In 2017, NASA’s Juno spacecraft spotted electrons being fired down into Jupiter’s atmosphere at up to an enormous 400,000 volts, giving rise to the planet’s glowing auroras. Now, space scientists, using data from the Keck Observatory in Hawai’i. have revealed the previously unknown mechanism behind Jupiter’s atmospheric heating. The international team of astronomers have created the most-detailed yet global map of the gas giant’s upper atmosphere, confirming for the first time that Jupiter’s powerful aurorae are responsible for delivering planet-wide heating.
The Keck Observatory Discovery
“We first began trying to create a global heat map of Jupiter’s uppermost atmosphere at the University of Leicester,” said lead author Dr. James O’Donoghue, a researcher at the Japanese Space Agency (JAXA). “The signal was not bright enough to reveal anything outside of Jupiter’s polar regions at the time, but with the lessons learned from that work we managed to secure time on one of the largest, most competitive telescopes on Earth some years later.”
Jupiter is not a failed star, it’s a highly successful dwarf planet,” Tweeted O’Donoghue.
“Using the Keck telescope we produced temperature maps of extraordinary detail,” explains O’Donoghue. “We found that temperatures start very high within the aurora, as expected from previous work, but now we could observe that Jupiter’s aurora, despite taking up less than 10% of the area of the planet, appears to be heating the whole thing.
“This research started in Leicester and carried on at Boston University and NASA before ending at JAXA in Japan. Collaborators from the University of Leicester, the Japanese Space Agency (JAXA), Boston University, NASA’s Goddard Space Flight Center and the National Institute of Information and Communications Technology (NICT) working together made this study successful, combined with data from NASA’s Juno spacecraft in orbit around Jupiter and JAXA’s Hisaki spacecraft, an observatory in space.”
The long-standing puzzle at the Top of Giant Planets
“There has been a very long-standing puzzle in the thin atmosphere at the top of every Giant Planet within our solar system,” explained Dr. Tom Stallard at the University of Leicester. “This paper describes how we have mapped this region in unprecedented detail and have shown that, at Jupiter, the equatorial heating is directly associated with auroral heating. With every Jupiter space mission, along with ground-based observations, over the past 50 years, we have consistently measured the equatorial temperatures as being much too hot.”
“This ‘energy crisis’ has been a long standing issue—do the models fail to properly model how heat flows from the aurora, or is there some other unknown heat source near the equator?”
Jupiter is shown below 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. (J. O'Donoghue (JAXA)/Hubble/NASA/ESA/A. Simon/J. Schmidt)
“This paper describes how we have mapped this region in unprecedented detail and have shown that, at Jupiter, the equatorial heating is directly associated with auroral heating.”
Aurorae occur when charged particles are caught in a planet’s magnetic field. These spiral along the field lines towards the planet’s magnetic poles, striking atoms and molecules in the atmosphere to release light and energy.
Jupiter’s moon, Io creates most powerful aurora in the Solar System
On Earth, this leads to the characteristic light show that forms the Aurora Borealis (in the North) and Australis (in the South). At Jupiter, the material spewing from its volcanic moon, Io, leads to the most powerful aurora in the Solar System and enormous heating in the polar regions of the planet.
( NASA/JPL/University of Arizona)
Although the Jovian aurorae have long been a prime candidate for heating the planet’s atmosphere, observations have previously been unable to confirm or deny this until now.
Previous maps of the upper atmospheric temperature were formed using images consisting of only several pixels. This is not enough resolution to see how the temperature might be changed across the planet, providing few clues as to the origin of the extra heat.
Researchers created five maps of the atmospheric temperature at different spatial resolutions, with the highest resolution map showing an average temperature measurement for squares that are two degrees longitude ‘high’ by two degrees latitude ‘wide’. The team scoured more than 10,000 individual data points, only mapping points with an uncertainty of less than five per cent.
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.
Gas Giant’s Dynamic Aurorae
In an email to The Daily Galaxy about the possible variability of the waves of energy from the fast-changing polar auroras, astronomer Tom Stallard wrote: “We’ve been struggling to understand how you can move the vast supply of auroral heating from the polar regions of Jupiter’s upper atmosphere down to the equator, and one way that was hypothesized in the past was that with enough variability, you might create waves of heating in the atmosphere that carry that heat away from the pole. When you drop a pebble in a lake, the energy from the stone hitting the water moves away from where the stone landed, moving through the water. The water itself doesn’t move very far, mostly in a circular motion on the surface, and yet the wave gets carried to the shore, where it crashes and drops its energy.
“But models of this process,” continued Stoddard, “suggest that the amounts of heat carried in this way seem fairly slight, perhaps an added 15 C at the peaks. That’s not enough to heat the equator. But we show that these waves must be much stronger – the equatorial regions are clearly heated up when there have recently been dynamic aurora – so we’re already aware that something is up. But in the map from this period of increased heating is a long thin region of heating, well away from the aurora. To me, this looks exactly like a wave of heat, caught in the act of flowing from the aurora – and it is a lot hotter than the surrounding – more like 150 C hotter.
Aurora’s Brightness the Clue
“So what we’ve found,” summarizes Stoddard, “is that when Jupiter’s aurora are bright, they dramatically change the temperatures in the region around the aurora, and that this appears to allow the flow of a lot more energy away from the aurora then even the models that predicted these waves calculated were possible. It’s a startling result that shows how dynamic Jupiter’s aurorae are, and showcases how difficult it has been to fully appreciate the complex nature of how energy is moved around in Jupiter’s upper atmosphere.”
Planetary research at the University of Leicester spans the breadth of the Jovian system, from the planet’s magnetosphere and atmosphere, out to its diverse collection of satellites.
Source information: O’Donoghue, J. et al, Global upper-atmospheric heating on Jupiter by the polar aurorae, Nature (2021). DOI: 10.1038/s41586-021-03706-w
Avi Shporer, formerly a NASA Sagan Fellow at the Jet Propulsion Laboratory (JPL) currently with the MIT Kavli Institute for Astrophysics and Space Research via Thomas Stallard and University of Leicester