Images taken with the Hubble Space Telescope document the emergence of a large dark spot on Neptune for the first time, researchers report in a new study.
Like Jupiter's Great Red Spot, the large dark spots of Neptune are storms that arise from areas of high atmospheric pressure. In contrast, storms on Earth form areas of low pressure.
Scientists have seen six dark spots on Neptune over the years. Voyager 2 identified two storms in 1989. Since Hubble observed four more of these storms in 1990.
In the new study, planetary scientists Hubble analyzed photos of the ice giant that had been taken in recent years and saw the growth of a new Great Dark Spot, which became visible in 2018.
Investigating cloud clouds that surfaced two years before the new Great Dark Spot, researchers conclude that dark spots are much deeper in the atmosphere of Neptune than previously thought.
Hubble images also helped researchers determine how often Neptune gets dark spots and how long they last. The new findings give the scientists insights into the inner workings of the little-understood giant ice planets, but also have an impact on the study of exoplanets of similar size and composition.
"If you study the exoplanets and want to understand how they work, then you know we really need to understand our planets," said planetary scientist NASA's Planetarium Space Flight Center in Greenbelt, Maryland, and principal author of the new one Study published today in the AGU journal Geophysical Research Letters . "We have so little information about Uranus and Neptune."
A History of Dark Spots
Scientists first saw a huge dark spot on Neptune in 1989 when NASA's Voyager 2 probe flew past the mysterious blue planet. As the spaceship passed, it shot images of two giant storms brewing in the southern hemisphere of Neptune. Scientists called the storms "The Great Dark Spot" and "Dark Spot 2".
Just five years later, the Hubble Space Telescope made sharp images of Neptune that revealed both the Earth-sized Great Dark Spot and the smaller Dark Spot 2 had disappeared.
"It was certainly a surprise," said Simon. "We were used to seeing Jupiter's Great Red Spot, which had probably been there for more than a hundred years."
A new Great Dark Spot was released in 2018 on Neptune, which in size and shape was almost that of Voyager in 1989. Simon and her colleagues analyzed Hubble images of a smaller dark spot that appeared in 2015 when they saw small, bright white clouds in them discovered in the region in which later would appear the Great Dark Spot 2018.
"We were so busy tracking this little 2015 storm that we did not expect another big one so quickly," Simon said.
The high clouds are made of methane ice crystals that give them their characteristic bright white color. Scientists suspect that these methane clouds accompany the storms, which form dark spots, and hover above them as lenticular clouds cover high mountains on the earth.
Lifetime of a Spot
Simon and co-authors Michael Wong and Andrew Hsu at the University of California Berkeley tracked the methane clouds from 2016 to 2018. The clouds were the brightest in 2016 and 2017 before The New Great Dark Spot became visible.
Computer models of Neptune's atmosphere have shown that the brighter the companion's clouds are, the brighter the storm. That these white clouds had appeared two years before the Great Dark Spot, and that they had lost their brightness when they became visible, suggests that dark spots might be much deeper in the atmosphere of Neptune than previously thought.
Simon, Wong, and Hsu Also used images from Hubble and Voyager 2 to determine how long these storms are and how often they are occurring. They report in a second study, published today in the Astronomical Journal that they suspect new storms every four to six years on Neptune. Each storm can last up to six years, though the lifetime is likely after two years.
Dark against Red
The new findings show how the dark, dark spots of Neptune are different from Jupiter's. Great red spot. The Great Red Spot has been observed since at least 1830 and could be up to 350 years old. Thin jet streams on Jupiter prevent the Great Red Spot from breaking up and changing latitude. it's about Jupiter, but it does not move north or south.
But Neptune's winds are in much broader bands around the planet, so that storms like the Great Dark Spot drift slowly across latitudes. These storms usually float between western equatorial windjets and eastward currents in the higher latitudes before being torn apart by strong winds.
The planetary scientists hope to next investigate changes in the shape of the vortex and wind speed in storms that form dark spots. "We have never directly measured winds in the dark vortexes of Neptune, but we estimate that the wind speeds are 100 meters per second, much like the wind speeds in Jupiter's Great Red Spot," Wong said. More frequent observations with Hubble would give a clearer picture of the evolution of the storm systems on Neptune, he said.
Hubble reveals dynamic atmospheres of Uranus, Neptune
A. A. Simon et al., Creation of a New Large Dark Spot on Neptune in 2018, Geophysical Research Letters (2019). DOI: 10.1029 / 2019GL081961