Basketball-Sized Meteorites Strike the Surface of Mars Every Day

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 eventually ended when its solar panels were covered in dust. But some of its instruments performed well, including SEIS, the Seismic Experiment for Interior Structure.

SEIS gathered Mars seismic data for more than four years, and researchers working with all of that data have determined a new meteorite impact rate for Mars.

SEIS was designed to probe Mars’ interior structure by measuring seismic waves from Marsquakes and impacts. It measured over 1300 seismic events. There’s no way to absolutely measure how many of them were from impacts, but scientists working with the data have narrowed it down.

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

“This is the first paper of its kind to determine how often meteorites impact the surface of Mars from seismological data.”

Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich and co-Principal Investigator for the NASA Mars InSight Mission.

Though SEIS was an effective instrument, it couldn’t always tell what each seismic event was. Only a handful of the events it detected were powerful enough to determine their location. However, six events in close proximity to the InSight lander were confirmed as meteorite impacts because they were correlated with acoustic atmospheric signals that meteors make when they enter Mars’ atmosphere. The six events belong to a larger group called very high-frequency (VF) events.

While the source process for a typical marsquake measuring magnitude 3 takes several seconds, an impact-generated quake takes much less time because of the collision’s hypervelocity. These are the VF events.

During about three years of recording time, InSight and SEIS detected 70 VF events. 59 of them had good distance estimates, and according to the researchers, a handful of them were “higher quality B VF events,” meaning their signal-to-noise ratios are strong. “Although a non-impact origin cannot be definitively excluded for each 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 Mars’s impact frequencies. The researchers say that between 280 and 360 meteoroids about the size of basketballs strike Mars each year and excavate craters greater than 8 meters (26 ft) in diameter. That’s almost one every day at the upper end. “This rate was about five times higher than the number estimated from orbital imagery alone. Aligned with orbital imagery, our findings demonstrate that seismology is an excellent tool for measuring impact rates,” Zenhäusern said in a press release.

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

Orbital images and models based on preserved lunar craters have been the main tools used by planetary scientists to infer impact rates. The data from the Moon was used to extrapolate Mars’ impact rate. But there are problems with that method. Mars has more powerful gravity and is closer to the source of most meteors, the asteroid belt.

That means more meteoroids strike Mars than the Moon, and that had to be calculated somehow. Conversely, Mars has widespread dust storms that can obscure craters in orbital images, while the lunar surface is largely static. Mars also has different types of surface regions. In some regions, craters stand out; in others, they don’t. Trying to accurately account for that many differences when extrapolating impact rates from the Moon to Mars is challenging.

This work shows that seismometers can be a more reliable way to understand impact rates.

“We estimated crater diameters from the magnitude of all the VF-marsquakes and their distances, then used it 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 happen annually on the whole surface of Mars,” Wójcicka explained.

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

These results extend beyond Mars. Understanding Mars also helps us understand the wider Solar System. “The current meteoroid impact rate on Mars is vital for determining accurate absolute ages of surfaces throughout the Solar System,” the authors write in their paper. Without accurate surface ages, we don’t have an accurate understanding of the Solar System’s history.

Now we know that an 8-metre (26-feet) crater is excavated somewhere on Mars’ surface almost daily, and a 30-metre (98-feet) crater is a monthly occurrence. But it’s about more 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 human missions safer.

“The higher overall number of impacts and the higher relative number of small ones found in our study show that meteoritic impacts might be a substantial hazard for future explorations of Mars and other planets without a thick atmosphere,” the authors write in their conclusion.

This study is a win for InSight and SEIS and for the researchers who pieced this together.

“This is the first paper of its kind to determine how often meteorites impact the surface of Mars from seismological data – which was a level one mission goal of the Mars InSight Mission,” says Domenico Giardini, Professor of Seismology and Geodynamics at ETH Zurich and co-Principal Investigator for the NASA Mars InSight Mission. “Such data factors into the planning for future missions to Mars.”