The planet Neptune is the most distant giant planet from the sun. Despite this, Neptune’s atmosphere is extremely dynamic, with meteorological phenomena evolving over a surprising range of timescales.
Neptune’s thick atmosphere is made up mostly of hydrogen and helium with just a little bit of methane. With the most powerful zonal winds in the solar system, Neptune’s highest clouds evolve so rapidly that the planet’s appearance can change dramatically over the course of days.
By gathering data from multiple observatories over almost two decades and combining it, an international team of scientists has revealed unexpected changes in the atmospheric temperatures of Neptune. The study suggests that Neptune is cooler than previously thought.
The study used observations in thermal-infrared wavelengths beyond the visible light spectrum, effectively sensing heat emitted from the planet’s atmosphere. After analyzing the data, scientists determined a more complete picture of trends in Neptune’s temperatures than ever before.
The data was gathered in the form of thermal infrared images from the European Southern Observatory’s Very Large Telescope and Gemini South telescope in Chile, together with the Subaru Telescope, Keck Telescope, and the Gemini North telescope, all in Hawai’i, and spectra from NASA’s Spitzer Space Telescope.
Neptune’s thermal brightness gradually dropped since reliable thermal imaging began in 2003. This indicates that the globally-averaged temperatures in Neptune‘s stratosphere have declined by roughly 8 degrees Celsius (14 degrees Fahrenheit) between 2003 and 2018.
Dr. Michael Roman, Postdoctoral Research Associate at the University of Leicester and lead author of the paper, said: “This change was unexpected. Since we have been observing Neptune during its early southern summer, we would expect temperatures to be slowly growing warmer, not colder.”
Dr. Glenn Orton, Senior Research Scientist at JPL and co-author of the study, noted: “Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes.”
On the other hand, at Neptune’s south pole. The data reveals some different changes. Observation from Gemini North in 2019 and Subaru in 2020 reveals that Neptune’s polar stratosphere warmed by roughly 11˚C (~20˚F) between 2018 and 2020. Such polar warming was never observed before on Neptune.
However, the cause of unexpected stratospheric temperature changes remains elusive. The results are challenging scientists’ understanding of Neptune’s atmospheric variability.
Dr. Roman stated, “Temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools. But random variability in weather patterns or even a response to the 11-year solar activity cycle may also affect.”
“The 11-year solar cycle has been previously suggested to affect Neptune’s visible brightness. the new study reveals a possible but tentative correlation between the solar activity, stratospheric temperatures, and the number of bright clouds seen on Neptune.”
Leigh Fletcher, Professor of Planetary Science at the University of Leicester, will lead such observations with JWST’s suite of instruments allocated time. Professor Fletcher, also a co-author of this study, said: “The exquisite sensitivity of the James Webb Space Telescope (JWST) ‘s mid-infrared instrument, MIRI, will provide unprecedented new maps of the chemistry and temperatures in Neptune’s atmosphere, helping to identify the nature of these recent changes better.”
- Michael T. Roman et al. Subseasonal Variation in Neptune’s Mid-infrared Emission. DOI: 10.3847/PSJ/ac5aa4