One of the simplest ways to date the surface of another world is to count its craters. A landscape with a lot of them suggests it’s relatively old because lots of time would be needed for enough meteorite impacts to occur. A world with fewer craters means the opposite is true—the surface is likely younger. Assuming an average impact rate, planetary scientists can calculate an age.
This technique is particularly useful on the Moon and Mars, for which we have detailed imagery of the surface. A fresh analysis of impacts on Mars, however, suggests our neighboring planet gets hit by meteorites more often than we thought, meaning our crater counting has been off.
“It implies a younger surface,” said Natalia Wójcicka, a planetary scientist at Imperial College London who coled the research published in Nature Astronomy.
Wójcicka and her colleagues came to this conclusion using data from a now-defunct NASA lander called InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport), which operated on Mars from 2018 to 2022. InSight carried the first seismometer to the Martian surface, which it used to measure marsquakes—the Martian equivalent of earthquakes. These rumblings are caused by the planet’s shrinking crust, meteorite impacts, and other events.
“We’re missing a lot of newly forming craters.”
Throughout the mission, the lander detected 1,319 marsquakes. Eight of these were definitively linked to meteorite impacts when scientists looked at before and after images from spacecraft orbiting Mars and found craters appearing around the same time as the seismic waves.
Wójcicka and her colleagues, however, determined that dozens more marsquakes may have been caused by impacts but haven’t been linked to a crater. The frequency of the seismic waves produced by these quakes matches that expected from a meteor slamming into the surface.
“If the signal is very high frequency, that indicates the source was moving very fast,” said Wójcicka, “and we know that planetary impacts are very fast processes.”
The discovery points to a livelier impact history than scientists thought. The team identified 58 marsquakes from InSight’s data that could have been caused by impacts. “The rate is higher than the rate derived from orbital images,” Wójcicka said, by a factor of up to 10. “We’re missing a lot of newly forming craters.”
A Younger Surface
On the basis of the amplitude of the seismic signals, each impact the lander recorded would have been produced by an incoming object between 3 and 40 meters (10–130 feet) across hitting the surface.
Though the exact location of most of the impacts is unknown, except for those also seen in orbital imagery, the seismic signals also suggest most of the meteorites struck within 2,600 kilometers (1,600 miles) of the lander. That allowed the researchers to estimate that 280–360 new craters larger than 8 meters (26 feet) in size formed on Mars every year.
Simone Marchi, a planetary scientist from the Southwest Research Institute, said he was “impressed with the detail of the analysis” and the modeling. “It provides an independent constraint on the present impact rate on Mars,” he said.
If Mars is hit by meteorites more often than we thought, its surface could be younger than scientists have assumed because the number of craters being counted would have been produced more recently than previously calculated. “If you assume 10 craters is a billion years, but actually you’re getting 15 craters every billion years, the surface ends up being younger,” said Benjamin Fernando, a planetary seismologist at Johns Hopkins University who was part of a team that performed a concurrent analysis of InSight’s marsquakes published in Science Advances. Wójcicka was a coauthor on that study.
The exact difference in age isn’t likely to be dramatic but could be up to about 2 million years in some locations depending on crater counts, Wójcicka said. A revised age could have “wider consequences for the geological history of Mars,” she said, such as the known rate of volcanism.
“Perhaps the impact rate around InSight has been higher than the background rate.”
However, Marchi said he was not sure about that analysis because InSight’s signals would have corresponded to relatively small craters up to tens of meters across. “Typically, for dating older surfaces, we want to use larger craters,” he said, on the order of hundreds of meters to kilometers in size.
InSight’s increased crater count might have another explanation, Fernando said. More abundant impacts could be the result of a recent event in space temporarily increasing the number of meteorites hitting the surface near the lander.
“Perhaps the impact rate around InSight has been higher than the background rate,” Fernando said. “Maybe something broke up in Mars orbit and bits of it have been slowly decaying down onto the surface,” such as an asteroid or comet.
It’s also hard to be sure all 58 of the events Wójcicka and her colleagues identified are impacts. The seismic data from the six marsquakes linked to known craters had a noticeable “chirp” caused by acoustic waves as the meteorite traveled through the air. Many of the other quakes, however, “don’t have chirps,” Fernando said. “So we’re just basing our analysis of seismic signals on waves propagating in the ground.”
With InSight now inoperative, we’re unlikely to learn more from the surface any time soon. But further observations from space could provide some answers. “There could be something more dramatic going on,” Fernando said, such as material being blasted off the two moons of Mars. “The thing to do will be to find more fresh craters,” he said.
—Jonathan O’Callaghan (@astro_jonny), Science Writer