The large dark spot (upper center) and its smaller companion (upper right) on Neptune, as seen by the Hubble Space Telescope in 2020. The large dark spot (discovered in 2018) changed direction unexpectedly and went to it drifts her down her path. The smallest storm is believed to be a fragment of the largest storm. Image via NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
Like its giant brothers Jupiter, Saturn, and Uranus, Neptune has massive storms in its turbulent atmosphere that can be seen in the most powerful telescopes on Earth. Scientists have been tracking one of Neptune’s great points, first seen by the Hubble Space Telescope in September 2018. This time, the storm did something unforeseen. He changed direction, saving himself from the expected disintegration, at least for now. The startling developments were announced by Hubble astronomers on December 15, 2020 and presented at the fall meeting of the American Geophysical Union on the same day.
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This storm of Neptune is wider than the Atlantic Ocean, with 7,403 km in diameter. It is Neptune’s fourth dark storm seen by Hubble since 1993. Previously, two other similar storms, including Neptune’s Great Dark Spot, had been seen up close by the Voyager 2 spacecraft in 1989 during Neptune’s historic flyby. Unfortunately, they had disappeared before Hubble came on the scene in 1990 and they could take a look.
Here is the smallest dark spot which, according to scientists, is probably a fragment that broke the largest dark spot. Image via NASA / ESA / STScI / MH Wong (University of California, Berkeley) / LA Sromovsky / PM Fry (University of Wisconsin-Madison).
The change of direction of this new storm was not the only unusual thing about it. He also had a notable missing of smaller, brighter, leadier clouds, which had often been seen over the dark spots of Neptune. According to scientists, those clouds disappeared when the dark spot stopped moving south. Bright clouds are made up of methane ice crystals that form when gases deviate upward over dark spots and freeze into crystals.
The large dark spot also had a smaller companion, called a “dark spot jr.” – First detected in January 2020, which astronomers believe may have broken from the main storm. This smaller point ended up disappearing. As Michael Wong of the University of California, Berkeley explained:
We are excited about these observations because this smaller dark fragment is potentially part of the dark spot disruption process. This is a process that has never been observed. We have seen some other dark spots fade and disappear, but we have never seen anything interrupted, although it is predicted in computer simulations.
The new largest storm began its life in the northern hemisphere of Neptune and then began to gradually drift south to the southern hemisphere of the planet. As seen in the observations of other storms of this type, this is where they begin to disintegrate. But not this time. Instead, it was observed that the storm slowed down and changed direction in August 2020, retreating northward from where it had come from. This was the first time this behavior was noticed in 30 years of Hubble observations.
Michael Wong of the University of California, Berkeley, lead author of the new article on the dark spots of Neptune. Image through the University of California, Berkeley.
Interestingly, the smallest dark spot appeared about the same time as the larger storm reversed course and began to return in the direction from which it had come. This newest point was approximately 6,276 km in diameter and appeared next to the largest point facing the equator of Neptune. This was the area where some computer simulations suggested that a larger site disruption could occur, but the timing did not seem to match. Wong said:
When I first saw the small place, I thought the larger one was being interrupted. I didn’t think another vortex would form because the little one is further away towards the equator. Therefore, it is located within this unstable region. But we cannot prove that the two are related. It remains a complete mystery.
It was also in January [2020] that the dark vortex stopped its movement and began to move north again. Perhaps pouring this fragment was enough to keep it from moving toward the equator.
Neptune’s storms are similar to those of other giant giants and ice giants (Neptune is an ice giant) and can grow in size to swallow the largest hurricanes or cyclones on Earth, becoming as large as Earth itself. Unlike terrestrial hurricanes, they 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 this effect weakens as storms approach the equator and eventually the storms disintegrate.
This is the general scenario that unfolds as Neptune’s storms unfold and eventually break. But in this case, the storm did not reach the “death zone” beyond the equator. As Wong pointed out:
It was really exciting to see how this one acted like it was supposed to act and suddenly it just stops and takes a turn back. It was amazing.
The Great Dark Spot of Neptune, as seen by Voyager 2 in 1989. The white, leady clouds above are made up of methane ice crystals. Image via NASA / JPL / Wikipedia.
It is not yet known if the dark spot jr. it also survived, but scientists continue to analyze Hubble data. The new observations also provide valuable clues about the overall behavior of Neptune’s atmosphere.
Hubble has played a central role in observing these points on Neptune, especially since no other spacecraft has yet visited the planet since Voyager 2. According to Amy Simon, at NASA’s Goddard Space Flight Center:
We wouldn’t know any of these last dark spots if it weren’t for Hubble. We can now follow the great storm for years and see its full life cycle. If we didn’t have Hubble, we might think that the Great Dark Spot seen by Voyager in 1989 is still on Neptune, just like Jupiter’s Great Red Spot. And, we would not have known of the other four points that Hubble discovered.
Until (hopefully) a new mission is launched, telescopes like Hubble will continue to observe Neptune’s mysterious dark spots.
Conclusion: A large, dark storm over Neptune changed direction unexpectedly, saving itself from the approaching destruction in the process. It may also have produced a smaller accompanying storm.
Source: NDS-NDS-2018: The dark vortex that would not die
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