Images obtained by the Hubble Space Telescope (HST) have confirmed the presence of a "dark vortex" in the atmosphere of Neptune, a rare type of feature that can persist for years on the eighth and farthest known planet from the Sun in the Solar System.
The vortex, the first to be observed on Neptune in the 21st century, was first seen in September 2015 by the HST, a space-based telescope launched into low Earth orbit in 1990 and operated by the U.S. National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA).
"Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains," research astronomer Mike Wong, who led the team that analyzed the HST data, was quoted as saying by a news release from University of California, Berkeley, on Thursday. He first announced the discovery in an astronomical telegram dated May 17.
On Neptune, dark vortices are associated with bright patches of high-altitude clouds created when air is diverted upward and the gases freeze out. They are high-pressure systems more similar to mid-Atlantic eddies on Earth or the high-pressure regions that periodically bring dry, warm weather to the U.S. West Coast, rather than to cyclones like typhoons or hurricanes.
Together with "companion clouds," dark vortices were first discovered in 1989 in high-resolution images of Neptune from Voyager 2, a space probe launched by NASA in 1977 to study the outer planets.
Bright clouds were again seen on Neptune starting in July 2015. Observers wondered whether these clouds were large convective storms, or bright companion clouds following an unseen dark vortex. In September, images from the HST revealed a dark spot close to the location of the bright clouds that had been tracked by observers on the ground, including amateurs and professional astronomers at the Keck Observatory in Hawaii.
The vortices discovered so far exhibit surprising diversity, in terms of size, shape, oscillatory behavior, drift rates along the meridian and meandering from latitude to latitude, according to the UC Berkeley release. In general, Neptune's dark vortices come and go on much shorter timescales compared to similar anticyclones on Jupiter, which evolve over decades on the fifth planet from the Sun.
Questions remain as to how dark vortices originate, what controls their drift and oscillation, how they interact with the environment and how they eventually dissipate. And measuring the evolution of the new dark vortex is expected to extend knowledge of both the dark vortices themselves and the structure and dynamics of the surrounding atmosphere.
