Scientists have long wondered why more bowl-shaped craters from asteroid strikes haven’t been found on Mars, despite the Red Planet sitting next to the asteroid belt and its sparse atmosphere being just 1 percent as thick as Earth’s. A new study suggests roughly 300 basketball-sized meteorites pockmark Mars’ surface every year, raising previous estimates by five times.
The findings are based on data from NASA’s InSight lander, which for over four years listened to Mars’ seismic shakes and probed the planet’s geological history. The researchers studied seismological data recorded by InSight’s onboard seismometer, which scientists say is capable of hearing the slightest rumbles on Mars. They found that “there isn’t a place on Mars that is more likely to be hit than any other place,” says study lead author Natalia Wójcicka of the Imperial College London, adding that future robotic and crewed missions to Mars could benefit from these findings while selecting landing spots and base camps.
The new impact rate is five times higher than estimates published a decade ago and based on satellite images, rather than on-the-ground data. “That suggests that we miss a lot of craters by just looking at the surface of the planet because we don’t image all of Mars all the time,” says Wójcicka.
Moreover, many craters left behind by such impacts have never been seen by orbiting satellites, the researchers found. “We were certainly very excited,” says Wójcicka. “We almost didn’t believe it.”
Rumbles on Mars
The seismometer onboard InSight, called the Seismic Experiment for Interior Structure, or SEIS, recorded about 1,300 marsquakes during its four-year mission. The instrument by itself wasn’t able to distinguish which rumbles were due to meteorites, or where on Mars they occurred. But a group of signals share similar properties, key of which is that most of their energy is above 4 hertz, consistent with a shallow source like a meteorite strike rather than a quake deep within Mars, says Wójcicka.
Between 2018 to 2022, InSight heard 70 so-called “very high-frequency” events, all of which were likely created by meteorite crashes, according to the new study, which was published last month in Nature Astronomy.
By studying the properties of these signals, Wójcicka and her team predict between 280 and 360 basketball-sized meteorites land across Mars each year and leave behind craters bigger than 26 feet (8 meters) in diameter. Although this crater size is within the range scientists can spot in satellite images, “we don’t have a systematic way that we take images of areas,” says Wójcicka, so it’s difficult to associate craters with the events InSight heard. But “[the signals] are similar enough to each other that at least more if not all of them could be impact related, but we don’t have confirmation of that because we haven’t seen the craters.”
Spotting craters
Satellites don’t capture all of Mars all the time, and often there aren’t repetitive images of the same region. This limits the amount of comparison scientists can do of a region before a meteorite strike versus after, when the crater or any material excavated by the meteorite could be spotted in images. The task is made harder by the fact that sifting through the images almost always requires a human eye, says Wójcicka.
“It does take a lot of training from the human side to find those craters in the images,” she says. “The smaller it gets, the more you’re likely to miss it.”
Last month, a different team of scientists spotted eight football field-sized impact craters puncturing Mars’ surface that were not previously seen by satellites. Six were near InSight’s landing spot and two were among the largest ever spotted, suggesting “the planet is getting hit much more frequently than we can see using imaging alone,” Ingrid Daubar of Brown University in Rhode Island, who led the companion study, said in a statement.
Impacts this large are expected to occur only every few decades, perhaps even just once in a lifetime, Daubar said. But his team found they had occurred just 97 days apart. “It could just be a crazy coincidence, but there’s a really, really small likelihood that it’s just coincidence,” he said. “What’s more likely is that either the two big impacts are related, or the impact rate is a lot higher for Mars than what we thought it was.”
Piecing together Mars’ history
Unlike Earth, Mars lacks active plate tectonics, the continually shifting chunks of crust that trigger earthquakes when they grind together. The planet rumbles from deep within nonetheless, primarily driven by the world shrinking and cooling
Conventionally, scientists use the number of craters as “cosmic clocks” to date a planet’s surface, with older surfaces pockmarked with more craters than younger ones. Because higher impact rates mean less time is needed to accumulate the same number of craters, better constraints of impact rates can help scientists fine-tune their gauges of how old the martian surface is. This, in turn, can reveal when the last major event that erased previous craters, such as a volcanic eruption, occurred on Mars.
And if the impact rate on Mars is different than thought, “this is going to require us to rethink some of the models the science community uses to estimate the age of planetary surfaces throughout the entire solar system,” Daubar said. And by understanding what happened on Mars, scientists can better understand the history of our own planet, he added.
“This is important for understanding our solar system, what’s in it and what the population of impacting bodies in our solar system looks like — both as hazards to the Earth and also historically to other planets,” said Daubar.
Related: Probe counts space rock impacts on Mars
