In the grand scheme of understanding hard things, understanding Earth is pretty easy. We can go out. We can dig, drill, and sample. While robots are starting to be able to poke at the surfaces of other worlds, they are limited and give us views of only a few miles along the surface and a few inches beneath.

Luckily, we occasionally get some free samples of Mars sent our way during impacts that can dig deep into the red planet. Basically, an asteroid hits Mars, the impact tosses chunks of Mars outward at escape velocity, and some lucky fragments get sent on a collision with Earth.

The bulk of these massive impacts occurred early in our solar system’s history, during a period called the Great Heavy Bombardment, during which – as the name implies – all the worlds were heavily bombarded by asteroids, comets, and other small and large objects, including the occasional now-dead planet.

After one such impact, a set of rocks took off on a flight through space that would bring them to crash into northwestern Africa. This would have happened only thousands of years ago and may have been seen by humans. For the interceding millennia, the black stones from space lay in the desert, undisturbed.

About ten years ago, these rocks happened to get scooped up and sliced up by researchers trying to figure out what kind of black rock randomly hangs out in the desert. The answer, as you may have guessed, was a Martian rock. Each meteor was made of a blend of material from Mars that had been crushed and then packed together. This diversity of material holds a record of all sorts of different events in Mars’s past. As described by Aaron Covosie: It’s truly a big smorgasbord of the early processes on Mars.

In this mixture were zircon crystals — 66 grains of zircon to be exact. Viewed under an electron microscope, these crystals showed irregular structures that indicated the crystal experienced an intense shock that compressed it like an accordion. We’ve seen similar crystals, called shock-twinned zircon, on Earth, and, as Cavosie explains: On Earth, nearly all occurrences of shock-twinned zircon are found in rocks from the central area of impact sites where pressures are highest.

This implies that these rocks are from the central part of an impact like those that formed Sudbury crater in Canada or Chicxulub in the Yucatan Peninsula.

It is super cool to get proof that impacts on Mars could affect zircon the same way that has happened on Earth. What is less cool are all the questions this sample can’t answer. Exactly when did it form? Not sure. The original zircon formed 4.45 billion years ago according to radioisotope dating, so the impact was sometime in the last 4.45 billion years, which isn’t helpful information. Where did the impact occur? The best we can say is “on Mars.”

Still, it’s cool. And it is more reason to want more robots digging on Mars.

More Information

Martian Meteorites Reveal Evidence of a Large Impact (Eos)

“Impact and habitability scenarios for early Mars revisited based on a 4.45-Ga shocked zircon in regolith breccia,” Morgan A. Cox et al., 2022 February 2, Science Advances