Clouds on Neptune might be created by the sun, strangely

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As distant as it is, orbiting nearly 3 million miles from the sun, we know a surprising amount about the atmosphere and weather conditions on Neptune. Dramatic storms have been observed there including by the Voyager 2 spacecraft which passed by in the 1980s, which saw dark spots surrounded by white clouds of frozen methane. However, astronomers are now faced with a puzzle about these storms and why they seem to be appearing and disappearing over time.

Researchers recently used Hubble and other telescopes to observe Neptune’s clouds to investigate a mystery: why sometimes the planet had plentiful clouds in its atmosphere and at other times had barely any. In 2019, the level of clouds dropped dramatically and it wasn’t clear why.

“Even now, four years later, the most recent images we took this past June still show the clouds haven’t returned to their former levels,” said lead researcher Erandi Chavez of the Center for Astrophysics | Harvard-Smithsonian in a statement. “This is extremely exciting and unexpected, especially since Neptune’s previous period of low cloud activity was not nearly as dramatic and prolonged.”

This sequence of Hubble Space Telescope images chronicles the waxing and waning of the amount of cloud cover on Neptune. This long set of observations shows that the number of clouds grows increasingly following a peak in the solar cycle – where the Sun’s level of activity rhythmically rises and falls over an 11-year period. NASA, ESA, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley)

The team found a link between the amounts of clouds and the solar cycle, which is an 11-year pattern of activity which the sun goes through. At certain times the number of sunspots and solar flares from the sun increases, which sends more ultraviolet (UV) radiation out into the solar system. This radiation seems to affect the clouds on Neptune, as the research shows that over 30 years of data more clouds are present two years after the peak of the solar cycle. The researchers think that this two-year lag is due to the chemical processes which begin in the planet’s atmosphere and need time to produce clouds.

“These remarkable data give us the strongest evidence yet that Neptune’s cloud cover correlates with the Sun’s cycle,” said senior researcher Imke de Pater. “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 researchers want to continue tracking the planet’s cloud activity to understand how the sun affects the clouds, and whether the clouds will reappear from their current low levels.

“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 fellow researcher Carlos Alvarez of the Keck Observatory . “Advances in technology and observations have enabled 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 is published in the journal Icarus.

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