Astronomers capture surprising changes in Neptune’s temperatures

This composite shows thermal images of Neptune taken between 2006 and 2020. The first three images (2006, 2009, 2018) were taken with the VISIR instrument on ESO’s Very Large Telescope, while the 2020 image was taken by the COMICS instrument on the Subaru telescope. (VISIR was not operational in mid-late 2020 due to the pandemic). After the planet’s gradual cooling, the South Pole appears to have become dramatically warmer in the last few years, as shown by a bright spot at the bottom of Neptune in the images from 2018 and 2020. Credit: ESO / M. Novel, NAOJ / Subaru / Comics

An international team of astronomers has used terrestrial telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track Neptune’s atmospheric temperatures over a 17-year period. They found a surprising drop in Neptune’s global temperatures followed by a dramatic warming at its south pole.

“This change was unexpected,” said Michael Roman, a postdoctoral fellow at the University of Leicester, UK, and lead author of the study published today in The Planetary Science Journal. “As we observed Neptune during its early southern summer, we expected temperatures to slowly become warmer, not colder.”

Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts about 40 years, with a Neptune year lasting 165 Earth years. It has been summer time in Neptune’s southern hemisphere since 2005, and astronomers were eager to see how temperatures changed after the southern summer solstice.

Astronomers looked at nearly 100 thermal-infrared images of Neptune, taken over a 17-year period, to put together overall trends in planetary temperature in more detail than ever before.

Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures

Observed changes in Neptune’s thermal-infrared brightness, a measure of the temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness of Neptune’s stratosphere with the time of all existing images taken with terrestrial telescopes. Lighter images are interpreted as warmer. Corresponding thermal-infrared images (top) at wavelengths of ~ 12 viserm show Neptune’s performance in 2006, 2009, 2018 (observed by the European Southern Observatory’s Very Large Telescopes VISIR instrument) and 2020 (observed by COMICS instruments). The South Pole appears to have warmed dramatically over the past few years. Credit: Michael Roman / NASA / JPL / Voyager-ISS / Justin Cowart.

These data showed that despite the beginning of the southern summer, most of the planet had gradually cooled over the last two decades. The global average temperature for Neptune decreased by 8 ° C between 2003 and 2018.

Astronomers were then surprised to discover a dramatic warming of Neptune’s south pole over the last two years of their observations, with temperatures rising rapidly by 11 ° C between 2018 and 2020. Although Neptune’s hot polar vortex has been known for many years, so rapid polar warming has never before been observed on the planet.

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

Astronomers measured Neptune’s temperature using thermal cameras that work by measuring the infrared light emitted by astronomical objects. For their analysis, the team combined all existing images of Neptune collected over the last two decades by terrestrial 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 southern summer.

Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures

Neptune seen in visible light (center) and thermal-infrared wavelengths (right), in 2020. The center image combines several images from the Hubble Space Telescope, while the thermal-infrared image on the right was taken from the Subaru telescope in Maunakea, Hawaii. In the thermal-infrared, Neptune’s warm south pole shines brighter than ever before. Credit: Michael Roman / NASA / ESA / STSci / MH Wong / LA Sromovsky / PM Fry.

Because Neptune is about 4.5 billion kilometers away and is very cold, the planet’s average temperature is around -220 ° C, and it is no easy task to measure its temperature from Earth. “This type of study is only possible with sensitive infrared images from large telescopes like VLT that can observe Neptune clearly, and these have only been available for the last 20 years or so,” said co-author Leigh Fletcher, a professor at the University of Leicester.

About a third of all images taken came from the VLT Imager and Spectrometer for mid-infrared (VISIR) instrument on ESO’s VLT in Chile’s Atacama Desert. Due to the telescope’s mirror size and height, it has a very high resolution and data quality that provides 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 with the Subaru Telescope, the Keck Telescope and the Gemini North Telescope, all in Hawai’i.

Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures

Voyager 2 image of Neptune, captured in August 1989. Credit: NASA / JPL-Caltech / Kevin M. Gill.

Because Neptune’s temperature variations were so unexpected, astronomers do not yet know what could have caused them. They may be due to changes in Neptune’s stratospheric chemistry or random weather patterns or even the solar cycle. More observations will be needed over the coming years to explore the causes of these fluctuations. Future terrestrial telescopes like ESO’s Extremely Large Telescope (ELT) could observe temperature changes like these in more detail, while NASA / ESA / CSA’s James Webb Space Telescope will provide unprecedented new maps of chemistry and temperature in Neptune’s atmosphere.

“I think Neptune itself is very exciting for many of us because we still know so little about it,” Roman says. “It all points to a more complicated picture of Neptune’s atmosphere and how it changes over time.”


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More information:
“Sub-seasonal variation in Neptune’s mid-infrared emission” The Planetary Science Journal (2022). DOI: 10.3847 / PSJ / ac5aa4

Citation: Astronomers Capture Surprising Changes in Neptune’s Temperatures (2022, April 11) Retrieved April 11, 2022 from https://phys.org/news/2022-04-astronomers-capture-neptune-temperatures.html

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