The dark storm that was discovered on Neptune in 2018 was spinning to its demise by moving towards the equator. There is where these storms go to die, vanishing in the turbulent atmosphere. But this new storm was just not ready to go. Observations from January to August 2020 has shown that the vortex is once again moving North.
In 30 years’ worth of Hubble observations and four dark spots tracked, this is the first time that such a course reversal was witnessed. The storm, which is wider than the Atlantic Ocean, might have accomplished this feat by shedding a fragment.
"We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process," lead researcher Michael H. Wong of the University of California at Berkeley, said in a statement. "This is a process that's never been observed. We have seen some other dark spots fading away and they're gone, but we've never seen anything disrupt, even though it’s predicted in computer simulations."
The Dark Spot is 7,200 kilometers (4,600 miles) having shrunk down from its original size and Dark Spot Jr (as the fragment is informally named) was estimated to be 6,300 kilometers (3,900 miles) in diameter. Both are powered by Coriolis forces. As a planet rotates, its atmosphere is subjected to such forces, creating storms rotating clockwise in the northern hemisphere.
These forces are weakest at the equator, so when a storm moves there it loses energy and dissipates. It is also why it’s puzzling that Dark Spot Jr formed so close to the equator unlike any other storm previously witnessed.
"When I first saw the small spot, I thought the bigger one was being disrupted," Wong added. "I didn't think another vortex was forming because the small one is farther towards the equator. So it's within this unstable region. But we can't prove the two are related. It remains a complete mystery.
"It was also in January that the dark vortex stopped its motion and started moving northward again," Wong added. "Maybe by shedding that fragment, that was enough to stop it from moving towards the equator."
The findings were presented at the fall meeting of the American Geophysical Union. The team is continuing to analyze follow-up data to understand the fate of the Dark Spot and Dark Spot Jr. These observations would have not be possible without the Hubble Space Telescope and Outer Planet Atmospheres Legacy (OPAL) program.
"We wouldn't know anything about these latest dark spots if it wasn't for Hubble," OPAL project lead Amy Simon of NASA's Goddard Space Flight Center explained. "We can now follow the large storm for years and watch its complete life cycle. If we didn't have Hubble, then we might think the Great Dark Spot seen by Voyager in 1989 is still there on Neptune, just like Jupiter's Great Red Spot. And, we wouldn't have known about the four other spots Hubble discovered."