Something weird is happening to Neptune's temperature

It should be getting hotter, and it isn't.
Mert Erdemir

The temperature of Neptune has been showing some abnormalities recently. In 2019, for example, NASA released images of Neptune going through massive storms during the summer period of the planet's southern hemisphere. And now, an international team of astronomers has detected unexpected changes in the temperature of Neptune. 

Michael Roman at the University of Leicester and his colleagues have used ground-based telescopes, including the European Southern Observatory's Very Large Telescope (ESO's VLT) to track the temperature changes of Neptune since the first relatively detailed measurements that were made in 2003.

The observations have finally demonstrated that the temperature in the planet’s southern hemisphere has been surprisingly declining although the measurements were taken in the planet's summer, according to a study published in The Planetary Science Journal today.

To measure the planet's temperature, the astronomers used thermal cameras that function by measuring the infrared light emitted from astronomical objects. And then they gathered all the images of Neptune over the last two decades and investigated infrared light emitted from a layer of Neptune’s atmosphere called the stratosphere. This led to a picture demonstrating the temperature and its variations during part of the southern summer of Neptune.

Something weird is happening to Neptune's temperature
Thermal images of Neptune taken between 2006 and 2020. | Souce: M. Roman/ESO

“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester and the lead author of the study. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”

Dramatic temperature changes

Just like other planets in the Solar system, Neptune also experiences seasons as it orbits the Sun. But these seasons are unlike the ones on the Earth since they last around 40 Earth years.

The southern hemisphere of the planet is going through summer since 2005. In addition to the fact that the southern hemisphere has been the cooling by 46 °F between 2003 and 2018, it is also reported that the temperature has rapidly risen by 51 °F between 2018 and 2020

“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” says Glenn Orton, a senior research scientist at Caltech’s Jet Propulsion Laboratory and the co-author of the study.

The study sheds light on how dramatically the temperature of the planet can change. The reason behind these rapid fluctuations is also intriguing since very little is known about the 8th planet. Therefore, more observations will be needed over the coming years to explain what's going on with the temperature of the planet.

We present an analysis of all currently available ground-based imaging of Neptune in the mid-infrared. Dating between 2003 and 2020, the images reveal changes in Neptune's mid-infrared (∼8–25 μm) emission over time in the years surrounding Neptune's 2005 southern summer solstice. Images sensitive to stratospheric ethane (∼12 μm), methane (∼8 μm), and CH3D (∼9 μm) display significant subseasonal temporal variation on regional and global scales. Comparison with H2 S(1) hydrogen quadrupole (∼17.035 μm) spectra suggests that these changes are primarily related to stratospheric temperature changes. The stratosphere appears to have cooled between 2003 and 2009 across multiple filtered wavelengths, followed by a dramatic warming of the south pole between 2018 and 2020. Conversely, upper-tropospheric temperatures—inferred from ∼17 to 25 μm imaging—appear invariant during this period, except for the south pole, which appeared warmest between 2003 and 2006. We discuss the observed variability in the context of seasonal forcing, tropospheric meteorology, and the solar cycle. Collectively, these data provide the strongest evidence to date that processes produce subseasonal variation on both global and regional scales in Neptune's stratosphere.

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