On 26 November 2018, the NASA InSight lander successfully set down on Mars in the Elysium Planitia region. Seventy Martian days later, the mission’s seismometer SEIS began recording the planet’s vibrations. 

A team of researchers and engineers at ETH Zurich, led by Professor Domenico Giardini, delivered the SEIS control electronics and is responsible for the Marsquake Service. In collaboration with the Swiss Seismological Service at ETH Zurich, this team oversees the daily interpretation of the data transmitted from Mars.

Results of the mission in the first months of operation on Mars have now been published in Nature Geosciences.

Models calculated on Piz Daint

InSight recorded 174 events until the end of September 2019. Since then, the measurements have continued. In total, more than 450 marsquakes have been observed, which have not yet been analyzed in detail. This accounts for one event a day on average.

The data allows researchers to observe how seismic waves travel through the planet and reveal its internal characteristics – similar to how x-rays are used in medical tomography.

Before InSight landed, researchers had developed a wide range of possible models to represent the internal structure of the red planet. Scientists from the Institute of Geophysics calculated the seismic wave propagation for more than 100 different Mars models on CSCS supercomputer “Piz Daint”. The new marsquake data are already enabling them to refine their understanding of the structure of the planet and to reduce uncertainties.

Interpreting marsquake data 

Marsquakes are similar to the seismic events we see on Earth, although they are generally of smaller magnitude. 

The 174 registered marsquakes can be categorized in two families: One includes 24 low-frequency events with magnitudes between 3 and 4, with waves propagating through the Martian mantle. A second family of marsquakes comprises 150 events with smaller magnitudes, shallower hypocentral depth, and high frequency waves trapped in the Martian crust.

“Marsquakes have characteristics already observed on the Moon during the Apollo era, with a long signal duration of 10 to 20 minutes due to the scattering properties of the Martian crust,” explains Giardini.

In general, however, he says, interpreting marsquake data is very challenging. In most cases, it is only possible to identify the distance but not the direction from which the waves are arriving.

InSight landed on a thin, sandy layer

InSight opens a new era for planetary seismology. The SEIS performance has so far exceeded expectations despite harsh conditions. The surface of Mars is characterized by temperatures ranging from minus 80 to 0 degrees Celsius every day and by strong wind oscillations. 

Indeed, wind shakes the InSight lander and its instrumentation during the day, leading to a high level of ambient noise. However, at sunset the winds calm down, allowing recording of the quietest seismic data ever collected in the solar system. As a result, most seismic events detected on Mars by SEIS occurred in the quiet night hours.

This challenging environment also requires researchers to carefully distinguish between seismic events and signals originating from movements of the lander, other instruments, or atmospheric-induced perturbances.

The hammering by the HP3 instrument (another InSight experiment) and the close passage of whirlwinds (dust devils), recorded by SEIS, allow mapping of the physical properties of the shallow soil layers just below the station.

We now know that SEIS landed on a thin, sandy layer reaching a few meters deep, in the middle of a 20 meter-wide ancient impact crater. At greater depths, the Martian crust has properties comparable to Earth’s crystalline massifs but appears to be more fractured. The propagation of the seismic waves suggest that the upper mantle has a stronger attenuation compared to the lower mantle. 

Evidence for tectonic stress

InSight landed in a rather quiet region of Mars, and no events near the station had been recorded until now. The three biggest seismic events were located in the Cerberus Fossae region about 1500 km away. It is a tectonic graben system, caused by the weight of the Elysium Mons, the biggest volcano in the Elysium Planitia area.

This provides strong evidence that seismic activity on Mars is not only a consequence of the cooling and shrinking of the planet but also induced by tectonic stress. The total seismic energy released on Mars lies between that of Earth and the Moon.

Combined with other InSight measurements, SEIS meaningfully contributed data to better understand meteorological processes on Mars. The instrument’s sensitivity to both wind and atmospheric pressure allowed identifying meteorological phenomena characteristic of Mars, including the many dust devils that pass by the spacecraft every afternoon.