For the first time since arriving at Saturn orbit in 2004, NASA’s Cassini Solstice mission has imaged the distant blue dot of Uranus past Saturn’s majestic rings.
The “ice giant” world is located nearly 29 astronomical units (or AU, where 1AU is equivalent to the average orbital distance from the Earth to the sun) from Cassini, which places it nearly on the opposite side of the sun when this picture was taken on April 11. The image of Uranus has been brightened 4.5 times by the Cassini imaging team.
Seen through the combined red, blue and green filters of the mission’s narrow-angle camera, the familiar bright visible light blue hue of Uranus is obvious. This is caused by the presence of methane in the Uranian atmosphere, a compound that absorbs red light in the visible spectrum, so we see preferentially blue light reflecting off the atmosphere.
Both Uranus and its more distant sibling Neptune are often referred to as ice giants as their atmospheres contain quantities of water, ammonia and methane that are typically found in a frozen state in the outer solar system. Their composition varies greatly with Jupiter and Saturn, both of which orbit closer to the sun and are composed mainly of hydrogen and helium.