A fainter Uranus emerges in the latest image from the Hubble Telescope

With the vernal equinox passing a few days ago, we in the northern hemisphere can expect the warmer days of spring while those in the southern hemisphere will begin to feel the chill of fall.

The seasons also change on other planets, neither does Uranus, which is essentially flipped on its side. Photographs from the Hubble Space Telescope, released on Thursday, provide more detail for astronomers to examine changing climatic conditions on the strange ice giant.

Studying the seasons of Uranus takes some time. A year on the distant bluish gas giant – the time it takes Uranus to circle the sun once – is 84 Earth years.

“It’s so long that no human being 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 suggest that Uranus’ northern hemisphere, now emerging into sunlight, is brighter than the southern hemisphere, which Voyager 2 observed when it passed in 1986.

“Is it due to a difference in cloud thickness?” said Dr. Hammel. “Cloud chemistry? Dynamics in clouds triggered by sunlight? A complicated combination of all of the above? We honestly don’t know. We are slowly accumulating enough data to begin to untangle these differences.

The European Space Agency, which is collaborating with NASA on the Hubble Telescope, has 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 with clouds of methane ice crystals circled the mid-northern latitudes. Eight years later, a haze appeared over the North Pole that resembled the smog of polluted cities, with several small storms near the edge of the polar haze. (Look at the thin ring to get an idea of ​​how Uranus’ orientation has changed.)

Causes could include changes in winds and chemical processes.

The planet’s northern hemisphere summer solstice – when the sun shines almost directly at the north pole and most of the southern hemisphere is in darkness – will occur in 2028.

Images from Hubble, as well as 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 in a once-a-decade priorities survey, possibly including an orbiting spacecraft and an atmospheric probe.

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