Jupiter's upper atmosphere is way hotter than it should be

By Rachel Feltman

Jupiter's Great Red Spot and three of its four largest moons. Photo / AP
Jupiter's Great Red Spot and three of its four largest moons. Photo / AP

Jupiter's Great Red Spot has fascinated scientists for hundreds of years.

The centuries-old, hurricane-like storm - once three times as large as Earth, but now around half that size - runs on the kind of time and size scale that reminds us just how tiny our own world really is. According to a study published today in Nature, the Great Red Spot, or GRS, may help solve a longstanding Jovian mystery: The upper atmosphere of this gaseous planet is much, much hotter than it should be, and tumultuous winds down below may be to blame.

Jupiter, the king of the solar system by measures of both age and size (and even by mass, which is pretty impressive given that it's made of gas) is five times farther away from the sun than Earth is. But somehow, its upper atmosphere is nearly as hot as our own.

"We call this question the energy crisis, and it's been going on since the late 70s," said Boston University's James O'Donoghue, lead author of the new Nature study.

"It's relentless."

By rights, O'Donoghue explained, Jupiter's upper atmosphere should be around -100 degrees Fahrenheit (-73C). Instead, the temperatures average from between 800 to 1700 (425 to 925C) - a massive discrepancy. So if the sun's rays aren't to blame, what's causing things to heat up?

Jupiter has the most tumultuous magnetic field of any planet in the solar system. The charged particles that zip up and down its poles - nearly at the speed of light - are so powerful that Nasa's recently inserted Juno probe needed a 180kg titanium shield to protect its computer components from being shredded. That activity could explain why the poles get as hot as they do - around 1700F - but, O'Donoghue said, "the rest of the planet really has no excuse."

He and his colleagues used the Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii, to look at heat signatures across the entire upper atmosphere, hoping that creating a global temperature map would help them pick out trends. Sure enough, the atmosphere just above the Great Red Spot stood out as a great big hotspot, reaching temperatures of over 2400F - the highest ever detected on the planet.

The researchers believe the energy that heats the upper atmosphere may come up from below, with the GRS providing an extreme example. The rest of the atmosphere's heat may come from Jupiter's swirling bands: Some move hundreds of miles per hour in one direction while bands of cloud below move just as quickly in the opposite direction. That turbulence could produce gravity waves (which aren't the same thing as the gravitational waves produced by black hole collisions and the like - gravity waves form when fluids meet and gravity tries to restore equilibrium between them, like when the wind meets the ocean and creates a wave or tsunami) that carry energy up into the atmosphere.

You can picture the GRS as swirling tea being stirred in a cup: If you start stirring in the opposite direction, you'll see and hear splashes as a result of the interference. GRS flows against the grain of the atmosphere's movement and creates "splashes" like these. They collide with the upper atmosphere, losing their energy by moving the molecules they encounter. Temperature is actually just the movement of molecules and atoms against on another, so by making more movement these waves turn up the heat.

"It's not that the GRS makes the entire planet hot," O'Donoghue said. "But we wanted to find some connection between the upper and lower atmosphere."

If the atmosphere hundreds of miles above the GRS is heated by its roiling clouds, the same effect could be happening to a lesser extent everywhere on the planet. It could even be happening on other worlds. All of the gas giants have this same "energy crisis," and the same quick rotations and fast wind speeds that could create this effect. Jupiter - being the closest and the largest of the crew - is the easiest to study.

"This question really highlights our failure to understand the basic physics of these atmospheres," O'Donoghue said. "It's just really shocking that we don't understand something so basic."

Now that Nasa's Juno probe is in orbit, the king of the planets could get a lot less mysterious. O'Donoghue's work will continue to mostly rely on ground-based telescopes, but he hopes that some of the probe's data will help inform this energy mystery. An infrared instrument on board could help confirm the GRS temperature readings, and might be able to detect hotspots over other, smaller storms. Juno, which will use its instruments to probe below Jupiter's cloud cover for the first time ever, could also help answer basic questions about those swirling gasses: We don't know how deep the GRS or the rotating bands go, or whether there's a rocky core hiding beneath them.

- Washington Post

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