Neptune boasts some of the strangest weather in the solar system. The sun’s eighth planet holds the record for the fastest winds observed on any world, with speeds cutting through the atmosphere upward of 1,100 miles per hour, or 1.5 times the speed of sound. Scientists still don’t know exactly why its atmosphere is so tumultuous. Their latest glimpse of Neptune provided even more reason to be confused.
The Hubble Space Telescope identified a storm in 2018, a dark spot some 4,600 miles across. Since that time, it appears to have drifted toward the equator but then swooped back up north, according to the latest Hubble observations. It also has a smaller companion storm, nicknamed Dark Spot Jr., that scientists think might be a chunk that broke off the main storm. These inky vortexes stand out against the dizzying cerulean blue of the planet, but while they’re dazzling to see, their life spans are short, making them even more challenging to study.
This is not the first time Neptune’s dark spots have behaved so strangely. When the Voyager 2 spacecraft flew past the planet in 1990, (still the only spacecraft to do so) it observed two storms. One was the original Dark Spot, a large vortex about the size of the Earth. It too had a companion, a smaller, fast moving storm nicknamed Scooter. The first observed Dark Spot also seemed to move south and then back to the north.
“When we were tracking the great dark spot with Voyager, we saw it oscillating up and down in longitude,” said Heidi Hammel, a member of the imaging team of the Voyager 2 space probe and currently the vice president for science at the Association of Universities for Research in Astronomy. “We had enough time on Voyager, that we were able to track the feature for something like four to five months leading up to the flyby. That storm was huge, a big monster,” as big as planet Earth.
But by the time the Voyager team were able to get time with the Hubble telescope to observe the storms again, some four years later, they were gone. Astronomers estimate the average life span of a Neptune storm is anywhere from two to five years, and its longevity might also depend on its size. That’s a contrast with the Great Red Spot of Jupiter, our outer solar system’s other best known storm, which shrinks at times, but has been churning consistently for at least hundreds of years.
Neptune’s dark vortexes dive deep inside the planet — imagine them as the canopy of a very tall tree with roots that stretch down to the icy world’s core. This long connection can move the storm around in every direction, allowing it to drift south with the winds or get yanked back up to the north. But as these large storms drift south toward the planet’s equator, where the wind fields are even stronger, they can get torn apart.
Because astronomers only get one shot a year to use Hubble to look at Neptune, it’s difficult to really monitor the temperamental atmosphere. So by the time scientists spot new storms, we only have a few chances to observe them before they’ve vanished.
“This whole idea that they disappear is one of the more puzzling aspects about them,” Dr. Hammel said.
Until humans can get an orbiter around the planet to better understand the life cycle of these storms, we are left with more questions than answers about this blue beauty. Will Dark Spot and Dark Spot Jr. survive? Check back in 2021 to find out.