The Great Red Spot visible on Jupiter is a swirling anticyclonic storm, the largest of its kind in the Solar System – large enough to fit our entire planet within. Now, a new analysis has revealed the winds around the edge of the spot are mysteriously speeding up.

Using data collected from the Hubble Space Telescope, researchers looked at the patterns of the storm from 2009 to 2020, measuring a wind speed increase in the outer edge of 8 percent over that time.

That's an increase of a little less than 2.5 kilometers (1.5 miles) per hour for every year that measurements were taken – not huge, but significant, and only measurable because of the high-resolution imagery that Hubble is able to capture.

wind speedsOuter and inner ring wind speeds. (Wong et al, Geophysical Research Letters, 2021)

"When I initially saw the results, I asked 'Does this make sense?' No one has ever seen this before," says planetary scientist Michael Wong, from the University of California, Berkeley.

"But this is something only Hubble can do. Hubble's longevity and ongoing observations make this revelation possible."

The researchers put together a new kind of data analysis approach to reach their conclusions, using modeling software to assess up to tens of thousands of different wind vectors (individual directions and speeds) for each time that Hubble was able to photograph Jupiter.

With the colored clouds on the edges of the storm now reaching speeds of up to 640 kilometers (398 miles) per hour in a counter-clockwise direction, the next question is what's causing the acceleration – and there's no easy answer to that.

Some possibilities were ruled out by the study and the collected data though: neither changes in the wind shear (or angle) from the surrounding atmosphere, nor changes in temperature across different altitudes are thought to be responsible. For now, the underlying cause eludes experts.

"That's hard to diagnose, since Hubble can't see the bottom of the storm very well," says Wong. "Anything below the cloud tops is invisible in the data, but it's an interesting piece of the puzzle that can help us understand what's fueling the Great Red Spot and how it's maintaining its energy."

We know from previous research that the Great Red Spot is shrinking in size over time, which is another mystery to be solved: until more direct and more detailed measurements of the gigantic storm can be taken, we're going to have a lot of unanswered questions.

Astronomers have been observing Jupiter's most recognizable feature since the 17th century, with continuous observations recorded since 1878. Across that time it's become more circular, as well as getting smaller.

And even though the Hubble Space Telescope can only spot objects bigger than 170 kilometers (106 miles) in size, that's still a fantastically high resolution when it comes to mapping space and charting what's happening on Jupiter.

"Since we don't have a storm chaser plane at Jupiter, we can't continuously measure the winds on site," says planetary scientist Amy Simon, from the NASA Goddard Space Flight Center in Maryland.

"Hubble is the only telescope that has the kind of temporal coverage and spatial resolution that can capture Jupiter's winds in this much detail."

The research has been published in Geophysical Research Letters.