The Unanticipated Vanishing of Neptune’s Clouds

A photograph of Neptune, taken using the Keck II telescope on June 21, 2023, reveals a striking lack of clouds with the exception of the area near the southern pole. Credit goes to researchers Imke de Pater, Erandi Chavez, Erin Redwing (UC Berkeley), and W. M. Keck Observatory.

For the first time in three decades, Neptune’s clouds have all but disappeared. Scientists have observed a potential connection between the planet’s cloud behavior and solar cycles, a puzzling discovery considering Neptune’s great distance from the Sun. Current research endeavors to elucidate this enigma.

Images from 1994 to 2022 of the massive blue planet, taken from Maunakea in Hawaiʻi Island through W. M. Keck Observatory’s lens and space views from NASA’s Hubble Space Telescope, show the clouds nearly gone, except near the south pole. These observations, published in the journal Icarus, also highlight a relationship between the disappearance of Neptune’s clouds and the solar cycle – a finding that is unexpected, as Neptune, being the most remote major planet from the Sun, receives only 1/900th of the sunlight Earth receives.

Cloud Observations and Unexpected Discoveries

A group of astronomers from the University of California (UC) Berkeley found that the normal profusion of clouds at the icy giant’s mid-latitudes began to diminish in 2019.

Imke de Pater, emeritus professor of astronomy at UC Berkeley and the senior study author, expressed surprise at how swiftly the clouds on Neptune vanished. She noted that cloud activity declined within several months.

Dramatic transformations in Neptune’s appearance were seen from late 2019 and remained until June 2023. Images captured with the NIRC2 and adaptive optics system on the Keck II Telescope demonstrated numerous cloud features in latitudinal bands before 2002 through late 2019. After that, clouds seemed nearly absent except near the south pole. These images were displayed using an Asinh function that, like a log-scale display, reduces the contrast between features; if displayed on a linear scale, only the most luminous features would be seen. Credit: Imke de Pater, Erandi Chavez, Erin Redwing (UC Berkeley)/W. M. Keck Observatory.

“Even after four years, the photographs we took last June revealed that the clouds have not reverted to previous levels,” stated Erandi Chavez, a Harvard University graduate student at the Center for Astrophysics, who spearheaded the study as an undergraduate astronomy student at UC Berkeley. “This is incredibly thrilling and unforeseen, particularly as Neptune’s last period of reduced cloud activity was not nearly as intense or enduring.”

Methods of Observing Neptune’s Clouds

To track Neptune’s appearance over time, Chavez and her team examined images from 1994 to 2022, using Keck Observatory’s Near-Infrared Camera (NIRC2), paired with its adaptive optics system (since 2002), as well as observations from Lick Observatory (2018-2019) and the Hubble Space Telescope (since 1994). Recent Keck Observatory observations have been supplemented by images from Keck Observatory’s Twilight Observing Program and Hubble Space Telescope images from the Outer Planet Atmospheres Legacy (OPAL) program.

Solar Cycle and its Influence on Neptune’s Clouds

The data uncovered a fascinating pattern between alterations in Neptune’s cloud cover and the solar cycle, an 11-year period during which the Sun’s magnetic field inverts, causing fluctuations in solar radiation levels. When the Sun radiates more intense ultraviolet (UV) light, particularly strong hydrogen Lyman-alpha emission, Neptune sees a rise in clouds about two years later. The research team also discovered a positive correlation between the quantity of clouds and the brightness of the ice giant reflected by sunlight.

De Pater stated, “These extraordinary data provide us the most compelling evidence thus far that Neptune’s cloud cover is correlated with the Sun’s cycle.” She added, “Our findings reinforce the hypothesis that when the Sun’s UV rays are sufficiently powerful, they may initiate a photochemical reaction that forms Neptune’s clouds.”

A sequence of Hubble Space Telescope images depicts the fluctuation in Neptune’s cloud cover. This near-30-year series of observations illustrates that the number of clouds tends to grow following a solar cycle’s peak – an 11-year rhythmic rise and fall in the Sun’s activity level. The vertical axis plots the Sun’s level of ultraviolet radiation, and the 11-year cycle is plotted from 1994 to 2022 along the bottom. The Hubble observations at the top clearly show a link between cloud abundance and solar activity peak. The chemical changes are driven by photochemistry, occurring high in Neptune’s upper atmosphere, and require time to form clouds. Credit: NASA, ESA, LASP, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley).

The connection between the solar cycle and Neptune’s cloudy weather is derived from 2.5 cycles of cloud activity observed over 29 years. During this period, the planet’s reflectivity rose in 2002 (reaching brightness maxima), dimmed in 2007 (brightness minima), brightened again in 2015, and darkened in 2020 to the lowest level ever seen, corresponding with most of the clouds’ disappearance. The alteration in Neptune’s brightness caused by the Sun seems to align relatively with the appearance and disappearance of clouds on the planet.

Further investigation is needed to unravel this correlation due to other complex factors; for instance, a UV sunlight increase could produce more clouds and haze, but also darken them, thus decreasing Neptune’s overall brightness. Storms arising from Neptune’s deep atmosphere can affect cloud cover but are unrelated to photochemically-produced clouds, which might complicate correlation studies with the solar cycle. Continual observation of Neptune is required to determine how long the current near-absence of clouds will persist.

Neptune’s Turbulent Atmosphere

This discovery contributes to the intriguing observations of the blue planet’s highly active and turbulent atmosphere, characterized by methane clouds swept by supersonic winds – the most rapid wind speeds recorded in our solar system. One of the earliest and most prominent images was taken by NASA’s Voyager 2 spacecraft during its 1989 Neptune flyby, uncovering a massive storm system called the “Great Dark Spot.” Additional storms and dark spots have been detected since, notably a large equatorial storm in 2017 and a significant dark spot at northern latitudes in 2018.

Carlos Alvarez, staff astronomer at Keck Observatory and co-author of the study, commented, “It’s captivating to utilize telescopes on Earth to investigate the climate of a world over 2.5 billion miles away from us.” He continued, “Technological advancements, as well as our Twilight Observing Program, have empowered us to constrain Neptune’s atmospheric models, crucial for understanding the correlation between the ice giant’s climate and the solar cycle.”

Continual Research and Future Probes

The research group continues to monitor Neptune’s cloud behavior. The recent images captured in June 2023 were taken concurrently with NASA’s James Webb Space Telescope (JWST), capturing near- and mid-infrared images.

De Pater observed more clouds in the latest images, especially at northern latitudes and high altitudes, as anticipated from the observed rise in solar UV flux over the past approximately two years.

The amalgamated data from JWST and Keck will provide a more coherent understanding of the ice giant’s atmosphere, along with its delicate link to the Sun. Future missions to Neptune, which is being contemplated by NASA and the European Space Agency, could validate these findings.

NASA’s Outer Planet Atmospheres Legacy program (OPAL), the Heising-Simons Foundation, the NSF, the Gemini Observatory, and the UC Berkeley Chancellor’s Fund have funded the research behind the paper published in Icarus, which consists of a collaboration between the University of California, Berkeley, and W. M. Keck Observatory.

The discovery about Neptune’s disappearing clouds not only reveals exciting scientific aspects but also provides critical insights into planetary atmospheres and their connections to cosmic factors like the Sun’s activity. The ongoing and future observations will undoubtedly shape our understanding of these profound planetary phenomena.

Frequently Asked Questions (FAQs) about solar cycle’s influence on Neptune’s clouds

More about solar cycle’s influence on Neptune’s clouds

• Icarus Journal
• W. M. Keck Observatory
• NASA’s Hubble Space Telescope
• University of California, Berkeley
• James Webb Space Telescope (JWST)