Neptune’s clouds have disappeared, and scientists think they know why

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Astronomers have been puzzling over a mystery on Neptune, and now they think they have unlocked its secret.

The ice giant’s ghostly, cirrus-like clouds largely disappeared four years ago. Today, just a patch hovers over the planet’s south pole.

Thanks to an analysis of nearly three decades’ worth of Neptune observations captured by three space telescopes, scientists have determined the ice giant’s diminished clouds may indicate that shifts in their abundance are in sync with the solar cycle, according to a recent study published in the journal Icarus.

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle,” said senior study author Imke de Pater, professor emeritus of astronomy at the University of California, Berkeley, in a news release. “Our findings support the theory that the Sun’s (ultraviolet) rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds.”

During the solar cycle, the level of activity in the sun’s dynamic magnetic fields waxes and wanes. The magnetic field flips every 11 years as it becomes more tangled like a ball of yarn, according to NASA. When there is heightened activity on the sun, more intense ultraviolet radiation bombards the solar system.

With data from NASA’s Hubble Space Telescope, the W.M. Keck Observatory in Hawaii and the Lick Observatory in California, scientists observed 2.5 cycles of cloud activity over the 29-year period of Neptune observations — during which the planet’s reflectivity increased in 2002 and dimmed in 2007. Neptune brightened again in 2015, before darkening in 2020 to the lowest level ever seen. That’s when most of the cloud cover faded away.

“Even now, four years later, the most recent images we took this past June still show the clouds haven’t returned to their former levels,” said the study’s lead author Erandi Chavez, a doctoral student at the Center for Astrophysics, Harvard & Smithsonian, in a statement.

The findings are “extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged,” Chavez added.

A surprising correlation

The authors also found that two years after the cycle’s peak, more clouds appeared on Neptune — and that the more clouds there were, the brighter Neptune was from the sunlight reflecting off it. That connection was “surprising to planetary scientists because Neptune is our solar system’s farthest major planet and receives sunlight with about 0.1% of the intensity Earth receives,” according to NASA. The findings also contradict the idea of the clouds being affected by Neptune’s four seasons, which each last about 40 years.

“This is a very interesting paper and a very nice piece of good, old-fashioned detailed detective work,” said Patrick Irwin, a professor of planetary physics at the University of Oxford who wasn’t involved in the study, via email. “This new paper covers a longer time frame than previous studies and shows a convincing correlation of the observed cloud cover with the solar UV brightness.”

But there is a two-year time lag between the solar cycle’s peak and Neptune’s increased abundance of clouds. The authors think this gap could be explained by the photochemistry that occurs high in the planet’s upper atmosphere, which takes time to produce clouds.

The relationship between increased brightness of the sun and cloud formation could be due to the generation of ionized molecules that can act as cloud condensation nuclei and help initiate condensation, Irwin said.

“It’s fascinating to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away from us,” said study coauthor Carlos Alvarez, a staff astronomer at Keck Observatory, in a statement. “Advances in technology and observations have enabled us to constrain Neptune’s atmospheric models, which are key to understanding the correlation between the ice giant’s climate and the solar cycle.”

The research team is still monitoring Neptune’s cloud activity since more UV light could also darken the planet’s clouds, lowering its overall brightness, the authors said.

Additionally, Neptune storms rising from the deep atmosphere do influence the planet’s cloud cover — but aren’t related to clouds formed in the upper atmosphere. That variable could interfere with studies looking at correlations between photochemically produced clouds and the solar cycle. More research could also suggest how long the near absence of clouds on Neptune might last.

These pursuits, in turn, could not only expand astronomers’ knowledge of Neptune but also help researchers better understand the many exoplanets outside the solar system believed to have characteristics similar to the ice giant, according to NASA.

The study also “underlines the need to keep monitoring the solar system planets,” Irwin said. “It’s only by observing these planets at regular intervals that it is possible to build up a long-term, reliable dataset to probe for these periodic variations.”

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