Jupiter, the largest planet in our solar system, has never been short on mystery. But one question that has proven particularly puzzling is why the planet's upper atmosphere sizzles away at extraordinarily high temperatures when it is so far from the Sun. Decades have gone by without a plausible explanation, but scientists have now come across evidence suggesting the enigmatic Great Red Spot is acting as the supplier of this mysterious heat.

Researchers have been aware of the stifling heat in Jupiter's upper atmosphere, or thermosphere, for around four decades, since NASA's Voyager spacecraft passed through the area and ground-based telescopes took its temperature in the 1970s. At around 500 miles (800 km) above the planet's stormy surface, the thermosphere has temperatures of more than 1,000° F, (537° C), which wouldn't look out of place if found in the same part of our atmosphere on Earth.

Here these temperatures can be explained by solar energy, which heats the upper atmosphere to such high temperatures. But the gas giant is more than fives times the distance from the Sun as we are, so why on Jupiter is it so hot up there? This has perplexed scientists since they discovered the obscene temperatures, with electrical currents from aurorae one explanation floated but then later ruled out, causing some to label the mystery an "energy crisis." But now an international team of researchers believe they have come up with the answer.

Astronomers from Boston University and England's University of Leicester were using a telescope in Hawaii to measure levels of non-visible infrared light emitted by the planet to produce a heat map of its upper atmosphere. But partway through the task, Boston's James O'Donoghue was caught off guard by an unexpected discovery.

The research scientists found a hugely irregular temperature spike 500 miles above the Great Red Spot. At 1,600 Kelvin (2,420° F/1,327° C), it was almost twice as hot as the surrounding atmosphere, leading O'Donoghue and his colleagues to suspect that the wild hurricane below was heating the planet's upper atmosphere.

"When I first looked at it, I just didn't believe it," says O'Donoghue. "It took a good month of work to convince ourselves. But it's undeniable, really. This is possibly the smoking gun that tells you that there is some interaction between Jupiter's lower atmosphere and the upper atmosphere."

It is still early days in terms of understanding how exactly this interaction plays out, but the researchers have some ideas. They believe that within the giant storm in the Great Red Spot are two kinds of turbulent energy forces: acoustic waves and gravity waves. When these two waves collide they then travel upwards through other atmospheric layers to dump their collective energy into the thermosphere, kind of like waves crashing onto the beach.

Similar phenomena could be playing out on other gas giants in our solar system, Saturn, Uranus and Neptune, which also feature thermospheres hundreds of degrees hotter than the science has told us they should be.

The researchers are now continuing to build their temperature map of the upper atmosphere, looking to establish further links between hot spots in the thermosphere and the wild weather down below. They hope this will answer some of the questions they have, such as why the hot spot only appears over the north half of the Great Red spot, and whether it is constantly hot or also goes through cool periods.

"Energy transfer to the upper atmosphere from below has been simulated for planetary atmospheres, but not yet backed up by observations," O'Donoghue says. "The extremely high temperatures observed above the storm appear to be the smoking gun of this energy transfer, indicating that planet-wide heating is a plausible explanation for the 'energy crisis.'"

The research was published in the journal Nature.

Source: NASA, Boston University (1), (2)

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