A Paler Uranus Emerges in the Latest Hubble Telescope Image

With the passage of the vernal equinox a few days ago, those in the northern hemisphere can look forward to the warmer days of spring, while those in the southern hemisphere are beginning to feel the chills of fall.

The seasons change on other planets as well, no more so than on Uranus, which is essentially tilted on its side. Photos from the Hubble Space Telescope released Thursday provide astronomers with more details to study the changing climatic conditions on the strange ice giant.

Studying the seasons of Uranus takes a while. A year on the distant, bluish gas giant—the time it takes for Uranus to orbit the Sun once—is 84 Earth years.

“This is so long that no human can hope to study it directly,” said Heidi B. Hammel, vice president for science at the Association of Universities for Research in Astronomy.

dr Hammel notes that although Uranus was discovered 242 years ago, sophisticated instruments did not exist then, and even electronic detectors capable of accurately measuring the planet’s brightness did not exist until the 1950s.

Long-term brightness measurements since then have suggested that the northern hemisphere of Uranus, which is now emerging into the sunlight, is brighter than the southern hemisphere observed by Voyager 2 when it passed in 1986.

“Is this due to different thicknesses of the clouds?” said Dr. Mutton. “The chemistry of the clouds? The dynamics in the clouds triggered by sunlight? A complicated combination of all this? We honestly don’t know. We are slowly gathering enough data to work out these differences.”

The European Space Agency, which works with NASA on the Hubble Telescope, offered a comparison between what Uranus looked like in 2014 – seven years after its vernal equinox – and an image taken last year.

In 2014, several storms carrying clouds of methane ice crystals orbited the northern mid-latitudes. Eight years later, a haze resembling the smog of polluted cities appeared over the North Pole, with several small storms at the edge of the polar haze. (Look at the thin ring to get a sense of how Uranus’ alignment has shifted.)

The causes could be changes in the winds and chemical processes.

The summer solstice of the planet’s northern hemisphere – when the sun shines almost directly on the North Pole and almost all of the southern hemisphere is in darkness – will occur in 2028.

Images from Hubble, the James Webb Space Telescope and the Keck Observatory in Hawaii will help astronomers better understand what is changing on Uranus and why.

Last year, planetary scientists agreed to put a mission to Uranus at the top of their list during a decennial poll of priorities, perhaps including an orbiting spacecraft and an atmospheric probe.

“The more we learn about Uranus now,” said Dr. Hammel, “the more focused and scientifically productive this mission will be.”