Using ground-based telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), an international team of astronomers tracks Neptune’s atmospheric temperatures over a 17-year period. They found a surprising drop in Neptune’s global temperatures followed by dramatic warming at its south pole.
“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study published on April 11, 2022, in The Planetary Science Journal. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”
Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts around 40 years, with one Neptune year lasting 165 Earth years. It has been summertime in Neptune’s southern hemisphere since 2005, and the astronomers were eager to see how temperatures were changing following the southern summer solstice.
Astronomers looked at nearly 100 thermal-infrared images of Neptune, captured over a 17-year period, to piece together overall trends in the planet’s temperature in greater detail than ever before.
These data showed that, despite the onset of southern summer, most of the planet had gradually cooled over the last two decades. The globally averaged temperature of Neptune dropped by 8 °C between 2003 and 2018.
The astronomers were then surprised to discover dramatic warming of Neptune’s south pole during the last two years of their observations when temperatures rapidly rose 11 °C between 2018 and 2020. Although Neptune’s warm polar vortex has been known for many years, such rapid polar warming has never been previously observed on the planet.
“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” says co-author Glenn Orton, a senior research scientist at Caltech’s Jet Propulsion Laboratory (JPL) in the US.
The astronomers measured Neptune’s temperature using thermal cameras that work by measuring the infrared light emitted from astronomical objects. For their analysis, the team combined all existing images of Neptune gathered over the last two decades by ground-based telescopes. They investigated infrared light emitted from a layer of Neptune’s atmosphere called the stratosphere. This allowed the team to build up a picture of Neptune’s temperature and its variations during part of its southern summer
Because Neptune is roughly 4.5 billion kilometers away and is very cold, the planet’s average temperature reaches around –220°C, measuring its temperature from Earth is no easy task. “This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so,” says co-author Leigh Fletcher, a professor at the University of Leicester.
Around one-third of all the images taken came from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on ESO’s VLT in Chile’s the Atacama Desert. Because of the telescope’s mirror size and altitude, it has a very high resolution and data quality, offering the clearest images of Neptune. The team also used data from NASA’s Spitzer Space Telescope and images were taken with the Gemini South telescope in Chile, as well as with the Subaru Telescope, the Keck Telescope, and the Gemini North telescope
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