Astronomers Catch the Formation of a Dark Vortex on Neptune

Jupiter’s Great Red Spot gets most of the attention when it comes to gas giant spots, but astronomers are particularly interested in what’s been happening on Neptune lately. After seeing several dark vortexes appear in the ice giant’s clouds over the decades, scientists have finally been able to watch one form, and it was a big one. Understanding how these storms appear could be key to understanding the behavior of Neptune’s atmosphere.

NASA announced the initial detection of the dark vortex and a white cap on Uranus earlier this year — both were detected as part of the agency’s Outer Planet Atmospheres Legacy (OPAL) program. Now, there’s a full report available on that vortex.

Scientists got their first look at the intermittent dark spots on Neptune when Voyager 2 made a fly-by in 1989 (above right). During that quick pass, the probe saw two vortexes, one of which was the largest on record until now. Since Hubble came online in the early 1990s, NASA has detected a further four dark vortexes on Neptune. The vortexes are huge (the new one is the size of Earth), but they don’t last been long — they come and go over the course of a few years. Compare that to Jupiter’s red spot, which had been churning for centuries at least.

Previous dark vortexes were discovered fully formed, but astronomers happened upon this one when it was barely visible. Michael Wong and Andrew Hsu from the University of California Berkeley were looking at Hubble images from the last dark spot in 2017 when they noticed some small, bright clouds nearby. Those clouds would soon darken and grow, becoming the 2018 vortex (above left).

Those white clouds were methane ice crystals, and modeling of Neptune’s atmosphere suggests these bright companion clouds become brighter as the depth of the storm increases. The researchers also believe that wind speeds at the bottom of the vortex are about four times as strong as at the surface. Down there, winds are moving at 100 meters per second (328 feet per second), which is equal to the Great Red Spot on Jupiter.

NASA plans to continue tracking the 2018 spot for as long as possible. Unlike the Great Red Spot, Neptune’s vortexes drift around, making them hard to follow from Earth. Ideally, scientists would like to have a satellite in orbit of Neptune, but the current vortex won’t last longer than six years — that’s the record for other such Neptunian storms. Although, another will be along eventually. On average, a new dark vortex appears every four to six years.

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