The large dark spot (upper 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 – unexpectedly changed direction and drifted back as it came . The smaller storm is presumably 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 shaky atmosphere that can be seen in the most powerful telescopes on earth. Scientists have been watching one of Neptune’s large patches, first seen by the Hubble Space Telescope in September 2018. This time, the storm did something unexpected. It changed direction, at least for the time being, saved from the expected disintegration. The surprising development was announced on December 15, 2020 by Hubble astronomers and presented on the same fall during the fall meeting of the American Geophysical Union.
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This Neptune storm is wider than the Atlantic Ocean, and it is 7403 kilometers across it. This is the fourth dark storm on Neptune seen by Hubble since 1993. Previously, two other similar storms, including Neptune’s Great Dark Spot, were seen up close by the Voyager 2 spacecraft in 1989 during Neptune’s historic flight. Unfortunately, they disappeared before Hubble appeared on the scene in 1990, and were able to watch.
Here is the smaller dark spot, which according to the scientists is probably 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 was not the only unusual thing about it. It was also striking lack of smaller, bright, feathery clouds, often seen above Neptune’s dark spots. According to the scientists, the clouds disappeared when the dark spot no longer moved south. The clear clouds consist of methane crystals that form when gases are diverted upwards over the dark spots and freeze into crystals.
The large dark spot also had a smaller companion – the name “dark spot jr.” – first observed in January 2020, which according to astronomers broke off from the main storm. This smaller place has finally disappeared. As Michael Wong of the University of California, Berkeley explained:
We are excited about these observations, as this smaller dark fragment is possibly part of the dark spot disruption process. It is a process that has never been observed. We’ve seen some other dark spots disappear, and it’s gone, but we’ve never seen anything disrupt, although it’s predicted in computer simulations.
The new larger storm began its life in the northern hemisphere of Neptune and then gradually drifted southward to the southern hemisphere of the planet. As seen in observations of other such storms, it begins to disintegrate. But not this time. Instead, by August 2020, the storm had slowed down and changed direction and moved north from where it came from. It was the first time such behavior had been noticed in 30 years of Hubble observations.
Michael Wong at 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 as the larger storm reversed its course and began to return in the direction from which it came. This newer place was about 6,976 km (6,900 miles) wide and appeared on the side of the larger place that looked at Neptune’s equator. This was the area where some computer simulations suggest that a disruption of the larger space may occur, but the timing does not seem to match. Wong said:
When I saw the little dot for the first time, I thought the bigger one would be interrupted. I did not think that a vortex would form again because the small one is further to the equator. It is therefore within this unstable region. But we can not prove that the two are related. It remains a complete mystery.
It was also in January [2020] that the dark vortex had stopped moving and started moving north again. Perhaps by shedding the fragment, it was enough to stop it from moving to the equator.
Neptune’s storms are similar to those of other gas giants and ice giants (Neptune is an ice giant) and can become large to easily dwarf the largest hurricanes or cyclones on earth and become as large as the earth itself. Unlike terrestrial hurricanes, these are high-pressure systems that form at mid-latitudes before migrating closer to the equator. In the northern hemisphere, they rotate clockwise due to the Coriolis effect. But the effect weakens as the storms approach the equator, and the storms disintegrate.
This is the general scenario that unfolds as Neptune storms develop and then eventually break apart. But in this case, the storm did not quite reach the ‘death zone’ outside the equator. As Wong noted:
It was really exciting to see this one act as if it had to act and then it suddenly stops and swings back. It was amazing.
The Great Dark Spot on Neptune as seen by Voyager 2 in 1989. The white feathery clouds above consist of methane crystals. Image via NASA / JPL / Wikipedia.
It is not yet known whether dark spot jr. also survived, but scientists continue to analyze Hubble’s data. The new observations also provide valuable clues as to how Neptune’s atmosphere behaves in general.
Hubble has played a central role in observing these spots on Neptune, especially since no other spacecraft has visited the planet since Voyager 2. yet. According to Amy Simon at NASA’s Goddard Space Flight Center:
We would not know anything about these latest dark spots if it were not for Hubble. We can follow the great storm for years now and witness its complete life cycle. If we did not have Hubble, we would think the Great Dark Spot that Voyager saw in 1989 is still there on Neptune, just like Jupiter’s Big Red Spot. And we would not have known about the four other places Hubble discovered.
Until a new mission (hopefully) is finally launched, telescopes like Hubble will observe the mysterious dark spots of Neptune.
In short: A great, dark storm on Neptune unexpectedly changed direction and saved itself from impending destruction in the process. It could also have caused a smaller companion storm.
Source: Neptune’s NDS-2018: The Dark Vortex That Wouldn’t Die
Via NASA
