Every day, basketball-sized meteorites hit the surface of Mars

NASA’s InSight Mars lander faced some challenges during its time on the red planet’s surface. Its mole instrument struggled to penetrate the compacted Martian soil, and the mission ultimately ended when its solar panels became covered in dust. However, some of its instruments worked well, including SEIS, the Seismic Experiment for Interior Structure.

SEIS has collected seismic data on Mars for over four years, and researchers who analyzed this data have determined a new meteorite impact rate on Mars.

SEIS was designed to study the internal structure of Mars by measuring seismic waves from marsquakes and impacts. Over 1,300 seismic events have been measured. There is no way to determine with certainty how many of these were due to impacts, but scientists working with the data have narrowed it down.

Their results are published in a new research paper entitled “An estimate of the impact rate on Mars using statistics of very high frequency Marsquakes,” published in Nature Astronomy. The lead authors are Géraldine Zenhäusern and Natalia Wójcicka from the Institute of Geophysics at ETH Zurich and the Department of Earth Science and Engineering at Imperial College London, respectively.

Although SEIS was an effective instrument, it could not always tell what seismic event was. Only a handful of the events it detected were strong enough to pinpoint their location. However, six events in close proximity to the InSight lander were confirmed to be meteor impacts because they correlated with acoustic atmospheric signals that meteors produce when they enter the Martian atmosphere. The six events belong to a larger group called very high frequency (VF) events.

While the formation process of a typical magnitude 3 Marsquake takes several seconds, an impact-triggered quake takes much less time due to the hypervelocity of the collision. These are the VF events.

During the roughly three years of recording, InSight and SEIS detected 70 VF events. 59 of these had good distance estimates, and according to the researchers, a handful of these were “higher quality B-VF events,” meaning their signal-to-noise ratio is strong. “Although a non-impact origin cannot be definitively ruled out for every VF event, we show that the VF class as a whole is plausibly caused by meteorite impacts,” the authors explain in their paper.

This led to a new estimate of the impact frequency on Mars. The researchers say that between 280 and 360 basketball-sized meteoroids hit Mars each year, digging craters over 8 meters in diameter. That’s almost one per day at the top end. “This rate was about five times higher than the number estimated from orbital images alone. Together with orbital images, our results show that seismology is an excellent tool for measuring impact rates,” Zenhäusern said in a press release.

Impact rates on various bodies in the solar system are one way to determine the age of their surfaces. Earth’s surface is young because the planet is so geologically active. It’s also much easier to study Earth in more detail, for obvious reasons. But for bodies like the Moon and Mars, impact rates can tell us the age of various surfaces, leading to a more thorough understanding of their history.

Orbital images and models based on preserved lunar craters are planetary scientists’ primary tools for estimating impact rates. Data from the moon have been used to extrapolate Mars’ impact rate, but this method is problematic. Mars has stronger gravity and is closer to the source of most meteors, the asteroid belt.

This means that more meteoroids hit Mars than the Moon, and that had to be accounted for somehow. Conversely, Mars has extensive dust storms that can obscure craters on orbital images, while the lunar surface is largely static. Mars also has different types of surface regions. Craters are noticeable in some regions, and not in others. Trying to accurately account for so many differences when extrapolating impact rates from the Moon to Mars is challenging.

This work shows that seismometers can be a more reliable method for measuring impact rates.

“We estimated crater diameters based on the magnitude of all VF marsquakes and their distances, and then calculated how many craters formed around the InSight lander over the course of a year. We then extrapolated these data to estimate the number of impacts that occur annually across the entire Martian surface,” Wójcicka explained.

“While new craters are best seen on flat and dusty terrain, where they really stand out, this type of terrain covers less than half of Mars’ surface. However, the sensitive InSight seismometer was able to hear every single impact within range of the landers,” Zenhäusern said.

These findings extend beyond Mars. Understanding Mars also helps us understand the entire solar system. “The current meteoroid impact rate on Mars is critical for accurately determining the absolute age of surfaces throughout the solar system,” the authors write in their paper. Without accurate surface ages, we do not have an accurate understanding of the history of the solar system.

Today we know that an 8-meter crater is excavated somewhere on the Martian surface almost every day, and a 30-meter crater occurs monthly. But there’s more to the issue than just crater size. These hypervelocity impacts create blast zones that dwarf the crater itself. The blast zones can easily be 100 times larger than the crater, so a better understanding of impact rates can make robotic missions and future manned missions safer.

“The higher overall number of impacts and the higher relative number of smaller impacts found in our study indicate that meteorite impacts could pose a significant threat to future exploration of Mars and other planets without dense atmospheres,” the authors write in their conclusion.

This study is a success for InSight and SEIS and for the researchers who put it together.

“This is the first study of its kind to use seismological data to determine how often meteorites hit the surface of Mars – which was a first-stage mission goal of the Mars InSight mission,” says Domenico Giardini, professor of seismology and geodynamics at ETH Zurich and co-leader of NASA’s Mars InSight mission. “Such data will be used to plan future missions to Mars.”