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Only satellites with a mean radius greater than 100 km have been included. The letter "R" in the period column indicates a retrograde orbit.

Only satellites with a mean radius greater than 100 km have been included. The letter "R" in the period column indicates a retrograde orbit.

formed from the tidal disruption of a satellite and will thus eventually dissipate. Hence, it is purely serendipitous that Saturn's ring system should be so spectacular at this particular moment in the solar system's history, when it can be observed by the people of Earth.

1.2.3 Uranus

Uranus and Neptune are a good deal smaller than their larger siblings Jupiter and Saturn, and a good deal denser, being composed mostly of icy materials, with a much less massive envelope of molecular hydrogen/helium. The greeny-blue color of Uranus (Figure 1.9, see color section), and the blue color of Neptune arises from the greater abundance of red-absorbing methane in the observable atmospheres of these planets and also in the nature of the particles comprising the main observable cloud deck of these planets, which preferentially absorb wavelengths longer than 0.6 ^m.

The observable atmospheres of both Uranus and Neptune are much colder than Jupiter and Saturn, and so ammonia is expected to condense very deep in the atmosphere, but at levels accessible to microwave remote sensing. However, the deep abundance of ammonia inferred from ground-based microwave observations is so low that it would seem that most of the ammonia combines at even deeper levels with other molecules to form perhaps aqueous ammonia or ammonium hydrosulphide (NH4SH) clouds, or perhaps even a water-ammonia ionic ocean well below the main

Figure 1.6. Saturn's rings behind Saturn. The rings appear to bend as they approach Saturn due to Saturn's atmosphere refracting the light.

observed upper cloud deck. The composition of the main cloud deck is unknown, but is most probably hydrogen sulfide (H2S). Above this main cloud deck, a second very much thinner cloud deck of methane (CH4) ice is observed at restricted locations on these planets.

The atmospheric circulation of all the giant planets is driven by both solar and internal heating. However, for Uranus the mean global internal heat flux is at most 6% of the solar flux and thus the dominant circulation must be that forced by the uneven distribution of sunlight over the planet, by which the atmosphere attempts to revert to a barotropic state, where temperature is constant on constant pressure surfaces. This thermal forcing is complicated by the fact that Uranus' large obliquity of 98° means that during the course of its orbit Uranus receives direct sunlight over both poles, as well as the equator. In fact, even though both poles experience a night half a Uranian year long, on average they receive 50% more sunlight per unit area than the equator. The Voyager 2 flyby in 1986 occurred soon after the northern winter solstice when the South Pole was facing almost directly towards the Sun and the North Pole was in complete darkness. If there were no meridional (i.e., latitudinal) circulation, we would have expected there to have been a significant pole-to-pole temperature gradient of the order of 10 K. However, the infrared spectrometer on Voyager 2 found there to be almost no temperature difference and hence the atmosphere appears to efficiently redistribute absorbed solar heat.

Although only a very low-contrast belt/zone structure was seen on Uranus by Voyager 2 and the convective overturning of Uranus' atmosphere thus appeared sluggish, occasional small white clouds were observed intermittently, probably resulting from deep convection cells transporting methane-rich air high into the atmosphere, where it condenses, and these discrete clouds could be tracked to determine the zonal wind speeds at the cloud condensation level. Similar cloud events are seen in the atmosphere of Neptune, although these have much higher contrast and appear to be much more vigorous than those seen on Uranus. The zonal wind structure of Uranus shows none of the rapid latitudinal structure associated with belts and zones seen on Jupiter and Saturn. Instead the structure appears fairly symmetric with midlatitude winds blowing at 200 m/s in the prograde direction and equatorial winds blowing at 100 m/s in the retrograde direction, opposite to that of Jupiter and Saturn.

The occasional storm clouds seen by Voyager 2 became noticeably more vigorous as Uranus approached its northern spring equinox in 2007; in addition, a distinct bright zone appeared at 45°S, which appears to have faded following the equinox, with a new bright zone appearing at 45°N. We shall return to this in Chapter 5.

The large obliquity of Uranus may be evidence of an off-center impact by a single planet-sized body into Uranus towards the end of its accretion phase. The fact that Uranus' compact and regular satellite and ring system closely shares this obliquity suggests that the unusual spin vector was imparted early, some 4.6 Gyr ago. It has even been speculated that this cataclysmic event may have extinguished the internal heat source by effectively turning the planet inside-out causing the planet to release most of its internal energy soon after formation rather than gradually like the other giant planets! The larger satellites of Uranus (R > 100 km) are listed in Table 1.2c.

1.2.4 Neptune

Although Neptune is farther from the Sun than Uranus, and thus receives less sunlight, its bolometic temperature is very similar to that of Uranus, indicating a strong source of internal heat. In fact, its ratio of emitted thermal/absorbed solar flux is the highest of any of the giant planets. Neptune appears bluer than Uranus and has some of the most active meteorology and global variability of any of the giant planets (Figure 1.10, see color section).

Table 1.2c. Major satellites of Uranus.

Satellite

Mass (kg)

Radius (km)

Density (gem ~3)

P (days)

Ag

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