An international team of researchers, co-led by ETH Zurich and Imperial College London, has used seismic data to produce the first estimate of the global meteorite impact rate on Mars. Their findings indicate that between 280 and 360 meteorites strike the planet each year, creating impact craters larger than 8 meters in diameter. Geraldine Zenhaeusern, who co-led the study, noted: “This rate was about five times higher than the number estimated from orbital images alone. In line with orbital images, our findings show that seismology is an excellent tool for measuring impact rates.”
Seismic “beep” heralds 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 station’s vicinity had previously been identified as meteor impacts (Garcia et al., 2023) – a process made possible by the recording of a specific atmospheric acoustic signal generated when meteorites enter the Martian atmosphere. Now, Zenhäusern, ETH Zurich, co-leader Natalia Wójcicka, Imperial College London, and the research team have found that these six seismic events belong to a much larger group of marsquakes, the so-called very high frequency (VF) events. The source process of these quakes occurs much more rapidly than a tectonic marsquake of comparable magnitude. While a normal magnitude 3 quake on Mars lasts a few seconds, an impact-generated event of the same magnitude lasts only 0.2 seconds or less, due to the hypervelocity of the collision. By analyzing the spectra of the marsquakes, an additional 80 marsquakes were identified that are now believed to have been caused by meteoroid impacts.
Their search began in December 2021, a year before the dust accumulating on the solar panels ended the InSight mission, when a large, remote earthquake recorded by the seismometer sent a broadband seismic signal bouncing across the planet. Remote sensing associated the quake with a crater 150 meters (492 feet) wide. To confirm this, the InSight team worked with the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) to search for other fresh craters that matched the timing and location of the seismic events detected by InSight. The teams’ detective work paid off, and they were fortunate to find a second fresh crater measuring over 100 meters (320 feet) in diameter. However, smaller craters formed when basketball-sized meteorites hit the planet and are thought to be much more common have remained elusive. Now the number of meteorite impacts is being re-estimated based on the occurrence of these special high-frequency earthquakes.
First meteorite impact percentage from seismic data
About 17,000 meteorites fall to Earth every year, but unless they streak across the night sky, they are rarely noticed. Most meteors disintegrate as they enter Earth's atmosphere, but on Mars the atmosphere is a hundred times thinner, exposing the surface to larger and more frequent meteorite impacts.
Until now, planetary scientists have relied on orbital images and models derived from well-preserved meteorite impacts on the Moon, but extrapolating these estimates to Mars has proven challenging. Scientists had to account for Mars’ stronger gravity and its proximity to the asteroid belt, both of which mean more meteorites strike the Red Planet. On the other hand, regular dust storms result in craters that are much less well-preserved than those on the Moon, and therefore not as easy to detect with orbital images. When a meteorite strikes 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 crater diameters based on the sizes of all VF marsquakes and their distances, and then used these 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 the impacts that occur annually on the entire surface of Mars.”
Zenhäusern adds, “While new craters are best seen on flat, dusty terrain where they really stand out, this type of terrain covers less than half of the Martian surface. However, the sensitive InSight seismometer was able to hear every single impact within the lander's range.”
Understanding the age of Mars and future missions
Like the lines and wrinkles on our faces, the size and density of craters caused by meteorite impacts reveal clues about the age of different parts of a planetary body. The fewer craters, the younger the region of the planet. For example, Venus has almost no visible craters because its surface is constantly being reworked by volcanism, while Mercury and the Moon are heavily cratered with their old surfaces. Mars falls between these examples, with some old and some young regions distinguishable by the number of craters.
New data show that an 8-meter (26-foot) crater occurs somewhere on the Martian surface almost every day, and a 30-meter (98-foot) crater occurs about once a month. Because hypervelocity impacts create blast zones easily 100 times larger in diameter than the crater, knowing the exact number of impacts is important for the safety of robotic missions, as well as for future human missions to the Red Planet.
“This is the first paper of its kind to use seismological data to determine how often meteorites hit the surface of Mars – which was a level one mission objective of the Mars InSight mission,” said Domenico Giardini, professor of seismology and geodynamics at ETH Zurich and co-principal investigator for the NASA Mars InSight mission. “Such data will 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.
