The clouds on Neptune perform a surprise disappearing act

The clouds observed on Neptune have almost completely disappeared for the first time in over three decades of measurements. With the exception of the south pole, clouds are almost nonexistent in images of the huge blue planet taken between 1994 and 2022 from Maunakea on Hawaii Island by the W. M. Keck Observatory and from orbit by NASA's Hubble orbit Telescope.

The data, which were reported in the journal Icarus, also point to a relationship between the solar cycle and Neptune's evaporating clouds. This is an unexpected finding considering that Neptune is the most distant main planet from the sun and only gets 1/900th of the sunlight that Earth receives on a daily basis.

2019 saw the mid-latitudes of the ice giant lose some of its usual profusion of clouds, according to scientists at the University of California (UC) Berkeley.

Imke de Pater, senior author of the study and emeritus professor of astronomy at UC Berkeley, remarked, "I was shocked by how swiftly clouds dissipated on Neptune. "We basically noticed a decline in cloud activity within a few months."

"Even four years later, the images we took this past June showed the clouds haven't returned to their former levels," said Erandi Chavez, a PhD student at the Center for Astrophysics at Harvard University who oversaw the project as an undergraduate astronomy student at UC Berkeley. This is really exciting and surprising, especially considering how brief and mild the prior low cloud activity on Neptune was.

Chavez and her team used images from the Hubble Space Telescope (since 1994) and the second-generation Near-Infrared Camera (NIRC2) at Keck Observatory, along with observations from Lick Observatory (2018–2019) and the Hubble Space Telescope (since 1994), to track the evolution of Neptune's appearance.

In recent years, photos from the Keck Observatory's Twilight Observing Program and Hubble Space Telescope photographs from the Outer Planet Atmospheres Legacy (OPAL) program have been used to supplement the observations made by the Keck Observatory.

The information showed an intriguing correlation between variations in Neptune's cloud cover and the solar cycle, which is the time during which the sun's magnetic field alternates directions every 11 years and results in variations in solar radiation levels. About two years later, additional clouds start to emerge on Neptune as a result of the sun's increased ultraviolet (UV) light emissions, particularly the powerful hydrogen Lyman-alpha emission. The scientists also discovered a link between the quantity of clouds and the brightness of the ice giant as reflected sunlight.

According to de Pater, "These extraordinary data provide us with the strongest evidence yet that Neptune's cloud cover correlates with the sun's cycle." The hypothesis that the sun's UV radiation, when powerful enough, may be starting a photochemical process that creates Neptune's clouds is supported by our observations.

The 2.5 cycles of cloud activity seen throughout the 29-year period of Neptunian observations are used to infer the relationship between the solar cycle and the cloudy weather pattern on Neptune. During this time, the planet's reflectivity increased in 2002 (brightness maxima), decreased in 2007 (brightness minima), brightened once more in 2015, darkened to the lowest level ever seen in 2020, which coincided with the disappearance of the majority of the clouds.

The variations in Neptune's brightness brought on by the sun seem to rise and set roughly in time with the arrival and departure of clouds from the planet.

Given the intricacy of other aspects, additional research is needed to fully understand this association. For instance, while an increase in UV rays may increase clouds and haze, it may also darken them, lowering Neptune's total brightness. Storms on Neptune that rise up from the deep atmosphere have an impact on the cloud cover but are unrelated to clouds that are created by photochemistry, which may make connection studies with the solar cycle more difficult. In order to determine how long the present near-absence of clouds will endure, more observations of Neptune are required.

This finding adds to the thrilling discoveries of the extraordinarily turbulent atmosphere of the blue-hued world, which includes methane clouds being thrashed around by supersonic winds—the highest wind speeds yet observed in our solar system. During its 1989 flyby of Neptune, NASA's Voyager 2 spacecraft obtained one of the first and most spectacular photos, depicting a gigantic storm system known as the "Great Dark Spot." Since then, more storms and dark spots have been observed, including a major storm in the equatorial region in 2017 and a sizable dark spot in the northern hemisphere 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 from us," said Carlos Alvarez, a staff astronomer at Keck Observatory and a co-author of the study. We have been able to restrict Neptune's atmospheric models, which are essential to understanding the relationship between the ice giant's climate and the solar cycle, thanks to technological advancements as well as our Twilight Observing Program.

The study group is still monitoring the cloud activity around Neptune. The most current pictures were taken in June 2023 at the same time as NASA's James Webb Space Telescope (JWST) was taking pictures in the near- and mid-infrared.

According to the reported increase in solar UV output during the previous two years, "we have seen more clouds in the most recent images, in particular at northern latitudes and at high altitudes," said de Pater.

The combined JWST and Keck Observatory data will allow for more in-depth research into the physics and chemistry that underlies Neptune's dynamic appearance, which might improve astronomers' knowledge of both exoplanets and Neptune.