Saturns Weather and the Great White Spots

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To the beginner, Saturn's globe appears fairly featureless when viewed through the eyepiece. Although Saturn does have equatorial, tropical, and temperate belts and zones (Figure 14.1), there is nothing on the disc that stands comparison with the Jovian Great Red Spot or even the giant planet's North and South Equatorial Belts. Undoubtedly, the huge amount of internal heat produced by Jupiter, combined with its closer proximity to the Sun, is responsible. Heat is the driving force behind weather systems and the ammonia crystals in Saturn's atmosphere form at higher levels, tending to give the planet its relatively featureless appearance. Features more than 3,000 kilometers in diameter are very rare in Saturn's atmosphere. However, an interesting phenomenon is that when the rings are near their maximum tilt with respect to the Sun, either the northern or southern hemispheres lie in the shadow of the rings. This cools the shadowed hemisphere and, when it reemerges into sunlight, the colder hemisphere often has a bluish tint, similar to that of colder Uranus and Neptune.

Despite the initially bland appearance of the planet, the experienced visual observer will easily spot some of the more distinctive features revealed in the best webcam images. Saturn's equatorial zone, like that of Jupiter, has a bright, creamy

Figure 14.1. Saturn's rings, divisions, and belt nomenclature. Webcam image and diagram: M. Mobberley.

appearance compared to areas at higher latitudes. Darker, brownish, SEB and NEB belts, either side of the Equatorial zone are visible at higher latitudes, but these in no way compare to the Jovian SEB and NEB. However, with webcams and modern image processing techniques, a wealth of subtle detail can be revealed on the best nights by the best observers. The polar regions are especially interesting when viewed by the Hubble space telescope (Figure 14.2) or the world's most advanced amateur astronomers. At the precise position of the poles, within the south or north polar regions (SPR and NPR) a tiny dark circle can be resolved, like a bulls-eye on a dartboard. This is usually surrounded by concentric dark bands of various subtle colorations. In the last few years the whole polar region of the southern hemisphere has sometimes taken on a greenish hue, but, closer to the pole, dark red/brown, dark blue, and even dark yellow collars have been seen. The images of Damian Peach have revealed these colors in webcam sequences taken from 2002 to 2005 and consultation with professional astronomers using Hubble have confirmed that these colors are real and do change on a short timescale. As a rough generalization, in 2002/2003 the main polar cap collar had a distinctly greenish hue, but in 2004/2005 it was mainly an orange/red color. However, in 1999 this SPC collar was a really deep red color, which faded out, and became pale blue in 2000. Prior to the webcam revolution only Hubble regularly revealed these features.

According to professional astronomers, such color changes in Saturn's atmosphere are most probably due to small changes in the size of the aerosols at upper levels (sometimes in the stratospheric haze at pressures around 10 mbar, sometimes in the upper level of the tropospheric haze at the 70 mbar level). A size change from .5 to 1.5 microns at some latitudes can reproduce the color variability. In some cases it is also possible to have changes in composition (in particular in the stratosphere, due to photochemistry or polar particle bombardment), that change the refractive index of the particles.

Despite all this talk of subtle Saturnian features, now and again Saturn produces large storms in the equatorial zone that become very obvious even in small telescopes. These "Great White Spots" can be dramatic, brightening the entire Equatorial Zone all of the way around the planet. Major white spots have been observed in 1876, 1903, 1933, 1960, and 1990, i.e., at intervals very close to Saturn's orbital period around the Sun, of 29.4 years. On this basis the next major white

Figure 14.2. Saturn imaged by the Hubble space telescope on March 22, 2004. An image processed to supposedly show the natural colors of the planet. Image: NASA STScI/ESA.

spot might be expected in 2020. Without a doubt the most highly publicized Great White Spot was that of 1933. Not only was it spectacular, its discovery was made by a famous British stage and screen comedian, Will Hay, on August 3rd of that year! The spot, shown in Figure 14.3, remained visible for six weeks.

In recent years, from observations by Hubble and by leading amateur astronomers, it has become apparent that relatively small white spots, typically 2,000 to 3,000 kilometers in diameter can be detected by amateur telescopes of only 25 cm in aperture. Darker spots have also been imaged at higher latitudes, in the polar regions. Such spots are very low contrast features though and only the best amateur images taken under near-perfect seeing conditions can reveal them.

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