Tangled up in blues: Why Uranus and Neptune look so different

Roses are red. Neptune's deep blue.

Why, scientists wondered, isn't Uranus, too?

It is an intriguing question. Uranus and Neptune, the two outermost planets of our solar system, are both ice giants — cold worlds that are part gas, part ice, with similar chemical compositions.

They're not far off in mass, either, Uranus being 15 times that of Earth, and Neptune 17 times. And they're both about four times the size of Earth, Uranus being slightly larger.

Yet the two worlds look decidedly different. Uranus, as first revealed by NASA's passing Voyager 2 spacecraft in 1986, is a featureless light-blue blob. When the same spacecraft encountered Neptune in 1989, it revealed a world with the most powerful winds in the solar system, which rip through a royal blue atmosphere, with giant storms and even a mysterious dark spot. Why the difference?

Patrick Irwin, a planetary physicist at Oxford University, and colleagues have now developed an answer. They pieced together a detailed understanding of each world's atmosphere using the Gemini North telescope in Hawaii, the Hubble Space Telescope and other observations.

Both worlds are blue because they have methane in their atmospheres, which absorbs the color red from the sun's light. But a key middle layer of methane haze on Uranus appeared to be twice as thick as the layer on Neptune. It is the presence of this additional haze that leads to the different visages.

Imke de Pater, a planetary scientist at the University of California, Berkeley, said the finding made sense. "The methane abundance on the two planets is very similar," she said. "Something has to explain the difference in colors."

Why Uranus has a thicker haze layer than Neptune may be a result of a giant impact early in its life that knocked the planet on its side, said Leigh Fletcher, a planetary scientist at the University of Leicester in England and a co-author on the paper.