NASA’s InSight lander has detected the first known “marsquake.”

The spacecraft picked up the faint trembling of Mars’s surface on April 6, 128 days after landing on the planet last November. The quake is the first to be detected on a planetary body other than Earth or moon.

The shaking was relatively weak, the French space agency CNES said on April 23. The seismic energy it produced was similar to that of the moonquakes that Apollo astronauts measured in the late 1960s and early 1970s.

“We thought Mars was probably going to be somewhere between Earth and the moon” in terms of seismic activity, says Renee Weber, a planetary scientist at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “It’s still very early in the mission, but it’s looking a bit more moon-like than Earth-like,” she says.

It’s not yet clear whether the shaking originated within Mars or was caused by a meteorite crashing into the planet’s surface.

David Mimoun, a scientist with the mission at the Institut Supérieur de l’Aéronautique et de l’Espace in Toulouse, France, says that the signal is so weak that it would not have been detected on Earth. “It’s so small that at the beginning we were wondering if it was a quake or something else,” he says.

Ears to the ground

InSight heard the marsquake using a French-built instrument that contains three extremely sensitive seismometers nestled inside a dome to protect them from the wind. Mission scientists had previously observed vibrations caused by the Martian wind blowing overhead. But the seismic characteristics of the April 6 event show that it came from within the planet.

“This signal was not like anything we’d seen before,” says Mark Panning, a planetary seismologist at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Team scientists can’t tell where the quake originated. Determining that would allow them to trace how seismic energy radiated through the planet, and to begin to understand the planet’s interior structure—InSight’s main goal. The spacecraft is meant to operate for one Martian year, or nearly two Earth years. “We’ve got time,” says Panning. “In my ideal universe, Mars would be having giant marsquakes all the time.”

InSight detected three other possible marsquakes, on March 14, April 10, and April 11. But they were even fainter than the April 6 event and their source is still unclear.

The spacecraft is working on Elysium Planitia, near Mars’s equator. Mission controllers are still trying to figure out how to unstick its German-built heat probe. It became lodged on what is probably a buried rock in February, as it tried to hammer itself into the ground to measure temperatures there.

This article is reproduced with permission and was first published on April 23, 2019.