Seismic waves recorded by the Mars lander InSight have revealed the locations of three fresh impact craters on the Martian surface. A team led by Raphael F. Garcia from the Université de Toulouse was then able to assign visible impact traces to the signals measured between February and September 2021. As the working group writes in »Nature Geoscience«, the high-resolution images of the Mars Reconnaissance Orbiter found zones of dark material at all three locations that were not previously there in photos before the respective impacts. She was later able to assign a fourth fresh impact to other signals recorded by InSight.
A celestial body impacting Mars produces marsquakes in several ways, which the InSight lander can measure. On the one hand, the impact on the earth’s surface causes a small marsquake, on the other hand, impacting meteoroids produce violent explosions, called airbursts, on their way through the atmosphere. Some of the sound waves that are generated are also transmitted to the ground – actually, InSight would also have sensors to measure the sound directly, but they were switched off to save energy.
Since sound waves and earthquake waves travel at different speeds, the difference in their arrival times gives an indication of the distance to the source. In addition, the waves are polarized – they have a preferred direction of vibration that reveals roughly where the wave is coming from. According to the working group, with the help of this data it is possible to determine with an accuracy of 10 percent how far the origin of the waves is from the lander – the direction with an accuracy of 5 to 60 degrees, depending on how strong the signal is.
If you then combine the measurements of the impact and the associated airbursts, the location of the impact can be narrowed down more precisely via the trajectory. The images from the Mars Reconnaissance Orbiter show that the resulting craters are about four to twelve meters in size. The meteoroids still weighed a few dozen kilograms on impact, but had lost significant parts of their mass during the airbursts. However, it is not InSight’s job to reveal details of such impacts. Actually, the probe uses the earthquake waves generated by such hits to learn more about the internal structure of Mars.