Although giant dark storms (called dark spots) are not new on Neptune, but this is for the first time that scientists have observed a storm unexpectedly turning back towards the polar region after moving towards the equator.
Because of its massive distance from Sun, Neptune is very difficult to observe in much detail. It lies at an average thirty times that of Earth-Sun distance. It was in 1989 that two storms were discovered on ice giant when Voyager 2 made its Neptune flyby.
Since 1993 NASA’s Hubble Space Telescope has spotted four more such storms. Interestingly, Hubble remains the only telescope which can do so.
This particular storm vortex is called NDS-2018 and it was first discovered by Hubble Telescope in 2018. At that time, it was measured around 11,000 kilometres across.
In January 2020, it was again observed by Hubble while drifting southward towards ice giant’s equator. The storm was exactly behaving in a manner as predicted by computer simulations and previous observations.
These January observations also showed that NDS-2018 was not alone, it was accompanied by a smaller storm, now informally called Dark Spot Jr.
At that time larger storm was about 7,400 kilometres across and smaller one was about 6,275 kilometres across.
But, baffling scientists, August 2020 Hubble observations revealed that this particular storm did not behave like past observed storms. It stopped moving south and turned its way back northwards. While smaller storm was nowhere to be seen.
Usually, theses storms behave in a similar manner. They pop up at mid latitudes, stay around for about two years while moving towards the planet’s equator and then finally they disappear as the Coriolis effect that keeps them stable starts to weaken . Then about in four to six years another storm comes up.
It was in January that the storm started moving northward again. It is believed that Dark Spot Jr. may be a part of the bigger storm’s disruption process.
As of now scientists can not prove that Dark Spot Jr. broke off from the larger storm, but they think that shedding that part was sufficient to prevent the bigger storm from proceeding towards the equator.
Hubble telescope has been observing similar storms on ice giant for the past thirty years, but such unpredictable atmospheric behaviour was never seen before.
For more details: Neptune’s NDS-2018: The Dark Vortex That Would Not Die