Hubble watches a giant storm, three billion miles away, as it shrinks. The storm was once big enough to reach from Boston to Portugal.

So, NASA’s Hubble Space Telescope, through its pictures, proves that the dark storm is shrinking out of existence.

In 1989, NASA’s Voyager 2 spacecraft flew by Neptune. Back then, the legendary spacecraft observed large, dark storms inhabiting the distant planet’s atmosphere. Since then, scientists have monitored Neptune using the Hubble Space Telescope and seen new storms develop.

This latest storm was first seen in 2015 but is now shrinking.

The Neptunian storm swirls just like Jupiter’s Great Red Spot (GRS), in an anticyclonic direction. While swirling, it dredges up material from deep inside the ice giant planet’s atmosphere.

The dark spot material may be hydrogen sulfide, with the pungent smell of rotten eggs. Joshua Tollefson from the University of California at Berkeley explained, “The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere.”

The Dying Storm

Unlike Jupiter’s GRS, which has been visible for at least 200 years, Neptune’s dark vortices only last a few years. This is the first time someone takes a picture of it while dying.

“We have no evidence of how these vortices are formed or how fast they rotate,” said Agustín Sánchez-Lavega from the University of the Basque Country in Spain. “It is most likely that they arise from an instability in the sheared eastward and westward winds.”

This series of Hubble Space Telescope images taken over 2 years tracks the demise of a giant dark vortex on the planet Neptune. The oval-shaped spot has shrunk from 3,100 miles across its long axis to 2,300 miles across, over the Hubble observation period. Credits: NASA, ESA, and M.H. Wong and A.I. Hsu (UC Berkeley)

“It looks like we’re capturing the demise of this dark vortex, and it’s different from what well-known studies led us to expect,” Michael Wong, a researcher at the University of California at Berkeley and lead author on the new work, said in the statement. Previous simulations suggested that the vortex would drift toward the planet’s equator, and “once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”

But instead, it drifted toward the planet’s south pole and is quietly fading away. The vortex was 3,100 miles (5,000 kilometers) across the long axis when Hubble spotted it in 2015, and now it’s down to 2,300 miles (3,700 km) across.

“No facilities other than Hubble and Voyager have observed these vortices. For now, only Hubble can provide the data we need to understand how common or rare these fascinating Neptunian weather systems may be,” said Wong.

The new work was detailed Feb. 15 in the Astronomical Journal.

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Thumbnail image credit:  NASA