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Astronomers have been baffled by the mystery of Neptune, and now they think they’ve cracked its secret.
The ice giant’s ghostly cirrus clouds largely disappeared four years ago. Today, there is just a patch hovering over the planet’s south pole.
Thanks to an analysis of nearly three decades of observations of Neptune taken by three space telescopes, scientists have determined that the ice giant’s dwindling clouds may indicate that shifts in its abundance coincide with the solar cycle, according to A recent study Published in the journal Icarus.
“This impressive data gives us the strongest evidence yet that Neptune’s cloud cover is related to the sun’s cycle,” said lead study author Emke de Pater, professor emeritus of astronomy at the University of California, Berkeley. press release. “Our findings support the theory that (ultraviolet) sunlight, when strong enough, may trigger a photochemical reaction that results in Neptune’s clouds.”
During the solar cycle, the level of activity in the Sun’s dynamic magnetic fields waxes and wanes. The magnetic field fluctuates every 11 years, becoming more and more tangled like a spinning ball, according to NASA. When there is increased activity on the sun, more intense ultraviolet radiation bombards the solar system.
Using data from NASA’s Hubble Space Telescope, the W.M. Keck Observatory in Hawaii, and the Lick Observatory in California, the scientists observed 2.5 cycles of cloud activity over the 29-year period of Neptune observations — during which the planet’s reflectivity increased in 2002 and abated in 2007. Neptune brightened again in 2015, before darkening in 2020 to an all-time low. That’s when most of the cloud cover has cleared.
“Even now, four years later, the most recent images we took last June still show that clouds have not returned to their previous levels,” said the study’s lead author, Irandi Chavez, a PhD student at the Center for Astrophysics at Harvard University and Harvard University. Smithsonian, in a statement.
Chavez added that the results are “very exciting and unexpected, especially since the period prior to the activity of low clouds on Neptune was not exciting and long-term.”
The researchers also found that two years after the peak of the cycle, more clouds appeared on Neptune, and the more clouds there were, the brighter Neptune was due to the sunlight reflecting off it. This association was “surprising to planetary scientists because Neptune is the outermost large planet in our solar system and receives sunlight about 0.1% as intense as Earth,” according to NASA. The findings also contradict the idea that clouds are affected by Neptune’s four seasons, each of which lasts about 40 years.
“This is a very interesting paper and a very nice piece of good, antique, detailed detective work,” Patrick Irwin, professor of planetary physics at the University of Oxford, who was not involved in the study, said by email. “This new paper covers a longer time frame than previous studies and shows a convincing relationship between observed cloud cover and solar UV brightness.”
But there is a two-year time lag between the peak of the solar cycle and the abundance of clouds on Neptune. The authors believe that this gap could be explained by photochemistry that occurs in the planet’s upper atmosphere, which takes time to produce clouds.
Irwin said the connection between the increased brightness of the sun and cloud formation could be due to the generation of ionized particles that could act as cloud condensation nuclei and help initiate condensation.
“It’s great to be able to use telescopes on Earth to study the climate of a world more than 2.5 billion miles away,” study co-author Carlos Alvarez, an astronomer at the Keck Observatory, said in a statement. “Advances in technology and observations have enabled us to constrain models of Neptune’s atmosphere, which are key to understanding the relationship between the ice giant’s climate and the solar cycle.”
The research team is still monitoring Neptune’s cloud activity, the researchers said, because more ultraviolet radiation can also darken the planet’s clouds, reducing their overall brightness.
In addition, Neptune’s storms from the deep atmosphere affect the planet’s cloud cover, but are not associated with clouds in the upper atmosphere. This variable could interfere with studies looking at associations between photochemical clouds and the solar cycle. Further research could also indicate how long the near absence of clouds on Neptune might last.
These endeavors, in turn, can not only expand astronomers’ knowledge about Neptune, but also help researchers better understand the many exoplanets outside the solar system that are thought to have properties similar to ice giants, according to NASA.
Irwin said the study “also underscores the need to continue observing the planets of the solar system.” “Only by observing these planets at regular intervals is it possible to build a reliable, long-term data set to explore these cyclic variations.”
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