Just as it happens on the Moon or Mercury, the surface of Mars is full of craters, ‘scars’ of all sizes caused by countless asteroid and comet impacts throughout its long history. But how many meteorites fall in … current events on Mars? It’s hard to say, but an international team of researchers from Imperial College London and ETH Zurich in Switzerland have managed to obtain the most reliable estimate yet. And they have done so, for the first time, using seismic data.
Their findings, recently published in ‘Nature Astronomy’, indicate that between 280 and 360 meteorites hit the red planet each year, leaving impact craters more than 8 meters wide. According to Geraldine Zenhäusern, co-leader of the study, “this rate is almost five times higher than the number estimated from orbital images alone. “Our findings demonstrate that seismology is an excellent tool for measuring impact rates.”
A new type of ‘marsquake’
Thanks to data from the seismometer deployed during NASA’s InSight mission, researchers discovered that 6 seismic events recorded in the vicinity of the station had already been previously identified as meteorite impacts. And now, Zenhäusern and her team have discovered that these six seismic events were just the tip of the iceberg, as they actually belong to a much larger group of ‘marsquakes’, so-called ‘very high frequency events’ (VF ).
Shaking from these earthquakes occurs much faster than from a tectonic earthquake of similar magnitude. For example, while a ‘normal’ magnitude 3 earthquake on Mars usually lasts for several seconds, an event of the same intensity, but generated by an impact, lasts only 0.2 seconds or less. By analyzing the spectra of a large number of ‘marsquakes’ the researchers identified another 80 that had been caused by meteorite impacts.
Three years of work
The investigation began in December 2021, a year before dust accumulated on the solar panels put an end to the InSight mission. Then, seismic waves from a distant, high-intensity earthquake caused all of Mars to ‘ring’ like a bell. And remote sensing systems associated that earthquake with a 150-meter-wide crater. To confirm this, the InSight team requested the use of the context camera (CTX) of the Mars Reconnaissance Orbiter (MRO), and with it searched for other recent craters that coincide with the timing and location of the seismic events detected by InSight.
Detective work that paid off, as researchers were lucky enough to find a second, newly formed crater more than 100 meters wide. But smaller craters, those formed when basketball-sized meteorites hit the planet and which logically should be much more common, were still very difficult to detect. And that is precisely what Zenhäusern and his colleagues have now achieved.
Inaccurate data
About 17,000 meteorites fall to Earth each year, but unless they fly across the night sky, they are rarely detected. Most of them, in fact, disintegrate upon entering the atmosphere. But on Mars things are different, because there the atmosphere is 100 times thinner than Earth’s, which leaves the surface ‘defenseless’ and exposed to impacts from larger and more frequent meteorites.
Until now, planetary scientists have relied on images and orbital models developed from well-preserved meteorite impacts on the Moon, but extrapolating those estimates to Mars proved to be very tricky. Scientists had to take into account Mars’ greater gravitational pull and proximity to the asteroid belt, which means that more meteorites hit the Red Planet than the Moon. On the other hand, the numerous sandstorms that sweep across the Martian surface result in craters that are much less well preserved than those on the Moon and are therefore not as easy to detect from orbital images. But when a meteorite hits the planet, seismic waves from the impact travel through the crust and mantle, and can be picked up by seismometers.
According to Natalia Wójcicka from Imperial College and co-author of the study, “We estimated crater diameters from the magnitudes of all VF marsquakes and their distances, then used this to calculate how many craters formed around the InSight lander over the course of a year. And finally we extrapolated the data to estimate the number of impacts that occur annually across the entire surface of Mars.”
«Although the new craters can be seen better on flat, dusty terrain, where they really stand out – adds Zenhäusern – this type of terrain covers less than half of the surface of Mars. And the sensitive InSight seismometer was able to hear every impact within range of the lander.”
The age of the planets
Like the lines and wrinkles on our faces, the size and density of impact craters reveal valuable clues about the age of different regions of a planetary body. The fewer craters there are, the younger the region of the planet in which they are located. Venus, for example, has almost no visible craters because its surface is continually being reshaped by volcanism, and on Earth most of the craters that once existed have been erased by erosion and tectonic movement. But Mercury and the Moon, with their thin atmospheres and little geological activity, boast undisturbed, very old surfaces riddled with impact craters. Mars, on the other hand, is somewhere in between, with some regions old and others much younger.
According to the study, an 8-meter crater appears almost every day somewhere on Mars, while a larger one, 30 meters, appears approximately once a month.
“This is the first study to determine how frequently meteorites impact the surface of Mars from seismological data, which was one of the main goals of the Mars InSight mission,” said Domenico Giardini of ETH Zurich and principal investigator of that NASA mission. “These data influence the planning of future missions to Mars.”
A second study
Another different team of researchers, this time from Brown University, has also just carried out a similar study based on data from the InSight mission seismometer, and in an article just published in ‘Science Advances’ announces the discovery of 8 new impact craters that had not been seen by spacecraft in orbit. According to the study, Mars is being bombarded by space rocks at a much more frequent rate than previously thought, with impact rates between two and ten times higher than previously estimated, depending on the size of the rocks.
