Mysterious Temperature Changes on Frigid Neptune: From Sudden Cooling to Warming

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Scientists often expect one thing and find another.

The surprise this week has been brought by Neptune, the planet of the Solar System farthest from the Sun.

For 17 years, between 2003 and 2020, this icy giant was scrutinized with powerful ground-based telescopes to study its evolution.

The international team that has monitored it has seen very sudden changes in temperature whose causes they cannot yet explain, as they admit this Monday in an article published in

In a very short period of time such as 17 years, they detected a surprising drop in the global temperature of Neptune which was followed after a few years by "dramatic warming" at its south pole.

"It was an unexpected change.

Michael Roman, a postdoctoral research associate at the UK's University of Leicester and lead of this investigation.

To measure the temperatures of Neptune,

they point to, in this case, this planet located about 4,500 million kilometers away.

To analyze them, they combined all the images of Neptune taken over the last two decades with ground-based telescopes, focusing on the infrared light emitted from the stratosphere (one of the layers that makes up Neptune's atmosphere).

This allowed them to reconstruct their temperatures and their variations during part of their southern summer.

Neptune, captured by the VLT telescope.

Its atmosphere is made up of hydrogen, helium and methane.

The latter gives it that blue colorESO/P.

Weilbach (AIP)

Being so far from the Sun it is an extremely cold planet, with an

This distance also makes it difficult to study.

"This kind of research is only possible with sensitive infrared images taken with large telescopes like the VLT, which can see Neptune clearly. And we've only had such images for about 20 years," says Leigh Fletcher, a professor at the University of Leicester and co-author of the study.

Seasons that last 40 Earth years

Neptune orbits the Sun but due to its distance, its year lasts about 165 Earth years and a season, 40 Earth years.

For this reason, since 2005 it has been summer at the south pole of Neptune and that has been the season they have been monitoring during these 17 years covered by the study.

However,

so that between 2003 and 2018 the average temperature of the planet fell by 8ºC.

Later, between 2018 and 2020, they were again surprised to see how the south pole warmed up, as

The authors recall that although Neptune's warm polar vortex has been known for many years, such rapid polar heating has never before been observed on the planet.

"Our study covers less than half of a season on Neptune, so none expected to see such large and rapid changes," says co-author Glenn Orton, a scientist at NASA's Jet Propulsion Laboratory (JPL).

Among the possible explanations for this abrupt temperature change, astronomers consider possible changes in the chemistry of Neptune's stratosphere, random weather patterns or the solar cycle.

To try to clarify it, they will use the next and more powerful generation of telescopes: on the ground the

(ELT) that is being built in Chile, and in space the

that will be operational in a few months.

With it they hope to draw new maps of the chemistry and temperature of the Neptunian atmosphere.

THERMAL IMAGES

The series of thermal images of Neptune that illustrate the article are composed of a hundred snapshots taken between 2006 and 2020 and show the evolution of its temperatures during that period in great detail.

The first three (corresponding to 2006, 2009 and 2018) were taken with the VISIR instrument of the Very Large Telescope (VLT) of the European Southern Observatory (ESO) in the Chilean Atacama Desert, while the 2020 one was taken with the COMICS instrument of the Subaru telescope because,

.

Following gradual cooling, the south pole has warmed dramatically in recent years, as reflected in the bright spot in images captured between 2018 and 2020.

Images captured since 2003 by NASA's Spitzer Space Telescope, the Gemini South Ground Telescope in Chile, and the Kech and Gemini North telescopes, both in Hawaii, were also used.

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