In 2020, astronomers using NASA’s Hubble Space Telescope watched a mysterious dark vortex wider than the Atlantic Ocean on Neptune abruptly steer away from a “likely death” on the giant ice planet, some 30 years after NASA’s Voyager 2 probe flew past in 1989 after a nearly 3-billion-mile odyssey, snapping images of two giant storms brewing in Neptune’s southern hemisphere. NASA scientists dubbed the storms “The Great Dark Spot” and “Dark Spot 2.” Five years later, Hubble revealed both the Great Dark Spot and the smaller Dark Spot 2 had vanished.
Hubble first observed he formation of a Great Dark Spot on Neptune in 2018. Like Jupiter’s 350-year-old Great Red Spot, Neptune’s Great Dark Spots are storms that form in areas of high atmospheric pressure. Scientists have seen a total of six dark spots on Neptune over the years. Voyager 2 identified two storms in 1989. Since Hubble launched in 1990, it has viewed four more of these storms.
The storm, reports the Hubble Space Science Institute was born in the planet’s northern hemisphere began drifting southward toward the equator in 2019, where such storms are expected to be obscured from sight. To the surprise of observers, Hubble spotted the vortex unpredictably change direction by August 2020, doubling back to the north.
Equally as puzzling, the storm was not alone. Hubble spotted another smaller dark spot in January 2020 that temporarily appeared near its larger cousin — possibly a piece of the giant vortex that broke off, drifted away, and then disappeared in subsequent observations.
“We are excited about these observations because this smaller dark fragment is potentially part of the dark spot’s disruption process,” said Michael H. Wong of the University of California at Berkeley. “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.”
Hubble Tracks Them Down
The large storm, which is 4,600 miles across, is the fourth dark spot Hubble has observed on Neptune since 1993. Two other dark storms were discovered by the Voyager 2 spacecraft in 1989 as it flew by the distant planet, but they had disappeared before Hubble could observe them. Since then, only Hubble has had the sharpness and sensitivity in visible light to track these elusive features, which have sequentially appeared and then faded away over a duration of about two years each. Hubble uncovered this latest storm in September 2018.
Neptune’s dark vortices are high-pressure systems that can form at mid-latitudes and may then migrate toward the equator. They start out remaining stable due to Coriolis forces, which cause northern hemisphere storms to rotate clockwise, due to the planet’s rotation. (These storms are unlike hurricanes on Earth, which rotate counterclockwise because they are low-pressure systems.) However, as a storm drifts toward the equator, the Coriolis effect weakens and the storm disintegrates. In computer simulations by several different teams, these storms follow a more-or-less straight path to the equator, until there is no Coriolis effect to hold them together. Unlike the simulations, the latest giant storm didn’t migrate into the equatorial “kill zone.”
“It was really exciting to see this one act like it’s supposed to act and then all of a sudden it just stops and swings back,” Wong said. “That was surprising.”
“The Fragment” –Smaller, but Equally Dark Spot
The Hubble observations also revealed that the dark vortex’s puzzling path reversal occurred at the same time that a new spot, informally deemed “dark spot jr.,” appeared. The newest spot was slightly smaller than its cousin, measuring about 3,900 miles across. It was near the side of the main dark spot that faces the equator—the location that some simulations show a disruption would occur.
However, the timing of the smaller spot’s emergence was unusual. “When I first saw the small spot, I thought the bigger one was being disrupted,” Wong said. “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 researchers are continuing to analyze more data to determine whether remnants of dark spot jr. persisted through the rest of 2020.
Formation –Still a Mystery
It’s still a mystery how these storms form, but this latest giant dark vortex is the best studied so far. The storm’s dark appearance may be due to an elevated dark cloud layer and it could be telling astronomers about the storm’s vertical structure.
Another unusual feature of the dark spot is the absence of bright companion clouds around it, which were present in Hubble images taken when the vortex was discovered in 2018. Apparently, the clouds disappeared when the vortex halted its southward journey. The bright clouds form when the flow of air is perturbed and diverted upward over the vortex, causing gases to likely freeze into methane ice crystals. The lack of clouds could be revealing information on how spots evolve, say researchers.
In terms of composition, planetary scientist Wong wrote in an email to The Daily Galaxy, “we think these dark spots are visible because of slight changes in cloud altitude and/or cloud density. We need a mission to Neptune to find out more.”
The Hubble Images
Hubble snapped many of the images of the dark spots as part of the Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project, led by Amy Simon of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, that annually captures global maps of our solar system’s outer planets when they are closest to Earth in their orbits.
OPAL’s key goals are to study long-term seasonal changes, as well as capture comparatively transitory events, such as the appearance of dark spots on Neptune or potentially Uranus. These dark storms may be so fleeting that in the past some of them may have appeared and faded during multi-year gaps in Hubble’s observations of Neptune. The OPAL program ensures that astronomers won’t miss another one.
“We wouldn’t know anything about these latest dark spots if it wasn’t for Hubble,” Simon said. “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.”
The Hubble findings give scientists insights on the inner workings of the poorly-understood ice giant planets but also have implications for studying exoplanets of similar size and composition.
The composite picture below shows images of storms on Neptune from the Hubble Space Telescope (left) and the Voyager 2 spacecraft (right). The Hubble Wide Field Camera 3 image of Neptune, taken in Sept. and Nov. 2018, shows a new dark storm (top center). In the Voyager image, a storm known as the Great Dark Spot (GDS) is seen at the center. It is about 13,000 km by 6,600 km in size — as large along its longer dimension as the Earth. The white clouds seen hovering in the vicinity of the storms are higher in altitude than the dark material.
Differs from Jupiter’s Great Red Spot
“Although Neptune’s dark spots are anticylconic (high pressure) vortices just like the Great Red Spot (GRS) on Jupiter, there are two huge differences,” continued Wong.
The first, writes Wong: “Jupiter’s GRS is trapped at a constant latitude by Jupiter’s alternating global-scale east-west jets, while Neptune’s much broader retrograde equatorial jet allows dark spots to drift north-south.”
“Second,” continues Wong in his email to The Daily Galaxy, “the GRS is well over a century old, but work led by a UC Berkeley undergrad (Hsu et al. 2019) demonstrated that Neptune’s dark spots only persist for one to six years. We need higher-cadence observational data to understand the life cycles of Neptune’s dark spots.”
Planetary scientists hope to next study changes in the shape of the vortex and wind speed in storms that form dark spots. “We have never directly measured winds within Neptune’s dark vortices, but we estimate the wind speeds are in the ballpark of 328 feet (100 meters) per second, quite similar to wind speeds within Jupiter’s Great Red Spot,” said Wong. More frequent observations using Hubble will help paint a clearer picture of how storm systems on Neptune evolve, he said.
Image credits; NASA/ESA/GSFC/JPL