An international team of astronomers have used ground-based telescopes,
including the European Southern Observatory's Very Large Telescope (ESO's
VLT), to track Neptune's atmospheric temperatures over a 17-year period.
They found a surprising drop in Neptune's global temperatures followed by a
dramatic warming at its south pole.
"This change was unexpected," says Michael Roman, a postdoctoral research
associate at the University of Leicester, UK, and lead author of the study
published today in The Planetary Science Journal. "Since we have been
observing Neptune during its early southern summer, we expected temperatures
to be slowly growing warmer, not colder."
Like Earth, Neptune experiences seasons as it orbits the Sun. However, a
Neptune season lasts around 40 years, with one Neptune year lasting 165
Earth years. It has been summertime in Neptune's southern hemisphere since
2005, and the astronomers were eager to see how temperatures were changing
following the southern summer solstice.
Astronomers looked at nearly 100 thermal-infrared images of Neptune,
captured over a 17-year period, to piece together overall trends in the
planet's temperature in greater detail than ever before.
|
| Observed changes in Neptune's thermal-infrared brightness, a measure of 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 European Southern Observatory's Very Large Telescope's VISIR instrument), and 2020 (observed by Subaru's COMICS instrument). The south pole appears to have become dramatically warmer in just the past few years. Credit: Michael Roman/NASA/JPL/Voyager-ISS/Justin Cowart. |
These data showed that, despite the onset of southern summer, most of the
planet had gradually cooled over the last two decades. The globally averaged
temperature of Neptune dropped by 8 °C between 2003 and 2018.
The astronomers were then surprised to discover a dramatic warming of
Neptune's south pole during the last two years of their observations, when
temperatures rapidly rose 11 °C between 2018 and 2020. Although Neptune's
warm polar vortex has been known for many years, such rapid polar warming
has never been previously observed on the planet.
"Our data cover less than half of a Neptune season, so no one was expecting
to see large and rapid changes," says co-author Glenn Orton, senior research
scientist at Caltech's Jet Propulsion Laboratory (JPL) in the US.
The astronomers measured Neptune's temperature using thermal cameras that
work by measuring the infrared light emitted from astronomical objects. For
their analysis the team combined all existing images of Neptune gathered
over the last two decades by ground-based telescopes. They investigated
infrared light emitted from a layer of Neptune's atmosphere called the
stratosphere. This allowed the team to build up a picture of Neptune's
temperature and its variations during part of its southern summer.
Because Neptune is roughly 4.5 billion kilometers away and is very cold, the
planet's average temperature reaching around –220°C, measuring its
temperature from Earth is no easy task. "This type of study is only possible
with sensitive infrared images from large telescopes like the VLT that can
observe Neptune clearly, and these have only been available for the past 20
years or so," says co-author Leigh Fletcher, a professor at the University
of Leicester.
Around one third of all the images taken came from the VLT Imager and
Spectrometer for mid-InfraRed (VISIR) instrument on ESO's VLT in Chile's
Atacama Desert. Because of the telescope's mirror size and altitude, it has
a very high resolution and data quality, offering the clearest images of
Neptune. The team also used data from NASA's Spitzer Space Telescope and
images taken with the Gemini South telescope in Chile, as well as with the
Subaru Telescope, the Keck Telescope, and the Gemini North telescope, all in
Hawai'i.
Because Neptune's temperature variations were so unexpected, the astronomers
do not know yet what could have caused them. They could be due to changes in
Neptune's stratospheric chemistry, or random weather patterns, or even the
solar cycle. More observations will be needed over the coming years to
explore the reasons for these fluctuations. Future ground-based telescopes
like ESO's Extremely Large Telescope (ELT) could observe temperature changes
like these in greater detail, while the NASA/ESA/CSA James Webb Space
Telescope will provide unprecedented new maps of the chemistry and
temperature in Neptune's atmosphere.
"I think Neptune is itself very intriguing to many of us because we still
know so little about it," says Roman. "This all points towards a more
complicated picture of Neptune's atmosphere and how it changes with time."
Reference:
Sub-Seasonal Variation in Neptune's Mid-Infrared Emission" The Planetary
Science Journal (2022).
DOI: 10.3847/PSJ/ac5aa4
Tags:
Space & Astrophysics