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Only satellites with a mean radius greater than 100 km have been included.

Only satellites with a mean radius greater than 100 km have been included.

The vertical cloud structure appears to be similar to that of Uranus. Again ground-based microwave spectra detect very little ammonia at the expected ammonia condensation level suggesting that it combines with either water or H2S well below the observable cloud decks. Instead, the main cloud deck is again probably composed of H2S. A thinner methane haze is found at higher altitudes. Like Uranus, no clear belt/zone structure is evident, although the opacity of the methane cloud deck between 30° and 60°, north and south, has increased significantly in the last decade. Unlike Uranus, however, a number of storm systems were observed by Voyager 2 on Neptune in 1989, including the "Great Dark Spot" at southern midlatitudes. The GDS may have had a similar structure to Jupiter's GRS, but it was short-lived and had disappeared by the time of new Hubble Space Telescope observations in 1994. However, by 1995 a new dark spot had appeared at northern midlatitudes, which itself disappeared shortly after. These features are dark probably because of either a darkening or deepening of the main H2S cloud top at 3.8 bar. In addition, several smaller white clouds are intermittently seen all over the planet, but mainly at mid-latitudes, and allow estimation of zonal wind speed. The general structure of this seems similar to that of Uranus in that rather than a series of alternating easterlies and westerlies as is found on Jupiter and Saturn, the winds are strongly retrograde at the equator and then reverse direction slowly to become strongly prograde at latitudes of approximately 70° before returning to zero at the poles. However, the strength of the zonal winds on Neptune greatly exceeds those found on Uranus with the retrograde equatorial jet reaching speeds of 400 m/s, while the prograde subpolar jets are estimated to reach 200 m/s. Hence, Neptune has the largest range of atmospheric rotation periods of any of the giant planets. Why the equatorial jets of Jupiter and Saturn should be prograde, and those of Uranus and Neptune are retrograde is a mystery that will be returned to in Chapter 5.

Neptune's satellite system is less compact and organized than Uranus' and the largest satellite, Triton, is in highly inclined orbit, as can be seen in Table 1.2d, which lists the larger Neptunian satellites with a radius greater than 100 km. Voyager 2 observed Triton to have an extremely thin N2 atmosphere.

Satellite

Mass (kg)

Radius (km)

Density (gem"3)

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