Astronomers are witnessing a so-called “Great Dark Spot” in the midst of formation on Neptune for the first time, reports Mark Wall at Space.com.
When NASA’s Voyager 2 probe zipped past the planet Neptune for the first time in 1989, it captured images of two Great Dark Spots, massive storms rolling through the thick atmosphere of the ice giant that dissipated less than five years later. Since then, the Hubble Space Telescope has captured three more of the ephemeral dark vortices. Now, astronomers say they inadvertently captured images of the birth of another one, which they estimate is larger than Earth, the team reports in the journal Geophysical Research Letters.
The images were captured as part of the Outer Planet Atmospheres Legacy (OPAL) project, a long-term endeavor that uses Hubble to track seasonal changes on Neptune and Uranus. Astronomers were looking at images of a smaller dark spot that appeared on the planet in 2015 when they noticed small, bright-white methane clouds in the region of the planet that corresponded to the latest Dark Spot, which emerged last fall. They then tracked cloud formation in that area between 2016 and 2018, when the new Great Dark Spot became visible.
“The previous dark spots were all discovered fully formed, with no observations to compare with from prior to its discovery,” lead author Amy Simon, planetary scientist at NASA’s Goddard Space Flight Center and leader of the OPAL project tells George Dvorsky at Gizmodo. “In this case we have several years of Hubble data from before we saw the new spot. Tracing backwards, we can see hints of high clouds in that area in those earlier years.”
The formation of the dark spot over several years gives researchers insight into how the storms develop. The white, high-altitude methane clouds are believed to hover above the storms forming below them. Their presence years before the Great Dark Spot appears suggests that the storm develops deeper in Neptune’s atmosphere before reaching an altitude that can be viewed from the Hubble.
In a companion paper published in The Astronomical Journal, the same team calculated that the storms likely appear every four to six years. They estimate that each new storm can last up to six years max, but suggest a two-year lifespan is probably closer to the average. According to a NASA press release, the team ran computer simulations and found that about 70 percent of storms last one year and 85 to 95 percent of storms last two years. The team also estimates the winds blow at 328 feet per second, though that's a number they would like to refine.
Understanding how Neptune’s thick atmosphere works is actually pretty important for planetary scientists. Mini-Neptunes—which are about 2 to 3.5 times bigger than Earth—are one of the most common types of planets discovered orbiting distant stars.
“If you study the exoplanets and you want to understand how they work, you really need to understand our planets first,” Simon says in the press release. “We have so little information on Uranus and Neptune.”