New Class of Mars Quakes Reveals Daily Meteorite Strikes

An international team of researchers, co-led by ETH Zurich and Imperial College London, has obtained the first estimate of global meteorite impacts on Mars using seismic data. Their findings indicate that between 280 and 360 meteorites strike the planet each year, forming impact craters more than 8 meters (about 26 feet) in diameter.

Geraldine Zenhaeusern, who co-led the study, commented: “This rate is about five times higher than the number estimated from orbital imagery alone. Together with orbital imagery, our results demonstrate that seismology is an excellent tool for measuring impact rates.”

Seismic ‘chirp’ signals new class of earthquakes

Using data from the seismometer deployed during NASA’s InSight mission to Mars, researchers found that six seismic events recorded in the immediate vicinity of the station had previously been identified as meteor impacts – a process made possible by recording a specific atmospheric acoustic signal generated when meteorites enter the Martian atmosphere.

Now, Zenhäusern of ETH Zurich, co-leader Natalia Wójcicka of Imperial College London, and the research team have discovered that these six seismic events belong to a much larger group of Marsquakes, called very high frequency (VHF) events. The source process for these quakes occurs much faster than for a tectonic earthquake of similar size. While a typical magnitude 3 earthquake on Mars takes several seconds, an event generated by an impact of the same size takes only 0.2 seconds or less, due to the hypervelocity of the collision. By analyzing the spectra of the quakes, the team identified 80 additional quakes that are now thought to be caused by meteorite strikes.

Their search began in December 2021, a year before dust accumulated on the solar panels ended the InSight mission, when a large distant earthquake recorded by the seismometer reverberated a broadband seismic signal across the planet. Remote sensing matched the earthquake to a 150-meter-wide crater. To confirm this, the InSight team teamed up with the Mars Reconnaissance Orbiter’s (MRO) Context Camera (CTX) to search for other recent 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 recent crater more than 100 meters in diameter. However, smaller craters, formed when basketball-sized meteorites hit the planet and expected to be much more common, have remained elusive. Today, the number of meteorite falls is again estimated based on the frequency of these special high-frequency earthquakes.

First meteorite impact rate from seismic data

About 17,000 meteorites fall to Earth each year, but unless they streak across the night sky, they are rarely noticed. Most meteorites disintegrate when they enter Earth’s atmosphere, but on Mars, the atmosphere is 100 times thinner, exposing its surface to larger and more frequent meteorite impacts.

Until now, planetary scientists have relied on orbital images and inferred models of well-preserved meteorite impacts on the Moon, but extrapolating these estimates to Mars has proven difficult. Scientists had to take into account 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 sandstorms result in craters that are much less well preserved than those on the Moon and, therefore, less easy to detect with orbital imaging. When a meteorite hits the planet, seismic waves from the impact travel through the crust and mantle and can be picked up by seismometers.

Wójcicka explains: “We estimated the diameter of the craters from the magnitude of all the VF earthquakes and their distances, then we used this data to calculate the number of craters formed around the InSight lander at course of a year. We then extrapolated this data to estimate the number of impacts that occur each year across the entire surface of Mars. »

Zenhäusern adds: “Although the new craters are best seen on flat, dusty terrain where they really stand out, this type of terrain covers less than half of Mars’ surface. The sensitive InSight seismometer, however, could hear each impact in space within range of the lander.”

A look at the Mars era and future missions

Much like the lines and wrinkles on our faces, the size and density of craters resulting from meteorite impacts reveal clues about the ages of different regions of a planetary body. The fewer craters there are, 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 between these examples, with older and younger regions that can be distinguished by the number of craters.

New data shows that an 8-meter crater forms almost every day on the surface of Mars, and a 30-meter crater forms about once a month. Since hypervelocity impacts cause explosion zones easily 100 times larger in diameter than the crater, knowing the exact number of impacts is important for robotics safety, but also for future human missions to the red planet.

“This is the first study of its kind to determine the frequency of meteorite impacts on the surface of Mars from seismological data, which was a primary goal of the Mars InSight mission,” explains Domenico Giardini, professor of seismology and geodynamics at ETH Zurich and co-principal investigator of NASA’s Mars InSight mission. “This data is taken into account in planning future missions to Mars. »

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