For the first time in nearly three decades of observations, the clouds that usually grace Neptune's atmosphere have all but disappeared.
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This startling observation was made using images captured from Maunakea on Hawaiʻi Island through the lens of the W. M. Keck Observatory, and from space via NASA's Hubble Space Telescope. Remarkably, these images show that Neptune's clouds are nearly gone, with the exception of those at the south pole.
A Surprising Connection to the Solar Cycle
The observations, published in the journal Icarus, reveal a surprising connection between Neptune's disappearing clouds and the solar cycle. This is a significant find, given that Neptune is the farthest major planet from the sun and receives only 1/900th of the sunlight that Earth does. A team of astronomers led by the University of California (UC) Berkeley discovered that the abundance of clouds normally seen at Neptune's mid-latitudes began to fade in 2019.
Rapid and Unprecedented Disappearance
"I was surprised by how quickly clouds disappeared on Neptune," said Imke de Pater, emeritus professor of astronomy at UC Berkeley and senior author of the study. "We essentially saw cloud activity drop within a few months," she added. Erandi Chavez, a graduate student at Harvard University's Center for Astrophysics, who led the study when she was an undergraduate at UC Berkeley, noted that "even four years later, the images we took this past June showed the clouds haven't returned to their former levels."
The Sun’s Role in Neptune’s Clouds
The data revealed an intriguing pattern between changes in Neptune's cloud cover and the solar cycle, which is the period when the sun's magnetic field flips every 11 years, causing levels of solar radiation to fluctuate. The team 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.
Continued Observations and Future Research
The research team continues to track Neptune's cloud activity, with recent images taken in June 2023 showing more clouds at northern latitudes and at high altitudes. The combined data from the James Webb Space Telescope (JWST) and Keck Observatory will enable further investigations into the physics and chemistry that lead to Neptune's dynamic appearance.
This discovery adds to the exciting observations of Neptune's wildly active and chaotic atmosphere, which features methane clouds whipped around by supersonic winds—the fastest wind speeds recorded anywhere in our solar system.
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