An amateur astronomer searching through archival Galileo mission photographs discovered something that everyone else had missed: an obvious impact crater. Professional astronomers have now supplied proof that what Jesper Sandberg discovered was the genuine deal, an asteroid impact that Io’s volcanic activity has yet to destroy.
From the moment Galileo pointed his telescope at the Moon, it was clear that our satellite was riddled with craters. Although they were first considered to be the remnants of volcanoes – by far the most frequent type of crater known to scientists at the time – it was later discovered that they were the result of close collisions with space objects.
Once expeditions to other planets began, it became evident that, except for gas giants, being covered with craters is the Solar System norm. Most worlds, like our Moon, proudly flaunt their craters, with impacts stretching back billions of years on display. Most traces on Earth, Mars, and Europa have been erased by wind or water, but a few remain visible on each.
Io has been an outlier. It’s not because Jupiter never gets struck; in fact, Jupiter’s closeness to the asteroid belt means its moons are hit more frequently than others. However, with over 400 active volcanoes driven by Jupiter’s tidal flexing, craters on Io are quickly covered up by flowing lava. Nonetheless, it appears that one remains.
Sandberg discovered something strangely round-looking in Galileo data at 51.7 S, 117.1 W on a mesa’s (flat-topped hill’s) side. The picture can be viewed in four frames, indicating that there is no optical illusion.
On such a volcanic moon, the obvious assumption is that this is a volcano crater, but at the American Geophysical Union conference last week, Professor David Williams of Arizona State University and Dr Rosaly Lopes of NASA’s Jet Propulsion Laboratory presented evidence that an impact crater is the most likely explanation.
In photos obtained by Galileo on October 16, 2001, the crater is 7-8 pixels wide, with a resolution of 13 m/pixel, making the crater 92-104 meters (300-360 feet) in diameter. Aside from the roundness, the ejecta lines around the crater appear to be from a collision rather than an eruption.
The site may appear especially implausible because it is not isolated from Io’s constant volcanism. “The whole region subsequently has been undergoing gradational processes, perhaps including deposition of sulfurous materials from nearby plume eruptions and degradation including slumping of sulfurous plains deposits, sublimation, or other processes,” according to the researchers.
“I processed the Galileo SSI mosaics from that time,” Williams told Eos. I went back, lifted it, and examined up close – there’s an area that could easily be mistaken for an impact crater.”
If that’s the case, the key issue is whether Galileo (spacecraft, not scientist) just managed to capture it during a brief time of survival, or if there are more of them on Io than we imagined.
We don’t know if the crater is still there 20 years later, but that might change. The Juno mission was initially focused on Jupiter, and it has only lately changed its emphasis to the moons. Following flybys of Ganymede and Europa, it was Io’s turn in December 2022. However, it, and a second flyby last year, were at much larger distances than Galileo’s pass.
A closer flyby is scheduled on December 30 this year, followed by one on February 3, 2024. Although neither the Europa Clipper nor the Jupiter Icy Moons Explorer (JUICE) have Io as a primary target, it is feasible that one of them will approach near enough to check for other crater locations.
Lopes is a co-author of a preprint that maps Io’s volcanism during the last 40 years. According to this, volcanic activity has been focused in the southern trailing hemisphere since 2013. Despite its location in the south, Sandberg’s crater is in Io’s leading hemisphere, providing some possibility for survival.