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.
Ghostly Objects
Hubble was the first to observe Neptune’s Great Dark Spot formation 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. Over the years, scientists have identified six dark spots on Neptune. Voyager 2 detected two storms in 1989. Four more storms have been observed by Hubble since its launch in 1990.
According to the BBC, the storm is now over 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. Hubble, to the delight of many observers, saw the vortex change direction unpredictably by August 2020 and double back to the North.
The storm wasn’t the only one puzzling. 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. WongUniversity 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
This large storm is 4,600 miles wide and is the fourth dark spot Hubble observed on Neptune. Voyager 2’s spacecraft discovered two other dark storms in 1989 while passing by Neptune, but they disappeared before Hubble could see them. Hubble is the only one who has been able to see these features clearly and have had the sensitivity and sharpness in visible light for about two years. Hubble discovered this latest storm in September 2018.
Coriolis Forces
Neptune’s dark vortices are high-pressure systems that can form at mid-latitudes and may then migrate toward the equator. They remain stable at first due toCoriolis forces which cause northern hemisphere storms to rotate clockwise, due to the planet’s rotation. These storms rotate counterclockwise, unlike hurricanes that are located on Earth. As a storm moves towards the equator, however, the Coriolis effects weakens and the storm breaks apart. These storms are shown in computer simulations from several teams. They follow a straight line to the equator until there is no Coriolis effect. 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. It was approximately 3,900 miles smaller than its predecessor. 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 is still a 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.”
Researchers continue to analyze data to see if dark spot jr. remains through 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.
The absence of bright companion cloud around the dark spot is another unusual characteristic of the vortex. This was evident in Hubble images that were taken in 2018 when it was discovered. The vortex stopped its southward travel and the clouds vanished. When the flow of air is disturbed and diverted upwards over the vortex, bright clouds can form. The gases will likely freeze to methane ice crystals. Researchers believe that the lack of clouds could reveal information about how spots develop.
Planetary scientist is the best in terms of composition WongSend 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 took many of the photos of dark spots as part the “The Hubble Project”. 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.”
Hubble observations provide insight into the inner workings to the poorly understood ice giant planets. They also have implications for studying exoplanets with similar sizes and compositions.
Below is a composite image showing images of Neptune storms from the Voyager 2 and Hubble Space Telescope. A new dark storm is shown in the Hubble Wide Field Camera 3 photo of Neptune taken in Sept. and November 2018. The Voyager image shows a storm called the Great Dark Spot (GDS). It is about 13,000 km by 6,600 km in size — as large along its longer dimension as the Earth. The white clouds that hover in the area of storms are higher than the darker 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.”
Scientists from the Planetary Institute hope to investigate changes in the vortex shape and wind speed during storms that form dark patches. “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. He said that Hubble’s more frequent observations will allow for a better understanding of the evolution of storm systems on Neptune.
Avi Shporer Research Scientist with MIT’s Kavli Institute for AstrophysicsAnd Space Research via Space Telescope Science Institute, Michael Wong and American Geophysical Union.
Images courtesy NASA/ESA/GSFC/JPL
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Avi Shporer Research Scientist, MIT Kavli Institute for Astrophysics and Space Research. A Google ScholarAvi was once a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL). His motto, not surprisingly, is a quote from Carl Sagan: “Somewhere, something incredible is waiting to be known.”
