The NASA/ESA Hubble Space Telescope has captured images of a large, dark storm on Neptune shrinking out of existence.
This series of Hubble images taken over two years tracks the demise of a giant dark vortex on Neptune. The oval-shaped spot has shrunk from 3,100 miles (4,990 km) across its long axis to 2,300 miles (3,701 km) across, over the Hubble observation period. Image credit: NASA / ESA / M.H. Wong & A.I. Hsu, University of California, Berkeley.
Neptune’s dark storms were first discovered in the 1980s by NASA’s Voyager 2 spacecraft.
Since then, only Hubble has had the sharpness in blue light to track these elusive features that have played a game of peek-a-boo over the years.
Hubble found two dark storms that appeared in the mid-1990s and then vanished.
This latest storm was first seen in 2015, but is now shrinking.
Like Jupiter’s Great Red Spot (GRS), the storm swirls in an anti-cyclonic direction and is dredging up material from deep inside the ice giant planet’s atmosphere.
The elusive feature gives astronomers a unique opportunity to study Neptune’s deep winds, which can’t be directly measured. The dark spot material may be hydrogen sulfide.
“The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere,” said Dr. Joshua Tollefson, from the University of California, Berkeley.
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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 one that actually has been photographed as it is dying.
“We have no evidence of how these vortices are formed or how fast they rotate,” said Dr. Agustín Sánchez-Lavega, from the University of the Basque Country.
“It is most likely that they arise from an instability in the sheared eastward and westward winds.”
The dark vortex is behaving differently from what planet-watchers predicted.
“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,” said Dr. Michael Wong, from the University of California, Berkeley.
“Their dynamical simulations said that anticyclones under Neptune’s wind shear would probably drift toward the equator. We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”
But the dark spot, which was first seen at mid-southern latitudes, has apparently faded away rather than going out with a bang. That may be related to the surprising direction of its measured drift: toward the south pole, instead of northward toward the equator.
Unlike Jupiter’s GRS, the Neptune spot is not as tightly constrained by numerous alternating wind jets.
Neptune seems to only have three broad jets: a westward one at the equator, and eastward ones around the north and south poles. The vortex should be free to change traffic lanes and cruise anywhere in between the jets.
The findings are published in the Astronomical Journal.
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Michael H. Wong et al. 2018. A New Dark Vortex on Neptune. AJ 155, 117; doi: 10.3847/1538-3881/aaa6d6
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