The new scars on the face of the planet are the first impact craters ever detected from the explosion and crash of ejected meteoroids bombarding another planet. The findings will help scientists build a more accurate picture of how often Mars is hit by rocky debris from the solar system and improve their understanding of the deep internal structure of our planetary neighbor. “This is the first time we feel and hear an impact on another planet,” said Professor Rafael 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 landed on the barren expanse of Elysium Planitia in November 2018 in 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 investigated in detail. Knowing how fast acoustic and seismic waves travel through Martian air and rock, the team estimated how far from InSight the various meteorites hit the surface. Then they worked out the direction. The loud bang on impact sends sound waves racing over the surface in all directions. These deform the ground subtly, but Insight’s data was so sensitive that the team picked up the direction of the impact from the slight tilt of the seismometer as the sound wave passed by. The analysis allowed scientists to predict roughly where the incoming meteoroids hit the surface. To check for signs of fresh craters, they turned to images taken by NASA’s Mars Reconnaissance Orbiter. Before and after photos from this probe revealed new black patches on the ground – freshly formed craters near the expected impact points. A meteorite hit Mars on September 5, 2021 and unleashed three intense shock waves. The first came when it slammed into the Martian atmosphere at about 10 kilometers per second, creating a shock wave along its path. The space rock then exploded at an altitude of between 13 and 16 kilometers, creating multiple fragments. They then fell to the ground, creating a cluster of fresh craters several meters wide. The data is extremely valuable to planetary scientists studying the structure of the Martian 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 more cratered than younger ones. “If people want to know if a surface is older or younger, it’s important to know the impact rate, but we’re not there yet,” Garcia said. Details are published in Nature Geoscience.
