Neptune’s Dark Storm Spins Offshoot

Way back in 1989, we got our first good look at the planet Neptune when the Voyager 2 spacecraft did a quick reconnaissance flyby. Far from being a bland world like sister ice giant Uranus, Neptune was a bit more interesting. For one thing, it showed some interesting features called “dark spots” which turned out to be Neptune’s dark storms. There were other features in the upper atmosphere, but the Dark Spot and Dark Spot 2 really grabbed people’s attention. A few years later, Hubble Space Telescope took its first peek at the planet. By then, the storms had disappeared.

What happened? That’s a question planetary scientists have focused on ever since the flyby. And, since that time, several other similar types of features have come and gone in the Neptunian atmosphere. Hubble has been tracking those storms through a project called the Outer Planet Atmospheres Legacy (OPAL) program, in an effort to give more insight into these features.

These days there’s a relatively new storm in Neptune town, and it’s been around since at least 2018. That’s when Hubble took another look and found it. It was high in the northern hemisphere, and typical of such storms, it began drifting southward. In the past, when storms drift south, they tend to disappear. This latest storm could span the Atlantic Ocean if it occurred here on Earth.

Neptune’s Dark Storm Formation

What are these storms? They’re called dark vortices and are high-pressure systems that form, usually in the mid-latitudes. After a while, they drift toward the equatorial regions and start to lose stability. That’s because the Coriolis effect weakens the storm. Depending on the size and strength of the storm, it can withstand that effect for a while. Eventually, the storm just disintegrates. Storms that don’t end up in the equatorial region “kill zone” can remain stable for quite a while. Most of the storms that have been observed on Neptune follow the path to destruction. However, this latest one seems to be resisting the siren call of the equator.

Neptune’s Dark Storm Changes Course

This storm is acting differently. Instead of staying on a course south, it has turned back northwards. And, there’s another storm in the same general region. Is it a new storm? Or, did the 2018 storm spin off a clone? Those are the questions planetary scientists are looking to answer as they study this latest development.

It’s possible that as these storms head south toward dissolution, they tend to fragment and dissolve over time. Observers haven’t actually seen these disruptions in real time, so these two features give them a chance to see something happening more or less “in action” in the Neptunian atmosphere.

The mystery of how these storm features form still hasn’t been solved. Nor do planetary scientists have a strong feel for the structure of the storms. They ask “why are they ‘dark’?” It could be that there are high-altitude dark cloud layers. If that’s the case, then their existence could give a good insight into the vertical (top-to-bottom) structure of the storm itself. Perhaps there’s a more complex atmospheric chemistry answer. In any case, long-term studies, where the telescope studies the planets as often as possible, are the best way to understand the ephemeral nature of Neptune’s storms.

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