Meteorology Of Saturn General circulation and zonal structure

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Saturn emits 1.78 x more energy than it receives from Sun, but compared with Jupiter the overall energy emission is much less (only 1/3 that of Jupiter). Hence, it might be expected that Saturn should have a commensurately less vigorously overturning atmosphere. However, the atmosphere appears just as energetic as Jupiter's with strong zonal winds that reach speeds of 400 m s-1 in the eastward-flowing equatorial jet. Although the atmosphere is very dynamic, the appearance of Saturn is generally much more subdued than that of Jupiter, with the belt/zone structure being much less clear. The tropospheric cloud structure appears to be much more masked by tropospheric and stratospheric haze layers due both to the expected ammonia ice cloud deck condensing at deeper levels than in Jupiter's atmosphere and also due to the greater scale height of the Saturnian atmosphere (as outlined in Chapter 4). The belts and zones have similar names to those of Jupiter and the universally accepted naming convention is shown in Figure 5.21.

The obliquity of Saturn (26.7°) means that it is much more prone to seasonal effects (Figure 5.22, see color section) than Jupiter and the Voyager spacecraft found in 1980 and 1981 (at a time corresponding roughly to the Northern Spring Equinox) that the upper tropospheric temperature at the 210 mbar level was approximately 10 K warmer in the southern hemisphere than the northern hemisphere (Bezard et al., 1984; Conrath and Pirraglia 1983; Hanel et al., 1981, 1982), decreasing at higher pressures. The thermal response of the atmosphere at this level is estimated from the radiative time constant (Gierasch and Goody, 1969) to be roughly 5 years or equiva-

North Polar Region

South Temperare Zoii

Equatorial Zone

North Fo North Temperate Zoi

South Po

North Polar Region

South Temperare Zoii

North Fo North Temperate Zoi

South Po

North Equatorial Beit

South Equatorial Belt

South Polar Region Figure 5.2l. Standard Saturnian zonal nomenclature.

North Temperate Belt

South Temperate Belt

North Equatorial Beit

South Equatorial Belt

South Polar Region Figure 5.2l. Standard Saturnian zonal nomenclature.

lently -k/2 out of phase with solar forcing, which corresponds well to the observations. Like Jupiter, the variations in temperature with latitude were used to calculate vertical wind shear, which again was well correlated with cloud top zonal winds, indicating that the jets decay with height (Pirraglia et al., 1981; Smith et al., 1981, 1982). Prior to the arrival of the Cassini mission a similar north-south asymmetry in stratospheric temperature in 1995 was reported by Karkoschka (1998a) by analyzing the 890 nm methane absorption feature, whose width depends on temperature. Stratospheric temperature asymmetries consistent with the 5-year time lag have also been observed in the approach to and during the Cassini mission with ground-based observations (Greathouse et al., 2005; Orton and Yanamandra-Fisher, 2005; Yanamandra-Fisher et al., 2001) and by Cassini observations themselves (Flasar et al., 2005; Fletcher et al., 2007a, b) (Figure 5.23, see color section).

During its observations Cassini has found (Fletcher et al., 2007b) that the northern hemisphere is consistently cooler than the southern hemisphere, though the seasonal contrast again becomes smaller for larger pressures, where the radiative time constant for the thermal response of the atmosphere to changes in seasonal forcing is longer (Bezard et al., 1984; Conrath and Pirraglia, 1983). At the top of the upper troposphere at 100 mbar, the South Polar region is 10 K warmer than the North Polar region, which is consistent with the findings of Voyager (Hanel et al., 1982), bearing in mind that Cassini started observing prior to Saturn's Northern Spring Equinox in 2009, while the Voyager observations were made just after Saturn's previous Northern Spring Equinox. Cassini has observed smaller latitudinal structure superimposed onto this seasonal asymmetry, and zonal temperature minima occur symmetrically about the equator at planetographic latitudes of 58-60°, 46-47°, 30-31°, and possibly near 71°S, 83°S, and 74°N (Fletcher et al., 2007b), which coincide well with the zonal wind structure determined from cloud tracking (Porco et al., 2005), with eastward prograde jets appearing on the poleward side of temperature minima.

In the stratosphere at 1 mbar, no belt/zone variations are observed by Cassini at midlatitudes, but strong hemispherical asymmetry exists with the South Pole appearing 10 K to 12 K warmer than the equator and 25 K to 30 K warmer than the North Pole (Figure 5.23, see color section). In addition, although a temperature minimum is seen at the equator in the upper troposphere, in the stratosphere there is a local temperature maximum at the equator, which is 8 K to 10 K warmer than at 20°N or 20°S. Furthermore, Orton et al. (2008) find, from analyzing a long time series of ground-based observations, that the temperature of the equatorial stratosphere changes regularly with a period of —15 years, or half a Saturnian year. This suggests that the equatorial stratospheric temperature is forced by seasonal variations and has similarities with Earth's QBO and the QQO in Jupiter's equatorial stratosphere. During the Voyager observations, the stratosphere at ±20° was at its coldest relative to the equatorial belts, but during the early Cassini measurements it was only slightly colder.

Although seasonal effects are observed in stratospheric temperatures, the north-south symmetry of the zonal wind pattern observed at the cloud tops suggests that tropospheric circulation is most influenced by rotational forces and perhaps internal energy sources. Estimates of the zonal wind speeds near the equator of Saturn were determined via cloud top tracking by the Voyager spacecraft (Ingersoll et al., 1984; Smith et al., 1981, 1982) at two wavelengths (479 nm, 556 nm) and later reanalyzed by Sanchez-Lavega et al. (2000). Using ground-based observations in 1995-1997, Sanchez-Lavega et al. (1999) found that the winds in the Equatorial Zone (EZ) had slowed significantly since the Voyager epoch and the authors suggested that this might have been due to the Great White Spot (GWS) activity of 1990 and 1994 injecting significant westward momentum into the equatorial jet (Hueso and Sanchez-Lavega, 2004). However, since Voyager and later Cassini thermal measurements (Flasar et al., 2005) show that the equatorial winds slow with height, some uncertainty remained as to whether a real change in wind speeds had been seen or whether additional cloud opacity in the equatorial region in 1995-1997 and different filter characteristics meant that the later measurements were tracking clouds at a higher altitude, where the winds were slower. Hence, Sanchez-Lavega et al. (2004) reanalyzed HST observations at three different wavelengths (439 nm, 814 nm, 890 nm) to confirm their conclusion that the winds in the EZ had indeed slowed by about 150 ms—1 since the Voyager observations. The Cassini mission made new observations of the cloud top speeds upon Cassinis arrival in the Saturnian system in 2004 at 727 nm and 752 nm, and Porco et al. (2005) also found that winds in the equatorial region had slowed since the Voyager observations, although at other latitudes the winds appeared to have changed very little. Using the vertical wind shears determined by the Cassini CIRS experiment (Flasar et al., 2005) at ±5° latitude, Perez-Hoyos and Sanchez-Lavega (2006a) have shown that, by matching the vertical cloud structure to the limb darkening in the EZ observed by Voyager and later observations, all cloud-tracking observations since the Voyager mission are consistent with each other. Hence, they conclude that the equatorial winds at the 350mbar level have indeed physically dropped by almost 100 m s-1 since the Voyager epoch.

The periodic variations in temperature in the equatorial stratosphere reported by Orton et al. (2008) should, from thermal wind shear arguments, give rise to periodically alternating prograde and retrograde jets in the stratosphere either side of the equatorial band. At the beginning of the Cassini observations the winds should thus have increased with height above the tropopause to create prograde stratospheric jets. However, these prograde stratospheric jets should reverse as we approach the Northern Spring Equinox in 2009 (Fletcher, pers. commun.). Curiously, the Voyager observations (during which equatorial tropospheric winds were very strong) coincided with a period of maximum temperature contrast between stratospheric equatorial and near-equatorial temperatures, while HST and early Cassini determinations (when equatorial tropospheric winds were seen to be slower) coincided with a period of minimal temperature contrast. However, no regular periodic variation in equatorial upper-tropospheric temperatures has been seen and there is no known mechanism for variations in the thin stratosphere to affect conditions in the deeper, denser troposphere. Hence, the apparent correlation is probably just a coincidence.

One of the big surprises of the Cassini mission was the appearance of Saturn at 5 ^m, measured by the VIMS instrument. Jupiter's appearance at 5 ^m is to a good approximation closely anti-correlated with its visible appearance, with zonal regions which are bright at visible and near-IR wavelengths appearing dark at 5 ^m and belt regions, which are dark at visible and near-IR wavelengths, appearing bright at 5 ^m. At visible and near-IR wavelengths the large optical depths of tropospheric and stratospheric hazes in Saturn's atmosphere mask the structure of the planet's main clouds and thus the 5 ^m images were expected to show more detail, but Cassini VIMS has shown the deep belt/zone structure of Saturn to be far more complex than that of Jupiter with the latitudinal scale of the belts/zones appearing to be much finer (as can be seen in Figure 5.24, see color section).

The circulation near the poles of planets often reveals interesting dynamics that help to probe the general circulation. A notable example is the warm polar dipole seen near Venus' poles (Piccioni et al., 2007) and the poles of Saturn have proven to be equally intriguing regions. Sanchez-Lavega et al. (2002) used HST observations to show that a dark "cap" of radius 1,000 km to 2,000 km existed over the South Pole from 1997 to 2004 and that a prograde jet with winds of 90 ms-1 existed at 73°S. Keck imaging observations in 2004 at wavelengths between 8 ^m and 24 ^m (Orton and Yanamandra-Fisher, 2005) showed that Saturn had a warm South Polar cap and a compact hot point within 3° of the South Pole. These observations were consistent with a temperature rise at the 100mbar level of ~2K between 69°S and 74°S (planetocentric) and an increase of ~5K at 3mbar. The temperature was estimated to rise still further between ~87°S and 90°S by ^2.5K at the 100mbar level, correlated with a rise of K at 3mbar. While the asymmetry in Saturn's stratospheric temperatures is a known seasonal effect, due in part to solar heating of aerosols, photodissociation of methane, and thermal blanketing by dissociation products such as acetylene and ethane, Flasar et al. (2005) showed that the radiative time constant in the stratosphere is 9-10 years and thus the temperatures should lag the solar forcing by roughly a sixth of an orbital period or 5 years. The fact that Cassini CIRS found the South Polar stratosphere to be much warmer than would be expected with a simple radiative forcing model, suggests that dynamics also plays a part.

The zonal wind speeds determined by Voyager extended from ~70°S to 82°N. Measurements of zonal wind speeds were extended to nearly 80° S by HST (Sanchez-Lavega et al., 2004) and have been extended all the way to the South Pole by Cassini ISS (Sanchez-Lavega et al., 2006) to further explore the South Polar Vortex (SPV). A prograde jet with speeds of 90m s"1 was seen at 74°S and a second, much stronger prograde jet was seen at 87°S, with speeds of 160 ± 10 m s"1. Similar jets would be expected about the North Pole, but Cassini will be unable to extend wind measurements towards the North Pole until sunlight returns in the Northern Spring Equinox in 2009 to allow cloud tracking. Like Flasar et al. (2005), Sanchez-Lavega et al. (2006) found that the temperature inside the warm, cyclonic SPV was higher than would be expected by solar heating alone, implying a dynamical component that could be explained by the region downwelling at a rate of 1.4ms"1. In addition, from the thermal wind equation it was concluded that the wind speeds in the jets should decrease with altitude.

Fletcher et al. (2008a) have analyzed almost all of the Cassini CIRS observations made during Cassini's prime mission to compare and contrast the conditions at Saturn's North and South Poles. Such a study is only possible with Cassini CIRS data since ground-based observations can only ever observe the summer pole, while CIRS can observe both poles in the thermal-infrared and is not reliant on reflected sunlight. Fletcher et al. (2008a) find that warm cyclonic vortices are actually present at both poles (Figure 5.25, see color section) and both are surrounded by polar collars that are cool in the 70mbar to 300mbar pressure range, which themselves are surrounded by warmer polar belts (again between 70mbar and 300mbar) at 79°N and 76°S. Cyclonic warm features, such as the polar belts and hotspots, are related to horizontal convergence and subsidence of tropospheric air, whereas the cold polar zones are related to divergence of upwelling, suggesting upwelling all around the polar vortices and downwelling within. Since Fletcher et al. (2008a) found that the abundance of PH3 (whose mole fraction decreases with height due to photodissociation) was depleted within both vortices, this interpretation would seem sound. Differences between the poles are apparent, however, as the North Polar hotspot and cold collar seem more tightly confined than in the south.

Dyudina et al. (2008) used Cassini ISS observations to further characterize the SPV. Analyzing images at different wavelengths, Dyudina et al. conclude that the SPV is effectively clear of hazes in the upper troposphere down to the tops of tropospheric clouds at 2 bar to 3 bar, again consistent with the view that this is a region of downwelling. Furthermore, the region around the vortex is seen to have high levels of tropospheric haze, consistent with upwelling. Dyudina et al. find that the central haze-free "eye" (Figure 5.26) is surrounded by two cloud walls, one roughly 1 ° away from the pole with an oval shape and a second circular cloud wall

Figure 5.26. Cassini ISS image of Saturn's South Polar Vortex (SPV), recorded in November 2006 (after Dyudina et al., 2008). In this image the Sun is shining from the top and the shadows cast by the SPV cloud eye walls can be seen on the underlying clouds. Courtesy of NASA.

roughly 2° away from the pole. By measuring the length of the shadows cast by these walls, Dyudina et al. estimate that the outer eye cloud wall rises to ~40 km above the main cloud deck, while the inner cloud wall rises to a height of ~70 km, implying that it extends all the way to the tropopause at 100 mbar. No equatorwards or polewards mean motion was seen. Although stratospheric haze is seen to extend over the South Pole and become particularly UV absorbing (Chapter 4), the subsidence in the region leads to a clearing of the tropospheric haze layer allowing remarkably clear images of the tops of tropospheric clouds at 2 bar to 3 bar. This clearing can be seen in both

Cassini VIMS and ISS observations (Dyudina et al., 2008). Another curious feature in the South Polar region is that the small eddies surrounding the vortex core are seen to be dark in 5 ^m images, pointing to their large optical depths, but appear to be grouped into two classes according to their visible and near-IR reflectivities with one type appearing 2/3 the brightness of the other.

While the warm cyclonic vortices of Saturn's atmosphere are particularly striking, it may be that such phenomena are found in most giant planet atmospheres, and are related to the release of internal heat and convective overturning. As mentioned in Section 5.5.2, Pioneer 11 observations of Jupiter (Ingersoll, 1990) found the temperature above the cloud tops to be slightly higher at the North Pole than the equator, and a similar cyclonic polar hotspot has been observed at Neptune's South Pole (Orton et al., 2005), where the temperature at the 100mbar level is seen to be warm polewards of 70°S.

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Renewable Energy 101

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