Mars more seismically active than the Moon

NASA’s InSight Mars lander, which touched down in the Elysium Planitia region of Mars on 26 November 2018, has now detected hundreds of Martian earthquakes, some as large as magnitude 3 or 4.

Not that scientists are surprised that Mars is seismically active – the primary purpose of the InSight mission, after all, was to place a seismometer on Mars and use marsquakes to probe the Red Planet’s depths by seeing how their vibrations pass through them. 

But until InSight actually began detecting marsquakes, there was no proof that they even existed: only theory saying they should.

Now, however, we know that Mars is more seismically active than the Moon, although much less active than the Earth, says the mission’s principal investigator, Bruce Banerdt of NASA’s Jet Propulsion Laboratory (JPL), who is the lead author of the first of a suite of papers in the journal Nature Geoscience.

Most of Earth’s earthquakes are associated with plate tectonics, but on Mars, which has no plate tectonics, most are probably caused by the slow cooling of the planet’s interior. “As the planet cools, it contracts and the brittle outer layers fracture,” Banerdt says.

It is also possible that some of these quakes are caused by the cooling and shrinking of magma chambers, says Suzanne Smrekar, the mission’s deputy principal investigator, also of JPL. 

Two of the larger marsquakes, she says, have been traced to the Cerberus Fossae region, which has seen volcanic activity within the last 10 million years. “So it’s possible that there is magma at depth that is cooling,” she says.

So far, none of the marsquakes has been big enough to generate seismic waves powerful enough to allow scientists to probe all the way to the planet’s core. 

But scientists are nevertheless obtaining an increasingly interesting picture of the planet’s outer layers, says Philippe Lognonné of the University of Paris, France, and first author of another Nature Geoscience paper. 

To begin with, Lognonné says, the crust appears to have a discontinuity in the way it conducts seismic waves above and below a depth of about 10 kilometres – probably an indicator of the degree to which its top 10 kilometres have been fractured by asteroid impacts or ancient geological activity.

Also, he says, the propagation of seismic waves through the crust shows that it isn’t bone dry, like the crust of the Moon, but instead contains some degree of water – though how much is difficult to say.

“We do not have evidence either for or against concentrations of water, like an aquifer,” he says. “We just don’t have the kind of resolution to determine [that].”

Meanwhile, seismic data isn’t the only data being collected by InSight. Scientists are also using its radio signals to track tiny wobbles in Mars’s spin, hoping to use these to learn about its deep interior structure, Banerdt says.

Also, he says, the lander “is turning out to be an incredible weather station”, even though its weather-monitoring instruments were primarily intended to help seismologists compensate for vibrations caused by wind and air pressure fluctuations. “This is going to open up a whole new window of research on Mars,” he says.

Another interesting find is that the local magnetic field is 10 times stronger than expected. Not that this means that Mars has a magnetic field like the Earth’s says Catherine Johnson from Canada’s University of British Columbia, first author of a third paper.

Rather, she says, the field measured by InSight reflects the imprint of Mars’s ancient magnetic field, which was lost early in the planet’s history but is now frozen in magnetic particles contained in buried rocks that solidified when it was still strong.

What’s interesting, Johnson says, is that for the field to be present today, the rocks deep beneath the InSight landing site must never have heated up enough in the ensuing billions of years to lose their remnant magnetism – an important clue to the long-term evolution of the Martian interior.

Meanwhile, InSight continues to monitor marsquakes. The data in the Nature Geoscience papers, which cover the first 10 months of the mission’s operation, include 174 marsquakes. “We now have 450 in our catalogue,” Banerdt says.

Interestingly, he adds, the rate at which they are occurring has been increasing. “We don’t understand that at all,” he says, though noting it may have something to do with the planet’s orbit, or the slow progression of its seasons.

That, he says, is part of why the mission is slated to study the Red Planet for at least two years – one Mars year. “We’re wanting to see whether this is a cyclical effect, or if there’s something else going on,” he says.

Meanwhile, the mission is just beginning to produce important results. “We’re really in the Wild West of understanding,” Banerdt says. “As we start to learn what the data is telling us, we’ll start to answer questions a lot better.”