When Galileo observed the planet Mars with a telescope more than 400 years ago, it was recorded as little more than a blank orb, hanging in infinite darkness. In the four centuries since then, scientists have tried to fill in the blanks.
It wasn’t long after Galileo that Dutch astronomer Christiaan Huygens appeared and made a profound discovery about Mars. Observing the planet in 1659, Huygens noticed a large, dark area on its face, shaded into a heart-shaped spot on a sketch of the red planet. It was the first time humans had observed the surface features of another world.
Some 359 years later, in November 2018, NASA landed InSight on the Martian surface about 2,000 miles east of the spot, the eighth time the space agency had placed a robotic rover on the red planet. Their mission, which was recently extended to 2022, is to listen to “marsquakes” and understand what is happening beneath the surface of our cosmic neighbor.
in a Series of Three studies Published in the journal Science on Thursday, a global team of researchers describes the interior of Mars using data obtained by InSight’s seismometer, an instrument that responds to vibrations and noise under the surface of Mars. Analyzing a series of marsquakes, felt by InSight since 2019, researchers have been able to reveal the inner workings of another planet in our solar system for the first time, a breakthrough for planetary geoscience.
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Ear to the ground
The first planetary rattle detected by InSight’s seismometer, known as SEIS, in 2019 was like Huygens’ first tentative drawing. It revealed that Mars was more seismically active than the Moon, but not as active as Earth, and gave researchers a tantalizing first look at the kind of data InSight could collect.
A slice of SIX, a dome-shaped instrument found on the surface of Mars. The white outer layer protects the sensitive instrument from the environment, while the inner layer from organized chaos contains pendulums that measure vibrations and noise.
NASA / JPL-Caltech / CNES / IPGP
SIX (pictured right) is a dome-shaped instrument that was deployed shortly after InSight’s arrival on Mars. It rests on the Martian soil and, as NASA says, it is like a doctor’s stethoscope, listening to the “pulse” of the planet. It is an extremely sensitive piece of technology, which records the seismic waves that rumble and vibrate through the interior of the planet after an earthquake.
Its outer dome is a shield against the Martian environment, protecting SIX from winds and dust that could affect internal vibration measurements. The seismometer itself is a fairly simple device: it contains three weights, suspended like a pendulum, that can detect vibrations from different directions, such as when a seismic wave, generated by an earthquake, passes over them.
Previous research has shown that marsquakes are common, but they are not very powerful. Only a handful of records greater than magnitude 3 which, on Earth, can feel like a slight rumbling a few miles away, but is not loud enough to cause significant damage to structures and buildings. Most originate in the top layer of the planet’s crust, but the studies probed 10 that originated deeper below the surface.
Listening to the waves generated by these earthquakes is how researchers came to understand the bowels of Mars. The seismic waves moving through the interior of the planet are changed by the material with which they come in contact, allowing InSight to paint a picture of what is happening. in the land.
Ogres, onions and other planets.
The anatomy of a “differentiated” planet like Mars is, to borrow from a 20-year-old movie, like an onion (… or an ogre). It has layers. Although scientists have filled in the blanks regarding the characteristics of the surface, the atmosphere and the chemical composition of the soil, what is happening below the surface has been a mystery.
“From everything we know about Mars, most of it is confined to the upper meter,” says Gretchen Benedix, an astrogeologist at Curtin University in Australia who was not affiliated with the study. “It’s like looking at a gift and concentrating on the packaging.”
In the set of new studies, the researchers tested these layers for the first time by studying the waves shaking InSight’s SEIS. “This new information is like opening the gift to take a look,” says Benedix.
One of the studies, led by Brigitte Knapmeyer-Endrun, a geophysicist at the University of Cologne, used the data to study the planet’s top layer, known as the crust.
The top layer of the crust, which is made up of basalt rock from ancient lava flows, appears to be about 10 kilometers (6.2 miles) thick at most. But the InSight data revealed that another layer, about twice that size, lies just below it. Below that, Knapmeyer-Endrun said in a press release, it could be where the “mantle” begins, which would make the crust of Mars “surprisingly thin.”
A “selfie” of the InSight lander, taken on the surface of Mars.
NASA / JPL-Caltech
But the team also showed that there may be a third layer in the crust, extending the depth up to about 40 kilometers.
Then there is the Martian core, which threw up some surprises.
As shown in the image at the top, marsquakes can send vibrations all the way to the planet’s core, where they bounce off and launch toward SIX. These signs, like described in a study Led by Simon Stähler, a geophysicist at ETH Zurich in Switzerland, they were relatively weak but helped estimate how big the planet’s core is. And size matters here.
The mantle-core boundary appears to be a little less than 1,000 miles below the surface, which is larger than some studies have suggested. The suggestion, according to an accompanying article published in Science Thursday, is that the iron-nickel core is less dense than previously predicted, but is in a liquid state, as other studies have argued.
Why does the interior of Mars matter?
The return of seismology to Mars was described by University of Texas geophysicist Yosio Nakamura as “a new dawn” in a Nature Geoscience Commentary in 2020. The ability to detect seismic waves helps put some fundamental limitations on how the planet likely evolved over time and, according to Benedix, “tells us a lot about the thermal evolution of that planet.”
Heat emanates from a planet’s core during its formation and early evolution, and by understanding the composition of the core, researchers can hypothesize how Mars may have cooled over time. Combining this with other data, obtained from orbiting spacecraft and NASA and Chinese rovers, not only helps us understand Mars, but reveals how planets form, change, and develop throughout the solar system and potentially also outside of him.
InSight also attempted to take a direct measurement of the temperature below the red planet’s surface using a “burrowing mole.” But at first, when the mole tried to dig into the crazy soil of Mars, it got stuck. Heroic attempts by NASA engineers to free the mole were unsuccessful and, in January, was declared dead. However, InSight’s mission is not over: it will continue to listen to marsquakes until 2022. Although it only provides a single “ear,” so to speak, repeated observations should allow scientists to further refine their understanding of the interior of Mars.
In less than four centuries, we have gone from Huygens’ sketch of a heart-shaped spot on the face of Mars to understanding the very heart of Mars. Keep filling in the blanks.