For centuries, perhaps almost as far back as the first telescope, Jupiter has been marked by the presence of its mighty storm known as the Great Red Spot. Yet the spot is not a constant, having shrunk and grown over that time and possibly disappeared entirely. Its latest change, discovered through the power of the Hubble telescope, is that the winds in the outer parts of the Spot are slowly accelerating. As with so many aspects of this giant vortex, we don't know why.
The Spot spins counterclockwise roughly once every six Earth days (14 days on Jupiter). Given its immense size, that requires wind speeds of hundreds of kilometers an hour at the Spot's outskirts, known as the high-speed ring.
The Hubble Space Telescope has been checking the red spot regularly since 2009 using an instrument that can measure wind speeds at various locations. An analysis of those observations published in Geophysical Research Letters concludes the winds at the Spot's edge have ramped up 8 percent between 2009 and 2020. No one noticed this before because the increase has been too slow and steady for those checking in occasionally. Each year the outermost winds have increased their speed by less than 2.7 kilometers an hour (1.6 miles per hour). Over time, however, this has added up to an additional 25 km/h (15.5 mph) – a decent wind on its own. Oddly, however, the already slower speeds near the center have dropped off further.
"When I initially saw the results, I asked 'Does this make sense?' No one has ever seen this before," said first author Dr Michael Wong of the University of California, Berkeley in a statement. "But this is something only Hubble can do. Hubble's longevity and ongoing observations make this revelation possible."
In the time the Great Red Spot has been with us, other Jovian meteorological events have come, including a convective storm that came close to the Spot in 2017, causing a sudden spike in windspeeds.
Hubble can only see the cloud tops of the Spot, which sticks out 8 kilometers (5 miles) above the surrounding cloud tops, stepping down at the edges. Consequently, Wong admits he doesn't know what is going on below that might be fueling its acceleration. The paper rules out certain possible explanations, but Wong and co-authors are uncertain of the answer.
The Great Red Spot is the largest and most long-lasting meteorological event on a planet in our Solar System. Having been considered relatively constant for more than a century it began shrinking around the start of the 21st Century, losing half its width by 2004 and more since then. Even in its diminished state the Spot is so large the Earth could fit inside it, and it once could have fitted three Earths side by side. The paper says changes in wind shear cannot explain the shrinkage.
The first definite sightings of the spot date from 1830. However, a similar red marking in Jupiter's southern hemisphere was reported from 1665 to 1713. It is thought most likely this was the same storm we see today, but for a century it shrank beyond the reach of the telescopes of the time, only to rebound. Sadly, we don't know what the windspeeds were like at the time.