The upper atmosphere of Jupiter is hotter than expected from the amount of sunlight that it receives: the average temperature should be about minus 73 degrees Celsius; instead, the measured value soars to around 426 degrees Celsius. The source of this extra heat has remained elusive for 50 years, causing planetary scientists to refer to the discrepancy as an ‘energy crisis.’ Now, using data from NASA’s Juno spacecraft, the W.M. Keck Observatory and JAXA’s Hisaki satellite, researchers have discovered the likely source of this thermal boost.
Jupiter is shown in visible light for context underneath an artistic impression of the Jovian upper atmosphere’s infrared glow. The brightness of this upper atmosphere layer corresponds to temperatures, from hot to cold, in this order: white, yellow, bright red and lastly, dark red. The aurorae are the hottest regions and the image shows how heat may be carried by winds away from the aurora and cause planet-wide heating. Image credit: J. O’Donoghue, JAXA / Hubble / NASA / ESA / A. Simon / J. Schmidt.
Aurorae occur when electrically charged particles are caught in a planet’s magnetic field. These spiral along invisible lines of force in the magnetic field towards the planet’s magnetic poles, striking atoms and molecules in the atmosphere to release light and energy.
At Jupiter, material erupting from its volcanic moon, Io, leads to the most powerful aurora in the Solar System and enormous heating in the upper atmosphere over the polar regions of the planet.
The idea that the aurora could be the source of Jupiter’s mysterious energy had been proposed previously but observations have been unable to confirm or deny this, until now.
“We found that Jupiter’s intense aurorae, the most powerful in the Solar System, are responsible for heating the entire planet’s upper atmosphere to surprisingly high temperatures,” said Dr. James O’Donoghue, a researcher at the JAXA Institute of Space and Astronautical Science.
High-resolution temperature maps from the NIRSPEC (Near-InfraRed Spectrometer) and the 10-m Keck II telescope at the W.M. Keck Observatory, combined with magnetic field data from Hisaki and Juno spacecraft, allowed the scientists to catch the aurora in the act of sending what appears to be a pulse of heat toward Jupiter’s equator.
They created five maps of the atmospheric temperature at different spatial resolutions, with the highest resolution map showing an average temperature measurement for squares two degrees longitude ‘high’ by two degrees latitude ‘wide.’
They scoured more than 10,000 individual data points, only mapping points with an uncertainty of less than 5%.
Models of the atmospheres of gas giants suggest that they work like a giant refrigerator, with heat energy drawn from the equator towards the pole, and deposited in the lower atmosphere in these pole regions.
These new findings suggest that fast-changing aurorae may drive waves of energy against this poleward flow, allowing heat to reach the equator.
The observations also showed a region of localized heating in the sub-auroral region that could be interpreted as a limited wave of heat propagating equatorward, which could be interpreted as evidence of the process driving heat transfer.
“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,” Dr. O’Donoghue said.
The authors will continue to analyze the data and produce more maps.
Their goal is to catch Jupiter’s aurora spew another hot spot, this time observing it over a 2-3 day period so they can track its energy as it moves around the planet.
“Can we observe one of these features moving? Will it show the flow of auroral heat in action? How does this flow of energy then affect the surrounding magnetic fields that we now know are so complex?” said Dr. Tom Stallard, a researcher at the University of Leicester.
“It’s a thrilling set of research questions in a region of Jupiter’s ionosphere that, five years ago, we thought of as mundane.”
The new results were published in the journal Nature.
J. O’Donoghue et al. Global upper-atmospheric heating on Jupiter by the polar aurorae. Nature 596, 54-57; doi: 10.1038/s41586-021-03706-w