Neptune’s clouds make a surprise disappearing act

For the first time in nearly three decades of observations, the clouds seen on Neptune have almost disappeared. 1994-2022 images of the big blue planet captured from Maunakea on the island of Hawaii through the lens of the WM Keck Observatory, along with views from space using NASA’s Hubble Space Telescope, show that the clouds have almost disappeared except for the South Pole.

The observations, which are published in the magazine Icarusthey also reveal a connection between the disappearing clouds of Neptune and the solar cycle—A surprising finding given that Neptune is the farthest planet from the sun and receives only 1/900 of the sunlight we have on Earth.

A team of astronomers led by the University of California (UC) at Berkeley discovered that the abundance of clouds normally seen in the ice giant’s mid-latitudes began to fade in 2019.

“I was surprised by how quickly the clouds disappeared on Neptune,” said Imke de Pater, professor emeritus of astronomy at UC Berkeley and lead author of the study. “We basically saw the cloud activity go down within a few months.”

“Even four years later, the images we took last June showed that the clouds had not returned to their previous levels,” said Erandi Chavez, a graduate student at Harvard University’s Center for Astrophysics who lead the study when he was an astronomy student. at UC Berkeley. “This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not as dramatic and prolonged.”

To monitor the evolution of Neptune’s appearance, Chavez and his team analyzed images taken from 1994 to 2022 using the Keck Observatory’s second-generation near-infrared camera (NIRC2) combined with their system of adaptive optics (since 2002), as well as observations from the Lick Observatory. 2018-2019) and the Hubble Space Telescope (since 1994).

In recent years, the Keck Observatory observations have been supplemented by images taken as part of the Keck Observatory Twilight Observing Program and by Hubble Space Telescope images taken as part of the Outer Planet Atmospheres Program Legacy (OPAL).

The data revealed an intriguing pattern between changes in Neptune’s cloud cover and the solar cycle, the period in which the magnetic field of the sun it rotates every 11 years, causing solar radiation levels to fluctuate. When the sun emits more intense ultraviolet (UV) light, specifically strong Lyman-alpha hydrogen emission, more clouds appear on Neptune about two years later. The team also found a positive correlation between the number of clouds and the brightness of the ice giant based on sunlight reflected from it.

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the sun’s cycle,” de Pater said. “Our findings support the theory that the sun’s UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds.”

The connection between the solar cycle and Neptune’s cloud weather pattern is derived from 2.5 cycles of cloud activity recorded over the 29 years of Neptunian observations. During this time, the planet’s reflectivity increased in 2002 (brightness maximum), then dimmed (brightness minimum) in 2007, brightened again in 2015, then dimmed in 2020 to its lowest level ever observed, which is when most of the clouds disappeared. .

Changes in Neptune’s brightness caused by the sun seem to rise and fall relatively in sync with the ebb and flow of clouds on the planet.

However, more work is needed to unpack this correlation given the complexity of other factors; 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. Neptune’s storms that rise from the deep atmosphere affect cloud cover, but are not related to photochemically produced clouds and thus can complicate studies of correlation with the solar cycle. Continued observations of Neptune are also needed to see how long the current near cloudlessness will last.

This discovery adds to exciting observations of the blue world’s active and chaotic atmosphere, which features methane clouds that are whipped by supersonic winds – the fastest wind speeds recorded anywhere in our solar system. One of the earliest and most striking images was captured by NASA’s Voyager 2 spacecraft during its flyby of Neptune in 1989, revealing a massive storm system called the Great Dark Spot. Other storms i dark spots have been detected since then, notably a large equatorial storm in 2017 and a large dark spot at northern latitudes in 2018.

“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,” said Carlos Alvarez, Keck Observatory staff astronomer and co-author of the study. “Technological advances, as well as our twilight observing program, have allowed 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 continues to monitor Neptune’s cloud activity. The recent images taken in June 2023 were obtained at the same time that NASA’s James Webb Space Telescope (JWST) captured near- and mid-infrared images.

“We’ve seen more clouds in the most recent images, particularly at northern latitudes and high altitudes, as expected from the observed increase in solar UV flux over the past two years,” de Pater said.

The combined data from JWST and the Keck Observatory will enable further research into the physics and chemistry that lead to Neptune’s dynamic appearance, which in turn can help deepen astronomers’ understanding of not only Neptune, but also of the exoplanets.