At an average distance of 778 million kilometres (484 million miles) from the Sun, one would expect Jupiter to be a fairly chilly world. Based on the amount of sunlight reaching the giant planet, the upper atmosphere should be no warmer than a frigid 73 Celsius (minus 100 Fahrenheit). But the average temperature is actually a more hellish 426 C (800 F), prompting head scratching for the past 50 years.
But not any longer.
An international team of astronomers using data from NASA’s Juno spacecraft, the W.M. Keck Observatory and Japan’s Hisaki satellite was able to nail down the source of Jupiter’s surprising temperature, reporting their findings in the journal Nature.
“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 James O’Donoghue of the JAXA Institute of Space and Astronautical Science in Sagamihara, Japan.
Auroras are caused when electrically charged particles spiral along magnetic field lines and crash into a planet’s upper atmosphere near its poles. As incoming particles collide with atoms and molecules in the atmosphere, light and energy are released. On Earth, the collisions generate the northern and southern lights. But at Jupiter, the auroras are much more intense thanks to the contribution of material spewing from the volcanic moon Io.
While astronomers had previously consider the auroras a possible explanation for the atmospheric heating, earlier models indicated heat from the polar regions would not reach lower latitudes because of high-speed winds powered by Jupiter’s swift rotation.
But high-resolution temperature maps based on Keck observations of more than 10,000 data points, along with magnetic field data from Juno and Hisaki, showed what appears to be a powerful atmospheric heat pulse pushing down toward the equator.
“We’ve attempted this multiple times with other instruments but with Keck’s NIRSPEC (Near-Infrared Spectrograph), we were able to measure for the very first time the light from Jupiter all the way to the equator quickly enough that we can then map out the temperature and ionospheric density,” said Tom Stallard, a co-author of the paper at the University of Leicester.
Instead of seeing high temperatures only in the polar regions, the maps showed heat in the upper atmosphere was more widely spread out, gradually decreasing toward the equator.
“We also revealed a strange localised region of heating well away from the aurora – a long bar of heating unlike anything we’ve seen before,” said Stallard. “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.”