Gigantic

When searching for the unknown, classic physics wisdom holds that a bigger detector boosts the chances of discovery. A physicist is taking that advice to heart, advancing a bold plan to use none other than Ganymede—Jupiter’s largest moon—as a dark matter detector on an astronomical scale.

Dark matter refers to the “invisible” mass that supposedly constitutes 85% of the universe. There’s considerable evidence that dark matter exists, but it’s “dark,” meaning it doesn’t respond to light and very weakly interacts with other matter. The search for dark matter has tested the limits of physicists’ creativity, but a proposal by William DeRocco, a physicist at the University of Maryland, may be the most extraordinary yet. In a preprint submitted to arXiv, Rocco suggests that Ganymede’s craters may store evidence of dark matter particles, which spacecraft like NASA’s Europa Clipper or ESA’s JUICE could observe during their respective missions.

The paper, which has yet to be peer-reviewed, proposes that massive dark matter particles could have struck and penetrated Ganymede’s thick, icy surface, leaving deep, broad ruptures. Unlike the comparatively small-sized candidates for dark matter that ground-based detectors are searching for, these particles would be much larger. These extra-large dark matter particles would create “dark matter craters”—smaller dents on Ganymede’s surface comprised of distinctive minerals pulled to the surface from deep inside the moon’s oceans. 

“If you used something like ground-penetrating radar, you might be able to see this column of melted ice going all the way down through the ice,” DeRocco explained in an interview with New Scientist. Studying Ganymede’s surface with this proposal in mind could uncover some unexpected insights about cosmic dark matter, according to the paper.

In principle, the proposal sounds promising, Bradley Kavanaugh, an astrophysicist at the University of Cantabria in Spain who was not involved in the study, also told New Scientist. At the same time—like all dark matter experiments—there is still no definitive evidence that such heavy, massive dark matter particles actually exist.

If all of this sounds bonkers, I don’t blame you. Still, it’s important to remember that, as many physicists are keen to point out, solving a physics mystery often means testing bold, unconventional ideas. And while there’s no decisive evidence that this particular proposal is correct, there isn’t any evidence to discount it, either. We’ll have to wait and see if NASA or ESA takes up DeRocco’s idea, and if they do, whether Ganymede really does have a surface dotted with dark matter craters.

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