Neptune’s temperatures run oddly warm and cold

Neptune is the eighth planet outward from our sun. Far from the sun’s light and heat, it’s a forbiddingly cold world, with an average temperature of -364 degrees Fahrenheit (-220 degrees Celsius). But last week (April 11, 2022), an international team of astronomers said they’d measured a surprising drop in temperature in Neptune’s atmosphere, followed by a dramatic warming at the planet’s south pole. They’re perplexed and said these findings were unexpected.

The researchers used the European Southern Observatory’s Very Large Telescope and several other telescopes, both on Earth and in space, to make their discovery. They published the peer-reviewed results in The Planetary Science Journal on April 11 (open access).

Unexpected variations in Neptune’s temperatures

The variations in temperature were surprising and unexpected, as Michael Roman, lead author at the University of Leicester, U.K., stated:

This change was unexpected. Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.

The researchers made the discovery after examining nearly 100 thermal infrared images of Neptune. The images were taken over a 17-year period. They revealed something rather unusual: even though southern summer was beginning on the planet, most of the planet had gradually cooled over the last two decades. Overall, the global average temperature of Neptune dropped by 46 degrees Fahrenheit (26 degrees Celsius) between 2003 and 2018. Why?

Then, during the last two years of observations, the temperatures at Neptune’s south pole dramatically warmed. In fact, they rapidly rose 52 degrees Fahrenheit (29 degrees Celsius) between 2018 and 2020.

Neptune does have a relatively warm polar vortex in its atmosphere over its south pole. However, scientists had never observed this kind of rapid warming until now. As Glenn Orton, a senior research scientist at NASA’s Jet Propulsion Laboratory (JPL), noted:

Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes.

Thermal-infrared image analysis

How did the astronomers measure the temperature changes on Neptune? They used thermal cameras that measure the infrared light emitted from astronomical objects. Their study of the images was comprehensive, using all known images taken by ground-based telescopes over the past two decades. In particular, they examined Neptune’s stratosphere.

It was southern summer on Neptune during the observations. The atsronomers were able to build a more comprehensive picture of the temperature variations during this time period. Co-author Leigh Fletcher at the University of Leicester, U.K., said:

This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so.

Possible explanations for Neptune’s temperatures

Scientists don’t yet know for sure what is causing the unexpected changes in temperature. They’re considering various possibilities, such as stratospheric chemistry, random weather patterns or perhaps the solar cycle of the sun.

Only further observations will help to nail down the cause or causes of the temperature fluctuations. In particular, ESO’s Extremely Large Telescope (ELT) could observe such temperature changes in greater detail, and the newly-launched James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere. All of these observations will provide valuable clues as to just what is happening in Neptune’s chilly atmophere. There is still much to learn about Neptune, as Roman noted:

I think Neptune is itself very intriguing to many of us because we still know so little about it. This all points toward a more complicated picture of Neptune’s atmosphere and how it changes with time.

Multiple telescopes

VLT played a central role in obtaining the images used in the study, specifically the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument. But in addition, several other telescopes contributed to the analysis as well, including NASA’s Spitzer Space Telescope and the Gemini South telescope in Chile. Also, the Subaru Telescope, the W. M. Keck Observatory and the Gemini North telescope, all in located in Hawaii, played a part.

Due to its mirror size and altitude, VLT provided the clearest images of Neptune, comparable with ones from the Hubble Space Telescope.

A cold, distant world

At 2.8 billion miles (4.5 billion km) from the sun, Neptune is, unsurprisingly, an intensely cold place, all the time. Even with variations, the average temperature is still a bone-freezing -364 degrees Fahrenheit (-220 degrees Celsius).

Neptune does have seasons, like Earth. However, each season lasts the equivalent of 40 Earth years. It takes Neptunes 165 Earth years to complete one orbit around the sun. The summer season in Neptune’s southern hemisphere began in 2005.

NASA’s Voyager 2 spacecraft flew past Neptune and its moons on August 25, 1989. It is still the only spacecraft so far to have visited these distant worlds.

Bottom line: Neptune’s temperatures are surprisingly both cooler and warmer than expected, scientists say. Several telescopes from around the world and in space made observations for the new international study.

Source: Subseasonal Variation in Neptune’s Mid-Infrared Emission

Via ESO