The lander Mars observes deep layers below the surface, giving clues about the formation of the planet Science

The crust of Mars is thin, which suggests that the planet once cooled by a kind of plate tectonics.

NASA / JPL-Caltech

By Paul Voosen

Two years ago, NASA’s InSight spacecraft landed on the surface of Mars, in order to gather clues to the interior of the planet, from the tremors of distant earthquakes and deep heat flowing from the ground. It turned out that Mars had other ideas. Its sticky soil thwarted InSight’s heat probe, and in recent months the howling winds have deafened its sensitive seismometers. Most mysteriously, the planet was not shaken by large marches that could brightly illuminate its depths.

Despite these obstacles, a precious ambiance of small but clear earthquakes allowed the InSight team to see clues of boundaries in the rock, tens and hundreds of kilometers below. These are clues to the formation of the planet billions of years ago, when it was a hot ball of magma and heavier elements such as iron sank to form a core, while lighter rocks rose from the mantle to form. form a crust cover.

The results, which debuted this month at an online meeting of the American Geophysical Union (AGU), show that the planet’s crust is surprisingly thin, its mantle is colder than expected and its large iron core is still molten. The findings suggest that in its early days Mars efficiently shed heat – perhaps through an upward mantle rock pattern and lowering crust similar to the tectonics of Earth’s plates. “This may be evidence of a much more dynamic crust formation in the early days of Mars,” said Stephen Mojzsis, a planetary scientist at the University of Colorado, Boulder who is not affiliated with the mission.

The evidence was hard to come by. At the beginning of the mission, the winds were calm enough for the InSight seismometers, housed in a small dome on the surface, to hear a multitude of small earthquakes – almost 500 in total. But since June, the winds have shaken the surface hard enough to suffocate all but a handful of new earthquakes. However, frustratingly, the winds were not strong enough to sweep away the dust that darkened the ship’s solar panels and foreshadowed the end of the mission sometime in the next few years. Seismometers still work non-stop, but power constraints have forced the team to shut down a weather station when using the robot’s robotic arm. “We’re starting to feel the effects,” says Bruce Banerdt, principal investigator at InSight and a geophysicist at NASA’s Jet Propulsion Laboratory.

Meanwhile, the heat probe, about the length of a paper towel tube, is stuck in the ground that has compacted instead of collapsing as the rod has tried to penetrate. In the next month or two, they will try again to get the poll in, Banerdt said. “If that doesn’t work, we’ll call it a day and accept the disappointment.”

Perhaps the biggest disappointment is the lack of an earthquake larger than magnitude 4.5. The seismic waves of a large earthquake travel deeper, reflecting from the boundaries of the core and mantle and even encircling the planet on its surface. Multiple echoes of a large earthquake can only allow a single seismic station such as InSight to locate the source of the earthquake. But above magnitude 4, Mars fell silent curiously – an apparent violation of the laws of scaling that apply to Earth and the Moon, where 100 events of magnitude 3 correspond to 10 earthquakes of magnitude 4 and so on. “It’s a little strange,” says Simon Stähler, a seismologist on the ETH Zurich team. It could simply be that Mars’ defects are not large enough to sustain large blows or that its crust is not fragile enough.

But two moderate earthquakes, with magnitudes 3.7 and 3.3, were treasures for the mission. Followed up to Cerberus Fossae, deep cracks in the crust 1600 kilometers east of the landing site that were suspected to be seismically active, the earthquakes sent a compressive pressure wave (P), followed by lateral shear waves ( S), gushing towards the lander. Some waves were confined to the crust; others were reflected in the top of the mantle. The displacement compensations of the P and S waves indicate the thickness of the crust and suggest distinct layers inside it, said Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne, in an AGU presentation. The top layer may reflect hardened material in the first billion years of the planet, a period of intense asteroid bombardment, says Steven Hauck, a planetary scientist at Case Western Reserve University.

At a thickness of 20 or 37 kilometers, depending on the reflections that follow exactly the tip of the mantle, the Martian crust seems to be thinner than the continental crust of the Earth – a surprise. The researchers believed that Mars, a smaller planet with less internal heat, would have built a thicker crust, with heat escaping through a limited conduction and volcanic crises. (Although Mars is volcanic dead today, huge volcanoes dot their surface.) A thin crust, however, could mean that Mars lost heat efficiently, recycling its early crust, rather than building it, probably through a shape. rudimentary plate tectonics, says Mojzsis.

A handful of distant earthquakes, originating about 4,000 kilometers away, provided an additional clue. These waves traveled deep through the mantle and interacted with the transition zone of the mantle, a layer in which pressure turns the mineral olivine into wadsleyite. Analyzing the travel time of the waves that passed above, below and through the transition area, the team located its depth – and found it smaller than expected, an indication of a colder mantle. For the mantle to be so cold today, it suggests that convection – the swirling movements that drive tectonic plates on Earth and bring heat from the mantle to the surface – could have operated earlier, says Quancheng Huang, a doctor. student at the University of Maryland, College Park, who presented some of the results at the AGU meeting. “Plate tectonics is a very efficient way to cool a planet.”

A third scientific experiment aboard the InSight probes even deeper, using small Doppler shifts in radio broadcasts sent from Earth to the probe’s receivers to detect slight fluctuations in the planet’s rotation. The size and consistency of the planet’s iron core affect the oscillations, as raw eggs rotate differently than boiled ones. “We had about 350 hours of tracking,” says Véronique Dehant, a geophysicist at the Royal Observatory in Belgium. Preliminary results confirm that the core is liquid, with a radius compatible with previous estimates made by spacecraft that measure small variations in the planet’s gravity, Dehant reports in her AGU poster. These gravity estimates found a core with a radius of about 1,800 kilometers – occupying more than half the diameter of the planet.

Rebecca Fischer, a mineral physicist and modeler at Harvard University, is not surprised by the signs of a liquid core. “It would be a big surprise if it weren’t,” she says. Sulfur and other elements mixed with iron should help it stay melted as it cools, especially since salt prevents freezing. On Earth, convective motions in the molten outer core drive the magnetic dynamo. But on Mars, those movements seem to have stopped long ago – and without a magnetic field, the planet’s atmosphere was vulnerable to the Sun’s cosmic rays and leached water into space.

Banerdt hopes to sharpen this blurred image of the planet’s interior and believes calmer winds will make this possible soon. After two years on Earth, the first Martian year of the probe ends, and the silence of the first months of the mission returns. “We look forward to another pile of event detections,” says Banerdt. And, although the planet has not cooperated so far, maybe the Great One is ready to hit Mars like a gong – a reverberation that would eventually make everything clear.

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