Neptune boasts some of the strangest weather in the solar system. The sun’s eighth planet holds the record for the fastest winds observed in any world at speeds cutting through the atmosphere upward at 1,100 miles per hour or 1.5 times the speed of sound. Scientists still do not know exactly why the atmosphere is so turbulent. Their recent glimpse of Neptune gave even more reason to be confused.
The Hubble Space Telescope identified a storm in 2018, a dark place about 4,600 miles across. Since then, it appears to have drifted toward the equator, but then swung back north, according to recent Hubble observations. It also has a minor companion storm, nicknamed the Dark Spot Jr., which scientists believe may be a lump that broke the biggest storm. These soft vortices stand out against the planet’s dizzying cerulean blue, but while dazzling to look at, their lifespan is short, making them even more challenging to study.
This is not the first time Neptune’s dark spots are behaving so strangely. When the spacecraft Voyager 2 flew past the planet in 1989 (still the only spacecraft to do so) it observed two storms. One was the original Dark Spot, a large vortex around the Earth’s size. It also had a companion, a smaller, fast-moving storm nicknamed the Scooter. The first observed Dark Spot also appeared to be moving south and then back north.
“When we followed the large dark spot with Voyager, we saw it swing up and down in longitude,” said Heidi Hammel, a member of the Voyager 2-space probe imaging team and currently vice president of science at the Association of Universities for Research in Astronomy. “We had enough time on Voyager that we were able to track the function for something like four to five months up to flyby. That storm was huge, a big monster ”as big as planet Earth.
But when the Voyager team was able to make time with the Hubble Telescope to observe the storms again, some years later, they were gone. Astronomers estimate that the average lifespan of a Neptune storm is anywhere from two to five years, and its lifespan may also depend on its magnitude. It is a contrast to the great red spot of Jupiter, our outer solar system’s second best-known storm, which at times shrinks but has been churring consistently for at least hundreds of years.
Neptune’s dark vortices dive deep into the planet – imagine them as the canopy of a very tall tree with roots that stretch down to the icy core of the world. This long connection can move the storm around in all directions so that it can slide south with the wind or be moved back up north. But as these great storms drift south toward the planet’s equator, where the wind fields are even stronger, they can be torn apart.
Because astronomers only get one shot a year to use Hubble to look at Neptune, it’s hard to monitor the temperamental atmosphere. So when scientists discover new storms, we have little chance of observing them before they have disappeared.
“This whole idea of them disappearing is one of the weirder aspects of them,” said Dr. Hammel.
Until humans can get an orbiter around the planet to better understand the life cycle of these storms, we have more questions than answers about this blue beauty. Will Dark Spot and Dark Spot Jr. stay alive? Come back in 2021 to find out.