Dark vortex on Neptune changes direction unpredictably

(ORDO NEWS) — The most famous storm in the solar system is raging on Jupiter, but Neptune has just given scientists a new mystery.

While storm eddies are not uncommon on the distant ice giant, this is the first time it has veered back into the polar region after migrating to the equator.

Astronomers are still not sure how or why the storm changed course, but figuring it out could tell us more about Neptune’s atmospheric dynamics.

Neptune is actually quite difficult to see when compared to other planets in the solar system. It is far from the Sun, 30 times farther than it is to Earth, so it is difficult to see many details. So it wasn’t until 1989, when Voyager 2 flew by, that two storms were discovered on Neptune.

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

In general, their behavior was quite similar: storms appear in mid-latitudes, last for about two years, migrating to the equator, and then dissipate. Then, four or six years later, another one appears.

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

NDS-2018, as the name suggests, was first discovered in 2018. By then, it had grown over several years and was about 11,000 kilometers in diameter.

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

As it migrates, the Coriolis effect, which keeps the storm stable in mid-latitudes, is expected to weaken, gradually disappearing by the time the storm reaches the equator. Then, according to modeling and previous observations, NDS-2018 should have sunk into oblivion.

But observations in January revealed something odd – a slightly smaller dark spot that looks like a miniature version of a larger storm. The smaller dark spot had a diameter of about 6275 kilometers.

Then, in August of this year, when Hubble took another look at NDS-2018 (it’s a very busy telescope and can’t just stare at Neptune all the time), the storm went north again. The smaller dark spot is gone.

“We are encouraged by the observations because the smaller dark patch is potentially part of the dark spot destruction process,” Wong said.

“This is a process that has never been observed. We have seen several other dark spots disappear and they are gone, but we have never seen anything disturbed, even if predicted by computer simulations.”

It is impossible to know exactly what happened, but the appearance and subsequent disappearance of a small Dark Spot could be a clue. First, it was the side of NDS-2018 that was closer to the equator. According to the simulations, if something disrupts a Neptunian storm, it will happen there.

“When I first saw the small spot, I thought the larger one was crumbling. I didn’t think that another vortex would form, because the smaller one is closer to the equator. So he’s in this volatile area. It remains a complete mystery, ”Wong said.

“Also in January, the dark vortex stopped moving and began to move north again. Perhaps by discarding this fragment, it was enough to stop its movement towards the equator.”

There is still much we don’t know about Neptune’s dark spots. Compared to storm vortices on Saturn and Jupiter, they are pretty deserted in the middle. The clouds that we can detect are fluffy white clouds that appear at the edges, probably as a result of the freezing of gases into ice crystals of methane as they rise from lower altitudes.

NDS 2018 poses another mystery on that score: its fluffy white clouds disappeared as the storm changed direction.

The next time Hubble looks at Neptune as part of the Heritage Outer Planet Atmosphere program, scientists will indeed be very interested to see what is happening with NDS-2018.

The team’s research was presented at the fall 2020 meeting of the American Geophysical Union.

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