While humans have been struggling to control the Covid-19 pandemic, baking in record heat, and trying to figure out how not to run out of water, our spacecraft on Mars have been enjoying a rather more tranquil existence. (Not needing to breathe helps.) Parked on the Martian surface, the InSight lander is listening for marsquakes, while the Perseverance rover is rolling around in search of life.
This week, scientists are dropping an Olympus Mons of findings from the two brave robots. In three papers published today in the journal Science—each authored by dozens of scientists from around the world—researchers detail the clever ways they used InSight’s seismometer to peer deep into the Red Planet, giving them an unprecedented understanding of its crust, mantle, and core. It’s the first time scientists have mapped the interior of a planet other than Earth. And yesterday, another group of scientists held a press conference to announce early research results from Perseverance, and the next steps the rover will take to explore the surface of Jezero Crater, once a lake that could have been home to ancient microbial life.
Scientists still have a lot to learn about the Red Planet. “It’s built from similar building blocks as our own planet, but Mars looks very different,” says University of Cambridge global seismologist Sanne Cottaar, who penned a perspective paper in Science on the three new studies. “There’s lots of evidence that its evolution has been very different. And now forming this image of the layering of the planet will give us the tools to work out how this formed, how Mars came to be.”
Curiosities abound when comparing the two. Why, for instance, does Earth have a magnetic field, but Mars’ seems to have disappeared? Why are so many volcanoes spread all over Earth, while volcanoes are more localized—and bigger—on Mars? (At 374 miles in diameter and 16 miles high, Olympus Mons is the biggest known volcano in the solar system.) Its formation must have been cataclysmic, but the surface of Mars is now quiet; unlike Earth, it doesn’t seem to be volcanically active. (In May, though, scientists presented evidence of what they say is recent activity.) Only by peeking under the surface can scientists better understand these planetary oddities—and in doing so, better understand Earth’s own quirks as a fellow rocky planet.
But before we dive into today’s avalanche of scientific literature, we need a crash course on the workings of both Mars and its InSight observer. Compared to Earth, the Red Planet is geologically quite calm. Because our planet has plate tectonics—huge slabs of land that shift over the underlying mantle—the surface is positively popping with activity like volcanoes and catastrophic earthquakes. Mars lacks plate tectonics; it doesn’t have a plated surface, because its core formed and cooled off rapidly during its early days. Today it shakes with much smaller quakes that may come from the contracting of the planet as it continues to cool.
The InSight lander’s job is to detect these quakes with its seismometer, which it’s been doing since February 2019. The instrument provides scientists with extremely rich seismic data on two phenomena in particular: the P-waves and S-waves that marsquakes produce. “P-waves are compressional waves, like sound in air, and they are the fastest waves that we see moving through any planetary body,” says University of Cologne seismologist Brigitte Knapmeyer-Endrun, lead author on the paper that modeled Mars’ crust. “And then we have the secondary waves, the S-waves, the shear waves. The motion is more like if you pluck a string on a guitar and it swings.”
Critically, these S-waves are slower than P-waves, so when a quake pops off, they arrive at InSight’s seismometer a bit later. “This difference between the arrival of the P and S waves can give you an idea about what’s the location of the quake; how far it was away from your station,” says Knapmeyer-Endrun. The waves also differ in what mediums they can travel through, versus which ones they bounce off of. P-waves move through solids, liquids, and gases, while S-waves only travel through solids.
By analyzing the waves that reach InSight’s seismometer, scientists can get an idea of the composition of Mars’ insides. Since S-waves can’t travel through the liquid core, all of their energy bounces off the boundary between core and mantle. Think of it like binary code for computers: Just as two elements—ones and zeros—can combine to produce extremely complex programming, so too can two kinds of waves combine to produce a sophisticated picture of the Red Planet’s guts. “We also look at differences in arrival times, and then we can say, ‘OK, this tells us something about the thickness of the layer,’” says Knapmeyer-Endrun.
Using this technique, she and her colleagues were able to estimate the thickness of the crust. Previously, scientists had used satellites flying overhead to measure the differences in gravity and topography across the planet, and they had taken a stab at the crust’s thickness that way, landing on an estimate of a global average of 110 kilometers. “Now, with our measurements from inside, we can say that that’s definitely too much,” says Knapmeyer-Endrun. They now think the maximum figure for average thickness is 72 kilometers.
The researchers reckon that the crust is made of two or three layers. There’s a topmost layer that’s 10 kilometers thick, which InSight’s measurements revealed to be unexpectedly light, perhaps because it’s made of fractured rock left over from meteorite impacts. The layer below that goes down to about 20 kilometers. “Unfortunately, we are not sure what follows next, if it’s already the mantle or if we have a third layer in the crust. There’s some ambiguities that we haven’t resolved,” says Knapmeyer-Endrun. “We can definitely say that the crust is not as thick as has been predicted previously, and it has a lower density.”
Planetary seismologist Simon Stähler of ETH Zürich led the effort to characterize the hottest and innermost chunk of Mars’ interior—its core. Though they lack the ability to actually see inside the planetary center, Stähler’s team was able to extract some information just by analyzing the S-waves that bounce off the core-mantle boundary. These rumblings, unable to penetrate the liquid core, find their way back up to the Martian surface, where they are picked up by InSight’s receivers. “It takes a good 10 minutes,” Stähler says, from the time of the quake to the detection of the signal reflected by the core. By measuring this interval, his team was able to deduce how deep into the planet the waves are traveling, thus measuring the depth of the core itself: around 1,550 kilometers from the surface.
The researchers found the core density to be surprisingly low, at only about 6 grams per cubic centimeter, which is much lower than what they’d expected of an iron-rich center. “It’s still a bit of a mystery how the core is so light,” Stähler says. There must be lighter elements present, though exactly what those may be is unclear. He and his team eventually hope to detect P-waves produced by a marsquake originating directly across the planet from where InSight is parked. Since they can pierce through the core-mantle boundary, they will carry information about the core’s composition to the lander’s receiver. But for that to happen, Stähler says, “Mars has to play along and give us this one quake on the other side of the planet.”
In Stähler’s team’s paper, they report a core radius of 1,830 kilometers. Another team, led by ETH Zürich geophysicist Amir Khan, found that this size is so large it leaves little room for an Earth-like lower mantle, a layer that acts as a heat-trapping blanket around the core. Earth’s mantle is divided into two parts, with a so-called transition zone in between; the upper and lower levels are composed of different minerals. “The mantle of Mars is—can I say flippantly—a slightly simpler version of the mantle of Earth, simply in terms of the mineralogy,” says Khan, lead author on the paper describing the mantle.
Previous estimates of the core’s radius using geochemical and geophysical data hinted at the absence of a lower mantle, but scientists needed InSight’s seismological readings to confirm it. Without this layer, the Martian core likely cooled much more readily than Earth’s. This is key to understanding the evolution of the Red Planet, and in particular why it lost its magnetic field, a barrier that would have protected the atmosphere—and potential life—from harsh solar winds. Creating a magnetic field requires a temperature gradient between the outer and inner core, high enough to create circulating currents that churn the core’s liquid and give rise to a magnetic field. But the core cooled so fast that these convection currents died out.
Khan’s analysis also shows that Mars has a thick lithosphere, the rigid and cold part of the mantle. This might be a clue as to why the Red Planet doesn’t have the plate tectonics that drive the frenzy of volcanism on Earth. “If you have a very thick lithosphere, it’s going to be very difficult to break this thing up and create the exact equivalent of plate tectonics on Earth,” says Khan. “Maybe Mars had it very early on, but it’s certainly shut down now.”
While InSight eavesdrops on the interior vibrations of Mars, Perseverance has been rolling around its dusty surface looki
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