Storm vortices are quite common on distant ice giant Neptune, but this is the first time such a tempest has been recorded turning back on itself. Space experts remain unsure about how or why the storm changed course.
Storms were first discovered to be raging on Neptune in 1989 during NASA’s Voyager 2 flyby.
Since then, the Hubble Space Telescope has spotted four more such Dark Spots storms, named for their darker colour than the surrounding atmosphere.
The storms usually appear at mid-latitudes, last for approximately two years while travelling towards the equator, then disappear, before another arrives in roughly another five years.
However, the University of California’s Dr Michael Wong has revealed this storm, dubbed NDS-2018, was an intriguing exception.
He said: ”It was really exciting to see this one act like it’s supposed to act and then all of a sudden it just stops and swings back. That was surprising.”
When the storm was first discovered in 2018, it had been growing for several years and measured approximately 6,800 miles (11,000km) across.
When the storm observed it again in January 2020, the vortex was still behaving as anticipated, breezing southwards towards the equator from the northern mid-latitudes.
The Coriolis effect keeping the storm stable at mid-latitudes was then expected to weaken, gradually disappearing by the time the storm reached the equator.
According to simulations, and previous observations, NDS-2018 should have then started to fade.
However, January’s observations revealed something strange – the arrival of a miniature version of the larger storm.
NDS-2018 was, by that time, approximately 4,600 miles (7,400km) across, while the new storm, nicknamed Dark Spot Jr, was roughly 3,900 miles (6,275km) across.
Then, in August 2020, when Hubble took another look at NDS-2018, the storm was making its way north again, while Dark Spot Jr had disappeared.
Dr Wong said: ”We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process.
“This is a process that’s never been observed.
“We have seen some other dark spots fading away, and they’re gone, but we’ve never seen anything disrupt, even though it’s predicted in computer simulations.”
Although it remains a mystery what exactly happened, the appearance and subsequent disappearance of Dark Spot Jr is a clue.
The vortex by the side of NDS-2018 was closer to the equator and simulations suggests, if anything were to disrupt a Neptunian storm, this is where it would occur.
And Dr Wong added the fact Dark Spot Jr arrived offers another clue.
He said: ”When I first saw the small spot, I thought the bigger one was being disrupted.
“I didn’t think another vortex was forming, because the small one is farther towards the equator.
“So it’s within this unstable region. But we can’t prove the two are related. It remains a complete mystery.”