*Free* Martian meteor impact produces a seismic event that reveals new details on Mars’ interior

On 24 December 2021, NASA’s InSight lander recorded a large seismic event with a distinct signature rippling across the surface of Mars – one of the largest seismic events observed on the distant planet. Now, across a pair of studies, researchers show that this event, and another detected earlier that year, were caused by a meteor impact and use the surface waves produced by the collisions to untangle the structure of the Martian crust. The structure of a planet’s crust and mantle can provide important information about its origin and evolution. Seismic waves that propagate along the shallow surface of a planet, known as surface waves, can be used to map these structural features. However, to date, no such waves have ever been detected on any planet besides Earth. Although seismic tremors and meteor impacts on the red planet are not uncommon, they have been challenging to detect and characterize. However, the two seismic events that occurred during the latter half of 2021 were uniquely large. In one study, Liliya Posiolova and colleagues use images of the Martian surface taken aboard the Mars Reconnaissance Orbiter and show that these two events were caused by meteor impacts. These collisions, which resulted in two large impact craters (> 130-meters in diameter), sent surface waves reverberating across the planet. In the second study, Doyeon Kim and colleagues leveraged these surface waves – the first ever detected on Mars – to better understand the planet’s interior structure beneath the InSight lander. Kim et al. found that the crust here was more dense than previously inferred. These variations constrain models for the composition, formation, and thickness of the Martian crust. “The InSight lander is expected to end its operation by December 2022 because of dust accumulation on its solar panels,” write Yingjie Yang and Xiaofei Chen in a related Perspective. “However, continuing work on the already recorded seismic data should continue to deliver discoveries about the structure of Mars.”