Jupiter is five times as far from the Sun as Earth is, and as such should have an average temperature in its upper atmosphere of approximately negative 73 degrees Celsius. Instead, the planet’s temperate is an astonishing 426 Celcius, a figure that has left scientists puzzled for the past half-century.
“We found that Jupiter’s intense aurora, the most powerful in the solar system, is responsible for heating the entire planet’s upper atmosphere to surprisingly high temperatures,” said Japan Aerospace Exploration Agency’s (Jaxa) James O’Donoghue, using findings from Nasa’s spacecraft Juno in collaboration with Jaxa’s Hisaki satellite and astronomers from Keck Observatory in Hawaii.
The aurora on Jupiter are visible bursts of light that occur when charged particles interact with the gas giant’s atmosphere. This is the same phenomenon that creates the aurora borealis, better known as the northern lights, on Earth.
It was recently discovered that it was also the cause of huge bursts of X-rays that erupted from the planet every 40 minutes, producing hundreds of gigawatts of energy – an amount that would take one power station on Earth years to produce.
Scientists are now more confident that the volcanic moon Io has led to huge amounts of heat in the upper atmosphere. This is a theory that had been proposed previously, but without adequate observations it was unable to be confirmed.
High-resolution temperature maps from the Keck II telescope in Hawaii, combined with magnetic field data from Hisaki and Juno, meant scientists could see the aurora sending a pulse of heat towards the equator.
It was previously thought, based on planetary models, that winds caused by heat from the aurora and directed towards the equator were redirected westward due to the 43,000 kilometre per hour speed at which Jupiter spins, giving the planet a day less than 10 hours long, and preventing auroral energy from escaping at the polar regions.
The new observations contradict this, finding that the winds are weaker than expected. What’s more, highly detailed maps now suggest that heat in the upper atmosphere is widely distributed with only a slight decrease at the equator.
“We also revealed a strange localized region of heating well away from the aurora - a long bar of heating unlike anything we’ve seen before,” said Tom Stallard, Associate Professor in Planetary Astronomy at the University of Leicester, who co-authored a paper on the findings. “Though we can’t be sure what this feature is, I am convinced it’s a rolling wave of heat flowing equatorward from the aurora.”
Observations from Hisaki made other striking discoveries, including that Jupiter’s magnetic field is strongly influenced by solar winds (streams of high-energy particles from the Sun) and that the separate magnetic fields from the winds are compressed when they meet Jupiter’s.
The result is an enhanced aurora, greater than it would be if the Hawaiian observatory had recorded the planet at a different time.
“Juno’s magnetic field data provided us with a ‘ground truth’ as to where the aurora was: this information isn’t readily available from heat maps, as heat leaks away in many directions,” said O’Donoghue.
“Picture this like a beach: if the hot atmosphere is water, the magnetic field mapped by Juno is shoreline, and the aurora is ocean, we found that water left the ocean and flooded the land, and Juno revealed where that shoreline was to help us understand the degree of flooding.”
O’Donoghue added that it was “pure luck” that astronomers managed to capture this event. “If we’d observed Jupiter on a different night, when the solar wind pressure had not recently been high, we would have missed it!”
It is also thought that Saturn, Uranus, and Neptune also have an “energy crisis” in which their upper atmospheres are unexpectedly hot, and while there is evidence that Saturn’s aurora is behind this mystery too Uranus and Neptune are so far that a probe is likely needed to verify such claims, O’Donoghue also tweeted.
The research paper, “Global upper-atmospheric heating on Jupiter by the polar aurorae”, has been published in Nature.