The large dark spot (top center) and its smaller companion (top right) on Neptune as seen by the Hubble Space Telescope in 2020. The large dark spot – discovered in 2018 – changed direction unexpectedly and drifted back as it had come. The smaller storm is thought to be a fragment of the larger storm. Image via NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
Like its giant siblings Jupiter, Saturn, and Uranus, Neptune has massive storms in its churning atmosphere seen in Earth’s most powerful telescopes. Scientists have tracked one of Neptune’s great spots, first seen with the Hubble Space Telescope in September 2018. This time the storm did something unexpected. It changed direction and saved itself from the expected disintegration, at least for now. The surprising development was announced by Hubble astronomers on December 15, 2020 and presented the same day at the fall meeting of the American Geophysical Union.
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This Neptune storm is wider than the Atlantic Ocean, at 7,400 miles (7,403 km) wide. It is the fourth dark storm on Neptune that Hubble has seen since 1993. Previously, two other similar storms, including Neptune’s Great Dark Spot, had been seen up close in 1989 by the Voyager 2 spacecraft during Neptune’s historic fly-by. Sadly, they were gone before Hubble hit the scene in 1990 and were able to take a look.
Here’s the smaller dark spot, which – according to the scientists – is likely a fragment that broke off the larger dark spot. Image via NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
The change in direction of this latest storm wasn’t the only unusual thing about it. It also included a noticeable one lack of smaller, bright, airy clouds, often seen above Neptune’s dark spots. According to the scientists, those clouds disappeared when the dark spot stopped moving south. The bright clouds are composed of methane ice crystals that form when gases are directed upwards over the dark spots and freeze into crystals.
The large dark spot also had a smaller companion – called “dark spot jr.” – first spotted in January 2020, which astronomers believe broke off from the main storm. This smaller stain eventually faded. As Michael Wong of the University of California, Berkeley explained:
We are excited about these observations because this smaller dark fragment may be part of the dark spot disruption process. This is a process that has never been observed. We’ve seen some other dark spots fade, and they’ve disappeared, but we’ve never seen any disturbances, even though it’s predicted in computer simulations.
The new larger storm began life in Neptune’s northern hemisphere, then began to gradually drift southward toward the planet’s southern hemisphere. As can be seen in observations of other such storms, they start to disintegrate there. But not this time. Instead, the storm was observed to have slowed and changed direction in August 2020, back to the north of where it had come. This was the first time such behavior had been noted in 30 years of Hubble observations.
Michael Wong of the University of California, Berkeley, lead author of the new paper on Neptune’s dark spots. Image via University of California, Berkeley.
Interestingly, the smaller dark spot appeared around the same time that the larger storm changed course and started moving back in the direction it came from. This newer spot was about 6,276 km wide and appeared on the side of the larger spot facing Neptune’s equator. This was the area where some computer simulations suggested a disturbance to the larger site might occur, but the timing didn’t seem to match. Wong said:
When I first saw the small spot I thought the bigger one was being disturbed. I didn’t think there was another vortex because the little one is further towards the equator. So it is in this unstable region. But we cannot prove that the two are related. It remains a complete mystery.
It was also January [2020] that the dark vortex stopped moving and started moving north again. Perhaps by shedding that fragment, that was enough to keep it from going to the equator.
Neptune’s storms are similar to those of the other gas giants and ice giants (Neptune is an ice giant) and can grow in size to easily shrink the largest hurricanes or cyclones on Earth, becoming as large as Earth itself. Unlike terrestrial hurricanes, they are high-pressure systems that form in mid-latitudes before migrating closer to the equator. In the Northern Hemisphere, they rotate clockwise because of the Coriolis effect. But that effect gets weaker as the storms approach the equator, and the storms eventually disintegrate.
That’s the common scenario that plays out as Neptunian storms develop and eventually disintegrate. But in this case, the storm didn’t quite make it to the kill zone beyond the equator. As Wong noted:
It was really exciting to watch this one behave the way it should act and then suddenly it stops and waves back. That was surprising.
The large dark spot on Neptune, as seen by Voyager 2 in 1989. The white, airy clouds above are composed of methane ice crystals. Image via NASA / JPL / Wikipedia.
It’s not yet known if dark spot Jr. survived, but scientists are continuing to analyze Hubble’s data. The new observations also provide valuable clues as to how Neptune’s atmosphere generally behaves.
Hubble has played a pivotal role in observing these spots on Neptune, especially since no other spacecraft has visited the planet since Voyager 2. According to Amy Simon of NASA’s Goddard Space Flight Center:
We wouldn’t know about these last dark spots if it wasn’t for Hubble. We can now monitor the great storm for years and follow its full life cycle. If we didn’t have Hubble, we might think that the Great Dark Spot Voyager saw in 1989 is still there on Neptune, as is Jupiter’s Great Red Spot. And we wouldn’t have known about the four other spots Hubble discovered.
Until a new mission is (hopefully) finally launched, telescopes like Hubble will continue to observe Neptune’s mysterious dark spots.
In short, a large, dark storm on Neptune changed direction unexpectedly, saving itself from impending destruction in the process. It may also have caused a smaller accompanying storm.
Source: Neptune’s NDS-2018: The Dark Vortex That Not Die
Via NASA
