The InSight lander has been on Mars, gathering data for a thousand days now, working to give us a better understanding of the planet’s interior. It’s at Elysium Planitia, the second largest volcanic region on Mars. A newly-published paper based on seismic data from the lander shows something unexpected underground: a layer of sediment sandwiched between layers of lava flows.

Much of InSight’s media coverage has centred around the lander’s Heat Flow and Physical Properties Package (HP3), also called the Mole. Its job was to measure the heat coming from the planet’s interior to the surface. After an epic struggle to get the instrument working, NASA and the DLR ( German Aerospace Center) announced in January 2021 that the Mole’s mission was over before it got started.

But the Mole isn’t InSight’s only instrument. Its other main science instrument is called the Seismic Experiment for Interior Structure (SEIS.) SEIS is working fine and has already delivered some solid science. This new paper rests on seismic data from SEIS.

The InSight Lander's seismometer underneath its protective wind and thermal shield. Image Credit: NASA/JPL-Caltech
The InSight Lander’s seismometer underneath its protective wind and thermal shield. Image Credit: NASA/JPL-Caltech

The new paper is titled “The shallow structure of Mars at the InSight landing site from inversion of ambient vibrations.” It’s published in the journal Nature Communications, and the authors are an international team of scientists.

The team used seismic data from InSight’s SEIS lander to analyze the sub-surface structure at Elysium Planitia to a depth of 200 meters. Directly beneath the lander, they found a three-meter layer of sandy material that makes up the regolith. Beneath that, there’s a 15-meter layer of material described as coarse, rocky ejecta. That material is debris from a meteorite impact, thrown up into the air before falling back to Mars.

Then it gets interesting.

Below the ejecta layer, InSight found a 150-meter region of layered basaltic material made of solidified lava. That layer is in line with expectations. But there’s something unexpected between the basaltic rock. Between 30 to 75 meters, there’s a “seismic low-velocity zone” that the team interpreted as a layer of less dense sedimentary material.

This figure from the paper shows the near-surface region under the InSight Lander. The left and center panels show what different models predicted. Reference models 1 and 2 are pre-landing models. ML and MAP stand for Maximum Likelihood and Maximum A Posteriori models and the grey lines are an ensemble of all the models. On the right is the authors' interpretation of the seismic data. See the study for more detail. Image Credit: Hobiger et al 2021.
This figure from the paper shows the near-surface region under the InSight Lander. The left and center panels show what different models predicted. Reference models 1 and 2 are pre-landing models. ML and MAP stand for Maximum Likelihood and Maximum A Posteriori models, and the grey lines are an ensemble of all the models. On the right is the authors’ interpretation of the seismic data. See the study for more detail. Image Credit: Hobiger et al 2021.

InSight has made progress probing Mars’ deeper planetary structure, and it’s studied the shallow subsurface to a depth of 10 to 20 meters. But it hasn’t studied the first few tens or hundreds of meters before. This data is helping build a detailed model of the near-surface region that will help explain the formation of Elysium Planitia.

InSight’s mission focuses on the Martian sub-surface, so it landed at Elysium
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