According to new analyses of data from the Seismic Experiment for Interior Structure (SEIS) instrument deployed during NASA’s InSight mission, Mars likely has a 24- to 72-km- (15-45-mile) thick crust with a very deep lithosphere close to 500 km (311 miles); similar to the Earth, a low-velocity layer probably exists beneath the Martian lithosphere; the crust of Mars is likely highly enriched in radioactive elements that help to heat this layer at the expense of the interior; the Martian core is liquid and large, 1,830 km (1,137 miles), which means that the mantle has only one rocky layer rather than two like the Earth has. These results appear in three papers in the journal Science.
Schematic view of the propagation of seismic waves in the Martian crust and seismic conversions at the base of the discontinuity, at a depth of 10 km (6.2 miles). Image credit: IPGP.
Like Earth, Mars heated up as it formed from the dust and larger clumps of meteoritic material orbiting the Sun that helped to shape our early Solar System.
Over the first tens of millions of years, the planet separated into three distinct layers — the crust, mantle, and core — in a process called differentiation.
Part of InSight’s mission was to measure the depth, size, and structure of these three layers.
“When we first started putting together the concept of the mission more than a decade ago, the information in these papers is what we hoped to get at the end,” said InSight’s principal investigator Dr. Bruce Banerdt, a researcher at NASA’s Jet Propulsion Laboratory.
“This represents the culmination of all the work and worry over the past decade.”
The earthquakes most people feel come from faults caused by tectonic plates shifting.
Unlike Earth, Mars has no tectonic plates; its crust is instead like one giant plate. But faults, or rock fractures, still form in the Martian crust due to stresses caused by the slight shrinking of the planet as it continues to cool.
The members of the InSight team spend much of their time searching for bursts of vibration in seismograms, where the tiniest wiggle on a line can represent a quake or, for that matter, noise created by wind.
If seismogram wiggles follow certain known patterns (and if the wind is not gusting at the same time), there’s a chance they could be a quake.
The initial wiggles are primary, or P, waves, which are followed by secondary, or S, waves. These waves can also show up again later in the seismogram after reflecting off layers inside the planet.
“What we’re looking for is an echo,” said Dr. Amir Khan, a researcher at ETH Zurich.
“We’re detecting a direct sound — the quake — and then listening for an echo off a reflector deep underground.”
“Layering within the crust is something we see all the time on Earth,” said Dr. Brigitte Knapmeyer-Endrun, a researcher at the University of Cologne.
“A seismogram’s wiggles can reveal properties like a change in porosity or a more fractured layer.”
The InSight scientists found the Martian crust was thinner than expected and may have two or even three sub-layers.
It goes as deep as 20 km (12.4 miles) if there are two sub-layers, or 37 km (23 miles) if there are three.
Beneath that is the mantle, which extends 1,560 km (969 miles) below the surface.
At the heart of Mars is the core, which has a radius of 1,830 km. Confirming the size of the molten core was especially exciting for the authors.
“This study is a once-in-a-lifetime chance,” said Dr. Simon Stähler, a researcher at the ETH Zurich.
“It took scientists hundreds of years to measure Earth’s core; after the Apollo missions, it took them 40 years to measure the Moon’s core. InSight took just two years to measure Mars’ core.”
Amir Khan et al. 2021. Upper mantle structure of Mars from InSight seismic data. Science 373 (6553): 434-438; doi: 10.1126/science.abf2966
Brigitte Knapmeyer-Endrun et al. 2021. Thickness and structure of the Martian crust from InSight seismic data. Science 373 (6553): 438-443; doi: 10.1126/science.abf8966
Simon C. Stähler et al. 2021. Seismic detection of the Martian core. Science 373 (6553): 443-448; doi: 10.1126/science.abi7730