On the red planet next door, a plucky spacecraft is busy recording the fits and spasms of the world beneath its feet—even as it approaches the end of its life.
NASA’s InSight lander, which arrived on Mars in 2018, wasn’t designed to look outward—it was designed to feel the planet tremble, to measure its quakes and rumbles, and to help scientists map the Martian interior. But the lander has also picked up the jitters of a planet being pummeled by space rocks: InSight has detected the signatures of at least four meteors colliding with Mars, and scientists have spotted the resulting craters in images taken from orbit.
“These observations are very exciting,” says Elizabeth Silber of Canada’s Western University, who studies similar phenomena but was not involved in the InSight observations. “It is awesome that we have arrived at a technological point at which we're advanced enough to be able to detect and link seismic signatures to impact events on another planet.”
On Earth, scientists have made such a link only once. And on the moon, where more than a hundred impacts shook Apollo-era seismometers, none could be linked to any resulting craters. At Mars, scientists can now use the planet’s cosmic pummeling to make even better maps of its interior.
“The great advantage of these impacts is we know the location of the source,” says Raphael Garcia of the Aeronautics and Space Institute at the University of Toulouse in France, the lead author of a paper reporting the impacts, published today in the journal Nature Geoscience. “All the unknowns that remain are only the internal structure.”
But with its solar panels covered in dust, InSight’s time is running out and mission leaders suspect it might not make it through the end of the year.
“Our optimistic but plausible projections are taking us into January, but the more likely situation is that between now and then, we will get a spike in atmospheric dust and we may not be able to operate through that,” says the Jet Propulsion Laboratory’s Bruce Banerdt, InSight’s principal investigator.
InSight touched down on Mars in the blandest spot its team members could identify—a flat, sandy plain called Elysium Planitia near the planet’s equator.
“I’m very, very happy that it looks like we have an incredibly safe and boring-looking landing plain. That’s exactly what we were going for—it’s what the landing site selection people promised me," JPL’s Tom Hoffman, the InSight project manager, said in 2018, after it landed. "They promised me sandy with no rocks. But there’s one rock, so I might have to talk with them about that.”
Unlike the spacecraft that focus on dramatic Martian terrains—the planet’s massive volcanoes, dramatic rift valleys and polar ice caps—InSight’s job was to look beneath the surface. And that meant it needed no distractions, plus plenty of sunlight to power its instruments. Soon after landing, InSight unfurled its solar panels and deployed an exquisitely sensitive seismometer that would monitor the trembling planet. As Mars shakes, seismic waves bounce around and travel through the planet’s interior. Those waves carry information about the materials and boundaries they’ve moved through, so scientists can collect them and use those records to make a map of the Martian crust, mantle, and curiously large core.
Over nearly four Earth-years, InSight has measured more than 1,300 tremors. For much of the mission, the Marsquakes have been small; but over the past year, a handful of large quakes have jolted the planet, with the strongest—nearly a magnitude 5, something scientists had been hoping for—rolling through in May. Now, as the spacecraft’s power dwindles, mission leaders say the spacecraft lived a good life. With the exception of one hiccup, a heat probe that couldn’t burrow into the Martian soil as expected, the mission has met its objectives.
“We’ve been able to illuminate the interior structure of Mars for the first time, instead of having a fuzzy picture that is informed by analogy to the Earth, or the moon,” Banerdt says. “Mars is now a solidly understood planet. It’s not like we know everything about what’s going on inside, but we know what the basic building blocks of Mars are.”
In addition to deciphering the planet’s innards, scientists also wanted to measure the impact rate at Mars, or how frequently the planet gets smacked by incoming space rocks. For a while, it seemed like that might not happen—the team detected no impacts during the mission’s first two years.
But in a twist, it turns out that among those 1,300-plus tremors are a handful produced by crater-forming meteors. Three of these impacts occurred over six months in 2021—on February 18, August 31, and September 5. One had actually occurred earlier, in May 2020, producing a 36-foot-wide cluster of craters spotted from overhead by the Mars Reconnaissance Orbiter, which has surveilled the planet since 2006. But scientists hadn’t identified that fingerprint in the data until more recently.
Garcia and his colleagues identified the impacts based on telltale acoustic waves produced as meteors explode in the atmosphere as they streak toward a planet’s surface. On Mars, and at night, these sonic signals can travel for hundreds of miles through the bottom layer of chilled Martian atmosphere. By combining information from the acoustic waves and the impact-generated seismic waves, scientists can determine the meteor’s trajectory and eventual crash site.
When the three impacts occurred in 2021, Garcia says the resulting acoustic waves were so strong the team almost didn’t believe they were real. But they were real—and when the Mars Reconnaissance Orbiter snapped images of the potential crash sites, it found fresh craters. Later, when scientists went back through the data, they uncovered the acoustic signature of the event from 2020.
“That was iron-clad proof that these seismic records were caused by those cratering events,” Banerdt says.
The fresh craters are between 13 and 25 feet across, the work of objects so small they would likely burn up in Earth’s thicker, more protective atmosphere. By definitively linking a seismic signal with a resulting crater on another world, scientists can study the energetics of the collisions. And the calculated rate of impacts is in line with predictions, although the team isn’t sure why the latter three collisions appear to be clustered in time, and why there aren’t any obvious impacts during the mission’s first two years.
“We don’t really know why that is,” Banerdt says. “It’s definitely kind of mysterious.”
Now, after nearly four years on the Martian surface, InSight’s mission is ending. The spacecraft’s fate is in the hands of the Martian winds, which carry the planet’s vermillion sand aloft, sometimes whipping it into dust devils and planet-spanning storms. A thick layer of dust has already settled on the spacecraft’s solar panels, blocking the sunlight that powers the lander and its instruments. In May, as the spacecraft’s power dwindled, mission leaders estimated that InSight’s seismometer might take data through the end of the summer—but that depended on the Martian weather.
“This is a very dynamic season on Mars—it’s dust storm season,” Banerdt told National Geographic at the time. “Even if we don’t get a dust storm right here at InSight, there are dust storms kicking up all over the planet that inject stuff into the atmosphere that can affect the amount of solar energy we’re getting.”
“That said,” he continued, “we’ve gotten really good at finding change in the couch cushions.”
Since May, Mars has been kinder to the lander than expected.
“The dust has been holding very steady, maybe even dropping a little bit, and our power has actually been rising, just a little bit, over the last month or so,” Banerdt says. “But all of our historical data suggests the dustiness in the atmosphere will increase pretty significantly in the next month or so.”
If the team is lucky, InSight could continue taking the planet’s pulse through the end of the year—or perhaps a bit longer. And if they’re really lucky, a dust devil will spin through the lander’s perch in Elysium Planitia, clear off the solar panels, and allow the spacecraft to once again soak up the sunlight.
But that remains to be seen. Mars is, after all, a fickle world—a planet of promise and peril. It’s a world that had seduced scientists with hints of life while confounding efforts to detect it, a world with terrains that are tantalizingly Earth-like yet lethal to life as we know it.
Beneath those bewitching blue sunsets, not even robots can live forever.