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Uranus' Atmosphere: Layers of Icy Clouds

The seventh planet in the solar system — and the smallest of the gas giants — Uranus has an atmosphere more comparable with Neptune than with Saturn and Jupiter. Known as ice giants, the two most distant planets from the sun have atmospheres that are heavily dominated by ices. The bizarre orientation of the planet, which orbits the sun tipped on its side, reveals that its inner core has a stronger influence on its weather patterns than the distant star.

Bright Clouds on Uranus
The green and blue regions show where the atmosphere is clear, allowing sunlight to penetrate deep into Uranus. In the yellow and gray regions, a haze or cloud layer is reflecting sunlight away.
Credit: Erich Karkoschka (University of Arizona) and NASA

Atmospheric composition

Uranus' atmosphere is predominantly made up of hydrogen and helium. Unlike Jupiter and Saturn, these light gases dominate only the outer edges of the planet, but are not a significant contributor to the rocky interior.

The dull blue color of Uranus is caused by the presence of methane, which absorbs red light.

Traces of hydrocarbons are also present in the air around Uranus. Ices made up of water, ammonia, and possibly methane also exist in the atmosphere.

Atmospheric composition by volume:

  • Molecular hydrogen: 82.5%
  • Helium: 15.2%
  • Methane: 2.3%

Layering in the atmosphere

As on Earth, the atmosphere of Uranus is broken into layers, depending upon temperature and pressure. Like the other gas giants, the planet doesn't have a firm surface. Scientists define the surface as the region where the atmospheric pressure exceeds one bar, the pressure found on Earth at sea level.

Just above the "surface" of Uranus lies the troposphere, where the atmosphere is the densest. The temperature ranges from minus 243 degrees Fahrenheit (minus 153 degrees Celsius) to minus 370 F (minus 218 C) , with the upper regions being the coldest. This makes the atmosphere of Uranus the coldest in the solar system. Within the troposphere are layers of clouds — water clouds at the lowest pressures, with ammonium hydrosulfide clouds above them. Ammonia and hydrogen sulfide clouds come next. Finally, thin methane clouds lay on the top. The troposphere extends 30 miles (50 kilometers) from the surface of the planet.

Radiation from the sun and from space heats the stratosphere of Uranus from minus 370 F (minus 218 C) to minus 243 F (minus 153 C). The stratosphere contains ethane smog, which may contribute to the planet's dull appearance. Acetylene and methane are also present. These hazes help warm the stratosphere. Hydrocarbons are less abundant in the atmosphere of Uranus than they are of other giant planets, however. The stratosphere reaches almost 2,500 miles (4000 km) above Uranus.

The thermosphere and corona of Uranus reach temperatures of 1,070 F (577 C), although scientists are unsure as to the reason. Because the distance to Uranus from the sun is so great, the amount of heat coming from the star is insufficient to generate such high temperatures. Extending twice as far from the center of the planet as its surface, the far-reaching outer layers are unique to Uranus. They create a drag on the ring particles that orbit the planet.

Cloud patterns on Uranus

Although the planet looks to be a solid shade of blue, it contains stripes like Jupiter and Saturn. But the bands are faint, and are only seen with enhanced images. As with other gas giants, the zones form as gases within the region warm and rise, while in the belts, the gases fall back to the planet as they cool. In the belts, the winds blow east, while they travel west within the zones.

When Voyager 2 flew by the planet in 1986, it only observed 10 cloud patterns on the planet. As technology improved, higher resolution images taken from Earth revealed fainter clouds. The clouds, which exist primarily in the troposphere, are carried by winds reaching up to 560 miles per hour (900 kilometers per hour).

'Tis the season

Unlike other planets in the solar system, which spin along the same plane as the sun, Uranus, discovered in 1781, was knocked on its side by a collision soon after its formation. With its equator down, the planet appears to roll around the sun. This means that only one pole at a time faces the distant star, so that each pole spends half of its 84-Earth-year orbit in a dark winter and half in summer's light. (The planet also spins backwards as a result, so that if it rotated with its equator along the plane of the solar system, the sun would rise in the west rather than the east.)

On most planets, the equator receives the most sunlight, causing warm air to rise and move to the poles. But the equator of Uranus hardly ever faces the sun. Therefore, the warm air should rise from the pole that faces the sun, and fall back at the cooler pole. But the bands and zones of Uranus reveal otherwise. The planet's stripes rotate around the equator the same way they do on Jupiter and Saturn. Instead of being warmed by the sun, the interior heat of the planet appears to drive its weather.

— Nola Taylor Redd, SPACE.com Contributor

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