Nearly two decades of observations have shown that Neptune’s southern hemisphere has been slowly cooling down when it should be heating up, and we don’t know why
11 April 2022
Summer on Neptune seems to be cooling down. Observations dating back to 2003 show that the temperature in the planet’s southern hemisphere has been dropping, despite the fact that these measurements were taken in the early part of its long summer.
Michael Roman at the University of Leicester, UK, and his colleagues examined data from several of the world’s biggest telescopes to figure out how the temperature of Neptune has changed since the first relatively detailed measurements were made in 2003.
“Because we are observing Neptune in this southern summer, we basically expect temperatures to be getting slowly warmer in time,” says Roman. “But what we saw was that they dropped by about 8°C” over the course of 15 years, he says.
The observations also revealed a surprise near the planet’s south pole. Between 2018 and 2020, an area there warmed by about 11°C, an unexpectedly rapid change given that it takes Neptune more than 165 Earth years to complete a circuit around the sun. “A season on Neptune is over 40 years long, so we’d expect these changes to be a lot more gradual,” says Roman.
It is unclear what is causing these two opposite changes in Neptune’s atmosphere. The rapid warming could simply be due to weather – similar heating was observed on Saturn during the formation of a huge storm over its north pole – but the long-term cooling is probably more complicated than that, says Roman.
It could be related to the 11-year cycle of solar activity, which may affect the chemistry in the planet’s atmosphere, or it could be some seasonal process that we don’t fully understand.
Thanks to the length of Neptunian seasons, it might be a while before we can figure out what has caused these strange changes in its climate. “We have about 17 years of images that amount to roughly 100 high-quality pictures of Neptune, and this is all that currently exists – it’s less than half a single season,” says Roman. “We need decades more observations to really nail this down.”
Journal reference: Planetary Science Journal, DOI: 10.3847/PSJ/ac5aa4
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