Neptune's southern hemisphere has spent the last 17 years in its summer season, yet global temperatures have plummeted. It's a head-scratching finding that leaves astronomers with more questions than answers. "Since we have been observing Neptune during its early southern summer, we would expect temperatures to be slowly growing warmer, not colder," says Michael Roman, a postdoctoral research associate at the University of Leicester and lead author of the study published Monday in the Planetary Science Journal, in a statement. It's based on observations of the ice giant's thermal-infrared brightness, an indication of heat in the atmosphere, over nearly two decades—about half the length of the 40-year summer, per Space.com.
Researchers observed "a decline in Neptune's thermal brightness since reliable thermal imaging began in 2003, indicating that globally averaged temperatures in Neptune's stratosphere—the layer of the atmosphere just above its active weather layer—have dropped by roughly 8 degrees Celsius (14 degrees Fahrenheit) between 2003 and 2018," according to a release. Strangely, observations of Neptune's south pole suggest the polar stratosphere warmed by roughly 11 degrees Celsius, or 20 degrees Fahrenheit, in just two years between 2018 and 2020, "reversing the previous globally averaged cooling trend," per the release. It adds "such polar warming has never been observed on Neptune before."
Researchers actually know very little about Neptune's seasons, taking place over its 165-year orbit of the sun. They say the temperature variations could be linked to seasonal changes in Neptune's atmospheric chemistry. As the paper notes, per ScienceAlert, "photochemically produced hydrocarbons—primarily ethane and acetylene—are powerful infrared emitters that serve to cool the stratosphere." But the changes observed between 2018 and 2020 "appear surprisingly swift for seasonal response." According to Roman, "random variability in weather patterns or even a response to the 11-year solar activity cycle" may play a role, per the release. Researchers hope to learn more through observations powered by the James Webb Space Telescope later this year. (Read more Neptune stories.)