A giant storm that has been raging on Neptune for more than two years is slowly dying, and scientists can’t figure out why. The “ominous, dark storm” was first discovered in 2015 by NASA’s Hubble Space Telescope, which has been tracking its development ever since. Recent photos taken by Hubble revealed that the massive storm — once 3,100 miles across its long axis — is slowly shrinking and has lost a third of its length since it was first spotted.
The shrinking storm on Neptune is the fifth to be observed on this remote planet, the farthest in our solar system, and the first to ever be photographed as it’s dying. According to the latest Hubble images, the giant dark vortex has been reduced to 2,300 miles across its long axis, and is behaving in a rather puzzling way, which contradicts well-established scientific predictions.
“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 Michael H. Wong, planetary scientist at University of California, Berkeley.
Wong has recently published a paper analyzing the massive storm on Neptune as photographed by Hubble, and detailing its unpredictable behavior in comparison with other storms previously discovered on the ice giant three billion miles away.
The first giant dark vortex ever detected in the atmosphere of this windy planet was spotted almost three decades ago, during the Voyager 2 flyby in 1989. At the time, the Voyager 2 spacecraft came across two “immense dark storms” in the planet’s atmosphere, NASA notes in a news release.
Since then, the Hubble telescope has been keeping tabs on Neptune, and has found three other dark vortices: two in the mid-1990s, and this latest one in 2015. Given its high resolution in blue light, Hubble is the only one able to detect Neptune’s dark vertices (or dark spots) — low-contrast features that can only be seen at blue optical wavelengths.
This latest dark vortex discovered on Neptune is an anticyclone, and was first seen at mid-southern latitudes, which is why researchers named it SDS-2015, for “southern dark spot discovered in 2015.” NASA’s best guess is that Neptune’s dark spot is probably made up of hydrogen sulfide, which most likely makes the massive storm smell foul.
Yet what makes SDS-2015 so intriguing is its unexpected demise. Although its true that storms on Neptune typically last for just a few years (by comparison, Jupiter’s Great Red Spot, which is also an anticyclone, has been going on for at least two centuries), researchers had expected it to go out with a bang rather than silently fade away as shown by the Hubble images.
“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,” Wong explained in the news release.
Hubble Sees Neptune's Mysterious Shrinking Storm— Astrobiology Mag (@AstrobiologyMag) February 15, 2018
Three billion miles away on the farthest known major planet in our solar system, an ominous, dark storm is shrinking out of existence as... - https://t.co/BtTX9HHL3M pic.twitter.com/KfFpx6SRYL
However, unlike the first giant dark vortices of 1989, which inspired a series of simulations upon which astronomers still base their predictions, SDS-2015 has gradually drifted toward Neptune’s south pole, instead of going the opposite way, towards the equator. This mysterious behavior, which researchers are still trying to understand, may be what is causing the vortex to slowly shrink, NASA informs.
“The five Neptune dark spots exhibited surprising diversity, in terms of size, shape, companion cloud distribution, oscillatory behavior, meridional drift rates, and meandering,” Wong writes in his paper, featured on February 15 in the Astronomical Journal.
The storm has also brought with it an unexpected chance for astronomers to study Neptune’s deep winds, which — NASA points out — can’t be directly measured. The Hubble images revealed the dark vortex is cruising along three alternating wind jets that circle the planet: a westward one at the equator, and two eastward ones around the north and south poles. Neptune has the fastest winds in the entire solar system, reaching supersonic speeds, and its wind jets can be 10 to 15 times stronger than those found on Earth, Space.com reports.
Compared to Jupiter’s Great Red Spot, which is restricted by significantly more wind jets, Neptune’s dark spot can travel more freely “and should be free to change traffic lanes and cruise anywhere in between the jets,” shows NASA.