Voyager 2 image of Uranus taken 21 minutes before closest approach.
THE ATMOSPHERE OF URANUS
 
Even in close-up pictures taken by Voyager, Uranus is a featureless blue globe. The planet has no dark oval-shaped spots like the other gas giants. Contrast enhancement of the images, however, reveals some extremely tenuous clouds and very faint bands which encircle the planet.
 
Dust particles on the lens of Voyager caused some of the images to show dark ring markings which at first were thought to be features on the surface of Uranus. It was then realised that the rings appeared in the same place in each image.
 
Atmospheric composition
Uranus' atmosphere is estimated to comprise about 83% hydrogen (H2) , 15% helium (He)and 2% methane (CH4 ). As with Neptune, it is the small amount of methane which gives the planet its blue colour. Methane absorbs red light, and reflects blue light. Neptune has more methane than Uranus which is why it appears a more intense shade of blue.
 
The proportion of hydrogen to helium is the same as that which is detected in the Sun, suggesting that the planet is largely unchanged since its formation. It is very likely that other more complex compounds have found their way into the atmosphere, delivered by comets and meteorites during the early history of the Solar System. The amount of helium detected in Uranus' atmosphere is greater than that found in Saturn. Helium in Saturn is thought to have moved to greater depths. Oxygen, nitrogen, and sulphur are all thought to be present in greater abundances than the solar composition. They form compounds including water and ammonia (NH3), though little ammonia has been detected. Voyager's on-board ultraviolet spectrometer identified small quantities of hydrocarbon compounds including acetylene (C2H2) and ethane (C2H6 ). These are produced in the upper atmosphere by sunlight breaking apart methane molecules.
 

Atmospheric composition.

Atmospheric composition.
 
Temperatures
The lowest temperatures of Uranus' atmosphere are recorded at the 1 bar pressure level, where the temperature is 50 K (-223oC).
 
At the outer part of the atmosphere, called the exosphere, Voyager recorded very high temperatures (750 K or 477oC) Such extreme temperatures are explained by the absorption of shortwave radiation and the low emission of longwave radiation.
 

Temperature profile through Uranus'
atmosphere.
 
The temperature variation across the planet as measured by Voyager was quite small. Uranus moves around the Sun on its side, as if rolling around the Sun rather than spinning like a planet. At the time of the Voyager encounter the polar regions were receiving more energy from the Sun than the equator, but only a very small difference (1 K) in temperature between the polar and the equatorial regions was recorded.
 
Bands, winds and clouds
More recent pictures of Uranus made by the Hubble Space Telescope (HST) show more pronounced banding and a string of bright clouds. Banded patterns in the atmosphere are easily visible in false colour infrared images. The faint pink clouds which are seen in some of the HST pictures are white in the visible spectrum. Computer enhancement of Voyager imagery also reveals banding within Uranus' atmosphere.
 

Invisible wavelengths, in this case infra-red, are needed to clearly see structure in Uranus' atmosphere.
Voyager images need to be enhanced before atmospheric banding becomes apparent.
This recent false-colour imagery from the The Hubble Space Telescope shows bright clouds in Uranus' atmosphere.
 
Voyager images taken at intervals show the movement of the atmosphere's larger white clouds over a period of time. The clouds circled the planet in about 16 hours. From this information their speed was calculated.
 

Voyager 2 tracked large white clouds.
Large white clouds have been observed and tracked by the Hubble Space Telescope.
 
Uranus' strong winds follow a similar pattern to Neptune's. Positive velocities, whereby the winds move in the same direction as the rotating planet, are observed at mid-latitudes. Winds at mid-latitudes (55o North and South ) with a speed of 720 kilometres per hour have been recorded.
 
Like Neptune, but unlike Saturn and Jupiter, there are retrograde winds at the equator. These move in the opposite direction to the rotation of the planet at speeds approaching 360 kilometres per hour.
 
Spring
When Voyager flew past Uranus in 1986, the northern regions of the planet were in darkness. However, the planet has moved along in its orbit, and sunlight is now reaching parts of Uranus that have been in darkness for years - spring is on the way. What effect this will have on global atmospheric circulation remains to be seen.