Mars’s crust is thin, which suggests the planet once cooled itself through a kind of plate tectonics.
Two years ago, NASA’s InSight spacecraft alighted on the surface of Mars, aiming to glean clues to the planet’s interior from the shaking of distant earthquakes and deep heat leaking from its soil. Mars, it turned out, had other ideas. Its sticky soil has thwarted InSight’s heat probe, and in recent months howling winds have deafened its sensitive seismometers. Most mysteriously, the planet hasn’t been rattled by the large marsquakes that could vividly illuminate its depths.
Despite these hurdles, a precious clutch of small-but-clear quakes has enabled the InSight team to see hints of boundaries in the rock, tens and hundreds of kilometers below. They are clues to the planet’s formation billions of years ago, when it was a hot ball of magma and heavier elements like iron sank to form a core, while lighter rocks rose up out of the mantle to form a capping lid of crust.
The results, some debuting this month at an online meeting of the American Geophysical Union (AGU), show the planet’s crust is surprisingly thin, its mantle cooler than expected, and its large iron core still molten. The findings suggest that in its infancy, Mars efficiently shed heat—perhaps through a pattern of upwelling mantle rock and subducting crust similar to plate tectonics on Earth. “This may be evidence for a far more dynamic crust formation in Mars’s early days,” says Stephen Mojzsis, a planetary scientist at the University of Colorado, Boulder, who is unaffiliated with the mission.
The evidence has been hard won. Early in the mission, winds were quiet enough for InSight’s seismometers, housed in a small dome placed on the surface, to hear a multitude of small quakes—nearly 500 in total. But since June, winds have shaken the surface strongly enough to smother all but a handful of new quakes. Yet frustratingly, the winds have not been strong enough to sweep away dust that is darkening the craft’s solar panels and foreshadowing the mission’s end sometime in the next few years. The seismometers are still running nonstop, but power constraints have forced the team to turn off a weather station when using the lander’s robotic arm. “We are starting to feel the effects,” says Bruce Banerdt, InSight’s principal investigator and a geophysicist at NASA’s Jet Propulsion Laboratory.
Meanwhile, the heat probe, about the length of a paper towel tube, is stuck in soil that compacted instead of crumbling as the rod tried to delve in. Mission engineers have used the robotic arm to push the probe down and scrape dirt on top. In the next month or two, they’ll try once more to get the probe to burrow in, Banerdt says. “If that doesn’t work, we’ll call it a day and accept disappointment.”
Perhaps the biggest disappointment is the lack of a marsquake larger than magnitude 4.5. The seismic waves of a large quake travel more deeply, reflecting off the core and mantle boundaries and even circling the planet on its surface. The multiple echoes of a large quake can enable just a single seismic station like InSight’s to locate the quake’s source. But above magnitude 4, Mars has been curiously silent—an apparent violation of the scaling laws that apply on Earth and the Moon, where 100 magnitude 3 events correspond to 10 magnitude 4 quakes, and so on. “That is a bit weird,” says Simon Stähler, a seismologist on the team from ETH Zurich. It could simply be that Mars’s faults aren’t big enough to sustain big strikes, or that its crust isn’t brittle enough.
But two moderate quakes, at magnitude 3.7 and 3.3, have been treasure troves for the mission. Traced to Cerberus Fossae, deep fissures in the crust 1600 kilometers east of the landing site that were suspected of being seismically active, the quakes sent a one-two punch of compressive pressure (P) waves, followed by sidewinding shear (S) waves, barreling toward the lander. Some of the waves were confined to the crust; others reflected off the top of the mantle. Offsets in the travel times of the P and S waves hint at the thickness of the crust and suggest distinct layers within it, Brigitte Knapmeyer-Endrun, a seismologist at the University of Cologne, said in an AGU presentation. The top layer may reflect material ground up in the planet’s first billion years, a period of intense asteroid bombardment, says Steven Hauck, a planetary scientist at Case Western Reserve University.
At 20 or 37 kilometers thick, depending on whether the reflections accurately trace the top of the mantle, the martian crust appears to be thinner than Earth’s continental crust—a surprise. Researchers had thought that Mars, a smaller planet with less internal heat, would have built up a thicker crust, with heat escaping through limited conduction and bouts of volcanism. (Though Mars is volcanically dead today, giant volcanoes dot its surface.) A thin crust, however, might mean Mars was losing heat efficiently, recycling its early crust, rather than just building it up, perhaps through a rudimentary form of plate tectonics, Mojzsis says.
A handful of distant quakes, originating some 4000 kilometers away, provided a further clue. Those waves traveled deep through the mantle and interacted with the mantle transition zone, a layer where pressure transforms the mineral olivine into wadsleyite. By analyzing the travel time of waves that passed above, below, and through the transition zone, the team located its depth—and found it shallower than expected, an indication of a cooler mantle. For the mantle to be this cool today suggests that convection—the swirling motions that, on Earth, drive tectonic plates and carry heat from the mantle to the surface—might have operated early on, says Quancheng Huang, a Ph.D. student at the University of Maryland, College Park, who presented some of the results at the AGU meeting. “Plate tectonics is a very effective way of cooling a planet.”
A third science experiment aboard InSight probes deeper still, using tiny Doppler shifts in radio broadcasts sent from Earth to receivers on the probe to detect slight wobbles in the planet’s spin. The size and consistency of the planet’s iron core affect the wobbles, much as raw eggs spin differently from cooked ones. “We’ve had something like 350 hours of tracking,” says Véronique Dehant, a geophysicist at the Royal Observatory of Belgium. The preliminary results confirm that the core is liquid, with a radius compatible with previous estimates made by spacecraft measuring tiny variations in the planet’s gravity, Dehant reports in her AGU poster. Those gravity estimates have found a core with a radius of about 1800 kilometers—taking up more than half the planet’s diameter.
Rebecca Fischer, a mineral physicist and modeler at Harvard University, isn’t surprised at the signs of a liquid core. “It would be a pretty big surprise if it weren’t,” she says. Sulfur and other elements mixed with the iron should help it to remain molten while cool, much as salt prevents icing. On Earth, convective motions in the molten outer core drive the magnetic dynamo. But on Mars, those motions 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 to space.
Banerdt hopes to sharpen this fuzzy picture of the planet’s interior, and he thinks calmer winds will soon make that possible. After two Earth years, the probe’s first martian year is ending, and the quiet of the mission’s first months is returning. “We’re looking forward to another whole pile of event detections,” Banerdt says. And though the planet has not cooperated so far, perhaps the Big One is poised to strike Mars like a gong—a reverberation that would at last make all clear.
https://www.sciencemag.org/news/2020/12/mars-lander-spots-deep-layers-beneath-surface-offering-clues-planet-s-formation
2020-12-15 20:50:00Z
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