Jupiter's most distinctive feature - a giant red spot bigger than Earth - is shrinking, images from the Hubble Space Telescope released on Thursday showed.
The so-called "Great Red Spot" is a violent storm, which in the late 1800s was estimated to be about 25,000 miles (about 40,000 km) in diameter - wide enough for three Earths to fit side by side.
The storm, which is the biggest in the solar system, appears as a deep red orb surrounded by layers of pale yellow, orange and white. Winds inside the storm have been measured at several hundreds of miles per hour, NASA astronomers said.
By the time NASA's Voyager space probes flew by in 1979 and 1980, the spot was down to about 14,500 miles (22,500 km) across.
Now, new pictures taken by the Earth-orbiting Hubble space telescope show Jupiter's red spot is smaller than it has ever been, measuring just under 10,250 miles (16,100 kilometers) in diameter. It also appears more circular in shape.
Scientists aren't sure why the Great Red Spot is shrinking by about 621 miles (1,000 km) a year.
"It is apparent that very small eddies are feeding into the storm ... These may be responsible for the accelerated change by altering the (storm's) internal dynamics," Amy Simon, an astronomer with NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.
Simon and colleagues plan follow-up studies to try to figure out what is happening in Jupiter's atmosphere that is draining the storm of energy and causing it to shrink.
(Editing by Jeffrey Benkoe)