Jupiter's most distinctive feature, a giant red spot bigger than Earth, is shrinking, images from the Hubble Space Telescope released today showed.
The so-called "Great Red Spot" is a violent storm, which in the late 1800s was estimated to be 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 kilometres per hour, according to NASA astronomers.
By the time NASA's Voyager space probes flew by in 1979 and 1980, the spot was down to about 22,500km 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 16,100km in diameter.
It also appears more circular in shape.
Scientists are not sure why the Great Red Spot is shrinking by about 1,000km 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.
Ms 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.