Temperatures in Neptune’s atmosphere have surprisingly changed during the last two decades, according to new study.
The research, which was published today (Monday) in Planetary Science Journal, employed data in thermal-infrared wavelengths beyond the visible light spectrum to detect heat released by the planet’s atmosphere.
An multinational team of researchers, including experts from the University of Leicester and NASA’s Jet Propulsion Laboratory (JPL), integrated all previously collected thermal infrared photos of Neptune from various observatories over over two decades. These include the Very Large Telescope and Gemini South telescopes of the European Southern Observatory in Chile, as well as the Subaru Telescope, Keck Telescope, and Gemini North telescopes in Hawaii, and spectra from NASA’s Spitzer Space Telescope.
The researchers were able to offer a more thorough picture of changes in Neptune’s temperatures by analysing the data than ever before.
However, these collective datasets show a decline in Neptune’s thermal brightness since reliable thermal imaging began in 2003, indicating that globally-averaged temperatures in Neptune’s stratosphere (the layer of the atmosphere just above its active weather layer) have dropped by approximately 8 degrees Celsius (14 degrees Fahrenheit) between 2003 and 2018.
Dr. Michael Roman, Postdoctoral Research Associate at the University of Leicester and the paper’s primary author, stated:
“This shift was unanticipated.” Since we’ve been watching Neptune throughout its early southern summer, we’d anticipate temperatures to gradually rise rather than fall.”
Because Neptune has an axial tilt, it experiences seasons like Earth. However, due to its immense distance from the Sun, Neptune’s cycle around its host star takes over 165 years, therefore its seasons change slowly, lasting over 40 Earth-years each.
Dr. Glenn Orton, Senior Research Scientist at JPL and research co-author, stated:
“Because our data only span half of a Neptune season, no one expected to find huge and quick changes.”
However, observations from Neptune’s south pole show a distinct and shockingly substantial alteration. Observations from Gemini North in 2019 and Subaru in 2020 show that Neptune’s polar stratosphere warmed by around 11 degrees Celsius (20 degrees Fahrenheit) between 2018 and 2020, reversing the previous globally-averaged cooling trend. Such polar warming has never been witnessed previously on Neptune.
The reason of these unanticipated stratospheric temperature variations is unclear at this time, and the findings call into question scientists’ knowledge of Neptune’s atmospheric variability.
Dr. Roman went on:
“Temperature fluctuations may be linked to seasonal changes in Neptune’s atmospheric chemistry, which can affect how efficiently the atmosphere cools.”
“However, random fluctuation in weather patterns or even a reaction to the 11-year solar activity cycle might have an impact.”
The 11-year solar cycle (characterised by periodic variations in the Sun’s activity and sunspots) has previously been proposed to influence Neptune’s visible brightness, and the new study reveals a possible, but tentative, correlation between solar activity, stratospheric temperatures, and the number of bright clouds seen on Neptune.
Follow-up studies of temperature and cloud patterns are required in the coming years to further analyse any potential link.
The James Webb Space Telescope (JWST), which is slated to monitor both ice giants, Uranus and Neptune, later this year, will provide answers to these and other puzzles.
Professor of Planetary Science at the University of Leicester, Leigh Fletcher, will lead such observations using JWST’s array of equipment. Professor Fletcher, another research co-author, stated:
“The remarkable sensitivity of the space telescope’s mid-infrared sensor, MIRI, will offer unparalleled new maps of the chemistry and temperatures in Neptune’s atmosphere, assisting in the identification of the nature of these recent changes.”
GIANTCLIMES, a European Research Council grant to the University of Leicester, supported this investigation. This study previously identified long-term shifts in atmospheric temperatures and clouds on Jupiter and Saturn, as well as the first maps of Uranus’ stratospheric temperatures. GIANTCLIMES has laid the groundwork for future discoveries on all four big planets from JWST.
Thomas Greathouse (Southwest Research Institute), Julianne Moses (Space Science Institute), Naomi Rowe-Gurney (Howard University / NASA Goddard Space Flight Center), Patrick Irwin (Oxford), Arrate Antuano (UPV/EHU), James Sinclair (JPL), Yasumasa Kasaba (Tohoku University), Takuya Fujiyoshi (Subaru Telescope), Imke de Pater (UC Berkeley), and Heidi Hammel are also authors (Association of Universities for Research in Astronomy).