

Supplemental data supporting E. García-Melendo, S. Pérez-Hoyos, A. Sánchez-Lavega and R. Hueso c, Icarus 215 (2011) 62–74)

ISS imaging data acquired by the Cassini Imaging Science Subsystem (ISS) instrument (Porco, C.C. et al., 2004. Space Sci. Rev. 115, 363–497.) spanning 2004-2009 were used to retrieve Saturn’s zonal wind profile. These data span a portion of Saturn’s year, from 2 years after Saturn’s northern hemisphere winter solstice to close to its northern spring equinox. Approximately 300 Narrow and Wide-angle images were used with resolutions ranging from 160 km/pix to 30 km/pix.

Cassini images used to retrieve Saturn zonal winds.

Filters	Date interval	Number of Images	Resolution (km pix_1)	Subsolar latitude
CB2, MT1, MT2	2004/09/06-2004/09/13	64	60	_23.9
CB2, MT3	2005/02/25-2005/02/27	51	160	_22.5
CB2, MT3	2005/02/27	15	160	_22.5
CB2, MT2	2005/06/15-2005/06/16	54	140	_21.4
CB2, MT2	2006/10/11	9	30	_15.4
CB2, MT2, CB3, MT3	2008/02/25-2008/02/27	40	90	_8.1
CB2, MT3	2008/03/29	24	80	_7.7
CB2, MT2, CB3, MT3	2008/06/19-2008/06/20	24	70	_6.4
CB2	2008/11/29-2008/12/01	7	40	_3.9
CB2, MT2, CB3, MT3	2008/12/28	16	60	_3.5
MT2, MT3	2009/01/01-2009/01/12	8	60	_3.5

Data from methane absorbing bands at 890 and 727 nm and in their adjacent continuum wavelengths at 939 and 752 nm were used to investigate Saturn’s global circulation at 60 mbar to 250 mbar with the methane filters and at 350 mbar to 500 mbar with the continuum filters. Comparisons with zonal wind profiles obtained by Voyager 1 and 2 (1980–1981), Hubble Space Telescope, and ground-based telescopes (1990–2004) reveal similar structure with the most variation at the peak of the eastward jets and the equatorial region, including a region of positive vertical shear within 25 deg (centric) of the equator.

The characteristics of the filters are as follows. This table was derived from the Cassini ISS User’s guide that contains updated values relative to Porco, 2004.

Characteristics of the filters utilized

	Filter	Central Wavelength (nm)	Bandpass FWHM (nm)	Effective Wavelength (nm)	PSF FWHM (pixels)	PSF Dynam Range	Extended PSF?
Narrow Angle Camera	MT2	727.421	4.1124	727.415	1.34	8.30E+07	yes
Narrow Angle Camera	CB2	750.505	10.0129	750.495	1.39	7.70E+07	yes
Narrow Angle Camera	MT3	889.194	10.472	889.196			no
Narrow Angle Camera	CB3	937.964	9.54761	937.928			yes
Wide Angle Camera	MT2	728.452	4.00903	728.418	1.34	1.50E+08	Approx.
Wide Angle Camera	CB2	752.364	10.0044	752.354	1.38	3.60E+07	Approx.
Wide Angle Camera	MT3	890.34	10.0116	890.332	1.64	3.60E+07	Approx.
Wide Angle Camera	CB3	938.532	9.95298	938.445	1.77	2.10E+07	Approx.

These characteristics of ISS bandpasses and PSF properties for non-polarized filters, were adapted and updated from Porco et. al. (2004). Effective wavelength is the central wavelength of the bandpass convolved with the solar spectrum. Wavelength and bandpass values were derived from ground-based measurements of the CCD quantum efficiency and filter and optics transmission curves. Extended PSFs marked “Approx.” refer to filters where the extended regions have been approximated from an average of available filter data and scaled to match the existing core PSF

The Planetary Laboratory Image Analysis (PLIA) software was used to navigate the images (Hueso, R. et al., 2010. Adv. Space Res. 46, 1120–1138.). Using SPICE (Acton, Ch.H., 1995. Planet. Space Sci. 44, 65–70.) PLIA automatically fits an ellipse (using the IAU/IAG standard ellipsoid (Seidelmann, P.K. et al., 2007. Celestial Mech. Dyn. Astron. 98,155–180) with equatorial and polar radii at the 1 bar level defined as 60,268 km and 54,364 km) to the limb of a projected image of the planet and allows the user to check and modify the fit – this is especially significant where only a small part of the limb is visible.

Zonal velocities were measured by performing automatic one dimensional cross-correlation of brightness longitude scans as described in García-Melendo and Sánchez-Lavega (Icarus 152, 316–330. 2001), which assumes small cloud tracers sustain their identity for a saturnian rotation (approx. 10.6 hours). The images were mapped into cylindrical projections with a lat/long resolution of 0.1o planetocentric, allowing for longitudinal brightness scans as a function of latitude to be extracted. Pairs of scans were automatically compared by performing a one-dimensional cross-correlation. The shift required for maximum correlation was converted to zonal velocity based on the radius at that latitude as defined by the standard IAU ellipsoid.

Main error sources are navigation uncertainties, intrinsic variability of the cloud tracers, local dynamics effects such as waves, vortices, divergent movements and lack of well-defined features. For images with the lowest spatial resolution (160 km/pix at the subspacecraft’s point) a one-pixel navigation error translates to an uncertainty of +/-4 m/sec, increasing away from the meridian.

These data represent average zonal wind profiles spanning the period from 2004 to 2009. For information concerning variability during this epoch see Saturn’s zonal wind profile in 2004–2009 from Cassini ISS images and its long-term variability. E. García-Melendo, S. Pérez-Hoyos, A. Sánchez-Lavega and R. Hueso c, Icarus 215 (2011) 62–74.