An international team of researchers, co-lead by ETH Zurich and Imperial College London, have derived the first estimate of global meteorite impacts on Mars using seismic data. Their findings indicate between 280 to 360 meteorites strike the planet each year forming impact craters greater than 8 metres (about 26 feet) across. Geraldine Zenhaeusern who co-led the study commented, "This rate was about five times higher than the number estimated from orbital imagery alone. Aligned with orbital imagery, our findings demonstrate that seismology is an excellent tool for measuring impact rates."
Using data from the seismometer deployed during the NASA InSight Mission to Mars, researchers found that 6 seismic events recorded in the near proximity of the station had been previously identified as meteoric impacts (Garcia et al., 2023) - a process enabled by the recording of a specific acoustic atmospheric signal generated when meteorites enter the Martian atmosphere. Now, Zenhäusern, ETH Zurich, co-lead, Natalia Wójcicka, Imperial College London, and the research team have found that these 6 seismic events belong to a much larger group of marsquakes, so called;very high frequency (VF);events. The source process of these quakes occurs much faster than for a tectonic marsquake of similar size. Where a normal magnitude 3-quake on Mars takes several seconds, an impact-generated event of the same size takes only 0.2 seconds or less, due to the hypervelocity of the collision. By analysing marsquake spectra, a further 80 marsquakes were identified that are now thought to be caused by meteoroid strikes.
Their research quest began in December 2021, a year before accumulated dust on the solar panels put an end to the InSight mission, when a large distant quake recorded by the seismometer reverberated a broadband seismic signal throughout the planet. Remote sensing associated the quake with a 150-metre-wide crater. To confirm, the InSight team partnered with Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) to search for other fresh craters that would match the timing and location of the seismic events detected by InSight. The teams’ detective work paid off and they were lucky to find a second fresh; crater over a 100-metres (320 feet) in diameter. Smaller craters, however, formed when basketball-sized meteoroids strike the planet and which should be far more common, remained elusive. Now, the number of meteorite strikes is newly estimated by the occurrence of these special high-frequency quakes.
Approximately 17,000 meteorites fall to Earth each year, but unless they streak across the night’s sky, they are rarely noticed. Most meteors disintegrate as they enter Earth’s atmosphere, but on Mars the atmosphere is 100 times thinner leaving its surface exposed to larger and more frequent meteorite strikes.
Until now, planetary scientists have relied on orbital images and models inferred from well-preserved meteorite impacts on the Moon but extrapolating these estimates to Mars proved challenging. Scientists had to account for the stronger gravitational pull of Mars and its proximity to the asteroid belt, which both mean that more meteorites hit the red planet. On the other hand, regular sandstorms result in craters that are much less well-preserved than those on the Moon, and, therefore, not as easily detected with orbital imagery. When a meteorite strikes the planet, the seismic waves of the impact travel through the crust and mantle and can be picked up by seismometers.
Wójcicka explains, "We estimated crater diameters from the magnitude of all the VF-marsquakes and their distances, then used it to calculate how many craters formed around the InSight lander over the course of a year. We then extrapolated this data to estimate the number of impacts that happen annually on the whole surface of Mars."
Zenhäusern adds, "While new craters can best be seen on flat and dusty terrain where they really stand out, this typ of terrain covers less than half of the surface of Mars. The sensitive InSight seismometer, however, could hear every single impact within the landers' range."
Much like the lines and wrinkles on our face, the size and density of craters from meteorite strikes reveal clues about the age of different regions of a planetary body. The less craters, the younger the region of the planet. Venus, for example, has almost no visible craters because its surface is continually reworked by volcanism, while Mercury and the Moon with their ancient surfaces are heavily cratered. Mars falls in between these examples, with some old and some young regions that can be distinguished by the number of craters.
New data shows, an 8-metre (26-feet) crater happens somewhere on the surface of Mars nearly every day and a 30-metre (98-feet) crater occurs about once a month. Since hypervelocity impacts cause blast zones that are easily 100 times larger in diameter than the crater, knowing the exact number of impacts is important for the safety of robotic, but also future human missions to the red planet.
"This is the first paper of its kind to determine how often meteorites impact the surface of Mars from seismological data – which was a level one mission goal of the Mars InSight Mission,” says Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich and co-Principal Investigator for the NASA Mars InSight Mission. “Such data factors into the planning for future missions to Mars."
According to Zenhäusern and Wójcicka, the next steps in advancing this research involve the use of machine learning technologies to aid researchers in identifying further craters in satellite images and identifying seismic events in the data.
