Scientists scanning Jupiter's atmosphere have found a mysterious spike in temperature high above the Great Red Spot — that massive, swirling storm that has graced the planet's face for centuries.
The discovery, described in the journal Nature, may hint at a deeper connection between the dynamics of the gas giant's upper and lower atmosphere, and could shed light on the basic physics of such planets in our solar system and beyond.
For more than four decades, scientists have struggled to explain why the temperatures in parts of the giant planets' upper atmospheres can be hundreds of degrees warmer than expected — too warm to be explained by heating from the sun.
"Jupiter, Saturn, Uranus and Neptune are all far too hot in their upper atmospheres compared to the amount of sunlight they receive," said lead author James O'Donoghue, a planetary scientist at Boston University. "For example, Jupiter should be about 200 degrees Kelvin, whereas we consistently measure it to be over 1,000. So it's a massive discrepancy."
To find out where this heat was coming from, a team of scientists from Boston University and the University of Leicester used a spectrometer at the NASA Infrared Telescope Facility to create a map of the temperature distribution across the striped gas giant. To do so, they tracked emission lines from a triple-hydrogen ion found in Jupiter's atmosphere.
"It's kind of like it's a probe of the atmosphere," O'Donoghue said. "Wherever that molecule is, we can read its temperature and construct a map that way."
The scientists were expecting to see the polar atmosphere show up brightly, given that those regions have massive auroras energized by the planet's powerful magnetic field. But they were not expecting the Great Red Spot to pop out of the haze near the equator as well.
"We were actually surprised," O'Donoghue said. "We just wanted to make the maps first and then assess things later, but we luckily got the Great Red Spot in our data."
The Great Red Spot, which is currently about twice as wide as Earth, sits about 50 kilometers above the so-called surface (the depth at which the planet reaches Earth's sea-level pressure — Jupiter doesn't really have a surface). But the overheated layers in the upper atmosphere sit much higher, around 600 to 1,000 kilometers above the surface. There, the temperature above the storm came out to a whopping 1,600 degrees Kelvin. That means that whatever was happening in the Great Red Spot was having a significant effect far above it.
"It was unthinkable in ways, because you're communicating energy from 500 miles away, which sounds just crazy — I still think it's quite crazy, but it still seems to be the case," O'Donoghue said. "On the other hand, you're talking about a storm that is ... the largest in the solar system. Then it didn't sound crazy."
What could be causing this heating? It's possible that gravity waves might explain the heating, although they generally only heat an area by tens of degrees at best. (These are pretty mundane physical phenomena and not at all the same as gravitational waves, those Einsteinian ripples in space-time.)
It's also possible that acoustic waves, generated as the swirling storm rubs against atmospheric layers headed in the opposite direction, might be carrying energy into the atmosphere and heating it. Perhaps a combination of these two wave types "crashing" in the atmosphere like waves upon a beach causes the super-heated spots.
The Great Red Spot is just that — a spot — but it's possible that the many smaller storms raging across the gas giant's stripey body may also be heating the atmosphere, albeit to a less extreme degree. This could explain why the upper atmospheric temperature all around Jupiter is surprisingly high.
But finding evidence for acoustic heating (or whatever the mechanism may be) is going to be a little tougher to tease out above the smaller, less noticeable storms, said Steve Miller, a planetary scientist at University College London who was not involved in the paper.
"The fact that you see it over the Great Red Spot is because you get a big effect," he said, "but over other places you're going to be looking for things that are much more subtle."
Luckily, NASA's Juno spacecraft just entered Jupiter's orbit on July 4, and the data it gathers on the structure of the planet's deeper layers could shed some light on the dynamics at work in the atmosphere.
"I think Juno should definitely follow that up," Miller said.
What the exact dynamics are, for the moment, remains unclear.
"The lack of a solution points to a huge misunderstanding in our fundamental knowledge of how an atmosphere works," O'Donoghue said.
The scientists want to see if they can find similar hot spots hovering over smaller storms on the gas giant, and plan to analyze data from the Keck telescope that examines the atmosphere above the Great Red Spot in finer detail.
With Juno's depth measurements and more atmosphere measurements, "we'll have a complete picture of the altitude profile of the storm," O'Donoghue said, "and figure out basically how it works, really."
12:06 p.m. July 29: This story has been updated with comments by planetary scientist Steve Miller.