In 1989, a rapid fly-by of the Voyager 2 probe provided a fleeting glimpse of Neptune’s weather, including two oval dark spots that resembled giant hurricanes. The more prominent storm, dubbed a Great Dark Spot, was as large as Earth. The launch of the Hubble Space Telescope in 1990 allowed astronomers to regularly monitor Neptune’s atmosphere from a distance. A series of Hubble images has revealed, for the first time, how Great Dark Spots develop on Neptune.
Hubble’s high-definition images showed a notable Great Dark Spot in 2018, similar in size and appearance to the 1989 storm. White streaks near the spot appeared to be clouds of methane ice crystals hovering above the dark storm, similar to the lenticular clouds that cap high mountains on Earth. While studying Hubble images of a smaller dark spot that appeared in 2015, scientists from the NASA Goddard Space Flight Center and the University of California, Berkeley noted similar white streaks at the location where the 2018 spot would later appear. The significance of this evidence was only recognized in hindsight. “We were so busy tracking this smaller storm from 2015, that we weren’t necessarily expecting to see another big one soon,” said Amy Simon, lead author of the recently published findings.
Looking back through past Hubble images, the astronomers found that the precursor clouds rose in brightness through 2016 and 2017, then dimmed as the 2018 Great Dark Spot appeared. Computer simulations, which indicated that white companion clouds are brighter when storms are deeper, concluded that the 2018 Great Dark Spot began in 2015 to 2017, before it was obviously visible. The storm was deeper in Neptune’s atmosphere than previously thought. The researchers also estimated that large Neptunian storms appear every four to six years, last an average of two years, and can remain as long as six years.
Comparing Neptune’s storms with those on other planets helps illuminate the many factors that influence planetary weather. Hurricanes on Earth are smaller and last only days or weeks as they move over land, while Neptune’s deep atmosphere meets no well-defined rocky boundary to drain energy from a storm. Jupiter’s Great Red Spot—which is kept spinning by narrow jet streams that flank its north and south edges—is not only larger than Neptune’s spots, but far more long-lived (it has been visible since at least 1830). Neptune’s prevailing wind belts are much broader than its Great Dark Spot and do not affect it as much.
Further monitoring by Hubble is needed to fully describe the dynamics of powerful planetary storms. (Geophysical Research Letters)