Astronomers who have observed Neptune for the past 17 years using multiple ground-based telescopes have tracked an astonishing drop in global temperatures for the ice giant, which was then followed by a significant warming trend at the planet’s south pole.
Neptune, which orbits the Sun at a distance of 2.8 billion miles (4.5 billion km), goes through seasons like Earth – they last longer. A year on Neptune lasts about 165 Earth years, so one season can last about 40 years. It’s been summer in the southern hemisphere of Neptune since 2005.
Astronomers decided to track the temperatures of the planet’s atmosphere once the southern summer solstice occurred that year.
Nearly 100 thermal images of Neptune taken since then have shown that much of Neptune has gradually cooled, declining 14 degrees Fahrenheit (8 degrees Celsius) between 2003 and 2018.
“This change was unexpected,” said the study’s lead author, Michael Roman, a postdoctoral researcher at the University of Leicester, in a statement. “Since we observed Neptune at the beginning of the southern summer, we expected temperatures to rise slowly and no more.”
Then, there was a significant warming in the south pole of Neptune between 2018 and 2020, and temperatures rose by 20 degrees Fahrenheit (11 degrees Celsius). This warm polar vortex completely reversed any cooling that had occurred before.
This type of polar warming has not yet been observed on Neptune.
“Our data covers less than half of Neptune’s season, so no one expected to see big, rapid changes,” study co-author Glenn Orton, principal investigator at NASA’s Jet Propulsion Laboratory, said in a statement.
The images were taken with the European Southern Observatory’s Very Large Telescope and Gemini Southern Telescope in Chile, as well as the Subaru Telescope in Hawaii, the Keck Telescope and the Gemini North Telescope, along with data from NASA’s now retired Spitzer Space Telescope. Infrared light emitted from Neptune’s stratosphere, or the atmospheric band above the active weather layer, has helped astronomers detect temperature fluctuations.
Frosty Neptune has a temperature of minus 340 degrees Fahrenheit (minus 220 degrees Celsius), and astronomers still don’t know what caused these temperature changes.
At the moment, they considered that the unexpected changes may be due to a number of factors.
“The temperature changes may be related to seasonal changes in Neptune’s atmospheric chemistry, which may alter the efficiency of atmospheric cooling,” Roman said. “But random variation in weather patterns or even response to the 11-year solar activity cycle could also have an effect.”
More observations will be needed to truly explore these possibilities. The James Webb Space Telescope will observe Uranus and Neptune later this year. The space observatory’s mid-infrared instrument can determine the chemistry and temperatures in Neptune’s atmosphere and can determine the cause of the change.
Neptune is more than 30 times farther from the sun than Earth, and is the only planet in our solar system that cannot be seen with the naked eye from Earth. So far, NASA’s Voyager 2 spacecraft has only flown close to Neptune, which it did in 1989.
“I think Neptune itself is interesting to many of us because we still know so little about it,” Roman said. “All of this points to a more complex picture of Neptune’s atmosphere and how it changes over time.”
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