In its final weeks of operation, NASA’s InSight lander can claim one final achievement: It detected a series of meteor strikes, and scientists used its data to identify recent impact craters.

Planetary scientist at the University of Toulouse Raphael Garcia and colleagues published the results in a recent article in the journal natural geosciences.

What’s new – Mars has a planetary defense problem: Its thin atmosphere isn’t much of a barrier to anything but the smallest incoming meteors. Small space rocks, up to a few dozen kilograms, fall to the ground on Mars more often than on Earth because our atmosphere is 100 times denser. And when meteorites, a meteorite that survives entry into the atmosphere, fall on Mars, their impacts send seismic waves through the Martian crust.

Last year, NASA’s InSight lander recorded seismic waves from three small meteorites, between 10 and 43 kilograms each, that penetrated the Martian surface within a few hundred kilometers of the lander. By measuring when different types of waves arrived at InSight, Garcia and his colleagues tracked ground motion to meteorite impact sites.

Then NASA’s Mars Reconnaissance Orbiter, 250 kilometers (196 miles) above the Martian surface, followed Garcia and his colleagues’ instructions and photographed the newly dug craters. Recording seismic waves from a meteorite impact and then finding the impact crater to accompany the seismograph is something we’ve only been able to do once on Earth, and never on another world, and it could reveal new information about the subsurface of Mars. make-up.

Diving into the details — A Marsquake induced by a meteorite impact shakes the ground in two main ways. First, vibrations called P waves or compression waves accelerate outward from the epicenter; these waves cause the rock to stretch and compress in the same direction the wave travels, so the ground appears to shake back and forth. Slightly slower-moving vibrations, called S waves or shear waves, cause the rock to move up and down, so the ground appears to rise and fall.

InSight was even able to record sound waves from meteorites falling through Mars’ thin atmosphere and colliding at high speed with the ground, thanks to the way those sound waves interact with the surface as they travel. through the lower layers of the air.

“The pressure wave fronts in the acoustic wave push and pull on the ground,” says Garcia. Reverse“And so that makes tiny rotations of the seismometer in the direction of arrival of the acoustic waves.”

Because the three waves travel at different but predictable speeds, Garcia and his colleagues were able to measure when each wave arrived at InSight’s instruments and then use that to calculate the distance the waves had traveled, plus or minus 10 percent. . The researchers were also able to estimate the direction from which the waves had come.

With that information, the team operating the Mars Reconnaissance Orbiter knew where to look for new impact craters with the satellite’s context camera. The telltale features were “extended dark blast zones surrounding new impact craters that were not present in earlier images from the context camera,” Garcia and his colleagues wrote.

Once MRO located a crater in the right place, the satellite’s higher-resolution (25 cm/pixel) camera zoomed in for a more detailed view, so Garcia and his colleagues could measure the crater’s diameter, direction of where the impact came from and other details.

Why does it matter – Seismic waves moving through rock are the only way geologists can actually “see” underground features, such as rock formations, fault lines, and magma chambers. Geologists on Earth rely on data from seismometer networks to understand what’s going on deep in the Earth’s crust (and how worried we should or shouldn’t be about it).

And since the early 1970s, scientists have been trying to do the same thing on the Moon. The Apollo missions left behind seismometers to measure moonquakes, and in some cases, astronauts detonated explosives on the lunar surface to create their own seismic waves for instruments to measure. NASA also crashed some third-stage Saturn V rockets on the Moon to collect even more seismic data.

A big part of InSight’s mission was to do the same thing on Mars. Knowing the location of the impacts that InSight recorded “takes some unknowns out” of the process of mapping Mars’ subterranean structure from seismic data, Garcia says.

Whats Next – And that’s next on the agenda for Garcia and his colleagues: using InSight’s seismic data to create a detailed map of the Martian crust around InSight. That sheds some light on Martian geology, but could also help plan future missions.

Garcia says his team wants to try a similar study on the Moon as well, using an instrument called the Farside Seismic Suite.

We already know, thanks to Apollo, that seismic waves fade much faster on Mars than on the Moon: after an impact, the Moon’s rocky crust continues to “resonate” for an hour, while the shaking of a similar impact on Mars it tends to run out after about 15 minutes.

“It shows that the Moon is much more fractured than Mars,” says Garcia. “This is consistent with the fact that Mars still has volcanic activity (capable of filling in impact fractures), while this ability stopped long ago for the Moon, so impact fractures can’t. ‘repair’ ”

But more detailed information could also help plan future lunar missions.