Sound and Gravity Waves Combine to Heat Jupiter's Great Red Spot
The upper atmospheres of gas giants are unaccountably hot. Jupiter’s Great Red Spot may be the key to understanding why.
Concept art of the high temperatures above the Great Red Spot. Image: Dillon Yothers with Luke Moore
Jupiter's Great Red Spot is one of the most distinctive planetary features in the solar system. At roughly three times the size of Earth, this gargantuan Jovian storm has mesmerized observers for centuries, generating insights into our neighboring worlds, and inspiring new questions about their behavior and dynamics.
As if it needed more street cred, the Great Red Spot may also be the key to resolving a longstanding scientific dilemma, according to new research published Wednesday in Nature.
For decades, researchers have noted that the upper atmospheres of gas giants like Jupiter are much warmer than would be expected from solar heating alone, implying that an unknown source augments the Sun's warmth. This has become known as the giant-planet "energy crisis."
"For all of these [gas giant] planets, we have a problem explaining why their upper atmospheres are as hot as they are," Luke Moore, a Boston University research scientist and second author on the paper, told me over the phone. "Anytime there's a problem that we can't explain, it implies that there is some missing understanding in our science of these planets."
Previous studies proposed that energy produced by Jupiter's auroras might radiate out from the poles towards lower latitudes, but that was countered by evidence that auroral energy is "trapped" at high latitudes by Coriolis forces induced by Jupiter's rotation. Moore and his colleagues also put forth gravity waves as a heat source, but point out that previous modelling studies of this scenario led to "inconclusive results for Jupiter."
The team suggests instead that acoustic waves, generated by turbulence in the lower atmosphere, combine with gravity waves to create higher temperatures. The Great Red Spot is a particularly compelling case study of this phenomena, given its visible turbulence and high temperatures, which are hundreds of degrees Kelvin hotter than anywhere else on the planet.
"The Great Red Spot is so massive," Moore said. "That means you have more turbulence, and probably more wave generation and heating there."
Moore used the analogy of stirring tea in one direction, stopping, then stirring in the opposite direction as a way to describe the underlying dynamics that create these waves. Energy transferred vertically creates hotspots in the atmosphere, about 500 miles above the storm.
The new paper includes "direct observational evidence" of these localized heating processes in the Great Red Spot, obtained by the SpeX spectrometer on the NASA's Infrared Telescope Facility on Mauna Kea, Hawaii.
SpeX spectrometer observations. Video: James O'Donoghue, Luke Moore and NASA Infrared Telescope Facility (IRTF)/YouTube
"Since we can't explain temperatures on any of the giant planets, this gives us a clue into what may be the missing energy source at Jupiter," Moore told me. "It may just be easiest to spot above the Great Red Spot because that's the biggest storm in the solar system."
"The problem with going out to the other planets is that from Earth's perspective, they get farther and farther away and it's harder to make these same measurements."
That's why the team plans to follow up with more precise measurements from Mauna Kea's Keck Observatory, the Cassini spacecraft currently orbiting Saturn, and the Juno orbiter that recently arrived at Jupiter. "We expect to see smaller signatures around other storms and other locations on Jupiter, and also the other giant planets," Moore said.