Surface trembling detected on Mars for the first time

THE WOODLANDS, TEXAS—After months of delicate maneuvering, NASA’s InSight lander has finished placing its hypersensitive seismometer on the surface of Mars. The instrument is designed to solve mysteries about the planet’s interior by detecting the booming thunder of “marsquakes.” But just a few weeks into its run, the car-size lander has already heard something else: atmosphere-driven trembling that continually roils our red neighbor. If marsquakes are the drum solo, these microseisms, as they’re known, are the bass line.

The signal first became apparent in early February, as soon as the lander placed a protective shield over the seismometer, said Philippe Lognonné, a planetary seismologist at Paris Diderot University who heads the team that runs the instrument, in a talk here today at the annual Lunar and Planetary Science Conference. “We do believe that these signals are waves coming from Mars.” This is the first time, he said, that such microseisms have been detected on another planet.

On Earth, microseisms are ubiquitous, caused largely by the sloshing of the ocean by storms and tides. Mars, despite the dreams of science fiction writers, has no present-day oceans. Instead, this newly discovered noise is likely caused by low-frequency pressure waves from atmospheric winds that rattle the surface, inducing shallow, longer-period waves in the surface, called Rayleigh waves, Lognonné said.

Even though InSight has not yet detected a marsquake, the microseisms are an important indicator that the lander’s seismometer is working as hoped. In recent decades, seismologists have begun to see microseisms on Earth as not just a nuisance, but as a valuable tool for understanding features in the subsurface. This noise will be similarly valuable on Mars, Lognonné said, allowing the team’s seismologists to probe the rigid surface crust in the immediate vicinity around the lander.

But the seismometer has had little time to listen so far. Although the sand-filled crater where InSight landed, nicknamed “Homestead Hollow,” had little in the way of large rocks to complicate its placement, the deployment still took a month longer than planned, thanks to two delicate tasks. First, scientists had to carefully tweak the electric tether connecting the seismometer to the lander, in order to reduce noise coming off the lander. Then, they had to place a wind and heat shield over the instrument.

Since then, InSight has spent much of its time troubleshooting for its second instrument, a heat probe designed to burrow up to 5 meters below the surface. The robotic arm placed that instrument in mid-February. But soon after the probe began to hammer itself into the surface, its 40-centimeter-long “mole” got stuck on a rock or some other blockage just 30 centimeters down. Now, mission scientists have put the hammering on hold as they wait for the agencies’ engineers to evaluate their options. That will continue for several more weeks, said Bruce Banerdt, InSight’s principal investigator and a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California.

Although the microseisms are a thrill to hear, everyone working on InSight is waiting for the main event: their first marsquake. There’s no need to panic about not seeing one yet, Banerdt said. “Before we get nervous … [the mission is] exactly where we expected to be.” The team expects to detect about one marsquake a month, but these will likely come in clusters, not perfectly spaced out. Banerdt, who had been preparing this mission for decades, can be patient, he said. “The wait’s not completely over yet.”

*Clarification, 20 March, 12:25 p.m.: This story’s headline and text were changed to prevent confusion that it might refer to marsquakes, which, as the article states, have not yet been detected by InSight. Similarly, the spacecraft has not detected an analogue for a longer-period background “hum” seen on Earth.