For months, the spacecraft sat very still in the vast, empty expanse of a flat Martian plain, alone and undisturbed but for the thin whine of an alien wind.

Back on Earth, operators at NASA’s Jet Propulsion Laboratory were getting worried. The whole point of the InSight mission was to track the Red Planet’s quakes, shakes and tremors for clues about its interior. But the spacecraft hadn’t felt so much as a twitch in the ground beneath its three feet.

"The longer we went without a quake, the more questionable or doubtful we were becoming that we’d be able to do the science we came to Mars to do,” said Bruce Banerdt, principal investigator for the mission.

Then, on April 6, InSight’s seismometer picked up a faint, eerie rumble — the sound, scientists say, of what is probably the first quake recorded on the surface of another planet.

“Finally,” Banerdt said, “we know Mars is seismically active; we know it’s talking to us.”

Now, he continued, “it’s just a matter of being patient, waiting and listening, and collecting the quakes as they come along.”

Scientists have long sought to explain how Mars, which looked a lot like Earth when the two bodies formed 4.6 billion years ago, became the desolate desert world we see today. Most think the Red Planet’s defunct internal dynamo might be the culprit. Constant churning inside Earth generates a magnetic field that protects our surface from radiation and helps us hold on to our atmosphere, but evidence from ancient rocks suggest that Mars’s magnetic field faltered about 3 billion years ago.

InSight is designed to figure out what happened by using seismic waves — the subterranean ripples produced by quakes — as probes.

“It’s almost like an X-ray,” Banerdt said. The spacecraft’s exquisitely sensitive seismometer can track subtle changes in the waves as they pass through Mars’s crust, mantle and core.

“Every time you have a Mars quake, that gives you one more slice through the planet,” Banerdt said. “Eventually, we want to build up a number of these slices to create a 3-D picture of what’s inside.”

But it takes a quake to generate these waves — and none seemed to be happening.

Banerdt was asleep at his California home when a fresh batch of InSight data was relayed from the Martian surface to seismologists in Switzerland. The scientists noticed a consistent, persistent signal in the data. It was unlike the sporadic shaking they were used to seeing from gusts of wind and the creaks and crackles of the spacecraft warming and cooling beneath the sun.

By the time Banerdt woke, his phone screen bore an urgent text message: “Teleconference in half an hour."

“I jumped right out of bed,” he recalled. “It was exciting.”

The scientists traded PowerPoint slides and debated interpretations. In a recording released by NASA, in which the seismic wave frequencies have been sped up to make them audible to human ears, the signal sounds almost like an airplane taking flight.

Finally, they decided that the signal must have come from a very small quake — about magnitude 2 or 2.5. Dozens of temblors just like it occur in Southern California every day, utterly unnoticed amid the noise of human activity and ocean waves.

The Martian signal was also prolonged; it lasted nearly 10 minutes, unusual for a seismic event this small. This is probably a result of fractures in the Martian crust that cause signals to bounce around, like echoes in a labyrinthine cave.

The signal, which Banerdt described in a keynote address at the Seismological Society of America’s annual meeting Tuesday night, wasn’t large enough to function as the kind of “X-ray” scientists had hoped to find. But it is still “like catnip” to them, Banerdt said.

“This is a whole new planet that we’re opening up to seismology,” he said.

Models using the new discovery suggest that the spacecraft should witness as many as a few dozen quakes over the course of its two-year mission.

“It’s going to be a little bit sparse and tight. We’re going to have to really squeeze the data for all its worth,” Banerdt said. “But it’s putting us in the right ballpark to do that kind of science.”

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