Nearly two decades of observations have shown that the southern hemisphere of the planet Neptune has been slowly cooling when it is supposed to be heating up. Scientists don’t know why.
surprising changes
Considering that it is the farthest planet in our solar system, it has never surprised us that it is one of the coldest, with an average temperature of -207 ºC. However, new research published in The Planetary Science Journal reveals that the ice giant could be even colder than previously thought.
An international team of researchers (including scientists from the University of Leicester and NASA’s Jet Propulsion Laboratory) have analyzed thermal infrared images of Neptune from multiple observatories for nearly 20 years , including the European Southern Observatory’s Very Large Telescope , the Subaru Telescope, the Keck Telescope, the Gemini North Telescope, or NASA’s Spitzer Space Telescope.
The study used observations in thermal infrared wavelengths beyond the visible light spectrum, effectively detecting heat emitted by the planet’s atmosphere. Observations show that summer on Neptune appears to be getting cooler. The temperature in the planet’s southern hemisphere has been falling , even though these measurements were taken in the early part of its long summer.
Why do temperatures fluctuate?
This changing trend in Neptune’s temperatures is surprising. The collective data sets show a decrease in Neptune’s thermal brightness since reliable thermal imaging began in 2003, indicating that average temperatures in Neptune’s stratosphere have decreased by approximately 8 degrees Celsius between 2003 and 2018.
“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study. “Since we’ve been observing Neptune during the early austral summer, we expected temperatures to get warmer, not cooler .”
The reason for this temperature fluctuation is unknown . However, the researchers believe that changes in atmospheric chemistry could be behind the observed variations. The rapid warming could simply be due to weather (we already saw something similar on Saturn during the formation of a large storm over its north pole), but long-term cooling is likely to be more complicated than this, the scientists say.
“While methane absorbs sunlight and heats the atmosphere, photochemically produced hydrocarbons, primarily ethane and acetylene, are powerful infrared emitters that serve to cool the stratosphere,” the authors explain. “The balance between this radiative heating and cooling changes as the amount of photochemical hydrocarbons changes.”
Neptune takes more than 165 Earth years to complete a circuit around the sun, so the seasons last more than 40 Earth years, which is why it was so unexpected that these changes are so little gradual. Without going any further, between 2018 and 2020, an area of the planet warmed up to about 11 ° C , another surprise near the south pole of the planet. It is something that has never been observed.
“The temperature variations may be related to seasonal changes in Neptune’s atmospheric chemistry, which can alter how effectively the atmosphere cools,” explains Roman. “But random variability in weather patterns or even a response to the 11-year solar activity cycle could also have a major effect.”
What do we need to solve the puzzle?
More decades of observations. The James Webb Space Telescope, which is already ‘warming up’, is also expected to help in further monitoring Neptune’s temperature and cloud patterns. Webb will provide unprecedented maps of the planet, both the chemistry and the temperature in the atmosphere of the frigid planet.
“The exquisite sensitivity of the space telescope’s mid-infrared instrument, MIRI, will provide unprecedented new maps of the chemistry and temperatures in Neptune’s atmosphere, helping to better identify the nature of these recent changes,” concludes Leigh Fletcher, Professor of Planetary Sciences at the University of Leicester and also co-author of the work.
Referencia: “Sub-Seasonal Variation in Neptune’s Mid-Infrared Emission” The Planetary Science Journal (2022). DOI: 10.3847/PSJ/ac5aa4
