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An international team of astronomers has used ground-based telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to measure the atmospheric temperatures of Neptune over a period of 17 years. They found a surprising drop in Neptune’s global temperatures followed by an impressive warming at its south pole.
“This change was unexpected,” he says. Michael Roman, postdoctoral research associate at the University of Leicester, UK, and lead author of the study published in The Planetary Science Journal. “Since we’ve been observing Neptune during its early austral summer, we expected temperatures to slowly get warmer, not cooler.”
Like Earth, Neptune experiences seasons as it orbits the Sun.
However, a Neptune season lasts about 40 years, and a Neptune year lasts 165 earth years. Neptune’s southern hemisphere has been in summer since 2005, and astronomers were eager to see how temperatures changed after the southern summer solstice.
Astronomers looked at nearly 100 infrared thermal images of Neptune, taken over a 17-year period, to reconstruct overall trends in the planet’s temperature in unprecedented detail.
These data showed that, despite the start of the austral summer, most of the planet had gradually cooled over the past two decades. Neptune’s global average temperature dropped by 8°C between 2003 and 2018.
The team was surprised to discover in their observations over the last two years an impressive warming of Neptune’s south pole: temperatures rose rapidly by 11°C between 2018 and 2020. Although Neptune’s warm polar vortex has been known for many years, it has never such rapid polar warming has been observed previously on the planet.
“Our data covers less than half of a Neptune season, so no one expected to see big, fast changes,” says co-author Glenn Ortona senior researcher at Caltech’s Jet Propulsion Laboratory (JPL) in the United States.
The team measured Neptune’s temperature using thermal cameras that work by measuring infrared light emitted by astronomical objects. For their analysis, the team combined all existing images of Neptune collected over the past two decades by ground-based telescopes. They studied infrared light emitted from a layer of Neptune’s atmosphere called the stratosphere. This allowed the team to build a picture of Neptune’s temperature and its variations during part of its austral summer.
Because Neptune is about 4.5 billion kilometers away and very cold (with an average temperature reaching around -220°C), measuring its temperature from Earth is not an easy task. “This type of study is only possible with sensitive infrared images from large telescopes like the VLT, which can observe Neptune clearly, and these have only been available for the last 20 years or so,” says the co-author. Leigh Fletcherprofessor at the University of Leicester.
About a third of all images obtained come from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument installed on ESO’s VLT in the Atacama Desert, Chile. Due to the size and height of the telescope’s mirror, it has high resolution and high quality data, providing the clearest images of Neptune. The team also used data from NASA’s Spitzer Space Telescope and images taken with the Gemini South Telescope in Chile, as well as the Subaru Telescope, the Keck Telescope and the Gemini North Telescope, all in Hawaii.
Since Neptune’s temperature swings were so unexpected, the team doesn’t yet know what might have caused them. They could be due to changes in Neptune’s stratospheric chemistry, or random weather patterns, or even the solar cycle. More observations will be needed in the coming years to explore the causes behind these fluctuations. Future ground-based telescopes, such as ESO’s Extremely Large Telescope (ELT), could observe temperature changes like these in greater detail, while the James Webb Space Telescope NASA/ESA/CSA, will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere.
“I think Neptune itself is very intriguing to a lot of us because we still know so little about it,” says Roman. “All of this indicates that the picture we had of Neptune’s atmosphere and how it changes over time is more complicated than we imagined.”
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