In 2018, astronomers using the NASA / ESA Hubble Space Telescope saw a giant dark storm in Neptune’s Northern Hemisphere, which is 7,400 km (4,600 miles) wide. Observations a year later showed that the vortex was beginning to drift southward towards the equator, where such storms are expected to disappear from view. To the surprise of astronomers, Hubble saw that by August 2020 the vortex changed direction and doubled back to the north. At the same time as the stunning reversal of the spot, a new, slightly smaller dark feature appeared near its larger cousin and later disappeared.
Neptune’s dark eddies are high-pressure systems that can form at mid-latitudes and then migrate to the equator.
They remain stable because of the Coriolis forces, which cause storms in the Northern Hemisphere to turn clockwise, due to the planet’s rotation. However, if a storm drifts towards the equator, the Coriolis effect weakens and the storm disintegrates.
In computer simulations, these storms follow a more or less straight path to the equator, until there is no Coriolis effect to keep them together.
Contrary to the simulations, the recently spotted giant storm did not migrate to the equatorial kill zone.
“It was really exciting to see this one behave the way it should and then suddenly stop and swing back. That was surprising, ”said Dr. Michael H. Wong, an astronomer at the University of California at Berkeley.
The Hubble observations also revealed that the enigmatic reversal of the dark vortex’s path occurred at the same time that a new blot appeared.
The newest site was slightly smaller than its cousin and measures approximately 6,300 km in diameter.
It was near the side of the main dark spot facing the equator – the location some simulations show would cause a disturbance.
However, the timing of the smaller spot’s appearance was unusual.
“When I first saw the small spot, I thought the bigger one was being disturbed,” said Dr. Wong.
‘I didn’t think there would be another vortex because the little one is farther towards the equator. So it is in this unstable region. But we cannot prove that the two are related. It remains a complete mystery. “
“It was also in January that the dark vortex stopped moving and started moving north again,” he added.
“Perhaps by shedding that fragment, that was enough to keep it from going to the equator.”
“We wouldn’t know about these latest dark spots if it wasn’t for Hubble,” said Dr. Amy Simon, an astronomer at NASA’s Goddard Space Flight Center.
“We can now monitor the great storm for years and trace its full life cycle.”
If we didn’t have Hubble, we might think that the Great Dark Spot Voyager saw in 1989 is still present on Neptune, as is Jupiter’s Great Red Spot. And we wouldn’t have known about the four other spots Hubble discovered. “
The astronomers presented their results on the fall meeting of the American Geophysical Union.
Michael H. Wong et al. Neptune’s NDS-2018: The Dark Vortex That Don’t Die. AGU20, abstract # P066-0011