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European
Southern
Observatory

eso2206 -- Science Release

ESO telescope captures surprising changes in Neptune's temperatures

11 April 2022

 
[eso2206a]
  

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.

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 degC 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 degC 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 kilometres away and is very
cold, the planet's average temperature reaching around -220degC,
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."

More information

This research was presented in the paper "Sub-Seasonal Variation in
Neptune's Mid-Infrared Emission" published today in The Planetary
Science Journal (doi:10.3847/PSJ/ac5aa4).

The team is composed of M. T. Roman and L. N. Fletcher (School of
Physics and Astronomy, University of Leicester, UK), G. S. Orton (Jet
Propulsion Laboratory/California Institute of Technology, California,
USA), T. K. Greathouse (Southwest Research Institute, San Antonio,
TX, USA), J. I. Moses (Space Science Institute, Boulder, CO, USA), N.
Rowe-Gurney (Department of Physics and Astronomy, Howard University,
Washington DC, USA; Astrochemistry Laboratory, NASA/GSFC, Greenbelt,
MD, USA; Center for Research and Exploration in Space Science and
Technology, NASA/GSFC, Greenbelt, MD, USA), P. G. J. Irwin
(University of Oxford Atmospheric, Oceanic, and Planetary Physics,
Department of Physics Clarendon Laboratory, Oxford, UK), A. Antunano
(UPV/EHU, Escuela Ingernieria de Bilbao, Spain), J. Sinclair (Jet
Propulsion Laboratory/California Institute of Technology, California,
USA), Y. Kasaba (Planetary Plasma and Atmospheric Research Center,
Graduate School of Science, Tohoku University, Japan), T. Fujiyoshi
(Subaru Telescope, National Astronomical Observatory of Japan, HI,
USA), I. de Pater (Department of Astronomy, University of California
at Berkeley, CA, USA), and H. B. Hammel (Association of Universities
for Research in Astronomy, Washington DC, USA).

The European Southern Observatory (ESO) enables scientists worldwide
to discover the secrets of the Universe for the benefit of all. We
design, build and operate world-class observatories on the ground --
which astronomers use to tackle exciting questions and spread the
fascination of astronomy -- and promote international collaboration in
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today ESO is supported by 16 Member States (Austria, Belgium, the
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centre and planetarium, the ESO Supernova, are located close to
Munich in Germany, while the Chilean Atacama Desert, a marvellous
place with unique conditions to observe the sky, hosts our
telescopes. ESO operates three observing sites: La Silla, Paranal and
Chajnantor. At Paranal, ESO operates the Very Large Telescope and its
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telescopes, VISTA working in the infrared and the visible-light VLT
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engage with Chilean partners and society. 

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Contacts

Michael Roman
School of Physics and Astronomy, University of Leicester
Leicester, UK
Email: m.t.roman@leicester.ac.uk

Glenn Orton
Caltech's Jet Propulsion Laboratory (JPL)
Pasadena, California, US
Email: glenn.s.orton@jpl.nasa.gov

Leigh Fletcher
School of Physics and Astronomy, University of Leicester
Leicester, UK
Tel: + 44 (0)116 252 3585
Email: leigh.fletcher@le.ac.uk

Barbara Ferreira
ESO Media Manager
Garching bei Munchen, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 0
Email: press@eso.org

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[                    ]
About the Release

Release No.: eso2206
Name:        Neptune
Type:        Solar System : Planet : Feature : Atmosphere
Facility:    Very Large Telescope
Instruments: VISIR

Images

Thermal images of Neptune taken between 2006 and 2020
PR Image eso2206a
Thermal images of Neptune taken between 2006 and 2020
Thermal images of Neptune taken between 2006 and 2020 (different
layout)
PR Image eso2206b
Thermal images of Neptune taken between 2006 and 2020 (different
layout)
Neptune from the VLT and Hubble
PR Image eso2206c
Neptune from the VLT and Hubble
---------------------------------------------------------------------

Videos

Evolution of thermal images from Neptune
PR Video eso2206a
Evolution of thermal images from Neptune
---------------------------------------------------------------------
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