InSight mission: Mars unveiled

Before NASA’s InSight mission, little was known about the internal structure of Mars. Models were based solely on measurements collected by orbiting satellites or the analysis of Martian meteorites that had fallen to Earth. The thickness of the crust, based on gravity and topography measurements alone, was estimated at between 30 and 100 km. The values for the planet’s moment of inertia and density suggested a core with a radius of between 1,400 and 2,000 km. The details of the planet’s internal structure and the depth of the boundaries between crust, mantle and core, however, were completely unknown.

With the successful deployment of the SEIS experiment on the surface of Mars in early 2019, the mission scientists, including the 18 French co-authors involved and affiliated to numerous French institutions and laboratories, and their colleagues from ETH Zurich, the University of Cologne and the Jet Propulsion Laboratory in Pasadena, have collected and analysed one year’s worth of seismic data from Mars (i.e. almost two Earth years).

It should be noted that to determine a structural model, the time (of arrival) of the earthquake and its distance, more than one station is usually required. However, on Mars only one station is available to scientists, InSight. We therefore had to search for, identify and validate in the seismic recordings the signature of waves that interacted differently with the internal structures of Mars. These new measurements, coupled with mineralogical and thermal modelling of the internal structure, meant that the single station constraint could be overcome. A method that opens up a new era in planetary seismology.

A single station, multiple results

Another difficulty on Mars is its low seismicity and the seismic noise generated by its atmosphere: on Earth, earthquakes are much stronger and seismometers are better installed, in cellars or underground, which makes it possible to obtain an accurate image of the planet’s interior. So we had to pay particular attention to the data. “But even though Martian earthquakes have a relatively low magnitude of less than 3.5, the very high sensitivity of the VBB sensor and the very low noise at the start of the night enabled us to make discoveries that two years ago we thought were only possible with earthquakes of a magnitude greater than 4” explains Philippe Lognonné, professor at the University of Paris and scientific manager of the SEIS instrument at the IPGP.

The data, processed and transmitted to the scientists by CNES, IPGP and CNRS, was carefully cleaned of ambient noise (wind and deformation linked to rapid temperature changes) on a daily basis. The international Mars Quake Service (MQS) team recorded the seismic events on a daily basis: more than 600 were catalogued, of which more than 60 correspond to relatively distant earthquakes.
Of these, around ten contain information about the deep structure: “The direct seismic waves of an earthquake are a bit like the sound of our voice in the mountains, generating echoes. And it’s the echoes of these waves, coming from a reflection on the core or at the crust-mantle interface or even on the surface of Mars that we’ve been looking for in the signals because of their similarity to the direct waves” explains Philippe Lognonné.

An altered crust, a revealed mantle and a large liquid core

By comparing the behaviour of the seismic waves as they passed through the crust before reaching the Insight station, several discontinuities in the crust were identified: the first, observed at a depth of around 10 km, marks the separation between a highly altered structure, the result of very ancient fluid circulation, and a crust that is not very altered. A second discontinuity at around 20 km and a third, less pronounced at around 35 km, reveal the stratification of the crust beneath InSight: “We used all the latest analysis methods, both for tectonic earthquakes and environmental vibrations (seismic noise), to identify these discontinuities,” explains Benoit Tauzin, lecturer at the University of Lyon and researcher at the LGL-TPE.

In the mantle, the differences between the travel times of waves generated directly during an earthquake and those generated by the reflection of these direct waves on the surface were analysed. Using a single station, these differences allow us to determine the structure of the upper mantle, and in particular the variation in seismic velocities with depth. These variations in velocity are linked to temperature. “This allows us to estimate the heat flow on Mars, which is three to five times lower than on Earth, and to place constraints on the composition of the Martian crust, which is thought to concentrate more than half of the heat-producing radioactive elements present on the planet”, adds Henri Samuel, CNRS research officer at the IPGP.

Finally, in the third study, the scientists looked for waves reflected by the surface of the Martian core, the radius of which is one of the main results of the InSight mission. “To do this”, explains Mélanie Drilleau, research engineer at ISAE-SUPAERO, “we tested several thousand mantle and core models against the phases and signals observed”. Despite the low amplitudes of the signals associated with the reflected waves (known as ScS), an excess of energy is observed for nuclei with a radius of between 1790 km and 1870 km. Such a size implies the presence of light elements in the liquid core and has major consequences for the mineralogy of the mantle at the mantle/core interface.

Objectives achieved and new questions raised

After more than two years of Martian seismic monitoring, the first model of the internal structure of Mars has been obtained, right down to the core. Mars thus joins the Earth and the Moon in the club of telluric planets and satellites whose deep structure has been explored by seismology. And as is often the case in planetary exploration, new questions are being raised: is the alteration of the crust over the first 10 kilometres general or limited to the InSight landing zone? What impact will these initial models have on the theories of the formation and thermal evolution of Mars, particularly for the first 500 million years when Mars had liquid water on its surface and strong volcanism?

With the two-year extension of the InSight mission and the additional electrical power obtained following the cleaning of its panels by JPL, new data will consolidate and further improve these models.

About InSight and SEIS:

JPL manages the InSight mission on behalf of NASA’s Science Mission Directorate. InSight is part of NASA’s Discovery program, managed by the Marshall Space Flight Center (MSFC), a NASA facility in Huntsville, Alabama. Lockheed Martin Space in Denver built the InSight probe, including its cruise stage and lander, and supports spacecraft operations for the mission. CNES is the prime contractor for SEIS, and the Institut de Physique du Globe de Paris (University of Paris/IPGP/CNRS) has scientific responsibility for it. CNES is financing the French contributions, coordinating the international consortium (*) and has been responsible for the integration, testing and supply of the complete instrument to NASA. IPGP designed the VBB (Very Broad Band) sensors, tested them before delivery to CNES and is contributing to the operation of the VBBs on Mars.

SEIS and APSS operations are led by CNES within the FOCSE-SISMOC, with support from the Centro de Astrobiologia (Spain). SEIS data is formatted and distributed by the Mars SEIS Data Service of the IPG Paris, as part of the InSight National Observation Service, to which the LPG also contributes and, for Sismo activities at the School, GéoAzur. The daily identification of earthquakes is carried out by InSight’s Mars Quake Service, a collaborative operational service led by ETH Zurich to which seismologists from IPG Paris, the University of Bristol (UK) and Imperial College London (UK) also contribute.

Several other CNRS laboratories, including LMD (CNRS/ENS Paris/Ecole polytechnique/Sorbonne University), LPG (CNRS/Université de Nantes/Université d’Angers), IRAP (CNRS/Université de Toulouse/CNES), LGL-TPE (CNRS/Ecole normale supérieure de Lyon/Université Claude Bernard Lyon 1), IMPMC (Sorbonne University/Museum national d’Histoire naturelle/CNRS) and LAGRANGE (CNRS/Université Côte d’Azur/Observatoire de la Côte d’Azur) are working with IPGP and ISAE-SUPAERO to analyse data from the InSight mission. These analyses are supported by CNES and the French National Research Agency as part of the ANR MArs Geophysical InSight (MAGIS) project.

(*) In collaboration with SODERN for the production of the VBBs, JPL, the Swiss Federal Institute of Technology Zurich (ETH, Zürich, Switzerland), the Max Planck Institute for Solar System Research (MPS, Göttingen, Germany), Imperial College London and Oxford University have supplied the SEIS subsystems and are participating in the scientific exploitation of SEIS.

To find out more

• See the SEIS experiment website
• See the NASA InSight mission website

Ref:

• Thickness and structure of the martian crust from InSight seismic data, Knapmeyer-Endrun B. et al (2021), Science, DOI : https://science.sciencemag.org/cgi/doi/10.1126/science.abf8966
• Upper mantle structure of Mars from InSight seismic data, Khan A. et al (2021), Science, DOI : https://science.sciencemag.org/cgi/doi/10.1126/science.abf2966
• Seismic detection of the Martian core, Stähler S. et al (2021), Science, DOI : https://science.sciencemag.org/cgi/doi/10.1126/science.abi7730