Jupiter’s Great Red Spot as Heat Source

Speculating about what an advanced extraterrestrial civilization might do has kept us occupied for the last two days, with gas giants like Jupiter the primary topic of conversation. We don’t know if it’s possible to ignite a gas giant to provide new sources of energy. But with Juno getting ready to measure Jupiter’s aurorae, we’re looking at naturally produced energy today, and now we have interesting work on the planet’s Great Red Spot that comes out of Earth-based observations. The enormous storm turns out to be a key factor in heating Jupiter’s atmosphere.

And what a storm it is. We knew about the Great Red Spot as early as the 17th Century because its span — three Earth diameters — qualifies this highly visible maelstrom as the largest hurricane we know of. Winds can take six days to complete one circuit of the Great Red Spot, which has varied in size and color ever since it was discovered. It is now observed to span 22,000 km by 12,000 km in longitude and latitude, respectively.

The Great Red Spot gives us a source of energy to heat Jupiter’s upper atmosphere but thus far we have lacked evidence of its effect upon temperatures. Now an analysis based upon new infrared data is changing our view of temperatures high above Jupiter’s visible disk.

Image: Acquiring Jovian spectra. Bright regions at the poles result from auroral emissions; the contrast at low- and midlatitudes has been enhanced for visibility. Great Red Spot (GRS) emissions at mid latitudes are indicated by the red arrow. Additional info: The vertical dark line in the middle of the image indicates the position of the spectrometer slit, which was aligned along the rotational axis of Jupiter. Image shown is taken from the slit (slit-jaw imaging) using the “L-filter” (3.13 – 3.53 μm). Credit: J. O’Donoghue, NASA Infrared Telescope Facility (IRTF).

The results come from James O’Donoghue (Boston University) and colleagues, who report their findings on the matter today in Nature, using 2012 data from the SpeX spectrometer on NASA’s Infrared Telescope Facility in Hawaii. The issue caught the astronomers’ attention because at mid- to low latitudes, temperatures in Jupiter’s upper atmosphere are hundreds of degrees warmer than heating from the Sun can explain. We’re looking at non-Solar energy whose sources could be studied by creating heat maps of the entire planet.

This is what the O’Donoghue set out to do, realizing that what his team refers to as an ‘energy crisis’ occurs not just on Jupiter but on other giant planets as well. One explanation for Jupiter has been auroral heating mechanisms that pump energy into the upper atmosphere. But the low to mid-latitudes lack this kind of heat source and yet remain 600 K warmer than can be explained by solar heating. The paper makes the case for a different kind of source:

A more likely energy source is acoustic waves that heat from below (also via viscous dissipation); this form of heating requires vertical propagation of disturbances in the low-altitude atmosphere. Acoustic waves are produced above thunderstorms, and the subsequent waves have been modelled to heat the Jovian upper atmosphere by 10K per day and on Earth have been observed to heat the thermosphere over the Andes mountains. On Jupiter, acoustic-wave heating has been modelled to potentially impart hundreds of degrees of heating to the upper atmosphere. However, to the best of our knowledge, no such coupling between the lower and upper atmosphere has been directly observed for the outer planets, so vertical coupling has not been seriously considered as a solution to the giant-planet energy crisis.

The team found that high altitude temperatures on Jupiter are greater than expected when the Great Red Spot is directly below. In fact, the atmosphere above this region is hundreds of degrees hotter than anywhere else on the planet. The temperature spike above the Great Red Spot points to coupling between lower and upper atmosphere. The authors believe the heating is caused by atmospheric turbulence that rises because of the shear between the storm and surrounding atmosphere, with propagating waves depositing their energies high above.

Image: Turbulent atmospheric flows above the storm produce both gravity waves and acoustic waves. Gravity waves are much like how a guitar string moves when plucked, while acoustic waves are compressions of the air (sound waves). Heating in the upper atmosphere 800 kilometers above the storm is thought to be caused by a combination of these two wave types ‘crashing’ like ocean waves on a beach. Credit: Art by Karen Teramura, UH IfA, James O’Donoghue

Co-author Tom Stallard (University of Leicester) puts the work into the context of ongoing missions like Juno:

“This fantastic result, showing how the upper atmosphere is heated from below, was produced directly from Leicester’s 2012 observing campaign, which was designed to try and answer why Jupiter’s upper atmosphere is so hot. Juno will be measuring the aurora and its sources, and we expected the auroral energy to flow from the pole to the equator. Instead, we find the equator appears to be heated from plumes of energy coming from Jupiter’s vast equatorial storms.”

The paper is O’Donoghue et al., “Heating of Jupiter’s upper atmosphere above the Great Red Spot,” Nature, published online 27 July 2016 (abstract).