An international team of researchers combined orbital images with seismological data from NASA's InSight lander to obtain a new meteorite impact rate on Mars. Seismology also provides a new means of determining the density of Martian craters and the age of different regions of the planet.

An international team of researchers, jointly led by ETH Zurich and Imperial College London, obtained the first estimate of global meteorite impacts on Mars using seismic data. Their findings show that between 280 and 360 meteorites hit the planet each year, forming impact craters larger than 8 meters. Géraldine Zenhäusern from ETH Zurich, who co-led the study, stated: "This rate was about five times higher than the number estimated only from orbital images. Aligned with orbital images, our findings show that seismology is an excellent tool for measuring impact rates."

Seismic "chirp" signals a new class of quakes
Using data from the seismometer installed during NASA's InSight mission on Mars, researchers discovered that 6 seismic events recorded near the station were previously identified as meteorite impacts (Garcia et al., 2023) - a process enabled by capturing a specific acoustic atmospheric signal generated when meteorites enter the Martian atmosphere. Now, Zenhäusern from ETH Zurich, co-leader Natalia Wójcicka from Imperial College London, and the research team have found that these 6 seismic events belong to a much larger group of marsquakes, the so-called very high-frequency (VF) events. The original process of these quakes occurs much faster than that of tectonic marsquakes of similar size. While a normal magnitude 3 quake on Mars lasts several seconds, an impact event of the same size lasts only 0.2 seconds or less, due to the hypervelocity collision. By analyzing the spectrum of marsquakes, a further 80 marsquakes were identified, which are now believed to have been caused by meteoroid impacts.

Their research quest began in December 2021, a year before accumulated dust on the solar panels ended the InSight mission when a large distant quake recorded by the seismometer echoed with a broadband seismic signal through the planet. Remote observation linked the quake to a 150-meter-wide crater. To confirm, the InSight team partnered with the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) to search for other fresh craters that would match the timing and location of the seismic events discovered by InSight. Their detective work paid off, and they managed to find another fresh crater over 100 meters in diameter. Smaller craters, however, formed when basketball-sized meteoroids hit the planet and should be far more common, remained elusive. Now, the meteorite impact rate has been newly estimated with the emergence of these specific high-frequency quakes.

First meteorite impact rate from seismic data
About 17,000 meteorites fall to Earth each year, but unless they pass through the night sky, they are rarely noticed. Most meteors disintegrate upon entering Earth's atmosphere, but on Mars, the atmosphere is 100 times thinner, leaving the surface exposed to larger and more frequent meteorite impacts.

Until now, planetary scientists relied on orbital images and models derived from well-preserved craters formed by meteorite impacts on the Moon, but extrapolating these estimates to Mars has proven challenging. Scientists had to account for Mars' stronger gravitational pull and its proximity to the asteroid belt, both of which mean more meteorites hit the red planet. On the other hand, regular dust storms result in craters that are much less preserved than those on the Moon and are therefore harder to detect with orbital images. When meteorites hit the planet, seismic shock waves travel through the crust and mantle and can be recorded by seismometers, providing an entirely new way to measure the impact rate on Mars.

Wójcicka explains: "We estimated crater diameters from the magnitude of all VF-marsquakes and their distances, and then used this 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 occurring annually on the entire surface of Mars."

Zenhäusern adds: "While new craters are best seen on flat and dusty terrain where they really stand out, such terrain covers less than half of Mars' surface. The sensitive seismometer of InSight, however, could hear every single impact within the lander's range."

Insight into Mars' age and future missions
Just as lines and wrinkles on our face reveal clues about the age of different regions of the body, the size and density of craters from meteorite impacts reveal clues about the age of different regions of a planetary body. Fewer craters mean a younger part of the planet. For example, Venus has nearly invisible craters because it is protected by a dense atmosphere, and its surface is constantly being reshaped by volcanism, while the ancient surfaces of Mercury and the Moon are covered in craters. Mars falls between these examples, with some old and some young regions that can be differentiated by the number of craters.

New data shows that an 8-meter crater occurs somewhere on Mars' surface almost every day, and a 30-meter crater appears roughly once a month. Since hypervelocity impacts cause explosive 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.

"So, this is the first paper of its kind to determine how often meteorites hit the surface of Mars from seismological data - which was a level one mission of the InSight Mars mission," says Domenico Giardini, professor of seismology and geodynamics at ETH Zurich and co-leader for NASA's Mars InSight mission. "Such data play a role in planning future missions to Mars."

According to Zenhäusern and Wójcicka, the next steps in advancing this research include using machine learning technologies to help researchers identify further craters in satellite images and identify seismic events in the data.

Source: Eidgenössische Technische Hochschule Zürich