In its final months of operation, NASA’s InSight lander can claim one more achievement: it detected a series of meteorite impacts, and scientists used its data to pinpoint the fresh impact craters.
University of Toulouse planetary scientist Raphael Garcia and his colleagues published the results in a recent paper in the journal Nature Geosciences.
What’s New — Mars has a planetary defense problem: its thin atmosphere isn’t much of a barrier to anything but the smallest incoming meteors. Small space rocks, up to a few dozen kilograms, fall to the ground on Mars more often than they do on Earth because our atmosphere is 100 times denser. And when the meteorites — a meteor that survives atmospheric entry — do fall to Mars, their impacts send seismic waves rippling through the Martian crust.
Last year, NASA’s InSight lander recorded the seismic waves from three small meteorites — between 10 and 43 kilograms each — that plowed into the Martian surface within a few hundred kilometers of the lander. By measuring when different types of waves reached InSight, Garcia and his colleagues traced the ground shaking back to the meteorites’ impact sites.
Then NASA’s Mars Reconnaissance Orbiter, 250 kilometers (196 miles) above the Martian surface, followed Garcia and his colleagues’ directions and photographed the freshly-dug craters. Recording the seismic waves from a meteorite strike, then finding the impact crater to go with the seismograph, is something we’ve only been able to do once on Earth — and never on another world — and it could reveal new information about Mars’ underground makeup.
Digging Into The Details — An Marsquake induced by a meteorite impact shakes the ground in two main ways. First, vibrations called P waves or compression waves speed outward from the epicenter; these waves cause the rock to stretch and compress along the same direction the wave travels, so the ground seems to shake back and forth. Slightly slower-moving vibrations, called S waves or shear waves, cause the rock to move up and down, so the ground seems to rise and fall.
InSight even managed to record sound waves from the meteors’ plunge through Mars’ thin atmosphere and high-speed collision with the ground, thanks to the way those sound waves interact with the surface as they travel through the lowest layers of the air.
“The pressure wavefronts in the acoustic wave are pushing and pulling on the ground,” Garcia tells Inverse, “and so that makes tiny rotations of the seismometer in the arrival direction of the acoustic waves.”
Because all three waves travel at different, but predictable, speeds, Garcia and his colleagues could measure when each wave reached InSight’s instruments, then use that to calculate the distance the waves had traveled — give or take about 10 percent. The researchers also managed to estimate the direction the waves had arrived from.
With that information, the team that operates the Mars Reconnaissance Orbiter knew where to look for fresh impact craters with the satellite’s Context Camera. The telltale features were “extended dark blast zones surrounding new impact craters that were not present in previous Context Camera images,” wrote Garcia and his colleagues.
Once MRO located a crater in the right spot, the satellite’s higher-resolution (25 cm/pixel) camera zoomed in for a more detailed view, so Garcia and his colleagues could measure the crater’s diameter, the direction the impact had come from, and other details.
Why It Matters – Seismic waves moving through rock are the only way geologists can really “see” features underground, like rock formations, fault lines, and magma chambers. Geologists on Earth rely on information from networks of seismometers to understand what’s going on deep in Earth’s crust (and how worried we should, or shouldn’t, be about it).
And since the early 1970s, scientists have tried to do the same thing on the Moon. The Apollo missions left behind seismometers to measure Moonquakes, and in a few cases, astronauts actually detonated explosives on the lunar surface to create their own seismic waves for the instruments to measure. NASA also crashed a few Saturn V third-stage rockets into the Moon to collect even more seismic data.
A huge part of InSight’s mission was to do the same thing on Mars. Knowing the location of the impacts that InSight recorded “removes some unknowns” from the process of mapping out Mars’ underground makeup from seismic data, says Garcia.
What’s Next – And that’s next on the agenda for Garcia and his colleagues: using InSight’s seismic data to create a detailed map of the Martian crust around InSight. That sheds some light on Martian geology, but it could also help plan future missions.
Garcia says his team also wants to try a similar study on the Moon, using an instrument called the Farside Seismic Suite.
We already know, thanks to Apollo, that seismic waves fade out much faster on Mars than on the Moon — after an impact, the Moon’s rocky crust keeps “ringing” for up to an hour, while the shaking from a similar impact on Mars tends to peter out after about 15 minutes.
“It demonstrates that the Moon is much more fractured than Mars,” Garcia says. “This is consistent with the fact that Mars still has volcanic activity (capable of filling the fractures created by impacts), whereas this capability has stopped a long time ago for the Moon such that the fractures created by the impacts cannot be ‘repaired.’”
But more detailed information could also help plan future lunar missions.
Meanwhile, NASA is squeezing as much science as possible out of InSight in the time the lander has left, which could be a few more weeks or several more months. That depends on how long InSight’s power can hold out with its solar panels covered in dust.
“The uncertainty is because the lander was never designed to operate under the dusty conditions its in, and we never know exactly how the Martian weather will respond,” a NASA representative at the Jet Propulsion Laboratory tells Inverse. “We definitely anticipate a sharp decline in power in coming months. We’re just not entirely sure how sharp.”
An earlier version of this story reported that InSight was in its final weeks of operation.
