The disappearance of Neptune’s clouds

The Hubble Space Telescope image sequence demonstrates the rise and fall of cloud cover on Neptune. The changes are caused by photochemical processes taking place in Neptune’s upper atmosphere. In 1989, Voyager 2 provided the first close-up images of white cirrus-like clouds on Earth, seen high in Neptune’s atmosphere. Hubble picked up where Voyager left off, keeping a close eye on the planet every year. Credits: NASA, ESA, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley)

For the first time in nearly three decades of observations, Neptune’s clouds have all but disappeared. Images taken from 1994 to 2022 by the WM Keck Observatory, along with those from NASA’s Hubble Space Telescope, show that the classic white streaks, reminiscent of terrestrial cirrus clouds, are no longer visible, except for the south pole.

The observations, published in the journal Icarus, also suggest an apparent connection between the disappearance of Neptune’s clouds and the eleven-year cycle of solar activity: a surprising finding given that Neptune is the farthest planet from the Sun and receives only 1/900 of it. sunlight received by the Earth.

The authors of the discovery are a group of astronomers led by the University of California, Berkeley, who discovered that the clouds normally observed in the icy giant’s mid-latitudes began to fade rather quickly in 2019, within a few months. “Even four years later, the images we took last June showed that the clouds have not returned to their previous levels,” he says. Erandi Chavez of the Center for Astrophysics at Harvard University. “This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity has not been as dramatic or prolonged.”

To track the evolution of Neptune’s appearance, Chávez and his team analyzed images taken from 1994 to 2022 using Keck’s second-generation near-infrared camera (Nirc2), together with its adaptive optics system (since 2002), as well as observations made at the Lick Observatory (2018-2019) and those of the Hubble Space Telescope (since 1994). In recent years, Keck’s observations have been supplemented by images taken under the Twilight Observing Program and by Hubble Space Telescope images taken under the Outer Planet Atmospheres Legacy (Opal) program.

When all this data was compared, an intriguing correlation emerged between changes in Neptune’s cloud cover and the solar cycle, the period in which the Sun’s magnetic field changes every 11 years, causing fluctuations in radiation levels. solar. The researchers observed that when the Sun emitted more ultraviolet (UV) light, particularly the strong Lyman-alpha emission from hydrogen, more clouds appeared on Neptune about two years later. The team also found a positive correlation between the number of clouds and the gas giant’s brightness due to sunlight reflecting off it.

“These extraordinary data provide us with the strongest evidence that Neptune’s cloud cover is related to the cycle of the Sun,” he reports. Imke de Pater of UC Berkeley. “Our findings support the theory that ultraviolet rays from the sun, when strong enough, could trigger a photochemical reaction that produces Neptune’s clouds.”

The connection between the solar cycle and Neptune’s cloudy weather pattern has been inferred from 2.5 cloud activity cycles recorded over 29 years of Neptune observations. During this period, the planet’s reflectivity increased in 2002 (brightness maximum), dimmed in 2007 (brightness minimum), brightened again in 2015, and then decreased in 2020 to its lowest level on record, when most clouds they have disappeared. Changes in Neptune’s brightness caused by the Sun seem to rise and fall quite in sync with the presence of clouds over the planet.