The South Pole is hot - on Neptune

Warming island sends methane gas into the upper atmosphere

Neptune - taken by Voyager 2 © NASA / JPL
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A south pole that is not icy, but much hotter than the rest of the planet - astronomers have now discovered this on Neptune. Using the Very Large Telescope of the European Southern Observatory, they created the first temperature map of the outer planet of our solar system. It has now been published in the journal Astronomy & Astrophysics.

Neptune orbits the sun at a distance about 30 times greater than Earth. The intensity of the sun's rays, which still arrives on the planet, is correspondingly low: only one-hundredth of the solar energy that reaches the earth every day. But even this small amount is enough to produce extreme temperature differences on the planet's surface, as a new study has shown.

Southern summer produces heat island

An international team of astronomers used the VISIR spectrometer at the Unit Telescope of the European Southern Observatory in Chile to create for the first time a temperature map of the lowest atmospheric layer of Neptune. They hit a hot spot, which coincided exactly with the position of the Neptunian South Pole. While the average temperature on the planet is minus 200 degrees Celsius, it is ten degrees higher at the South Pole.

Temperature images of Neptune © VISIR / VLT / ESO

According to astronomers, these differences can be explained by the seasons on Neptune. A Neptune Year lasts 165 Earth years, summer has been around for about 40 years in the southern hemisphere of the planet. "The south pole of Neptune is currently facing the sun, similar to the earthly south pole during the summer in the southern hemisphere, " explains Glenn Orton, lead author of the study. "But on Neptune, the Antarctic summer lasts just 40 years instead of just a few months, during which time the sun's rays can produce large temperature differences between the regions with continuous sunlight and those with day-night variations."

The strong temperature differences are also the cause of the extremely violent winds on the planet. "Neptune has the strongest storms of any solar system planet, " Orton says. "Sometimes the wind blows here at more than 2, 000 kilometers per hour. It is certainly not a place where you would like to stay. "Display

Methane lock in the upper atmosphere

However, the heat island at the South Pole could have an effect, as it offers gas methane a chance to ascend from the deep atmosphere up into the Neptunian stratosphere. While methane is not the main constituent of the planetary atmosphere, it's hydrogen and helium, but it's methane that gives the planet its typical blue color. Because the gas absorbs the red part of the sunlight and reflects the blue back into space this makes the Neptune shimmer blue.

"Temperatures here are so high that methane gas, which normally freezes out of the upper atmosphere, can leak into these regions, " says Orton. "It also clears the long-standing riddle of where the stratosphere gets its methane supply."

The new temperature map also contains mysterious "hotspots" in the stratosphere of the higher latitudes, which are not found on any other planet in the solar system. According to the astronomers, they could arise wherever gas from deeper layers rises into the upper atmosphere.

(ESO, 19.09.2007 - NPO)