Photo: ESO / M. Roman, NAOJ / Subaru / Comics
An international team of astronomers has been monitoring temperatures in Neptune’s atmosphere for 17 years using ground-based telescopes, including the Very Large Telescope (VLT) at the European Southern Observatory (ESO). In doing so, they recorded a sudden decrease in the global temperature of the planet, followed by a sharp rise in the temperature of the Antarctic.
“This change was unexpected,” said Michael Roman, a postdoctoral researcher at the University of Leicester (UK) and lead author of the study, the results of which are published today in The Planetary Science Journal. “Since we observed Neptune during the early Australian summer, we expected temperatures to rise gradually rather than decrease.” During its orbit around the Sun, Neptune goes through several seasons, just like the Earth. But because the year of Neptune lasts about 165 Earth years, the season on Neptune lasts about 40 years.
It’s been summer in Neptune’s southern hemisphere since 2005, and astronomers have been eager to see how temperatures change in response to the southern summer solstice. To this end, they studied nearly 100 thermal images of Neptune, taken over a 17-year period, to chart the planet’s temperature trajectory in more detail than ever before. These data show that despite the onset of summer in Australia, most parts of the planet have gradually cooled over the past two decades. Between 2003 and 2018, the average temperature of Neptune fell by 8 degrees Celsius.
To their surprise, the astronomers then found that Neptune’s south pole had been rising sharply during the last two years of their observations. Between 2018, temperatures there rose by 11 degrees Celsius. Such rapid polar warming has not been observed on this planet before. “Our data covers less than half of the Neptune season, so no one expected to see big, rapid changes,” said study co-author Glenn Orton, senior researcher at the Jet Propulsion Laboratory (JPL) at the California Institute of Technology in the USA.
Astronomers have measured the temperature of Neptune using thermal imagers sensitive to infrared light emitted by astronomical objects. For their analysis, the team collected all current images of Neptune collected by ground-based telescopes over the past two decades. They examined infrared light emitted by a layer in Neptune’s atmosphere called the stratosphere. This allowed the team to visualize the planet’s temperature and its changes during part of the southern summer.
Since Neptune is about 4.5 billion kilometers away from us and is very cold (the average temperature of the planet is about -220 ° C), it is not easy to measure its temperature from Earth. Study co-author Lee Fletcher, a professor at the University of Leicester, said.
About a third of the images come from the VLT Imager and Medium Infrared Spectrometer (VISIR) on ESO’s Very Large Telescope in the Chilean Atacama Desert. Due to the large size of its mirror and the height at which it is created, this telescope can take very sharp images of Neptune. In addition, the team also used data from NASA’s Spitzer Space Telescope and images from the Gemini Southern Telescope in Chile, the Subaru Telescope, the Keck Telescope and the Gemini North Telescope in Hawaii.
Since the temperature changes on Neptune were so unexpected, astronomers don’t yet know why they happened. They could be due to changes in Neptune’s stratospheric chemistry, random weather patterns, or even the solar cycle. More observations will need to be made in the coming years to investigate the causes of these fluctuations. Future ground-based telescopes such as ESO’s Very Large Telescope (ELT) can monitor temperature changes like these in greater detail, while the James Webb Space Telescope of NASA, the European Space Agency and the Canadian Space Agency will map the chemistry and temperature in Neptune’s atmosphere with unprecedented accuracy.
“I think Neptune itself is very interesting to many of us because we know so little about it,” Roman says. “All of this points to a more complex picture of Neptune’s atmosphere, and how it changes over time.”
source: ESO
