Mars is shaking. After several months of apprehensive waiting on a quiet surface, NASA’s InSight lander has registered a sweet, small sound: the first marsquake ever recorded. On 6 April, the lander’s seismometer detected its first verifiable quake, NASA and its European partners announced today. The quake is tiny, so small that it would never be detected on Earth amid the background thrum of waves and wind. But Mars is dead quiet, allowing the lander’s sensitive seismometer to pick up the signal, which resembles similar surface ripples detected traveling through the moon’s surface after moonquakes. The quake is so small that scientists were unable to detect any waves tied to it that passed through the martian interior, defying efforts to estimate its exact location and strength, says Philippe Lognonné, a planetary seismologist at Paris Diderot University who leads the mission’s seismometer experiment. Still, it was gratifying to observe, he says. “It is the first quake. All the time, we were waiting for this.”

The detection is a milestone for the $816 million lander, kicking off a new field of “martian seismology,” added Bruce Banerdt, InSight’s principal investigator and a geophysicist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, in a news release. It proves Mars is seismologically active, and marks NASA’s return to planetary seismology after more than 4 decades. The mission is intended to peer through the planet’s rust-colored shell, gauging the thickness and composition of its crust, mantle, and core. But while on Earth, the lander was plagued by delay and cost overruns; since landing on Mars in a sand-filled hollow, the lander’s second instrument, a heat probe, got stuck soon after it began to burrow into the surface.

Scientists had good reason to believe that Mars hosted such quakes even though it lacks plate tectonics, the force that drives most earthquakes. The moon suggested so: Seismometers deployed by the Apollo program had detected quakes caused by meteorite impacts, the solar-driven thermal expansion of its crust, and the gravitational tug of Earth. But the frequency was unknown. The InSight team estimated it might see one a month, but that number could be much higher or lower. And so, after deploying the volleyball-size seismometer and its shield in early February, the researchers waited. The seismometer was working well, they found: It was picking up background vibrations, called microseisms, in the martian surface that were induced by wind. But still, as the weeks ticked by, no quakes.

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