Storms and vortices

The cloud features that are visible on Saturn appear to be the tops of active convection systems that push their way up into the overlying semitransparent region. While generally less active than Jupiter's atmosphere, a number of spots have been observed in Saturn's atmosphere from ground-based observations over many years (Sanchez-Lavega, 1982).

Like Jupiter, cloud tracking of the motion of eddies in Saturn's atmosphere suggests that it is the small eddies that drive the zonal jets and not vice versa, though Del Genio et al. (2007) estimate that Saturn's jets are driven with a smaller rate of energy conversion than Jupiter's. In addition, Del Genio et al. (2007) find that convection occurs preferentially in cyclonic regions, but unlike Jupiter, some convection is also seen in eastward jet regions.

While most features are small, large white spots occasionally form in one of the planet's zones, which rapidly expand (on timescales of days) in the east-west direction to girdle the whole planet before gradually subsiding (on timescales of months). Such Great White Spots (GWSs) have now been observed from the ground in all zones except the STZ. Although no such storm was observed during the Voyager encounters, an equatorial storm (or "Equatorial Disturbance") was observed by the Hubble Space Telescope (HST) in September 1990 at 4°N (Westphal et al. 1992), which had almost completely disappeared by June 1991. Subsequently, a new storm (Figure 1.5) was observed in September 1994 (Sanchez-Lavega et al., 1996), which lasted for more than a year (Sanchez-Lavega et al., 1999), disappearing in 1996. It is thought that these storms may affect the zonal winds of the equatorial zone (as was discussed in Section 5.6.1). Although the origin of these storms remains unknown, detailed observations suggest that they result from a sudden outburst of convective activity, presumably originating from disturbances deep below the visible cloud tops, which trigger rapid vertical convection and resultant ammonia cloud condensation (with possible thunderstorm-style deep vertical convection). These localized, thick, bright, high clouds then spread latitudinally and are subsequently torn apart by the strong latitudinal wind shear observed in Saturn's atmosphere and spread right around the planet before eventually settling. Previous storms in the EZ were observed in 1876 and 1933, and at first glance these major equatorial storms would seem separated by approx 57 years (2 Saturn years) and appear correlated with the northern hemisphere summer suggesting a link with solar forcing. If this is true then the next equatorial disturbance may be expected around the year 2047 (Beebe, 1997).

While the Voyager spacecraft did not observe a GWS, numerous anticyclonic ovals were observed in Saturn's atmosphere including, among others, Brown Spots 1, 2, and 3 at 42°N, "Anne's Spot'' (which had a reddish color) at 55°S, shown in Figure 5.27 (see color section), and the "UV Spot'' at 27°N. A North Polar spot (sometimes called "Big Bertha'') was also observed at 75°N, whose then apparent interaction with the North Polar Hexagon feature will be discussed in Section 5.6.3. These features were generally found to have the highest contrast in green-filtered images, although as the name suggests, the UV spot was most prominent at UV wavelengths. In addition to these regular ovals, a number of convective regions were also seen near 39°N (Figure 5.20), a region of cyclonic vorticity, which were seen to have a similar appearance to the plumes that appear in Jupiter's NEB (Smith et al., 1981, 1982; Sromovsky et al. 1983). It is possible that the convective events observed were triggered by the passage of a cyclonic white spot immediately to the south of the outbreaks.

Prior to the Cassini observation period, ground-based imaging of Saturn in the 5 ^m window (Yanamandra-Fisher et al., 2001) showed the main thermal emission to originate between 38°S and 49°S (planetocentric), coinciding with an eastward jet. Discrete dark (cold) features were apparent in this latitude band, with lengths of 30,000 km to 50,000 km, which are quite unlike anything that has been observed in Jupiter's atmosphere. However, such large features have not been seen in Cassini VIMS (Baines et al., 2005) observations, nor in more recent ground-based observations (Orton and Yanamandra-Fisher, 2005) so they would appear to be ephemeral.

Since Cassini started observing Saturn, many eddies and vortices have been seen, but none matches those previously seen by Voyager. Hence, such systems would appear to come and go in Saturn's atmosphere and are not long-lived as some of the vortices in Jupiter's atmosphere appear to be. Cassini has observed a number of sudden, convectively active storms near 35°S in Saturn's so-called "Storm Alley''. One, the "dragon" storm, is shown in Figure 5.28 (see color section), while another example, on Saturn's night side, but illuminated by "ringshine" (i.e., light reflected from Saturn's rings) is shown in Figure 5.29. Unlike Jupiter, lightning has not been directly detected on Saturn through observing lightning flashes on Saturn's night side. This is partly due to the fact that Saturn's night side is often not very dark due to the significant levels of light reflected from Saturn's rings (as seen in Figure 5.29) and is also due to the expected Saturnian water clouds lying at much greater depths (~20 bar) and thus being obscured by much greater optical depths of overlying clouds and hazes. However, lightning is indicated by radio wave emissions. Voyager 1 first detected such radio emissions known as Saturn Electrostatic Discharges (SEDs) (Warwick et al., 1981) in 1980. More recently, the Cassini RPWS instrument observed a sudden burst of SEDs in 2004 (Fischer et al., 2006), and a particularly large

Figure 5.29. A convective thunderstorm observed by Cassini ISS in Saturn's Storm Alley in January 2006. The storm is here seen on Saturn's night side and is illuminated by "ringshine" (i.e., sunlight reflected off Saturn's rings and onto Saturn's cloud tops). Courtesy of NASA.

outburst was detected in 2006 (Fischer et al., 2007). Comparing these observations with Cassini ISS observations of Saturnian storm systems, Dyudina et al. (2007) find that the SEDs are correlated with the massive storms seen at 35°S in Saturn's Storm Alley, which suggests strongly that these observations do indicate lighting activity in Saturn's atmosphere. Intriguingly, while optical lightning flashes are seen on Jupiter, there is no equivalent to Saturn's high-frequency SEDs, although whistler signals are seen, which are not detected on Saturn. Zarka (1985) suggests that the lack of high-frequency electrostatic disturbances for Jupiter is due to strong absorption of the radiowaves propagating through Jupiter's lower-ionospheric layers.

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