Stunning Images of Jupiter's Moon Io Are Provided by the Closest Flyby in 20 Years

There are no other worlds like Io, at least not in our solar system. Every 400 minutes, 400 active volcanoes erupt, turning it inside out. Beautiful photos of this Jupiterian moon captured by the Juno spacecraft during its maiden flyby are now available. Professional and amateur image processors are already bringing out subtleties that may have been overlooked in the original photos.

More than 400 years have passed since we first learned that Io was unusual. Astronomy was shocked to discover it and the other three Galilean satellites. People became aware of its red hue—the other three major moons of Jupiter are white—after the first shock of its existence subsided.

What makes it unique was discovered by the Voyager missions: the planet is continually being resurfaced by volcanic explosions. Io's surface is heavily sulfurous, unlike the majority of other gas giant moons, which are coated with ice. It is a planet that "looks like a pizza pie and smells like rotten eggs," according to Carl Sagan.

Whatever you think Io looks most like, you can decide for yourself. However, after speeding just 1,500 kilometers (930 miles) over Io's surface, Juno has returned some incredible photos that will be studied for years to come. Although Juno has been orbiting Jupiter for seven years, the majority of that time has been spent observing the planet. The moons have only recently come into focus.

There's more to the close passing than merely shooting high-definition surface photos. Jupiter's strong magnetic field surrounds both Io and Europa, forming donut-shaped rings of ions around both. Juno breezed through it. The information gathered should be significant from a scientific standpoint and utilized to set up JUICE and the Europa Clipper to make comparable safe passageways.

On February 3, Juno will pass near Io once more. Juno's orbital period was already shortened by Io's gravity from 38 to 35 days, and it will now only be 33 days after the next passing. Within a short time, planetary scientists will be able to compare the photographs and look for surface changes on Io thanks to that passage. Every other time Juno approaches Jupiter, there will be more flybys of Io, but none will be as near as these two.

Io's extreme volcanism is caused by its interior being continually stretched and compressed by both the gravitational pull of the surrounding moons and its fluctuating distance from Jupiter. These effects generate enough heat at Europa's more comfortable distance from the gravity well to melt the ice beneath the outer shell and form an interior ocean. As Io gets closer, it produces a lot more heat, which melts rocks and causes lava to erupt through the surface. Io is now the densest moon in the Solar System after losing nearly all of its water in the process.

Other than the gas giants, Io was believed to be the only planet in the Solar System devoid of impact craters due to its extreme volcanic activity. But as of last month, there was evidence to suggest that this is probably untrue. Amateurs and possibly pros will comb through the photos captured during these near passes in search of more examples.

An apparent impact crater was discovered by an amateur astronomer going through old Galileo mission photos that everyone else had overlooked. Professional astronomers have now shown that Jesper Sandberg's discovery of an asteroid impact that Io's volcanic activity has not yet eliminated is real.

Galileo saw that our satellite was cratered from the moment he first trained his telescope on the Moon. It was subsequently discovered that they were the result of quick interactions with space rocks, even though these were first believed to be the remnants of volcanoes, which were by far the most common type of crater the era's scientists knew from Earth.

except gas giants, it became evident when expeditions to other planets started that they were all covered in craters. Like our Moon, most worlds flaunt their craters, which date back billions of years, proudly. Most traces on Earth, Mars, and Europa have been worn away by wind or water, although some are still discernible on each planet.

The one exception has been Io. It's not that Jupiter never gets struck, because of its closeness to the asteroid belt, its moons sustain more damage than others. On Io, however, craters are quickly filled in by flowing lava due to the island's more than 400 active volcanoes, which are fed by Jupiter's tidal flexing. However, it appears that one is still alive.

In the Galileo data, Sandberg saw an object that appeared strangely circular at approximately 51.7˚S, 117.1˚W, which was on the side of a mesa, or flat-topped hill. There appears to be no optical illusion as the image is seen in four frames.

The most likely explanation for this impact crater on a volcanic moon of this kind is an impact crater, according to evidence presented last week at the American Geophysical Union conference by Dr. Rosaly Lopes of NASA's Jet Propulsion Laboratory and Professor David Williams of Arizona State University.

Galileo's October 16, 2001, photos show the crater to be 7-8 pixels across, with a resolution of 13 m/pixel due to the distance, meaning that the crater's diameter ranges from 92-104 meters (300-360 feet). Aside from their round shape, the ejecta lines surrounding the crater resemble collision-related lines rather than eruption-related ones.

The location may appear especially improbable given that it is undoubtedly not isolated from Io's constant volcanism. "The entire area has since undergone gradual changes, potentially involving the accumulation of sulfurous elements from adjacent plume eruptions and deterioration such as the collapse of sulfurous deposits on the plains, sublimation, or other procedures," the speaker states.

Still, Williams said to Eos, "I processed the Galileo SSI mosaics from that era. I turned around, lifted it, and examined it closely. Sure enough, there is an area that is easily consistent with an impact crater.

If so, the key question is whether Galileo—a spaceship, not a scientist—just so happened to catch it during a brief time of survival, or whether there are more of these kinds of objects on Io than previously believed.