Neptune Has Sprouted a New, Giant Dark Spot

A leopard may not be able to change its spots, but Neptune sure can!

Thanks to new images snagged by NASA’s Hubble Space Telescope, astronomers were able to confirm the presence of a dark vortex in the planet Neptune’s atmosphere. This is just the third time a dark vortex has been witnessed on the planet. One was spotted during a Voyager 2 flyby in 1989, and another by the Hubble Space Telescope in 1994. But this is the first time one has been observed on Neptune in the 21st century.

Dark vortices are high-pressure systems, or anticyclones, that are often made visible by bright “companion clouds.” These bright clouds form when the flow of air in the atmosphere is disturbed and lifted upward over the dark vortex, causing the gases to freeze — likely into methane ice crystals.

"Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains," said Mike Wong, an astronomer at the University of California at Berkeley, and leader of the team that analyzed the Hubble data, in a HubbleSite news release. "And the companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth."

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The bright clouds were spotted in July 2015 by several professional and amateur astronomers who suspected that these clouds might be bright companions of an unseen dark vortex. Unfortunately, it is not easy to see Neptune’s dark vortices because of the blue wavelengths they emit, so astronomers needed Hubble’s help. Only Hubble has a high enough resolution to see weather features on Neptune. Luckily, Hubble was able to confirm the elusive vortex on May 16.

The vortex measures approximately 4,800 kilometers (3,000 miles) across and is located in the planet's southern hemisphere, according to Space.com.

^{Hubble Space Telescope image confirms the presence of a dark vortex in the atmosphere of Neptune. The image at top right is a close-up of the feature. Photo credit: NASA, ESA, and Z. Levay (STScI)}

Surprisingly, Neptune’s dark vortices actually vary quite a bit in terms of their size, shape, and stability — they tend to change latitudes, and sometimes speed up or slow down. They also have short timescales compared to similar anticyclones seen on Jupiter, which can last for decades.

Although this is the first time the strange weather phenomenon has been seen in 22 years, researchers suspect that other dark vortices have likely passed through Neptune’s atmosphere unnoticed. For example, in 2015, astronomers reported seeing bright clouds on Neptune, but they couldn’t confirm whether there was a dark vortex too.

Now that astronomers, thanks to Hubble, have a better picture of a Neptunian dark vortex, they hope to get a better understanding of how dark vortices originate, how and why they move, and what ultimately leads to their demise.

A paper on the discovery has not yet been published.

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