Surface waves and crustal structure on Mars
We detected surface waves from two meteorite impacts on Mars. By measuring group velocity dispersion along the impact-lander path, we obtained a direct constraint on crustal structure away from the InSight lander. The crust north of the equatorial dichotomy had a shear wave velocity of approximately 3.2 kilometers per second in the 5- to 30-kilometer depth range, with little depth variation. This implies a higher crustal density than inferred beneath the lander, suggesting either compositional differences or reduced porosity in the volcanic areas traversed by the surface waves. The lower velocities and the crustal layering observed beneath the landing site down to a 10-kilometer depth are not a global feature. Structural variations revealed by surface waves hold implications for models of the formation and thickness of the martian crust. Description An insightful impact On 24 December 2021, the seismometer for the InSight mission on Mars detected a large seismic event with a distinct signature. Posiolova et al. discovered that the event was caused by a meteor impact on the surface of Mars, which was confirmed by satellite observations of a newly formed 150-kilometer crater. The surface nature and size of the impact allowed Kim et al. to detect surface waves from the event, which have yet to be observed on Mars. These surface waves help to untangle the structure of the Martian crust, which has various amounts of volcanic and sedimentary rock, along with subsurface ice, in different regions of the planet (see the Perspective by Yang and Chen). The characteristics of the impact itself are important because they provide a seismic fingerprint of an impact event that is different from the marsquakes observed so far. —BG A new crater formed on the surface of Mars was detected with the seismometer on the InSight mission.