All of Neptune's clouds have disappeared, astronomers announced this unexpected update on Thursday, August 17. For the first time in three decades, Neptune is almost completely cloud-free.
Scientists observed detailed images of the planet between the years 1994 and 2022, they noticed a strange pattern beginning in 2019. Around the planet's mid-latitudes, cloud coverage seemed to start fading. Eventually, all evidence of clouds totally vanished, Space.com reported.
Since 2019, scientists saw only one patch of wispy white drifting around the planet's south pole.
"I was surprised by how quickly clouds disappeared on Neptune," Imke de Pater, an emeritus professor of astronomy at the University of California, Berkeley and senior author of a study on the findings, said in a statement. "We essentially saw cloud activity drop within a few months."
The scientists decided to dig deeper. To monitor the evolution of Neptune's appearance, Chavez and her team analyzed Keck Observatory images taken from 2002 to 2022, the Hubble Space Telescope archival observations beginning in 1994, and data from the Lick Observatory in California from 2018 to 2019. They explained that Neptune's clouds are inextricably linked with the way our sun behaves during its 11-year-long activity cycle.
The scientist said that the pattern somewhat matches up with the sun's 11-year cycle of activity, albeit with a two-year lag between the sun's extremes and Neptune's. It seems that when the sun is most active, more clouds begin to form on Neptune, and when it is least active, Neptune's clouds dissipate, a New Scientist report said.
According to NASA's release, when it's stormy weather on the Sun, more intense ultraviolet (UV) radiation floods the solar system. The team found that two years after the solar cycle's peak, an increasing number of clouds appear on Neptune. The team further found a positive correlation between the number of clouds and the ice giant's brightness from the sunlight reflecting off it.
"These remarkable data give us the strongest evidence yet that Neptune's cloud cover correlates with the Sun's cycle," said de Pater. "Our findings support the theory that the Sun's UV rays, when strong enough, maybe triggering a photochemical reaction that produces Neptune's clouds."
Scientists discovered the connection between the solar cycle and Neptune's cloudy weather pattern by looking at 2.5 cycles of cloud activity recorded over the 29-year span of Neptunian observations. During this time, the planet's reflectivity increased in 2002 and then dimmed in 2007. Neptune became bright again in 2015, then darkened in 2020 to the lowest level ever observed, which is when most of the clouds went away.
The changes in Neptune's brightness caused by the Sun appear to go up and down relatively in sync with the coming and going of clouds on the planet. However, there is a two-year time lag between the peak of the solar cycle and the abundance of clouds seen on Neptune. The chemical changes are caused by photochemistry, which happens high in Neptune's upper atmosphere and takes time to form clouds.
"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 Carlos Alvarez, staff astronomer at Keck Observatory and co-author of the study. "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."
However, more work is necessary. For example, while an increase in UV sunlight could produce more clouds and haze, it could also darken them, thereby reducing Neptune's overall brightness. Storms on Neptune rising up from the deep atmosphere affect the cloud cover, but are not related to photochemically produced clouds, and hence may complicate correlation studies with the solar cycle. Continued observations of Neptune are also needed to see how long the current near-absence of clouds will last.
The research team continues to track Neptune's cloud activity. "We have seen more clouds in the most recent Keck images that were taken during the same time NASA's James Webb Space Telescope observed the planet; these clouds were in particular seen at northern latitudes and at high altitudes, as expected from the observed increase in the solar UV flux over the past approximately 2 years," said de Pater.
The combined data from Hubble, the Webb Space Telescope, Keck Observatory, and the Lick Observatory will enable further investigations into the physics and chemistry that lead to Neptune's dynamic appearance, which in turn may help deepen astronomers' understanding not only of Neptune but also of exoplanets since many of the planets beyond our solar system are thought to have Neptune-like qualities, NASA said.
The findings are published in the journal Icarus.
