The Atmosphere

Molecular hydrogen and atomic helium are the two main constituents of the Uranian atmosphere. Hydrogen is detectable from Earth in the spectrum of sunlight scattered by the planet's clouds. The ratio of helium to hydrogen was determined from the refraction (bending) of Voyager 2's radio signal by the atmosphere as the spacecraft passed behind the planet. Helium was found to make up 15 percent of the total number of hydrogen molecules and helium atoms, a proportion that corresponds to 26 percent by mass of the total amount of hydrogen and helium. These values are consistent with the values inferred for the Sun and are greater than those inferred for the atmospheres of Jupiter and Saturn. It is assumed that all four giant planets received the same proportions of hydrogen and helium as the Sun during their formation but that, in the cases of Jupiter and Saturn, some of the helium has settled toward their centres. The processes that cause this settling have been shown in theoretical studies not to operate on less-massive planets like Uranus and Neptune.

Methane absorbs strongly at near-infrared wavelengths, and it dominates that part of the spectrum of reflected light even though the number of methane molecules is only 2.3 percent of the total. Astronomers determined this estimate of methane abundance using Voyager 2's radio signals that probed to atmospheric depths at which the methane-to-hydrogen ratio is likely to be constant. If this constancy is characteristic of the planet as a whole, the carbon-to-hydrogen ratio of Uranus is 24 times that of the Sun. (Methane [CHJ comprises one atom of carbon and four of hydrogen.) The large value of the carbon-to-hydrogen ratio suggests that the elements oxygen, nitrogen, and sulfur also are enriched relative to solar values. These elements, however, are tied up in molecules of water, ammonia, and hydrogen sulfide, which are thought to condense into clouds at levels below the part of the atmosphere that can be seen. Earth-based radio observations reveal a curious depletion of ammonia molecules in the atmosphere, perhaps because hydrogen sulfide is more abundant and combines with all the ammonia to form cloud particles of ammonium hydrosulfide. Voyager's ultraviolet spectrometer detected traces of acetylene and ethane in very low abundances. These gases are by-products of methane, which dissociates when ultraviolet light from the Sun strikes the upper atmosphere.

On average, Uranus radiates the same amount of energy as an ideal, perfectly absorbing surface at a temperature of 59.1 kelvins (K; -353 °F, -214 °C). This radiation temperature is equal to the physical temperature of the atmosphere at a pressure of about 0.4 bar. Temperature decreases with decreasing pressure—i.e., with increasing altitude—throughout this portion of the atmosphere to the 70-mil-libar level, where it is about 52 K (-366 °F,

-221 °C), the coldest temperature in Uranus's atmosphere. From this point upward the temperature rises again until it reaches 750 K (890 °F, 480 °C) in the exosphere—the top of the atmosphere at a distance of 1.1 Uranian radii from the planetary centre—where pressures are on the order of a trillionth of a bar. The cause of the high exospheric temperatures remains to be determined, but it may involve a combination of ultraviolet absorption, electron bombardment, and inability of the gas to radiate at infrared wavelengths.

Voyager 2 measured the horizontal variation of atmospheric temperature in two broad altitude ranges, at 60-200 millibars and 500-1,000 millibars. In both ranges the pole-to-pole variation was found to be small—less than 1 K (1.8 °F, 1 °C)—despite the fact that one pole was facing the Sun at the time of the flyby. This lack of global variation is thought to be related to the efficient horizontal heat transfer and the large heat-storage capacity of the deep atmosphere.

Although Uranus appears nearly featureless to the eye, extreme-contrast-enhanced images from Voyager 2 and more-recent observations from Earth reveal faint cloud bands oriented parallel to the equator. The same kind of zonal flow dominates the atmospheric circulation of Jupiter and Saturn, whose rotational axes are much less tilted than Uranus's axis and thus whose seasonal changes in solar illumination are much different. Apparently, rotation of the planet itself and not the distribution of absorbed sunlight controls the cloud patterns. Rotation manifests itself through the Coriolis force, an effect that causes material moving on a rotating planet to appear to be deflected to either the right or the left depending on the hemisphere—northern or southern— being considered. In terms of cloud patterns, therefore, Uranus looks like a tipped-over version of Jupiter or Saturn.

The wind is the motion of the atmosphere relative to the rotating planet. At high latitudes on Uranus, this relative motion is in the direction of the planet's rotation. At equatorial latitudes the relative motion is in the opposite direction. Uranus is like Earth in this regard. On Earth these directions are called east and west, respectively, but the more general terms are prograde and retrograde. The winds that exist on Uranus are several times stronger than on Earth. The wind is 720 km (450 miles) per hour prograde at a latitude of 55° S and 400 km (250 miles) per hour retrograde at the equator. Neptune's equatorial winds are also retrograde, but those of Jupiter and Saturn are prograde. No satisfactory theory exists to explain these differences.

Uranus has no large spots like Jupiter's long-lived Great Red Spot or the Great Dark Spot observed on Neptune by Voyager 2 in 1989. Voyager's measurements of the wind profile on Uranus came from just four small spots whose visual contrast was no more than 2 or 3 percent relative to the surrounding atmosphere. Because the giant planets have no solid surfaces, the spots must represent atmospheric storms. For reasons that are not clear, Uranus seems to have the smallest number of storms of any of the giant planets.

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