Surprising changes in Neptune’s temperature detected

An international team of astronomers used ground-based telescopes, including the European Southern Observatory’s (ESO) Very Large Telescope (VLT), to measure Neptune’s Atmospheric Temperatures during a period of 17 years. They found a surprising drop in Neptune’s global temperatures, followed by impressive warming at its south pole.

“This change was unexpected,” he says. roman michaelpostdoctoral research associate at the University of Leicester, UK, and lead author of the study published today in The Journal of Planetary Sciences. “Since we observed Neptune during the early austral summer, we expected temperatures to slowly get warmer, not cooler.”

Neptune’s global average temperature dropped by 8°C between 2003 and 2018, which is unexpected considering that its austral summer began in 2005.

Like Earth, Neptune experiences seasons as it orbits the Sun. Nonetheless, a season of Neptune lasts about 40 years, and its year lasts 165 Earthlings. 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 have observed almost 100 infrared thermal images Neptune, captured over the span of 17 years, to reconstruct the planet’s overall temperature trends in unprecedented detail.

These data showed that, despite the onset of the austral summer, most of the planet had gradually cooled over the past two decades. Neptune’s average global temperature dropped by 8°C between 2003 and 2018.

The team was surprised to find in their observations over the past two years an impressive warming of the South Pole Neptune: Temperatures rose rapidly by 11°C between 2018 and 2020. Although Neptune’s hot polar vortex has been known for many years, such rapid polar warming has never been observed on the planet before.

“Our data covers less than half of a Neptune season, so no one expected to see big, rapid changes,” says co-author Glenn Ortonsenior researcher at Caltech’s Jet Propulsion Laboratory (JPL) in the United States.

The team was surprised to see impressive warming of Neptune’s south pole: temperatures rose rapidly by 11°C between 2018 and 2020.

The researchers recorded Neptune’s temperature using thermal cameras They work by measuring the infrared light emitted by astronomical objects. For their analysis, they combined all existing images of the planet collected over the past two decades by ground-based telescopes. They studied infrared light emitted by a layer of Neptune’s atmosphere called the stratosphere. This made it possible to build an image of Neptune’s temperature and its variations during part of the southern summer.

As Neptune is about 4.5 billion kilometers away and very cold (with a average temperature about -220°C) measuring your Earth temperature 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 have only been available in the last 20 years,” says the co-author. Leigh Fletcherprofessor at the University of Leicester.

ground and space telescopes

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 sharpest images of Neptune.

The team also used data from the Spitzer Space Telescope NASA and images taken with the Gemini South Telescope in Chile, as well as the Subaru Telescope, Keck Telescope and Gemini North Telescope, all in Hawaii.

These thermal variations could be due to changes in Neptune’s stratospheric chemistry, random weather patterns, or even the solar cycle.

Because Neptune’s temperature changes were so unexpected, the team still doesn’t 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) will be able to observe temperature changes like these in more detail, while the NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in orbit. from Neptune. atmosphere.

“I think Neptune itself is very intriguing to many of us because we still know so little about it,” says Roman. “All this indicates that the picture we had of Neptune’s atmosphere and how it changes over time is more complicated than we imagined.

Image of Neptune taken with the MUSE instrument installed on the Very Large Telescope (left) and another taken with the Hubble Space Telescope (right). They weren’t taken at the same time, so they don’t show the same characteristics on their surface. /ESO/P. Weilbacher (AIP)/NASA, ESA and MH Wong and J. Tollefson (UC Berkeley)


M. Roman et al. “Subseasonal Variation in Neptune’s Mid-Infrared Emission”. The Journal of Planetary Sciences2022

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