The structure of Mars’s crust has been determined for the first time, researchers say.
Based on analysis of “marsquakes” recorded by Nasa’s InSight mission, three studies provide clues to the composition of the Red Planet.
Researchers have reported preliminary findings from the mission and for the first time have begun to map the interior of a planet apart from Earth.
Beneath the InSight landing site, the crust is either approximately 20 kilometres or 39 kilometres thick, according to an international research team led by geophysicist Dr Brigitte Knapmeyer-Endrun at the University of Cologne’s Institute of Geology and Mineralogy and Dr Mark Panning at Jet Propulsion Laboratory, California Institute of Technology (Caltech).
Studying a planet’s interior layers – its crust, mantle and core – can reveal key insights into its formation and evolution, as well as uncovering any geomagnetic and tectonic activity.
The internal characteristics of Earth have been surveyed using such methods.
In the past, only relative differences in the thickness of Mars could be estimated, and additional assumptions were required to obtain absolute thicknesses. These values showed large scatter, depending on which assumptions were made.
Seismology replaces these assumptions with a direct measurement at the landing site, and calibrates the crustal thickness for the entire planet.
The independent data also allows researchers to estimate the density of the crust.
Dr Knapmeyer-Endrun, lead author of the paper published in Science, said: “What seismology can measure are mainly velocity contrasts. These are differences in the propagation velocity of seismic waves in different materials.
“Very similar to optics, we can observe phenomena like reflection and refraction.
“Regarding the crust, we also benefit from the fact that crust and mantle are made of different rocks, with a strong velocity jump between them.”
The crust’s structure can be determined precisely based on these jumps.
According to the data, at the InSight landing site the top layer is about eight kilometres thick, with a margin of two kilometres either way.
Below that, another layer follows to about 20 kilometres, with a margin of five kilometres.
Dr Knapmeyer-Endrun said: “It is possible that the mantle starts under this layer, which would indicate a surprisingly thin crust, even compared to the continental crust on Earth.
“Beneath Cologne, for example, the Earth’s crust is about 30 kilometres thick.”
There is a third layer on Mars, which would make the crust under the landing site around 39 kilometres thick, with a margin of eight kilometres.
That would be consistent with previous findings, but the signal from this layer is not essential to match existing data, the experts said.
In both cases they are unable to rule out the possibility that the entire crust is made of the same material known from surface measurements and from Martian meteorites.
The data suggests the uppermost layer is made up of an unexpectedly porous rock. There could also be other rock types at greater depths than the basalts seen at the surface.
The single, independent measurement of crustal thickness at the InSight landing site is sufficient to map the crust across the entire planet.
Measurements from satellites orbiting Mars provide a very clear picture of the planet’s gravity field, allowing the scientists to compare relative differences in crustal thickness to the measurement taken at the landing site.
The combination of this data provides an accurate map.
Data on the present-day structure of Mars can also provide information on how the planet evolved.
In a separate study, Simon Stahler of ETH Zurich and colleagues used the faint seismic signals reflected off the Martian core-mantle boundary to investigate the planet’s core.
They found that the relatively large liquid metal core has a radius of nearly 1,830 kilometres and begins roughly halfway between the surface and the centre of the planet, suggesting the mantle consists of only one rocky layer, rather than two, like in Earth.
The findings indicate that the iron-nickel core is less dense than previously thought and enriched in lighter elements.