© NASA/ESA/GSFC/JPL
Neptune’s dark storms were first captured by Voyager 2 in 1989 (left). In 2018, Hubble spied an entirely new storm system.
By analyzing Hubble images dating back to 2015, astronomers have discovered high-altitude clouds that formed years ahead of the visible storm, indicating it was already forming there, swirling beneath the clouds and haze. The telltale clouds are teaching astronomers more about how such storms form and evolve on all the giant outer planets.
Birth of a Storm
Neptune, like all the outer solar system planets, forms large and durable storms. While
Jupiter's Great Red Spot is infamous, Neptune's dark blue spots were unknown until Voyager 2 flew past in 1989, sending back pictures of two large storms on its surface. Jupiter's Great Red Spot has been visible for at least 190 years, and possibly since the 1600s. But when Hubble peered at Neptune in 1994, its storms had already vanished.
Since then, Hubble has spotted dark storms appearing and disappearing on Neptune, lasting only two years or so - though maybe up to six years - before dissipating again. Like hurricanes on steroids, Neptune's storms are dark vortexes of clouds racing at high speeds, each roughly the size of planet Earth. But Earth storms rarely last more than a few weeks, and form around low-pressure areas. On the giant planets, they instead form around regions of high-pressure.
"That makes them more stable to start," says Simon. "And there are no land masses. That's what breaks storms up on Earth." On Jupiter, the planet's jet streams lock its massive storm in place near the equator, where it has safely churned for centuries. On Neptune, wind patterns push the storms north or south where they get shredded by opposing wind currents within a few years.
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