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Extreme climate, Neptune summer grows colder but gets warmer at the south pole

The European Southern Observatory’s VLT (Very Large Telescope), accompanied by some other ground-based telescope, made a strange discovery about Neptune’s rapidly changing temperatures. The astronomers said that the planet’s global temperature saw a steep drop in summer while a contrasting rise in the south pole. They were expecting the temperature to slowly grow warmer and not colder.

The information collected points out a drop in the planet’s temperature by an average of 8°C between 2003 and 2018, despite the onset of the southern summer. What astonished the astronomers was when the temperature climbed to an average of 11°C between 2018 and 2020 in the south pole. Although the astronomers had previous knowledge of the planet’s polar vortex, the sudden rise in the temperature was not expected in such a short span.

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 captured by the COMICS instrument on the Subaru Telescope (VISIR wasn’t in operation in mid-late 2020 because of the pandemic). After the planet’s gradual cooling, the south pole appears to have become dramatically warmer in the past few years, as shown by a bright spot at the bottom of Neptune in the images from 2018 and 2020.

Michael Roman, the lead author of this study on ‘The Planetary Science Journal’ by ESO, remarked that since they observed Neptune from its early southern summer, the change was somewhat unexpected. The temperatures were expected to grow warmer, not colder.

The planet has been observed for 17 years now. Nearly 100 thermal-infrared images of Neptune have been captured and studied to pull together the trends and patterns of its temperatures and seasons. 

Neptunes Seasons

Neptune also experiences seasons at regular intervals as it revolves around the sun. Since its orbit diameter is much longer than Earth, it takes Neptune 165 earth years to complete one rotation while a single season lasts up to 40 years. It has been summer on the planet since 2005.

Glenn Orton, a senior research scientist at Caltech’s Jet Propulsion Laboratory (JPL) in the US and the paper’s co-author, says that the data collected covers less than half of the planet’s season; hence no one was waiting on a sudden yet large scale change. 

Explaining the long-standing problem of identifying the source of Neptune’s high stratospheric methane abundances, he says that the temperatures are so high that methane gas, which should be frozen out in the upper part of Neptune’s atmosphere, or the stratosphere, can leak out through this region.

The team first published the initial temperature maps of the lowest portion of Neptune’s atmosphere nearly a decade ago, showing that this region opens a gateway for methane to escape into the deep atmosphere.

Equipment used for the study

Neptune is 4.5 billion kilometers away from Earth, and the planet’s average temperature reaches around –220°C since only 1/900th as much sunlight reaches Neptune in comparison to us.

This study demands sensitive infrared images taken from telescopes like ESO’S VLT (Very Large Telescope) that have only been available for the past 20 years. An average of one-third of the planet’s images come from ESO’S VLT imager, and spectrometer for mid-InfraRed (VISIR) instruments in Chile’s Atacama Desert. The telescope offers a brilliant mirror size and altitude. It is very high resolution, and data quality has provided the most explicit image of Neptune so far. 

Temperatures have been studied using thermal cameras that measure the infrared light emitted from astronomical objects. The astronomers pieced together all existing images of the planet collected over the past two decades to investigate infrared light emissions from Neptune’s Stratosphere, which allowed them to build up a solid map of Neptune’s temperature understanding including the variations during part of southern summer.

Asides from VLT, NASA’s Spitzer Space Telescope has also contributed data to the team. Keck Telescope, Gemini North telescope, and Subaru Telescope (all in Hawai’i) have taken images for the study, accompanied by the Gemini South telescope in Chile.

In the near future, ground-based telescopes like ESO’S Extremely Large Telescope (ELT) are said to observe temperature changes in greater detail. NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere.

Neptune – a less researched planet

Neptune is one of the lesser researched planets in our solar system. Studies are conducted under continued progress. What led to a sudden pattern break of the cycle is unknown to the astronomers since the temperature variations were so unexpected. Assumptions say that it could have been due to the planet’s stratospheric chemistry or random weather patterns. Solar cycles can also be discussed in the list. 

Roman says that this development points towards a ‘more complicated picture of Neptune’s atmosphere and how it changes with time. He thinks Neptune is an intriguing subject since not many of us know much about it.



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