Nasa Finally Knows What Is Beneath the Surface of Mars

You might be familiar with diagrams of the internal structure of planets if you ever took a planetary science course, or if you've just cracked open a book on the solar system. The figures in those diagrams are actually "cartoons and guesses" despite using gravitational measurements, according to Mark Panning at NASA's Jet Propulsion Laboratory. Scientists only have a complete understanding of the structure of Earth.

Surface of Mars

Unlike other planets, we have measured Earth's insides using seismology. Until this week, this was not true: Marine biologists found Mars has a large liquid metallic core. Three papers published in Thursday's issue of Science support this conclusion.

NASA's InSight mission landed a seismometer on the Red Planet in 2018 that has recorded Marsquakes ever since. Project scientist John P. Pannin is part of the JPL team. InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) attempted to figure out Mars' inner structure using seismological and thermal measurements taken from an equatorial plain on Elysium Planitia.

Using InSight seismometer data, Panning and colleagues reached the first direct seismic confirmation of Mars' core as well as establishing the thickness and possible structure of the Martian crust.

In other words, our eyes are now trained on what's inside the body rather than just cartoons.

What’s new - In the study of Martian crust, scientists examined readings from two types of seismic waves. P-waves and S-waves conducted measurements of the crust's thickness and potential structure. Scientists can locate Marsquakes by comparing the arrival times of P-waves and S-waves at the InSight seismometer, as P-waves are faster than S-waves.

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The P-wave's main function is to transport heat throughout the earth, says Panning, but some of the energy is converted into an S-wave at physical transition points, such as when the P wave becomes a S wave. Researchers can determine when a layer has been crossed by measuring the difference between those two waves.

Nasa Finally Knows What Is Beneath the Surface of Mars

There were three "bump" patterns in the data that could equally be explained by two models.

According to one model, the crust beneath InSight is about 20 kilometers thick and has two layers. It can reach a thickness of 39 kilometers in the other case, as there are three layers of crust. According to Panning, extrapolating across the planet, the crust is an average of between 24 and 72 kilometers thick. "Both of these are at the thin end of what we expected before the mission," he says. The crust was previously estimated to be 100 kilometers thick by some models.

Panning says the study examining Mars's mantle measured seismic velocity to understand the mantle's consistency and temperature based on seismic waves arriving later than the P- and S-waves. Compared to models we considered before the data was available, it is necessary to keep the mantle relatively cool to match observed velocities, he said.

Those findings are consistent with the results of the crustal study, since a hotter mantle under a thinner crust would result in more volcanism than we see on Mars today, Panning said. Researchers also estimated that the crust of Mars contained 13 to 20 times as many radioactive elements as the mantle based on their observations of the cooler mantle.

Nasa Finally Knows What Is Beneath the Surface of Mars

Using ScS waves - S-waves bouncing off the core - Mars' core study defined the core's radii of about 1,800 kilometers and confirmed gravity measurements indicating its liquid nature. Unless a solid core produces a strong reflection, Panning says we would not have seen it. "The fact that the core of the object is liquid confirmed our assumption."

To be able to confirm the tiny ScS-waves' existence, multiple measurements had to be taken. Panning says that the core measurements were the most challenging to obtain. "None of these events is big. According to him, the biggest event we might see is a magnitude 3.8. There is a good chance that if you are close to that size, you wouldn't even notice it."

As a ground seismometer whose sensitivity can measure vibrations as small as atoms of hydrogen, Panning's measurements were made possible in part due to the dry environment of Mars. Seismometers always record seismic noise generated by oceans on Earth, he says. Compared to the best station on earth, our Mars seismometer is probably two orders of magnitude quieter."

The deep interior of Mars is really important for understanding Earth. As a rocky planet, Earth is similar to Mercury, Venus, and Mars, and understanding the formation and evolution of other rocky planets is crucial to understanding our own. A candidate for medical school who studies only one patient will not be very good, Panning says.

In addition to analyzing the data they have so far, Panning's team will keep recording Marsquakes because InSight will continue to function through 2022. In fact, they might even be able to determine whether two or three layers of the Martian are correct if they measure some quakes at a different range than their current measurements.

“The story is not over,” he says. “We have to wait and see.”

Martian Crust Study abstract: 

There is no absolute measurement of the thickness of Mars' crust, which allows us to reveal a planet's past formation and evolution. This study uses marsquake recordings and the ambient wave field to determine the structure of the crust underneath the Mars InSight landing site. We find that the observations were consistent with models with two or three subsurface interfaces based on seismic phases that are reflected and converted at those interfaces. If the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers. The average thickness of the martian crust is between 24 and 72 kilometers, which can be extrapolated from global gravity and topography maps. According to independent estimates of bulk composition and geodynamic constraints, the thicker model accounts for the abundances of heat-producing elements at the shallow surface, while the thinner model requires greater concentration at depth.

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