Jupiter's Great Red Spot - which has been mysterious shrinking for a century and a half - seems to have been growing taller, NASA scientists say.
The spot is one of the most defining features of the gas giant. It was once big enough to swallow three Earths with room to spare.
Nobody is sure how long the storm will continue to contract or whether it will disappear altogether.
However, the new study suggests that it has not all been downhill. The storm seems to have increased in area at least once along the way, and it is growing taller as it gets smaller.
"Storms are dynamic, and that is what we see with the Great Red Spot. It is constantly changing in size and shape, and its winds shift, as well," said Amy Simon, an expert in planetary atmospheres at NASA's Goddard Space Flight Centre in the US.
Keen observers have long been able to measure the size and drift of the Great Red Spot by fitting their telescopes with an eyepiece scored with crosshairs.
A continuous record of at least one observation of this kind per year dates back to 1878.
Researchers drew on this rich archive of historical observations and combined them with data from NASA spacecraft, starting with the two Voyager missions in 1979.
In particular, they relied on a series of annual observations of Jupiter that team members have been conducting with NASA's Hubble Space Telescope as part of the Outer Planets Atmospheres Legacy (OPAL) project.
The team traced the evolution of the Great Red Spot, analysing its size, shape, colour and drift rate.
They also looked at the storm's internal wind speeds, when that information was available from spacecraft.
The findings, published in the Astronomical Journal, indicates that the Great Red Spot recently started to drift westward faster than before.
The storm always stays at the same latitude, held there by jet streams to the north and south, but it circles the globe in the opposite direction relative to the planet's eastward rotation.
Historically, it has been assumed that this drift is more or less constant, but in recent observations, the team found the spot is zooming along much faster.
The study confirms that the storm has been decreasing in length overall since 1878 and is big enough to accommodate just over one Earth at this point.
However, the historical record indicates the area of the spot grew temporarily in the 1920s.
"There is evidence in the archived observations that the Great Red Spot has grown and shrunk over time," said Reta Beebe, an professor at New Mexico State University in the US.
"However, the storm is quite small now, and it is been a long time since it last grew," said Beebe.
Since the storm has been contracting, researchers expected to find the already-powerful internal winds becoming even stronger, like an ice skater who spins faster as she pulls in her arms.
Instead of spinning faster, the storm appears to be forced to stretch up. The change in height is small relative to the area that the storm covers, but it is still noticeable, researchers said.
In some ways, the mystery of the Great Red Spot only seems to deepen as the iconic storm contracts.
Researchers do not know whether the spot will shrink a bit more and then stabilise, or break apart completely.
"If the trends we see in the Great Red Spot continue, the next five to 10 years could be very interesting from a dynamical point of view," said Rick Cosentino from Goddard.
"We could see rapid changes in the storm's physical appearance and behaviour, and maybe the red spot will end up being not so great after all," said Cosentino.
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