These two pictures of Uranus—one in true color (left) and the other in false color—were compiled from images returned in 1986 from the narrow-angle camera of Voyager 2. The spacecraft was 9.1 million kilometers from the planet, several days from closest approach.
The picture on the left has been processed to show Uranus as human eyes would see it. It was assembled from images taken through blue, green, and orange filters. The darker shadings at the upper right of the disk correspond to the day-night boundary on the planet. The night side of the planet northern hemisphere of Uranus. (“Hold it,” I hear the Gentle Reader cry. “Shouldn’t the half of the northern hemisphere be in daylight.” No. Not on Uranus. The planet’s axis is tilted almost 90°.) The blue-green color results from the absorption of red light by methane gas in the planets deep, cold, and remarkably clear atmosphere.
The picture on the right uses false color and extreme contrast enhancement to bring out subtle details in the south polar region of Uranus. It uses images take through ultraviolet, violet, and orange filters shifted to the same blue, green, and red colors used to produce the picture at left. The very slight contrasts visible in true color are greatly exaggerated. The false color image reveals a dark polar hood surrounded by a series of progressively lighter concentric bands. One possible explanation is that smog, concentrated over the pole, is arranged into bands by zonal winds of the upper atmosphere. The bright orange and yellow strip at the edge of the planet’s limb is an artifact of the image enhancement.
Image Credit: NASA