Mars interior mapped using data from Nasa’s Insight probe
Image credit: reuters
Data from Nasa’s Insight rover has been used to determine the thickness of Mars’ crust, as well as provide estimates about the size of its large metal core.
InSight was sent to Mars in 2018 to study the internal structure of the Red Planet and shed light on how rocky planets, moons and meteorites in the solar system formed. It detected its first ‘Marsquakes’ in 2019 which proved that the planet was seismically active underneath the surface.
They estimated of the size of the planet’s core, the thickness of its crust and the structure of its mantle based on the analysis of seismic waves reflected and modified by interfaces in its interior.
It is the first-ever seismic exploration of the internal structure of a terrestrial planet other than Earth and an important step towards understanding the formation and thermal evolution of Mars.
Before Nasa’s InSight mission, the internal structure of Mars was somewhat poorly understood. Models were based only on data collected by orbiting satellites and on the analysis of Martian meteorites that fell to Earth.
On the basis of gravity and topographical data alone, the thickness of the crust was estimated to be between 30 and 100km. Values of the planet’s moment of inertia and density suggested a core with a radius of 1,400 to 2,000km. The detailed internal structure of Mars and the depth of the boundaries between the crust, mantle and core were, however, completely unknown.
To simultaneously determine a structural model, the arrival time of an earthquake and its distance, more than one monitoring station is usually required.
However, on Mars the scientists only have one station, InSight, so it was necessary to search the seismic records for the characteristic features of waves that had interacted in various ways with the internal structures of Mars, then identify and validate them.
These new measurements, coupled with mineralogical and thermal modelling of the planet’s internal structure, have made it possible to overcome the limitation of having a single station.
Another difficulty on Mars is its low seismicity and the seismic noise generated by its atmosphere. On Earth, earthquakes are much stronger, while seismometers are more effectively located in vaults or underground, making it possible to obtain an accurate image of the planet’s interior. As a result, special attention had to be paid to the data from Mars.
“Although Martian earthquakes have a relatively low magnitude, less than 3.5, the very high sensitivity of the VBB sensor combined with the very low noise at nightfall enabled Nasa to make discoveries that, two years ago, we thought were only possible with earthquakes with a magnitude greater than 4,” explained researcher Philippe Lognonné.
“The direct seismic waves from an earthquake are a bit like the sound of our voices in the mountains: they produce echoes. 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.”
The team is now considering what impact these models will have on theories of the formation and thermal evolution of Mars, in particular for the first 500 million years when Mars had liquid water on its surface and intense volcanic activity.
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