Mysterious shrinking storms of Neptune spotted by NASA's Hubble

Image courtesy: NASA, ESA, and M.H. Wong and A.I. Hsu (UC Berkeley)

New Delhi: NASA's eye in the sky – the Hubble Space Telescope – has observed that an ominous, dark storm in Neptune, the farthest known major planet in our solar system, is shrinking out of existence.

In the late 1980s, NASA’s Voyager 2 spacecraft was the first to discover Neptune's dark storms. Since then, only Hubble has had the ability to track them over the years.

According to NASA, 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.

(Video courtesy: NASA Goddard)

Similar to Jupiter's Great Red Spot (GRS), Neptune's storm swirls in an anti-cyclonic direction and is dredging up material from deep inside the ice giant planet’s atmosphere.

Since the deep winds of the planet can't be directly measured, this elusive feature presents scientists with a unique opportunity to study them

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.”

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, NASA said.

“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.”

The dark vortex is behaving differently from what planet-watchers predicted. However, the dark spot, which was first seen at mid-southern latitudes, has apparently faded away rather than going out with a bang.

As per the space agency, 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 (seen as bands in Jupiter’s atmosphere).

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 first images of the dark vortex are from the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of our solar system’s four outer planets.

Only Hubble has the unique capability to probe these worlds in ultraviolet light, which yields important information not available to other present-day telescopes.