The International Panel of Astronomers has used ground-based telescopes, including the European Southern Laboratory’s Largest Telescope (ESO’s VLT), to monitor Neptune’s atmospheric temperature over a 17 – year period. They found a surprising drop in Neptune’s global temperature, followed by dramatic warming at its South Pole.
“This change is unpredictable,” said Michael Roman, co-founder of the University of Leicester in the UK and co-author of the study, which was published today. The Planetary Science Journal. “As we have been observing Neptune since the beginning of its southern summer, we expected temperatures to rise slowly and not be cold.”
Like Earth, Neptune enjoys the seasons as it orbits the Sun. However, a Neptune season lasts about 40 years, and a Neptune year lasts 165 Earth years. Summer has been in the southern hemisphere of Neptune since 2005, and astronomers have been curious to see how temperatures change following the southern summer solstice.
Astronomers have seen nearly 100 hot-infrared images of Neptune captured over a 17-year period, combining the overall trends of the planet’s temperature in more detail than ever before.
These data show that despite the onset of the southern summer, much of the planet has gradually cooled over the past two decades. Neptune’s global average temperature dropped by 8 ° C between 2003 and 2018.
As temperatures rose rapidly to 11 degrees Celsius between 2018 and 2020, astronomers were surprised to see the dramatic warming of Neptune’s South Pole over the past two years. Although Neptune’s hot polar vortex has been known for many years, no such fast pole has ever been seen on a warming planet.
“Our data are less than half the Neptune season, so no one expects big and rapid changes,” said Glen Orton, associate professor and senior research scientist at Caltech’s Jet Propulsion Laboratory (JBL) in the United States.
Astronomers measured the temperature of Neptune using heat cameras that measured the infrared light emitted from astronomical objects. For their analysis, the team put together all the images of Neptune that have been collected by underground telescopes over the past twenty years. They studied infrared light emanating from Neptune’s atmosphere layer called the stratosphere. This allowed the team to create an image of Neptune’s temperature and its variations in a part of its southern summer.
Neptune is approximately 4.5 billion kilometers away and because it is so cold, the average temperature of the planet reaches -220 ° C, measuring its temperature from Earth is not an easy task. “This type of study is only possible with sensitive infrared images of large telescopes such as the VLT that can clearly observe Neptune, and these have only been available for the last 20 years,” says co-author Lee Fletcher. University of Leicester.
One-third of the images taken came from the VLT Imager and Spectrometer Mid-Infrared (VISIR) instrument at ESO’s VLT in the Atacama Desert in Chile. Due to the glass size and height of the telescope, it has a very high resolution and data quality, which provides clear images of Neptune. The team used data from NASA’s Spitzer Space Telescope and the Gemini South Telescope in Chile, as well as data from the Subaru Telescope in Hawaii, the Czech Telescope and the Gemini North Telescope.
Because Neptune’s temperature variations were so unexpected, astronomers still do not know what caused it. They may be due to Neptune’s stratospheric chemical changes or random weather patterns or the solar cycle. To explore the causes of these fluctuations, further observations will be needed in the coming years. Future ground-based telescopes such as the ESO’s Extremely Large Telescope (ELT) will be able to observe such temperature changes in more detail, while the NASA / ESA / CSA James Web Space Telescope will provide unprecedented new maps of chemistry and temperature in Neptune’s atmosphere.
“I think Neptune is very mysterious to many of us because we know very little about it,” Roman says. “All of this points to a more complex picture of Neptune’s atmosphere and how it changes over time.”
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“Sub-seasonal variation in Neptune’s mid-infrared emission” The Planetary Science Journal (2022) DOI: 10.3847 / PSJ / ac5aa4
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