Storms on Neptune seem to follow a pattern of forming, strengthening and then dissipating over the course of about two Earth years. But a Neptunian storm spotted in the planet’s atmosphere over two years ago has done something quite different: it has reversed course and is still going strong.
The storm, which is wider than the Atlantic Ocean, originated in the planet’s northern hemisphere and seen with the Hubble Space Telescope in 2018. Observations a year later showed that it began drifting southward toward the equator, where previous similar whirling storms went to die. But recent observations with Hubble spotted the vortex doubling back to the north in August of 2020.
Scientists say this new storm pattern is something previously unseen.
Back in 1989, Voyager 2 gave us the first close-up look at Neptune. From Earth, Neptune looked like a featureless marble. But to the surprise of astronomers, Voyager 2 showed us a dynamic and turbulent world of whirling storms. One giant storm in 1989 was dubbed the Great Dark Spot – in homage to Jupiter’s legendary Great Red Spot. The Great Dark Spot was churning in Neptune’s far southern hemisphere.
The next time we were able to see Neptune in closer detail was in 1994, when the Hubble Space Telescope turned towards the distant planet. Surprisingly, the mysterious spot had vanished. But shortly afterward, in 1995, Hubble spotted another dark storm appearing in Neptune’s northern hemisphere. Over the past three decades, Hubble has observed several more dark spots come and go.
But this storm is different. And equally puzzling, the storm was not alone. Hubble spotted another smaller dark spot in January this year that temporarily appeared near the larger storm. Scientists think the small storm may have been part of the giant vortex that broke off, drifted away, and then disappeared in subsequent observations.
“We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process,” said Michael H. Wong, planetary scientist at the University of California, Berkeley, in a statement. “This is a process that’s never been observed. We have seen some other dark spots fading away, and they’re gone, but we’ve never seen anything disrupt, even though it’s predicted in computer simulations.”
Another unusual feature of the dark spot is the absence of bright companion clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018. Apparently, the clouds disappeared when the vortex halted its southward journey. The bright clouds form when the flow of air is perturbed and diverted upward over the vortex, causing gases to likely freeze into methane ice crystals. The lack of clouds could be revealing information on how spots evolve, say researchers.
“We wouldn’t know anything about these latest dark spots if it wasn’t for Hubble,” said Amy Simon, who leads a Hubble project called Outer Planet Atmospheres Legacy (OPAL), which uses Hubble to monitor the outer planets in our Solar System. “We can now follow the large storm for years and watch its complete life cycle. If we didn’t have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter’s Great Red Spot. And, we wouldn’t have known about the four other spots Hubble discovered.”