Since You Wondered, Here’s Why Jupiter’s Great Red Spot Isn’t White

The giant cyclonic storm that swallowed Alaska last week has nothing on Jupiter’s Great Red Spot. The GRS is a cyclone, too, but one so immense it could gulp down the Earth in one shot and still have room for Mars. It’s been swirling for centuries, at the very least, and while it’s smaller than it used to be, nobody thinks it’s going away.

All of this is pretty well known to planetary scientists. What they don’t know is the answer to a very simple question: Why is the Red Spot, well, red? “There are some other places on Jupiter that are reddish,” says Kevin Baines of NASA’s Jet Propulsion Laboratory (JPL), “although they’re more of a reddish-brown.” The spot’s color, however, is pretty much unique and thus pretty mysterious. In fact, Baines adds, “back in the 1970’s, when we were trying to sell the Galileo mission to Congress, it really resonated that we were going to try and answer that question.”

Now Baines and two JPL colleagues may have finally done it — not with data from Galileo, which orbited Jupiter and its moons from 1995 to 2003, but from the Cassini probe, which took a few snapshots en route to Saturn. Those images, supplemented by laboratory experiments, suggest that the red color is just a thin dusting on the very top of swirling clouds that are otherwise white. “I call it the creme brulee model,” Baines says, “or the strawberry frosting model.”

Cassini was essential to solving the mystery because its instruments were sensitive to a broader range of light wavelengths than Galileo’s, and could thus show that the very center of the Red Spot is redder than the rest. The center is also at the highest altitude of what’s already an unusually high-altitude feature. “It reaches something like 50,000 feet higher than the surrounding clouds,” says Baines.

That exposes the swirling clouds to more intense ultraviolet light from the sun than most of Jupiter’s clouds. And when the JPL scientists did lab experiments to test the effects of ultraviolet rays on chemicals such as ammonia, acetylene and various hydrocarbons, which are abundant in Jupiter’s atmosphere, they got the same red colors seen on the giant planet itself. (They did eventually, anyway. At first, they did their tests with ammonium hydrosulfide, another chemical abundant on Jupiter, and recreated what Baines calls the “Great Green Spot. We were faked out,” he says.)

This isn’t the only evidence that the Spot’s red is created from above rather than coming from reddish gases upwelling from below, which is the leading alternate theory: There actually are some other tiny spots of red dotted around Jupiter, and they also coincide with clouds of unusually high altitude.

The Red Spot, in short, as a JPL press release cutely puts it, represents “a sunburn, not a blush,” on the face of the Solar System’s largest planet.