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When NASA’s InSight lander touched down on Mars in 2018, the mission team hoped the stationary spacecraft would be able to perform a checkup on the red planet’s interior. Now, InSight and its instruments have exceeded those goals, revealing the mysteries of the Martian crust, mantle and core that have eluded scientists until now.
It’s the first time we’ve been able to peer inside and map the interior of another planet beyond Earth. The InSight mission team was able to achieve this extraordinary feat by tracking marsquakes on the red planet – like the earthquakes we experience on Earth, just a little bit different.
InSight’s seismometer, called the Seismic Experiment for Interior Structure, detected 733 different marsquakes. Researchers analyzed 35 of them, which reached magnitudes between 3.0 and 4.0, to determine the thickness of the Martian crust, the depth of the planet’s mantle and most importantly, a confirmation that the frozen desert planet’s core is molten.
The findings were shared in three studies, all published Thursday in the journal Science.
Before InSight journeyed to Mars, all previous robotic exploration of the red planet involved studying its surface.
“When we first started putting together the concept of the mission more than a decade ago, the information in these papers is what we hoped to get at the end,” said Bruce Banerdt, InSight’s principal investigator at NASA’s Jet Propulsion Laboratory in Pasadena, California, in a statement. “This represents the culmination of all the work and worry over the past decade.”
Getting to know Mars through its quakes
Unlike the Curiosity and Perseverance rovers, InSight is confined to the place where it landed, unable to roam across the surface in search of intrigue. But the lander’s incredibly sensitive seismometer has the ability to detect marsquakes from hundreds and thousands of miles away. It doesn’t need mobility to study Mars.
When we experience earthquakes, it’s because the tectonic plates on Earth are shifting, moving and grinding against one another. So far, Earth is the only planet known to have these plates.
So how do quakes occur on Mars? Think of the Martian crust as a single giant plate. This crust has faults and fractures within it because the planet continues to shrink as it cools. This puts stress on the Martian crust, stretching and cracking it.
When the seismic waves of marsquakes traveled through different materials within the Martian interior, it allowed researchers to study the planet’s internal structure. This helps them to understand the mysterious Martian interior and to apply this research to learn how other rocky planets, including our own, are formed.
Seismograms collected by InSight are full of wiggles, and those wiggles can be noise caused by wind or vibrations from marsquakes.
“What we’re looking for is an echo,” said Amir Khan, lead author of the mantle study, and a scientist at the Institute of Geophysics at ETH Zurich and the Physics Institute at the University of Zurich, in a statement.
“The direct seismic waves from an earthquake are a bit like the sound of our voices in the mountains: they produce echoes,” said Philippe Lognonné, principal investigator for the seismometer and a professor at the University of Paris, in a statement. “And it was these echoes, reflected off the core, or at the crust-mantle interface or even the surface of Mars, that we looked for in the signals, thanks to their similarity to the direct waves.”
Billions of years ago, Earth, Mars and the other planets in our solar system formed from a disk of material around the sun, including clumps of dust and rocks. Planets are incredibly hot as they form. Over time, distinct layers emerged in Mars during those first millions of years, including the crust, mantle and core.
“Now seismic data has confirmed that Mars presumably was once completely molten before dividing into the crust, mantle and core we see today, but that these are different from Earth’s,” Khan said.
Earth has a thin crust of rock that envelops a thick rock mantle, which surround a core made largely of iron and nickel.
Data gathered by InSight helped researchers learn that the Martian crust, which is thinner than expected, goes about 12 miles (19.3 kilometers) deep. This crust may contain sublayers that extend it about 23 miles (37 kilometers) down from the surface.
“Layering within the crust is something we see all the time on Earth,” said Brigitte Knapmeyer-Endrun, lead author of the crust study and geophysicist at the University of Cologne, in a statement. “A seismogram’s wiggles can reveal properties like a change in porosity or a more fractured layer.”
Understanding how the Martian crust formed, in comparison with Earth’s, can help researchers understand another piece of why the planets in our solar system are so different from one another.
Underneath the crust is the mantle, which travels another 969 miles (1,559.5 kilometers) down before reaching the liquid metal core.
The researchers were able to confirm the size of the core, which has a larger-than-expected radius of 1,137 miles (1,830 kilometers), and determine that the core is molten. The liquid core contains iron and nickel, as well as lighter elements like sulphur, oxygen, carbon and hydrogen.
“This study is a once-in-a-lifetime chance,” said Simon Stähler, lead author of the core study and seismologist at ETH Zurich, in a statement. “It took scientists hundreds of years to measure Earth’s core; after the Apollo missions, it took them 40 years to measure the Moon’s core. InSight took just two years to measure Mars’ core.”
Earth has a molten outer core that surrounds a solid inner core. The InSight mission, which has been extended through 2022, will continue to seek data that may show if Mars is similar or different from our planet in this way.
InSight awaits the big one
Mars was once volcanically active. Volcanic regions are visible across the red planet today, thanks to images from orbiters.
Most of the large marsquakes detected by InSight came from one particular region: Cerberus Fossae. This region, which may have been volcanically active as recently as a few million years ago, is littered with boulder tracks, likely created when they moved due to marsquakes.
Meanwhile, other volcanic regions on Mars appear to be quiet. But InSight keeps listening and waiting for marsquakes with magnitudes larger than 4.0.
“We’d still love to see the big one,” said Mark Panning, co-lead author of the crust study and JPL’s research scientist in planetary interiors and geophysics, in a statement. “We have to do lots of careful processing to pull the things we want from this data. Having a bigger event would make all of this easier.”
InSight’s steady stream of data heading to scientists on Earth will end in about a year when the solar cells can no longer generate enough power. But researchers will be studying the detections made by InSight for decades to come in order to learn as much as possible about our enigmatic planetary neighbor.
“Mars still presents us with many mysteries, most notably whether it formed at the same time and from the same material as our Earth,” said study author Domenico Giardini, a professor of seismology and geodynamics at ETH Zurich, in a statement.