Neptune’s Temperature is Behaving Strangely

One of the great things about science is that it builds on itself over time.  Data collected decades ago is still valid and helps scientists spot trends that would otherwise be lost in the flurry of new data they are trying to collect.  And sometimes, that data holds something interesting.  Such is the case when a group of scientists took a look at the infrared data of Neptune’s atmosphere and found not one but two weird changes happening.

To put the changes in context, it would help to understand Neptune’s “weather.”  The planet does have an axial tilt, which means it also has seasons.  Since scientists started collecting valid data in 2003, it has gone through about half of a southern hemisphere summer, which, like Neptune’s other seasons, lasts 40 Earth years each.  

With that perspective, scientists didn’t expect to see much change in the temperature of Neptune’s atmosphere over even a 20-year observational period – but they did.  They saw an overall global cooling of about 8 ? through the planet’s stratosphere between 2003 and 2018.  Even more shockingly, they saw an 11 ? temperature increase across the southern pole over just two years between 2018 and 2020.  These results were published in April in the Planetary Science Journal.

Some visible and infrared measurements of Neptune’s atmosphere.
Credit – Michael Roman/NASA/ESA/STSci/M.H. Wong/L.A. Sromovsky/P.M. Fry

Observations of the planet’s stratosphere were completed with a wide variety of infrared telescopes over almost 20 years.  All of those data showed the same cooling trend over the beginning period of data, which confused scientists who had expected to see general warming due to the onset of summer, which would be expected to have higher temperatures than spring. 

In addition, the two-year jump in southern polar temperatures was more rapid than the researchers were expecting.  These observations were collected with only two telescopes – Gemini North in 2019 and Subaru in 2020 – but both confirmed the same dramatic change.  That large of a jump in just 5% of a season is a surprising leap and could indicate ever more extensive changes to come later in the season.

These two trends are obviously contradictory, and scientists are still trying to make sense of them.  Their best theory right now is that the seasonal changes throughout the southern summer might impact the planet’s atmospheric chemistry, which would then also affect how it cools or warms up.  Additionally, it could be affected by the 11-year solar cycle or random weather patterns on the planet itself that we don’t yet have good models for.

UT video explaining the recent increased interest in Neptune.

One of the other great things about science is that, while there are always old data sets to look over, there are also new ones to get excited about.  The researchers are most excited about what the mid-range infrared instrument (MIRI) on James Webb will be able to show them about Neptune’s atmospheric makeup.  That telescope continues to progress through its onboard stages, and one of the current paper’s authors, Leigh Fletcher of the University of Leicester, has some time allotted on it as part of the GIANTCLIMES European Research Council grant.   What new patterns MIRI reveals have yet to be determined, but they are sure to be interesting.

Learn More:
University of Leicester – Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures
Roman et al. – Subseasonal Variation in Neptune’s Mid-infrared Emission
UT – What is the Surface Temperature of Neptune?
UT – Neptune’s South Pole is the Warmest Place on the Planet

Lead Image:
Chart of changes in Neptune’s atmospheric temperature.
Credit – Michael Roman/NASA/JPL/Voyager-ISS/Justin Cowart