Late last year, astronomers discovered the sixth known dark vortex on Neptune. This time, however, scientists were able to record the formation of this hurricane over several years and provide new insights into these puzzling and short-lived weather patterns.
The discovery of the latest dark vortex from Neptune and the appearance of a dramatic white cap on Uranus, were announced earlier this year. The results were made possible by the Outer Planet Atmospheres Legacy (OPAL) program, a long-term side project of the Hubble program, which tracks annual changes in both ice giants. Two new research papers published today in Geophysical Research Letters and the Astronomical Journal contribute to a better understanding of the new surface feature of Neptune. This includes new insights into the formation of these dark spots, the frequency of their occurrence and the duration of their occurrence.  The large dark vortex was observed in 1989 by Voyager 2.
The new dark vortex on Neptune was discovered in the Hubble data collected in the fall of 2018. It's the Biggest Storm Ever since the system's Voyager 2 flyby appeared in 1989 on the planet. This is now the sixth dark spot on the Ice Giant, two of which were discovered by the NASA probe and four by Hubble. The new dark spot is practically the same size and shape as the big spot of 1989, about the size of the earth.
These surface elements are dramatic, but they do not last long and become clear after a few years. This is in stark contrast to the Great Red Spot on Jupiter – a mighty storm that has been hurting for over a century. Like the Great Red Spot, the dark eddies on Neptune appear in high pressure areas, while storms in low pressure areas form on Earth.
As the two new studies highlight, this is the first time that scientists observe the formation of a dark vortex on Neptune. Amy Simon, a researcher at the NASA's Goddard Space Flight Center, who leads the OPAL mission, and Michael Wong and Andrew Hsu from the University of California Berkeley examined Hubble images of Neptune's earlier dark vortex, which came to light in 2015 when they noticed little , bright clouds in the area where the stain 2018 would eventually emerge. This was a surprise for the team as they did not expect a new dark vortex so soon.
"All previous dark spots were fully formed, and there were no observations comparable to those before their discovery." Simon told Gizmodo. "In this case, we have Hubble data for several years before we saw the new spot. If we go backwards, we can see evidence of high clouds in the area in earlier years. "
Simon and her colleagues said that the associated clouds appeared in the same spot about two years before the new one. A dark spot appeared – an important indication that the storms are much deeper in the planet's atmosphere than they are usually accepted.
"Of course, every high cloud does not mean a new storm, but that tells us something was disturbing the atmosphere before we could see another storm," she said. "It is believed that these high clouds are like clouds that form when air rises above a mountain."
The white clouds are made of methane ice crystals that give them their distinctive bright white color. The brightest of these methane clouds were observed in 2016 and 2017. Using computer models, Simon and her colleagues found that the depth of the storm also increased the brightness of the clouds. These storms do not suddenly flash overnight. Rather, the educational process takes several years, according to the new research. The "drift rate and the genesis of the dark vortex point to a deep origin," the authors write in the new study.
The researchers estimate that the wind speeds at the bottom of the dark vortex are almost four times higher than at the surface. The deep winds move at 100 meters per second (328 feet per second), comparable to Jupiter's Great Red Spot. In contrast to the Jovian storm, which is relatively fixed, the dark vortexes drift on Neptune over latitudes. Finally, strong winds cause the storms to break apart and disappear.
In the Accompanying Work of the Astronomical Journal co-authored by the same trio, the researchers said that the dark vortexes appear on Neptune about every four to six years. Overall, the dark spots have a lifespan of at least one to two years, but not more than six years. Further observations in the coming years should further refine these assessments.
"Of course, we will continue to watch this storm, but the next step is to do computer-based deep disturbance simulations and find out if we can find the conditions that cause a storm to form and grow." Smith said to Gizmodo. "We will also follow the size, shape and movement of the storm to learn more about the deeper wind and temperature fields, as these can determine what we see at the cloud tops."
What would be good would be a satellite permanently stationed in orbit around Neptune (and Uranus for that matter). Hubble is great, but it's too far from the outer planets and often does other astronomical tasks. It's high time that NASA and other space agencies plan something like that. These outer planets are just too damn interesting to ignore.
[Geophysical Research Letters, Astronomical Journal]