It's the same for any other planet, and it's possible to peer deeper below the surface of Earth.
The first in-depth look at what lies beneath the red planet's surface has been provided by the InSight lander.
The seismometer on board InSight points to a shallow layer sandwiched between hardened rocks resulting from lava flows, going down to a depth of around 200 meters.
This could show us a lot about how Mars was formed, how it evolved over time, and the geological factors that are still in play today. The lava flows can be connected to the planet's volcanic past.
Géraldine Zenhusern is associated withETH Zrich.
The artist's impression shows the impact crater of the InSight lander.
The researchers explain in their paper that the shallow subsurface around the InSight landing site has been limited to the uppermost 10-20m using seismic-travel time measurements and ground compliance estimates.
The Elysium Planitia is a wide and flat plain on Mars. In order to figure out what was out of view underneath, the landers instruments measured the slight ambient vibrations of the ground, caused by the winds flowing over the surface of the planet.
The same technique was used to assess the earthquake risk on Earth. The waves on Mars were consistent with two layers of rocks, most likely basalt and a thinner, less dense layer of material in the middle.
According to the researchers, the uppermost layer of hardened lava is 1.7 billion years old, formed during the cold, arid period on Mars when there were relatively few meteorite and asteroid impacts.
During the Hesperian period, when there was a lot of volcanic activity on the Red Planet, the deeper layer was created. Mars has been molded into the planet we are seeing and exploring today.
"This helps to tie this to trying to figure out what the timing was between the various different activities," Bruce Banerdt, from the Jet Propulsion Laboratory at the California Institute of Technology, told Inverse.
The fact that you have a layer of rocks sandwiched between the two volcanic stones tells you that there was a pause in the volcanic activity.
It's not clear what the middle layer is made up of or how it was formed, but it's something that surprised the researchers. It's possible that there's some mixing with the basalts, but the accuracy of the readings decreases at lower depths.
It's important to work out if life ever existed on Mars, but it's also important to know more about Earth's history and evolution.
Knowing what's underneath the surface of Mars allows scientists to work out the best places to put landers, rovers, and (eventually) space stations in the future.
"While the results help to better understand the geological processes in Elysium Planitia, comparison with pre-landing models is also valuable for future landed missions, since it can help to refine predictions," says seismologist Brigitte Knapmeyer-Endrun from the University of Cologne in Germany
The research has been published.
