Hubble watches massive storm on Neptune reverse course

A dark vortex-like storm on Neptune discovered by the Hubble Space Telescope in September 2018 has reversed course to avoid an untimely demise on the blue ice giant.

The storm is 4,600 miles across — wider than the Atlantic Ocean — and formed in Neptune’s northern hemisphere. Hubble has kept an eye on the storm since its discovery more than two years ago, and astronomers watched the storm take a southern sojourn near the planet’s equator.

This is essentially the kill zone, where storms go to die on Neptune and vanish without a trace.

The vortex unexpectedly shifted north again, however, heading back to its point of origin in August 2020.

Neptune has had four storms — including this one — called dark spots (like the Great Red Spot on Jupiter), observed by Hubble over the years. The storms follow a similar pattern of appearing and disappearing over the course of two years.

Voyager 2 also glimpsed two dark storms on Neptune during its 1989 flyby of the planet — but those disappeared well before Hubble could observe them after launching in 1993.

What makes this vortex-like storm a showstopper is that astronomers have never seen a storm on Neptune double back.

Researchers also believe the storm actually sheered off a fragment of itself in the process. Hubble in January caught sight of a smaller dark spot, called “dark spot jr.,” that showed up next to the larger dark spot. (Yes, it’s all lowercase).

The smaller dark spot was likely once part of the massive storm that broke off and remained nearby before drifting away and disappearing.

The Hubble image was released on Tuesday and presented during the American Geophysical Union Fall 2020 Meeting, which took place virtually due to the pandemic.

“We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process,” said Michael H. Wong, planetary scientist at the University of California, Berkeley, in a statement.

“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.”

Watching the weather on Neptune

The images of Neptune returned by Voyager 2 and Hubble revealed that the ice giant is a brilliant blue, due to its atmosphere of hydrogen, helium and methane. But it’s a dark, frozen world with an average temperature of negative 392 degrees Fahrenheit and screaming winds that send frozen methane clouds across the planet at 1,200 miles per hour.

It’s the most distant planet in our solar system, about 30 times father from the sun than Earth is — and this distance makes noon on Neptune look like twilight on Earth.

The Great Dark Spot on Neptune, witnessed by Voyager 2, was so large that it could contain the Earth.

What researchers don’t understand much about is how these massive storms form, but they were able to study this dark spot in greater detail than previous storms.

These storms on Neptune behave differently than hurricanes on Earth. The dark spots are high-pressure systems that start out stable and rotate clockwise while hurricanes on Earth are low-pressure systems that rotate counterclockwise.

But this stability breaks down when storms near Neptune’s equator — except for the latest dark spot.

“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,” Wong said. “That was surprising.”

When this reversal happened, dark spot jr. appeared. This fragment was still quite large, measuring 3,900 miles across.

However, the timing of the smaller spot’s emergence was unusual.

“When I first saw the small spot, I thought the bigger one was being disrupted,” Wong said. “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.

“It was also in January that the dark vortex stopped its motion and started moving northward again,” Wong added. “Maybe by shedding that fragment, that was enough to stop it from moving towards the equator.”

While dark spot jr. has disappeared, researchers are looking for any surviving remnants of the smaller storm.

Hubble keeps an eye on the more distant planets in our solar system through its Outer Planet Atmospheres Legacy program. This long-term program observes the outer planets of our solar system when they are closest to Earth in their orbits each year.

By comparing observations of these planets year after year, scientists can track events like storms and seasonal changes.

“We wouldn’t know anything about these latest dark spots if it wasn’t for Hubble,” said Amy Simon, OPAL program lead investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement.

“We can now follow the large storm for years and watch its complete life cycle. If we didn’t have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter’s Great Red Spot. And, we wouldn’t have known about the four other spots Hubble discovered.”