It sounds like a subway train rushing by. Or a plane flying low overhead. But it's something much more exotic: In all likelihood, the first "marsquake" ever recorded by humans.
NASA's InSight mission detected the quake April 6, four months after the lander's highly sensitive seismometer was installed on the Martian surface.
Since then, the instrument has registered the howling winds on the red planet and the motions of the lander's robotic arm. But the shaking picked up this month is believed to be the first quake detected rumbling through Mars' interior.
"We've been collecting background noise up until now, but this first event officially kicks off a new field: Martian seismology!" Bruce Banerdt, InSight's principal investigator, said in a statement. Banerdt studies planetary seismology at the Jet Propulsion Laboratory in La Canada-Flintridge, Calif.
For many, it's been an agonizing wait.
"We all knew it was just a matter of time," said Renee Weber, a planetary scientist at NASA's Marshall Space Flight Center in Huntsville, Ala., and a member of InSight's science team.
Collaborators in Europe are the first to see the data when it's beamed down from the spacecraft, so Weber knew the news of a quake would come in the middle of the night for U.S. researchers.
"I always checked first thing before I get out of bed," she said in an interview. "Is today going to be the day?"
Researchers were elated when the quake finally happened.
"It's so exciting to finally have proof that Mars is still seismically active," Philippe Lognonne, a geophysicist at the Institut de Physique du Globe de Paris in France, said in a statement. Lognonne is the principal investigator for InSight's Seismic Experiment for Interior Structure, the official name for the seismometer.
Scientists want to study how seismic waves propagate through the planet to determine its structure and composition. This will give them clues about what its internal layers look like, which in turn will tell them about how the planet formed and help them determine the size of its core.
The April 6 seismic event was too small to do that. InSight also recorded three other possible marsquakes, which were even smaller.
Researchers don't yet know what caused the quakes. "At this point, we haven't ruled any mechanisms," Weber said.
Mars does not have tectonic plates that crash into each other -- the primary cause of quakes on Earth. One possible explanation is that the Martian crust is cracking as it cools.
However, Weber said the events will certainly teach scientists about the nature of seismic activity on Mars and the upper layers of the planet that the waves traveled through.
Before InSight landed, researchers hypothesized that Mars might fall somewhere between the Earth and the moon, where astronauts placed seismometers during the Apollo missions.
"The first few are looking a little more moon-like than Earth-like, but it's still very early," Weber said.
She's optimistic that InSight will eventually record more intense shaking -- either from a meteorite impact or an internal source -- that will give researchers a better view of the planet's deep interior.
The mission has a lifetime of two Earth years, though it will likely last longer if its solar powered instruments keep working.
"It's a waiting game," she said. "We just have to wait until the planet cooperates."
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