The storm is 4,600 miles wide – wider than the Atlantic Ocean – and formed in Neptune’s Northern Hemisphere. Hubble has been monitoring the storm since its discovery more than two years ago, and astronomers watched the storm make a southerly stay near the planet’s equator.
This is essentially the kill zone, where storms on Neptune die and disappear without a trace.
The vortex unexpectedly shifted north again, however, heading back to 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 appearance and disappear over the course of two years.
Voyager 2 also glimpsed two dark storms on Neptune during its 1989 flight over the planet – but they disappeared well before Hubble could detect them after launch in 1993.
What makes this vortex-like storm a showstopper is that astronomers have never seen a storm on Neptune twice.
Researchers also believe that the storm actually toned down a fragment of itself in the process. In January, Hubble spotted a smaller dark spot called “dark spot jr.” That appeared next to the larger dark spot. (Yes, they are all lowercase).
The smaller dark spot was likely once part of the massive storm that broke and stayed nearby before drifting and disappearing.
The Hubble image was released and presented on Tuesday at the American Geophysical Union Fall 2020 Meeting, which pretty much took place because of the pandemic.
“We are excited about these observations because this smaller dark fragment may be part of the dark spot disruption process,” Michael H. Wong, planetary scientist at the University of California, Berkeley, said in a statement.
“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.”
Looking at 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 the atmosphere of hydrogen, helium and methane. But it’s a dark, frozen world with an average temperature of -392 degrees Fahrenheit and howling winds sending frozen clouds of methane across the planet at 1,200 miles per hour.
It is the most distant planet in our solar system, about 30 times father to the sun than Earth – and this distance makes Neptune look like twilight on Earth.
The Great Dark Spot on Neptune, witnessed by Voyager 2, was so large it could contain Earth.
What researchers don’t understand much about is how these massive storms originated, but they were able to study this dark place 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 stably and rotate clockwise, while on Earth hurricanes are low-pressure systems that rotate counterclockwise.
But this stability breaks down with storms near Neptune’s equator – except for the last dark spot.
“It was really exciting to see this one behave the way it should and then suddenly stop and swing back,” said Wong. “That was surprising.”
When this reversal took place, dark spot Jr. appeared. This fragment was still quite large, measuring 3900 miles in diameter.
However, the timing of the smaller spot’s appearance was unusual.
“When I first saw the small spot, I thought the bigger one was being disturbed,” Wong said. “I didn’t think there was another vortex forming because the little one is further towards the equator. So it’s in this unstable region. But we can’t prove the two are related. It remains a complete one. mystery.
“It was also in January that the dark vortex stopped its movement and started moving north again,” added Wong. “Maybe getting rid of that fragment was enough to keep it from going to the equator.”
As dark spot Jr. has disappeared, researchers are looking for remnants of the smaller storm.
Hubble is monitoring the more distant planets in our solar system through the 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 such as storms and seasonal changes.
“We wouldn’t know about these latest dark spots if it wasn’t for Hubble,” said Amy Simon, OPAL Program Principal Investigator. at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement.
“We can now track the great storm for years and trace its entire life cycle. If we didn’t have Hubble, we might think that the Great Dark Spot that Voyager saw in 1989 is still present on Neptune, as is Jupiter’s Great Red Spot. And we wouldn’t have known about the four other places Hubble discovered. ”