Why a massive black spot suddenly appeared on Jupiter

October 4, 2019, 12:43 p.m.
A black spot on Jupiter.
Photo: Kevin M. Gill/NASA/JPL-Caltech/SwRI/MSSS

Jupiter doesn't need another beauty mark to remind us that it's a stunner.

Thanks to several NASA missions — including the Juno orbiter, the Voyager and Cassini flybys, and the Hubble telescope — we've already got plenty of postcards of the gas giant all dressed up in stripes and swirls.

But last month, scientists were baffled — at least, for a moment — when our solar system's largest planet appeared to sport a black spot.

In keeping with Jupiter's supersized nature, the spot was estimated to be about 2,200 miles across. As CNN notes, that's big enough to plunge an area from Las Vegas to New York City into complete darkness.

But it turns out the spot is actually a shadow from Io, one of 79 moons in the gas giant's entourage. The moon passed between the sun and Jupiter on Sept. 11 and the solar-powered Juno spacecraft was there to capture the breathtaking result.

Kevin M. Gill, a citizen scientist, enhanced the colors based on the JunoCam image.

According to NASA, Jupiter's moons frequently leave a temporary beauty mark on the gas giant, because Jupiter's axis is "not highly tilted relative to its orbit, so the Sun never strays far from Jupiter's equatorial plane."

As a result, the planet is often eclipsed by its moons.

"Io is so big and close that it more than blocks the sun," Katie Mack, an astrophysicist at Carolina State University, explains in a tweet. "It appears 4x as big as the Sun from Jupiter's perspective. And it's so close that the penumbra (fuzzy outer edge of shadow) is super thin."

Every now and then, one of Jupiter's moons will throw some shade at the planet, but more often it's the gas giant doing the shadow casting. It's so deep and dark that the shadow can be terminal for spacecraft. Just this week, the Juno spacecraft came dangerously close to falling on Jupiter's "bad side" — from which it likely would have never returned.

It would mean spending 12 hours in a dark, deep freeze — long enough to completely drain a spacecraft that relies on solar energy. That's exactly what would have happened to Juno on Nov. 3, according to Universe Today, leaving engineers with the dicey proposition of having to reboot the dead spacecraft.

Instead, NASA engineers conducted a 12.5-hour fuel burn, firing up Juno's thrusters to steer it clear of that deadening shadow. So Juno lived to see another sunrise on Jupiter, all while sending us dazzling, and occasionally baffling postcards along the way.