Plenty of high-schoolers look up to scientists, but sometimes things are flipped on their head, with those laboratory-imprisoned academics turning to the younger generation for a bit of help. As a brand-new study in Physical Review Letters reveals, this also applies to the science of impact craters.
Asteroid and meteorite impacts are pretty darn messy. Depending on their size, their momentum, the type of atmosphere they have to breach, the angle of impact, and the type of geology their careening into, you can get all kinds of craters, from the elliptical to the perfectly circular, all surrounded in a beautiful halo of debris.
Speaking of those halos, what of crater rays? These are incredibly common patterns of material thrown out by those powerful impact events: beautiful, radiating lines that look a lot like the spokes of a bicycle wheel. Although there are some geological clues as to how these form, it turns out that scientists weren’t quite sure.
Certain subjects are ripe for investigation using bench-top analog experiments. A few years back, yours truly used a mixture of granules and some compressed air to simulate some particularly bizarre volcanic eruptions, all in the comfort of a German laboratory.
Impact craters can also be generated in similar conditions. Get various ball bearings, set up your layers of sediment, and fire the balls into it to make your very own baby craters, complete with their own debris field.
Indeed, that’s what several teams of researchers have been doing to try to create crater rays, but curiously, they’ve had little success. Simulated impacts tend to generate concentric-ish circles of material excavated from the impact site, but not those radiating lines.
As spotted by Astronomy Magazine, a team from the Okinawa Institute of Science and Technology pondered on how to solve this conundrum. Searching for inspiration, they had a look at videos on YouTube showing students conducting far less high-tech versions of these impact experiments themselves.
As it happens, plenty of these student experiments – normally done as part of a school or university’s scientific outreach program – did, in fact, manage to generate crater rays. What, they wondered, was this witchcraft?
Studying the videos more closely, they realized that it had something to do with the experimental design.
Their own designs tended to make sure the surface of the simulated planet was perfectly smooth and homogenous, whereas these high-school-grade versions often used roughly distributed flour, leaving imperfections in the topology. Then, it clicked: that’s far more like what you’d observe on actual planets – an uneven surface with varying grain sizes and geologies.
The team took this approach to some new analog experiments in their own lab, using a hexagonal grid to create roughness in the surface.
Instead of a flat plateau, they created evenly spaced bumps in the material – something they also replicated in matching computer models. Dropping steel balls onto their new simulated planet, they found that the impacts generated those beautiful crater rays.
It appears that the gaps between the bumps break up the shockwave produced by the impact. Those bumps force the shockwave's energy to be focused more along the tiny valleys between them, which pushes out sediment in a more linear way.
This doesn't just explain how those radiating lines take shape; it also hints that we can understand far more about the surface geology of worlds scarred by ancient impact craters by taking a closer look at the rays themselves.
The team’s study explains that crater rays were first spotted by astronomer Johannes Hevelius back in 1647, when he made what was probably the first map of the lunar surface. Now, thanks to these nifty experiments, we may finally understand how they formed, 371 years later.
