Jupiter and Saturn should be freezing cold. Instead, they’re hot. Researchers now know why.

Living as they do in the distant, sun-forsaken reaches of the solar system, Jupiter and Saturn, the gas giants, and Uranus and Neptune, the ice giants, were always expected to be frosty realms. But when NASA’s Voyager spacecraft sailed past them in the late 1970s and 1980s, scientists found that all four worlds were running planetary fevers — a revelation as jarring as finding a bonfire inside your freezer.

Follow-up observations by ground-based telescopes and the Galileo and Cassini spacecraft demonstrated that their planet-wide fevers have persisted through time. Their planetary pyrexias are acute: Jupiter’s lower latitudes, for example, should be a frigid −110 degrees Celsius. Instead, the atmosphere there cooks at 325 degrees. What incognito incinerator is behind this? And how is this unknown heat source warming not just a single spot on the planet, but the entire upper atmosphere?

Scientists have tried to explain this “energy crisis,” but have remained “confused for about 50 years,” said James O’Donoghue, a planetary astronomer at the Japan Aerospace Exploration Agency. Now two papers have conclusively revealed where all that heat is coming from: Jupiter and Saturn’s northern and southern lights — their auroras.

The results come from detailed measurements of both gas giants’ upper atmospheres. Saturn’s atmospheric temperature was taken by the Cassini spacecraft during the maneuvers that ultimately plunged it into the planet; Jupiter’s was stitched together using a telescope atop a giant Hawaiian volcano. Both show that the atmospheres are hottest near the auroral zones below both magnetic poles. As you approach the equator, the temperature drops off. Clearly, the aurora is bringing the heat — and, as with a radiator, that heat decreases with distance.

This composite video shows Jupiter’s auroras as seen by the Hubble Space Telescope. The auroras were photographed in far-ultraviolet light and superimposed on images of the planet taken in visible light. NASA, ESA, J. Nichols (University of Leicester), and G. Bacon (STScI); Acknowledgment: A. Simon (NASA/GSFC) and the OPAL team

A solution to the energy crisis may have far-reaching ramifications. Planets — from those in our own solar system to those orbiting distant stars — don’t always keep their atmospheres. Many gassy envelopes are destroyed over time, in some cases turning giant worlds into tiny, uninhabitable husks. Researchers want to be able to distinguish these from habitable, Earth-like planets. If we hope to do so, said Zarah Brown, a researcher at the University of Arizona, “one of the major parameters that you would want to know is the temperature of the outer atmosphere, since that’s where gas is lost to space.”

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June 23, 2021