The seventh planet in the solar system — and the largest of the ice 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.
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.
"I think poor Uranus is misunderstood, actually," planetary scientist Amy Simon said on NASA's Gravity Assist podcast. "Uranus is very bland in appearance most of the time. It's kind of a pale blue planet. It's the real pale blue dot."
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 percent
- Helium: 15.2 percent
- Methane: 2.3 percent
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 mph (900 km/h).
Simon said that temperature is a large part of the reason for Uranus' blandness. The ice giant doesn't have a lot of heat. In fact, it is the only planet that doesn't give off more heat than it receives from the sun, she said. That slows down the rise and fall of heat that would otherwise drive storms.
"You don't get the equivalent of thunderstorms. So, you don't see the bright clouds on Uranus that you see on the other planets," Simon said.
While storms on Uranus aren't as numerous as they are on other worlds, that doesn't mean the planet doesn't have occasional activity. In 2014, seven years after the planet made its closest approach to the sun, astronomers spotted active weather spots on the ice giant.
"The weather on Uranus is incredibly active," astronomer Imke de Pater of the University of California, Berkeley, in a 2014 statement.
"This type of activity would have been expected in 2007, when Uranus's once every 42-year equinox occurred and the sun shined directly on the equator," astronomer Heidi Hammel of the Association of Universities for Research in Astronomy said at the time. "But we predicted that such activity would have died down by now. Why we see these incredible storms now is beyond anybody's guess."
Some of the storms were even large enough that they could be spotted by amateur astronomer.
"I was thrilled to see such activity on Uranus," French amateur astronomer Marc Delcroix said in the same statement. "Getting details on Mars, Jupiter or Saturn is now routine. But seeing details on Uranus and Neptune are the new frontiers for us amateurs, and I did not want to miss that."
Storms aren't the only bright spot on Uranus. NASA's Hubble Space Telescope snapped its first image of auroras on the planet in 2011. When a team led by an astronomer from Paris Observatory took a second look at the auroras using Hubble's ultraviolet capabilities, they "found themselves observing the most intense auroras ever seen on the planet," NASA said in a statement.
"By watching the auroras over time, they collected the first direct evidence that these powerful shimmering regions rotate with the planet."
'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.
"Because it's tilted over on its side, that means that, for example, the south pole wouldn't see sunlight for about 40 years," Simon said. "So, it's got really extreme seasons, which help to drive the weather."
(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 sunlight, the interior heat of the planet appears to drive its weather.