Dark Vortex on Neptune changes direction in an unprecedented phenomenon

Jupiter may have the most famous storm in the solar system, but that doesn’t mean it gets all the fun – and Neptune just gave scientists a doozy.

Although storm eddies are not uncommon on the distant ice giant, this is the first time that one has been observed returning to the Arctic after migrating to the equator.

Astronomers are still not sure how or why it changed course, but if we find out, we could tell more about Neptune’s atmospheric dynamics.

Neptune is actually quite difficult to see compared to the other planets in the solar system. It is far from the sun, at an average distance of 30 times that between the earth and the sun, making it difficult to distinguish many details. So it wasn’t until 1989, when Voyager 2 made its flyby, that two storms were discovered on Neptune.

Since then, Hubble – the only instrument capable of this – has observed and tracked four more of these storms, called Dark Spots, because of their darker hue than the surrounding atmosphere.

In general, their behavior is quite similar: they appear in the middle latitudes, they hang around for about two years as they migrate to the equator, and then they disappear. And then, four to six years later, another appears.

However, this storm, the fourth observed by Hubble, dubbed NDS-2018, was the exception.

“It was really exciting to see this one doing the way it should act and then suddenly it stops and waves back,” said planetary scientist Michael Wong of the University of California, Berkeley. “That was surprising.”

NDS-2018, as its name suggests, was first discovered in 2018. By then it had been growing for several years and had a diameter of approximately 11,000 kilometers (6,800 miles).

When Hubble observed it again in January 2020, it behaved as expected, migrating south towards the equator from the northern middle latitudes.

As it migrated, the Coriolis effect that kept the storm stable at mid-latitudes was expected to weaken and gradually dissipate by the time the storm reached the equator. According to simulations and previous observations, NDS-2018 should have fallen into obscurity.

But those January sightings revealed something strange – a slightly smaller dark spot like a miniature version of the larger storm. NDS-2018 was about 7,400 kilometers (4,600 miles) wide by then. Dark Spot Jr., as it was called, was about 6,275 kilometers (3,900 miles) wide.

Then in August this year, when Hubble took another look at NDS-2018 (it’s a very busy telescope and can’t just stare at Neptune), the storm moved north again. Dark Spot Jnr. was gone.

spot label(NASA, ESA, STScI, MH Wong / UC Berkeley and LA Sromovsky & PM Fry / University of Wisconsin-Madison)

“We are excited about these observations because this smaller dark fragment may be part of the dark spot disruption process,” said Wong.

“This is a process that has never been observed. We have seen some other dark spots fade, and they have disappeared, but we have never seen anything distorted, even though it is predicted in computer simulations.”

It’s impossible to know exactly what happened, but the appearance and subsequent disappearance of Dark Spot Jr. could be a clue. For starters, it was on the NDS-2018 side, closer to the equator. According to simulations, if something were to disrupt a Neptunian storm, it would happen there.

The fact that Dark Spot Jr. turned up when it happened could also be a clue.

“When I first saw the little spot, I thought the bigger one was disturbed. I didn’t think there was another vortex forming because the small one is farther towards the equator. So it’s in this unstable region. But we can’t prove that the two are related. It remains a complete mystery, “Wong said.

It was also in January that the dark vortex stopped moving and started moving north again. Maybe by dropping that fragment off that was enough to keep it from going to the equator. ‘

There is still much we do not know about Neptune’s dark spots. They are quite empty of clouds in the center, compared to storm eddies on Saturn and Jupiter. The clouds we can detect are fluffy white clouds that appear around the edges, likely as a result of gases freezing into methane ice crystals when lifted from lower heights.

At that point, NDS-2018 offers another mystery: The fluffy white accompanying clouds disappeared as the storm changed direction.

This could reveal more information about how Neptunian storms are evolving, the researchers say. In the meantime, they are also looking more closely at the available data to see if they can find more information about Dark Spot Jr., to check if the smaller storm, or parts of it, may have been revolving a bit longer.

When Hubble then waves its shiny eye to Neptune as part of the Outer Planet Atmospheres Legacy program, scientists will indeed be very interested to see what NDS-2018 does.

The team’s research was presented at the American Geophysical Union’s Fall Meeting 2020.