Jupiter’s energy crisis solved: Auroras roast upper atmosphere

Jupiter is five times farther from the sun than Earth. So why is its upper atmosphere so much hotter than ours? That’s the question of Jupiter’s energy crisis that scientists have been trying to answer for more than 50 years. On August 4, 2021, a team of scientists led by James O’Donoghue of JAXA announced they’ve solved the puzzle. O’Donoghue said:

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.

The peer-reviewed journal Nature published the team’s new study on August 4.

Jupiter’s energy crisis: origins

For 50 years, scientists have wondered why the upper atmosphere of Jupiter is so hot even though it’s so far from the sun. This fundamental puzzle has been referred to as Jupiter’s energy crisis. The other giant planets – Saturn, Uranus and Neptune – are similarly hotter than expected. So scientists sometimes call this problem the giant planet energy crisis.

Jupiter’s upper atmosphere isn’t just a little hotter than expected. It’s much hotter. Based solely on Jupiter’s distance from the sun, its upper atmosphere should be about -100 F (-73 C). But observations show Jupiter’s upper atmosphere instead soars to temperatures of 800 F (426 C).

While auroras were a key suspect in the case of the hot upper atmosphere on giant Jupiter, scientists weren’t sure how the aurora’s effect would work in light of Jupiter’s strong winds and fast rotation. Jupiter has incredibly fast winds blowing from east to west near the equator, plus it rotates faster than any other planet in our solar system. Computer modeling showed the east-west equatorial winds driven by Jupiter’s rapid rotation would overwhelm the hot auroral winds heading toward the equator and redirect them westward. Basically, they believed the planet’s fast rotation and speedy winds would gang up to prevent the auroral heating from spreading across the globe.

Yet this isn’t what the scientists found.

Auroras roast Jupiter’s upper atmosphere

The scientists used data from a trio of observatories: NASA’s Juno spacecraft in orbit around Jupiter, JAXA’s Hisaki satellite and the Keck Observatory in Hawaii.

These observatories collected data on Jupiter, its aurora and magnetic field (the trigger that produces auroras), which scientists then analyzed. The Keck observations, taken on two nights in April 2016 and January 2017, allowed scientists to trace electrically charged particles from the poles to the equator. Scientists made a map of temperatures in Jupiter’s upper atmosphere that include 10,000 individual data points. These high-resolution maps of the upper atmosphere showed the heat widely distributed, with temperatures only slowly tapering off from the poles toward the equator. Tom Stallard of the University of Leicester said:

We also revealed a strange localized region of heating well away from the aurora. It’s a long bar of heating unlike anything we’ve seen before. 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.

Crucially, the team’s observations from January 2017 showed a heat wave emanating from one of the auroral rings, which coincided with a high-pressure pocket of solar wind that collided with Jupiter. O’Donoghue added:

It was pure luck that we captured this potential heat-shedding 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!

The Hisaki satellite in Earth orbit has been monitoring Jupiter’s magnetic field and influences from the sun since 2013. Hisaki noted that during the Keck observations, the solar wind was high and creating enhanced auroras. Finally, Juno’s role was to help pinpoint just where the aurora was. As O’Donoghue described it:

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.

Spreading the news on Twitter

O’Donoghue took to Twitter to share the excitement of their discovery and elaborate on their findings.

With these new maps we saw temperature gradients for the first time, globally. We saw temperatures slowly decrease from the poles to the equator, indicating auroras are the cause of heating for the whole planet(!)

— Dr. James O'Donoghue (@physicsJ) August 4, 2021

We did some digging and found that during the Jan. 2017 observations, a high pressure pocket of solar wind had recently collided with Jupiter's magnetic field, compressing it and likely causing the aurora to be more powerful. We think this led to a planet-wide heating event! pic.twitter.com/EDfXl1YEur

— Dr. James O'Donoghue (@physicsJ) August 4, 2021

Bottom line: Auroras roast the upper atmosphere of Jupiter, creating much hotter temperatures than otherwise expected on the distant planet.

Read more about Jupiter’s unusual auroras: Jupiter X-Ray Aurora Mystery Solved, After 40 Years

Source: Global upper-atmospheric heating on Jupiter by the polar aurorae

Via NASA