With the help of several ground-based telescopes, including the European Southern Observatory's (ESO) Very Large Telescope (VLT), an international team of astronomers has analyzed Neptune's atmospheric temperatures over a period of 17 years and found that there is a surprising decrease in global temperatures of the planet followed by a drastic warming of its south pole.
"These variations are unexpected," said Michael Roman, a postdoctoral researcher at the University of Leicester, UK, and lead author of the study published in The Planetary Science Journal. "We've been observing Neptune since the beginning of its southern summer and we would expect temperatures to gradually rise rather than fall."
As on Earth, there are seasons on Neptune as the planet orbits the Sun, with the difference that a season on Neptune lasts for about 40 Earth years and a year lasts for 165 Earth years. It's been summer in Neptune's southern hemisphere since 2005, and astronomers wanted to see how temperatures varied after the austral summer solstice.
Astronomers analyzed nearly a hundred thermal infrared images of Neptune, captured over a period of 17 years, to understand general trends in the planet's temperature in more detail than has been achieved to date.
The data showed that, despite the onset of the austral summer, most of the planet has been gradually cooling down over the past two decades. Neptune's average global temperature dropped by 8°C between 2003 and 2018.
Astronomers were also surprised to discover a drastic warming at Neptune's south pole in the last two years, when temperatures rose rapidly, that is, 11º C between 2018 and 2020. Although Neptune's hot polar vortex has been known for a long time For many years, such rapid polar warming has never been observed before.
"Our data covers less than half a station on Neptune, so we weren't expecting to find such large and rapid variations," said co-author Glenn Orton, a researcher at the Jet Propulsion Laboratory (JPL), Caltech, in USA.
Astronomers measured Neptune's temperature with the help of thermal cameras, instruments that measure infrared radiation emitted by astronomical objects. For their analysis, the team combined all the images of Neptune that were captured by ground-based telescopes over the past two decades.
The astronomers analyzed the infrared radiation emitted by a layer of Neptune's atmosphere called the stratosphere, which allowed them to paint a picture of Neptune's temperature and its variations during part of its southern summer.
Since Neptune is about 4,5 billion away and very cold — with the planet's average temperature reaching around -220º C — measuring its temperature from Earth is extremely complicated.
“This type of study is only possible thanks to sensitive infrared images taken by large telescopes such as the VLT, which can observe Neptune very clearly, but these types of telescopes have only become available in the last 20 years or so,” said the co-author. -author Leigh Fletcher, professor at the University of Leicester.
About a third of all images were taken by the VISIR instrument (VLT Imager and Spectrometer for mid-InfraRed) mounted on ESO's VLT in the Chilean Atacama Desert. Due to the telescope's mirror size and altitude, the images are of very high resolution and quality, making them the sharpest images of Neptune ever taken.
The team also used data from NASA's Spitzer Space Telescope and images taken with the southern Gemini telescope in Chile, as well as the Subaru, Keck and northern Gemini telescopes, all located in Hawaii.
Because Neptune's temperature variations are so unexpected, astronomers still don't know where they came from. They could be due to variations in Neptune's stratospheric chemistry, or random weather patterns, or even the solar cycle. More observations will be needed over the next few years to explore the reasons for these fluctuations.
Future ground-based telescopes such as ESO's Extremely Large Telescope (ELT) will be able to observe temperature variations like these in greater detail, while the NASA/ESA/CSA James Webb Space Telescope will provide new maps of the temperatures and chemistry of Earth's atmosphere. Neptune.
“I find Neptune very intriguing because, in reality, we still know very little about it,” says Roman. "These results point to a very complex picture of Neptune's atmosphere and its variations over time."
Author European Southern Observatory
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