Hubble captures birth of giant storm on Neptune

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This is a composite picture showing images of storms on Neptune from Hubble (left) and Voyager 2 (right).

Images captured by the Hubble Space Telescope show the formation of a "great dark spot" on Neptune for the first time, reported scientists in a new study. A brand new Great Dark Spot appeared on Neptune in 2018, nearly equivalent in size and shape to the one Voyager saw in 1989.

The Hubble images also helped the space scientists understand how often Neptune gets dark spots and their duration. Researchers have, so far, spotted six such Spots on Neptune since 1989, when Voyager 2 identified the first two. The research documents pictures taken of Neptune in 2018 by Hubble, which, upon examination, just so happened to capture the birth of the planet's latest great spot.

Simon says that discoveries on Neptune will have implications for those studying exoplanets in our galaxy that are similar in size to the ice giants.

Amy Simon, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, has been studying Neptune over the past several years to get a better grasp on how storms form on the icy planet.

Scientists first saw a Fantastic Dark Spot on Neptune in 1989, when NASA's Voyager 2 probe flew past the mysterious blue planet. The spacecraft zoomed by and saw two giant storms in the southern hemisphere. Three years later, a dark storm was spotted where the white clouds had been forming, the researchers noted. The Hubble image shows a new dark storm (top center).

"It was certainly a surprise", Simon said.

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A new Great Dark Spot appeared on Neptune in 2018, almost identical in size and shape to the one Voyager saw in 1989. They were actually monitoring another, smaller storm in Neptune from 2015 when the new one emerged. The white color of the clouds is from the methane ice crystals, and they were the brightest in 2016 and 2017, leading up to the full storm in 2018. The methane clouds likely float above Great Dark Spots the way "lenticular" clouds hover over tall mountains here on Earth, study team members said. In 2018, Hubble spied an entirely new storm system.

Like Jupiter's Great Red Spot, Neptune's Great Dark Spots are storms that form from areas of high atmospheric pressure. Each storm lasts up to six years before it disappears, although a two-year lifespan is more likely, according to the study. 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.

The new findings show how Neptune's Great Dark Spots differ from Jupiter's Great Red Spot. Compare that to Jupiter's red spot, which had been churning for centuries at least.

Jupiter's storms endure as they're caged in by thin jet streams, which keep them from changing latitude (north-south) and hold them together. But Neptune's Great Dark Spots wander around the planet much more freely before they're finally ripped apart by high-altitude winds, the researchers said.

Neptune, like all the outer solar system planets, forms large and durable storms.

Founded in 1919, AGU is a not-for-profit scientific society dedicated to advancing Earth and space science for the benefit of humanity.

Michael H. Wong, Andrew I. Hsu: University of California Berkeley, Berkeley, California, U.S.A.

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