It was a "faint rumble" but it meant something big—the first seismic signal detected on the surface of a planetary body other than our home planet and moon. We have NASA's Martian InSight lander to thank. The lander has been listening for quakes that could shed light on Mars' guts since its robotic arm deposited a shielded seismometer on the western side of Elysium Planitia last December, a few weeks after the lander touched down. The key moment—hear it here—came April 6, 128 days into the mission. "This particular Marsquake—the first one we've seen—is a very, very small one," says the mission's chief scientist, Bruce Banerdt. "You wouldn't even notice this one in your day-to-life." Adds researcher Tom Pike, per the BBC: "There are a lot of uncertainties" but "it's probably only a Magnitude 1 to 2 event, perhaps within [62 miles] or so."
The tremor—which lasted 10 minutes, per National Geographic—could indicate movement inside the planet or a meteorite impact. "Interestingly, InSight's scientists say the character of the rumble reminds them very much of the type of data the Apollo sensors gathered on the lunar surface," per the BBC. The team is also investigating lesser signals detected March 14, April 10, and April 11 but can't yet confirm they were seismic events. "When you've got one, you don't know whether you were just lucky, but when we see two or three we will have a better idea" about the activity within the planet, Pike tells the BBC. Yet one confirmed "Marsquake" seems enough for InSight's seismometer boss Philippe Lognonné. "It's so exciting to finally have proof that Mars is still seismically active," he says, per Space.com. "We've been waiting [for] months." (Read more Mars stories.)