Look just about anywhere in the solar system — from Mercury, to the moons of the gas giants, to the icy rocks of the Kuiper belt — and you’ll find worlds pocked with impact craters.
“Impact cratering, in terms of the geology of the planets, is the most pervasive process that there is,” said Brandon Johnson, assistant professor of earth, environmental and planetary sciences. “I use numerical models called hydrocodes to understand the impact process. The goal is to try to understand how impacts affect the evolution of ancient planets and their crusts.”
It’s a field that Johnson got into a bit by accident. He majored in physics as an undergrad and started graduate school at Purdue to study condensed matter. But one evening at a party, he met Jay Melosh, a renowned geophysicist and impact crater expert. The two started talking, and within a few days Johnson started in a whole new direction.
“We had a really interesting conversation,” he said. “I ended up deciding to switch fields and go into impact cratering. It’s one of the best decisions that I’ve ever made.”
Johnson completed his Ph.D. at Purdue with Melosh and went on to work as a postdoctoral researcher at MIT with Maria Zuber, a planetary scientist and Brown graduate.
The hydrocode models Johnson uses take the basic physics of an impactor slamming into a planet and combine it with data on how different rock types deform under stress. “What we do is alter unknown parameters like temperature, impactor size, and crustal thickness and try to see what best recreates what’s observed in these structures,” Johnson said.
Lately, Johnson has been using his models to study what are known as multi-ring basins, vast impact features found on the Moon and elsewhere. One of the best examples is Orientale basin, an impact scar 900 kilometers in diameter located on the western edge of the Moon’s nearside.
“These multi-ring basins are among the biggest things that have changed the surface of the Moon,” Johnson said. “We’re trying to understand how they form. These are ancient impacts, so they can tell us about what the Moon was like very early on, when it was warmer and more active.”
But the Moon isn’t the only place that has multi-ring basins, and Johnson is interested in studying them elsewhere in the solar system. One place of particular interest is Jupiter’s moon Europa, which is known to have a liquid ocean churning beneath its frozen outer shell.
“There are two of these multi-ring basins on Europa,” Johnson said. “Based on how big they are, the impacts should have probed to the depths where this ocean is. So they should be able to tell us how thick the ice shell of Europa was at the time these big craters formed.”
Johnson has also used his impact models to peer back in time to the very start of our solar system nearly 4.5 billion years ago. He was lead author of recent Nature paper on the origin of chondrules, small spheres of previously molten material contained in about 90 percent of meteorites found on Earth. Johnson’s hydrocodes suggest that chondrules were formed by colossal impacts that happened during our solar system’s infancy. The finding has the potential to change the way scientists look at the early solar system.
Scientists have long interpreted chondrules to be the earliest kernels of large planetary bodies. Some of these tiny spheres, the theory goes, gathered more and more material, eventually snowballing into asteroids and protoplanets. But Johnson’s work suggests chondrules are not planetary kernels. Rather, they’re debris left over from collisions of large bodies that were already there.
“Depending on which one you think is right,” Johnson said, “it could really change how we look at the early solar system.”
Johnson says he’s looking forward to continuing this work at Brown. In addition to his research, he’ll be teaching classes the geophysics of impact cratering, the formation of planets and their satellites, and planetary surface processes.
“Brown has a world-renowned planetary program,” he said. “Having colleagues that are leaders in the field is a huge draw for me. So is having the opportunity to work with students who are really smart and capable.”