Researchers have located fresh craters on Mars using shock waves caused by lumps of space rock as they tear through the sky and slam into the ground.
The new scars on the face of the planet are the first impact craters ever traced from the bang and crash of hurtling meteoroids bombarding another planet. The findings will help scientists build a more accurate picture of how often Mars is battered by the solar system’s rocky detritus and refine their understanding of the deep internal structure of our planetary neighbour.
“This is the first time we have felt and heard an impact on another planet,” said Prof Raphael Garcia, a planetary seismologist at the Higher Institute of Aeronautics and Space at the University of Toulouse.
To see if they could find craters produced by incoming meteoroids on Mars, the researchers examined seismic waves recorded by Nasa’s InSight lander between May 2020 and September 2021. The probe touched down in the barren expanse of Elysium Planitia in November 2018 on a mission to investigate the planet’s structure, crust and impact activity.
Scientists expected InSight to detect between one and 100 impacts every five Earth years using a sensitive seismometer deployed on the Martian surface. The seismic data recorded by the probe included four impact events that the researchers explored in detail.
By knowing how fast acoustic and seismic waves travel through Martian air and rock, the team estimated how far away from InSight the various meteoroids struck the surface. They then worked out the direction.
The loud bang on impact sends acoustic waves racing over the surface in all directions. These deform the ground imperceptibly, but the Insight data was so sensitive that the team picked up the direction of the impact from the slight tilt in the seismometer as the acoustic wave swept over.
The analysis allowed the scientists to predict roughly where the incoming meteoroids crashed into the surface. To check for signs of fresh craters, they turned to images taken by Nasa’s Mars Reconnaissance Orbiter. Before and after pictures from that probe revealed new black patches on the ground – freshly formed craters close to the expected impact sites.
One meteoroid reached Mars on 5 September 2021 and unleashed three stark shock waves. The first came when it slammed into the Martian atmosphere at about 10 kilometres a second, creating a shock wave along its trajectory. The space rock then exploded at an altitude of between 13 and 16km, producing multiple fragments. These then thumped into the ground, creating a cluster of fresh craters several metres wide.
The data is enormously valuable for planetary scientists studying the structure of Mars’s crust because the source of the seismic waves can be pinpointed to the crater. But impact craters are also used as cosmic clocks, with older surfaces on planets and moons pocked with more craters than younger ones.
“If people want to know if a surface is older or younger, it is critical to know the impact rate, but we are not there yet,” said Garcia. Details are published in Nature Geoscience.
