What lurks within the Red Planet? Although only a tenth as massive as Earth, Mars looks to have once been habitable like our own world, leading scientists to wonder whether such similarity cuts to the cores of both planets. In its innards, is Mars still a shrunken mirror of Earth, or is the interplanetary resemblance only crust-deep?
Tantalizing hints have been gleaned from gravitational data provided by past missions. But now the interior of Mars has been revealed as never before, thanks to unprecedented measurements from NASA’s InSight lander. Shortly after reaching the Martian surface in late 2018, InSight has been monitoring seismic waves rippling through the planet and using the echoing reflections of these “marsquakes” to map the subsurface. Only Earth and its moon have previously been subjected to such deep scrutiny. The results show a world both like and unlike our own and offer a thrilling second data point in a vast universe of rocky orbs. “InSight is kind of like the first telescope looking into the interior of the planet,” says Michael Meyer, lead scientist of NASA’s Mars Exploration Program at the agency’s headquarters.
InSight (Interior Exploration Using Seismic Investigations, Geodesy and Heat Transport) is not your typical Mars mission. Whereas others, such as the recently landed Perseverance, were sent to scientifically rich destinations that may have once supported life, InSight’s landing zone in Elysium Planitia was decidedly mundane, described by some as a “parking lot.” Flat and smooth—nearly featureless save for scattered rocks and impact craters—the site was the perfect place for the stationary lander to study the Martian interior. The Seismic Experiment for Interior Structure (SEIS) instrument, provided by France’s space agency and place gently on the surface by InSight’s robotic arm in December 2018, was encased in a domed shield, allowing it to detect waves moving through Mars without interference from wind or dust storms. storms. SEIS “can see motions on the order of atomic-sized vibrations,” says Andrew Lazarewicz of the Massachusetts Institute of Technology, who took part in a 1976 attempt to detect seismic waves with a seismometer on NASA’s Viking 2 lander.
In a series of papers published today in the journal Science, researchers describe how they used this instrument to trace seismic waves caused by dozens of detected marsquakes through the Martian interior. These events were possibly caused by meteorites hitting the planet’s surface or even by the stirrings of magma (some were localized to nearby Cerberus Fossae, a geologic formation displaying signs of recent volcanic activity). At less than magnitude 4 on the moment magnitude scale, all of these quakes were so small that they would be barely noticeable on Earth. But SEIS registered them clearly, allowing researchers to track their reverberations through the interior of Mars, all the way down to its core, revealing what was going on inside.
Simon Stähler of the Institute of Geophysics at the Swiss Federal Institute of Technology Zurich and his colleagues measured the waves’ reflections off the core to calculate its size and bulk composition. They found that it is likely 1,830 kilometers in radius, several hundred kilometers larger than predicted. And the strength of the reflected waves suggested they were bouncing off a core mostly composed of molten iron and nickel. The size of the core was a “surprise,” Stähler says. “People were assuming it must be on the order of 1,500 or 1,600 kilometers,” based on the fact that, kilogram for kilogram, Mars is a bit less dense than Earth, and the core would be expected to be mostly iron and nickel, which is heavier than rock. Instead the results show that the ratio of Mars’s core radius to its planetary radius is similar to that of Earth—which counterintuitively means the relatively low-density Martian core must be enriched with other elements, such as sulfur and oxygen, that are comparatively less abundant in our planet’s core. Why Mars’s core would have a different composition than ours is unclear. “If you assume that Mars was made from the same building blocks as Earth, then it is not so easy to explain,” Stähler says.
Moving outward, Amir Khan of the Institute of Geophysics and his colleagues used the seismic waves to probe Mars’s mantle, the region between the planet’s core and surface crust. Although Earth has an insulating liquid lower mantle layer that sits above its core, there is no such feature on our neighboring world. “That lower mantle does not exist on Mars,” Khan says. Instead, above the core, the lower mantle of Mars resembles the upper mantle of Earth, which then gives way to a higher layer, colder and more brittle, called the lithosphere. Mars’s lithosphere, the study shows, is about 500 kilometers in thickness, compared with Earth’s approximately 250-kilometer-thick lithosphere. Such a thick lithosphere, Khan says, could be why Mars lacks plate tectonics today. This unearthly configuration of subsurface layers could also explain how the Red Planet lost its heat because, unlike Earth, it lacks an insulating liquid mantle layer above its core.
At the surface, Brigitte Knapmeyer-Endrun of the University of Cologne in Germany and her colleagues measured the thickness of the Martian crust. They found two possibilities for the crust under InSight: One interpretation of the data suggests a two-layer crust like that of Earth with a thickness of 20 kilometers. The other hints at the presence of three layers totaling 39 kilometers in thickness. For the planet as a whole, the researchers estimate a crustal thickness of up to 72 kilometers, several dozens of kilometers thinner than predicted. If accurate, that estimate could be an important window into the fundamental differences between how Earth and Mars first formed. “Most of the crust is really old and is from really early on the planet, whereas on Earth, we have a lot of recycling going on due to plate tectonics,” Knapmeyer-Endrun says.
The results as a whole reveal intriguing differences between Earth and Mars. “What they’ve done with this single instrument is remarkable,” Lazarewicz says. Despite being rocky worlds that arose in relatively close proximity to the sun, these two planets may not have formed in the same way. They could have, say, coalesced from different mixes of materials that circulated in the disk of gas and dust that surrounded the young sun. Additionally, if InSight manages to seismically probe Mars’s inner core during its mission, that could help settle the long-standing mystery of how the planet lost its protective magnetic field, an event that is thought to have occurred perhaps four billion years ago and that may have allowed solar winds to sweep away much of the world’s atmosphere.
It was not until 1889 that we made our first measurements of seismic waves passing through Earth’s mantle, getting a glimpse at our own world’s interior. Now, more than a century later, we have our first comparative measurements for another planet in the universe, although these may be but a teaser of what is yet to come as scientists delve deeper into InSight’s data. “Now that we know how large the core is, and we know more about the crust and mantle, we can reinterpret the events we’ve detected so far in light of the interior model we have now,” Stähler says.