Massive storms are the norm on Neptune, as they are for many planets in the solar system – but astronomers have just observed one of the ice giant’s storms suddenly and dramatically change course without explanation.
The occurrence marks the first time a storm has been observed on the ice giant that reversed course and returned toward the polar region from which it came, instead of heading toward the equator and disappearing into the ether.
Researchers do not yet know how or why this happened, but the answer could provide fascinating insights into the atmospheric dynamics of the celestial outlier as well as its distant cousins in other solar systems.
An average distance from the Sun, Neptune is more than thirty times Earth’s orbit, making it extremely difficult to observe in any detail.
A flyby by the Voyager 2 spacecraft in 1989 detected two storms raging on the planet’s surface. Hubble has since discovered four more of these megastorms, known as “dark spots.”
These “spots” typically occur at mid-latitudes, linger for about two years as they slowly move toward the equator, and then disappear without a trace until a new storm takes their place about four to six years later.
This latest storm observed by Hubble, dubbed NDS-2018 after the year of its discovery, has proven to be an exception to the rule.
With a diameter of about 11,000 kilometers, NDS-2018 moved south from northern mid-latitudes toward the equator, where it should have slowly dissipated according to earlier observations and the latest models.
Instead, it split in two, with the original storm now measuring 7,400 kilometers and what is now affectionately known as “Dark Spot Jr.” measuring 6,275 kilometers across.
When Hubble looked again in August, Jr. had disappeared and the original Dark Spot was inexplicably moving back north.
“It was really exciting to see this spot behave as it should, and then suddenly it stops and swings back,” said planetary scientist Michael Wong of the University of California at Berkeley.
“This is a process that has never been observed before. We’ve seen some other dark spots fade and disappear, but we’ve never seen anything break off, even though it’s been predicted in computer simulations.”
The timing (January) and placement (in the “unstable region” of the atmosphere) of the second dark spot may give a clue as to what could have caused it, but scientists can’t be sure given the difficulty of observing the distant icy world. They will simply have to wait until Hubble can turn its gaze back to the planet to find out what’s really going on.