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Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures

April 11, 2022
Last updated: July 1, 2023

Mid-infrared observations with the Subaru Telescope and other telescopes have revealed how temperatures in Neptune’s atmosphere have unexpectedly fluctuated over the past two decades.

Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures Figure1

Figure 1: Neptune as seen in visible light (center) and thermal-infrared (right), in 2020. The visible light image combines multiple images from the Hubble Space Telescope, while the thermal-infrared image was taken by the Subaru Telescope. The left picture shows the disk geometry in 2020, when the Neptune's Antarctic side is facing towards the Earth. In the Subaru Telescope's image, Neptune’s warm south pole glows more brightly than it has ever been seen before. (Credit: Michael Roman/NASA/ESA/STSci/M.H. Wong/L.A. Sromovsky/P.M. Fry)

An international team of researchers, including ones from the University of Leicester, NASA’s Jet Propulsion Laboratory (JPL), Tohoku University, and Subaru Telescope, examined all existing thermal infrared images of Neptune gathered from multiple observatories over almost two decades. These include 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. The observations at thermal-infrared wavelengths effectively sense heat emitted from the planet’s atmosphere.

By analyzing the data, the researchers were able to reveal a more complete picture of trends in Neptune’s temperatures than ever before.

Neptune is cooler than we thought: Study reveals unexpected changes in atmospheric temperatures Figure2

Figure 2: Observed changes in Neptune’s thermal-infrared brightness, a measure of the temperature in Neptune’s atmosphere. The plot shows the relative change in the thermal-infrared brightness from Neptune’s stratosphere with time for all existing images taken by ground-based telescopes. Brighter images are interpreted as warmer. Corresponding thermal-infrared images (top) at wavelengths of ~12 µm show Neptune’s appearance in 2006, 2009, 2018 (observed by the VLT’s VISIR instrument) and 2020 (observed by the Subaru Telescope’s COMICS). (Credit: Michael Roman/NASA/JPL/Voyager-ISS/Justin Cowart)

But to the researchers’ surprise, these collective datasets show a decline in Neptune’s thermal brightness from 2003, when reliable thermal images started to become available, to 2018, indicating that globally-averaged temperatures in Neptune’s stratosphere – the layer of the atmosphere just above its active weather layer – have dropped by roughly 8 degrees Celsius.

"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." says Dr. Michael Roman, Postdoctoral Research Associate at the University of Leicester and lead author on the paper.

Neptune has an axial tilt, and so it experiences seasons, just like Earth. However, given its great distance from the Sun, Neptune takes over 165 years to complete an orbit, and so its seasons change slowly, lasting over 40 Earth-years each.

Dr. Glenn Orton, Senior Research Scientist at JPL and co-author on the study, noted "Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes."

Yet, at Neptune’s south pole, the data reveal a different and surprisingly dramatic change. A combination of observations from Gemini North in 2019 and the Subaru Telescope in 2020 show that Neptune’s polar stratosphere warmed by roughly 11˚C between 2018 and 2020, reversing the previous globally averaged cooling trend. The temperature in Neptune's atmosphere appears to have become dramatically warmer in just the past few years. Such polar warming has never been observed on Neptune before.

The cause of these unexpected stratospheric temperature changes is currently unknown, and the results challenge scientists’ understanding of Neptune’s atmospheric variability.

"Temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools," continues Dr. Roman, "but random variability in weather patterns or even a response to the 11-year solar activity cycle may also have an effect."

The 11-year solar cycle (marked by periodic variation in the Sun’s activity and sunspots) has been previously suggested to affect Neptune’s visible brightness, and this new study reveals a possible, but tentative, correlation between the solar activity, stratospheric temperatures, and the number of bright clouds seen on Neptune.

The Subaru Telescope's COMICS observations that played an important role in this study were made in 2011, 2012, and in 2020. In particular, the July 2020 image that led to the discovery of the rapid temperature increase, amid one of the earlier peaks of the COVID-19 pandemic, was obtained with this instrument's "Final Light." Dr. Yasumasa Kasaba (Tohoku University) and Dr. Takuya Fujiyoshi (Subaru Telescope), who conducted the final COMICS observations, said, "Since this was the last opportunity, we carefully acquired data from Jupiter, Saturn, Uranus, and Neptune. We are truly surprised and pleased to have obtained this valuable discovery from them." Observations in the thermal infrared with Japanese telescopes will be continued by the University of Tokyo's Atacama 6.5-m telescope TAO, which is currently under construction in Chile.

Long-term follow-up observations are needed to confirm the tentative correlation between the solar activity and the state of Neptune's stratosphere newly discovered in this study. Neptune, along with Uranus, is a member of the "ice giants," which are formed by the accumulation of icy bodies like comets and have relatively low gas content. These two planets have not yet been explored by orbiters, and are attracting international attention as targets for the next generation planetary explorer. The James Webb Space Telescope's MIRI mid-infrared instrument, which is scheduled to observe Uranus and Neptune at the end of this year, will be the first to make such follow-up observations. Led by Dr. Leigh Fletcher (University of Leicester, UK), who is also a co-investigator of this study, MIRI is expected to provide unprecedented new data on the chemistry and temperature of Neptune's atmosphere.

Video: Thermal images of Neputune, taken between 2006 and 2021, show Neptune gradually cooling down, before a dramatic heating of its south pole in the last few years. These images were taken by the VLT's VISIR instrument. (Credit: ESO/M. Roman)

These results appeared on April 11, 2022 as Roman et al. "Sub-Seasonal Variation in Neptune’s Mid-Infrared Emission" in the Planetary Science Journal.

About the Subaru Telescope
The Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan, National Institutes of Natural Sciences with the support of the MEXT Project to Promote Large Scientific Frontiers. We are honored and grateful for the opportunity of observing the Universe from Maunakea, which has cultural, historical, and natural significance in Hawai`i.

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