A new class of Martian tremors reveals daily meteorite impacts

An international team of researchers, jointly led by ETH Zurich and Imperial College London, has derived the first estimate of global meteorite impacts on Mars using seismic data. Their findings suggest that 280 to 360 meteorites hit the planet each year, creating impact craters larger than 8 meters (about 26 feet) in diameter.

Geraldine Zenhaeusern, co-leader of the study, noted: “This rate was about five times higher than the number estimated from orbital images alone. Consistent with orbital images, our findings show that seismology is an excellent tool for measuring impact rates.”

Seismic “chirps” signal a new class of earthquakes

Using data from a seismometer deployed during NASA’s InSight mission to Mars, researchers found that 6 seismic events recorded in close proximity to the station were previously identified as meteor impacts—a process made possible by recording a specific acoustic atmospheric signal generated when meteorites enter the Martian atmosphere.

Now Zenhäusern of ETH Zurich, co-leader Natalia Wójcicka of Imperial College London and a 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 shocks occurs much faster than a tectonic marsquake of similar size. Where a normal magnitude 3 earthquake on Mars lasts a few seconds, an event created by an impact of the same magnitude lasts only 0.2 seconds or less, due to the hypervelocity of the impact. By analyzing marsquake spectra, the team identified another 80 marsquakes that are now believed to be caused by meteoroids.

Their research quest began in December 2021, a year before dust build-up on the solar panels ended the InSight mission, when a large, distant earthquake recorded by the seismometer triggered a broadband seismic signal across the planet. Remote sensing linked the earthquake to a 150-meter-wide crater. For confirmation, the InSight team teamed up with the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) to search for additional 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 100 meters (320 ft) in diameter. However, smaller craters, formed when basketball-sized meteoroids hit the planet and should be much more common, have remained elusive. Now the number of meteorite impacts is newly estimated according to the occurrence of these special high-frequency shocks.

First meteorite impact velocity 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 meteors break up when they enter Earth’s atmosphere, but on Mars, the atmosphere is 100 times thinner, so its surface is subject to larger and more frequent meteorite impacts.

Until now, planetary scientists have relied on images and orbital 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 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 by orbital imagery. When a meteorite hits a planet, seismic impact waves travel through the crust and mantle and can be picked up by seismometers.

Wójcicka explains: “We estimated crater diameters from the size of all VF-marsquakes and their distances, then used that 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 occurs annually over the entire surface of Mars.”

Zenhäusern adds, “While the 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 could hear every single impact within range of the lander.”

A look into the age of Mars and future missions

Like the lines and wrinkles on our face, the size and density of meteorite impact craters reveal clues about the age of different regions of the 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 reworked by volcanism, while Mercury and the Moon, with their ancient surfaces, are heavily cratered. Mars falls between these examples, with some old and young regions that can be distinguished by the number of craters.

The new data shows that an 8-meter (26-foot) crater occurs somewhere on the surface of Mars almost every day, and a 30-meter (98-foot) crater occurs about once a month. Since hypervelocity impacts produce 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 robotics as well as future human missions to the Red Planet.

“This is the first work of its kind to use seismological data to determine how often meteorites hit the Martian surface – which was the goal of the first stage of the Mars InSight Mission,” says Domenico Giardini, professor of seismology and geodynamics at ETH Zurich. co-principal investigator for NASA’s Mars InSight mission. “Such data influences the planning of future missions to Mars.”

According to Zenhäusern and Wójcic, next steps in advancing this research include the use of machine learning technologies to help researchers identify additional craters in satellite images and identify seismic events in the data.