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Seismic clues from Marsquakes suggest liquid water and life potential beneath the surface

Are subterranean lifeforms viable on Mars? A new interpretation of Martian seismic data by scientists Ikuo Katayama of Hiroshima University and Yuya Akamatsu of Research Institute for Marine Geodynamics suggests the presence of water below the surface of Mars. "If liquid water exists on Mars," Katayama says, "the presence of microbial activity" is possible.

This analysis is based on seismic data from SEIS (Seismic Experiment for the Interior Structure), deployed from NASA's lander that landed on Mars in 2018 (Fig. 1). This robotic lander is unique because it was able to use its robotic arm to place a seismometer on the surface of Mars. The instrument, which contains the seismometer, uses the seismic waves naturally generated on Mars from Marsquakes or meteorite impacts to scan the planet's interior (Fig. 1).

When a Marsquake or meteorite impact occurs on Mars, SEIS can read the energy emitted as , , and to create an image of the planet's interior (Fig. 2). Scientists can use P-waves and S-waves to determine a lot about the rocks that make up Mars, including the density of the rocks or potential composition changes within the rocks.

For example, S-waves cannot travel through water and move at a slower speed than P-waves. Therefore, the presence, absence, and arrival time of S-waves can determine what the subsurface looks like. Moreover, P-waves can travel faster through higher-density material and slower through less dense material, so their velocity can help determine the density of the material the wave is traveling through, as well as if there are any changes in density along its path. The seismic data collected with SEIS shows a boundary at 10 km depth and 20 km depth from measured discrepancies in seismic velocity.

This boundary has previously been interpreted as sharp transitions in the porosity (the percentage of open space in a rock) or chemical composition of the Martian interior. However, Katayama and Akamatsu have interpreted these cracks as potential evidence for water within the Martian subsurface. The seismic data indicate a boundary between dry cracks and water-filled cracks in the Martian subsurface (Fig. 3). To test their hypothesis, they measured the seismic velocity passing through rocks with the same structures and composition of a typical Martian crustal rock under wet, dry, and frozen conditions.

A typical Martian rock is similar to the diabase rocks from Rydaholm, Sweden, due to their evenly sized plagioclase and orthopyroxene grains. In the lab, Katayama and Akamatsu measured P-wave and S-wave velocity using a piezoelectric transducer, which uses "electrical energy . . . as a wave source" that "monitor[s] seismic wave energy" on dry, wet, and frozen diabase samples. Experimentation revealed that the seismic velocities of the dry, wet, and frozen samples are significantly different, which supports the interpretation that the boundary at 10 km and 20 km could be from a change from dry rock to wet rock.

These laboratory experiments back up Katayama and Yuya's hypothesis that the boundary measured by seismic data indicates a transition from dry rock to wet rock rather than a change in porosity or chemical composition. The findings, therefore, provide compelling evidence for the existence of liquid water beneath the surface of Mars. "Many studies suggest the presence of water on ancient Mars billions of years ago," Katayama explains, "but our model indicates the presence of liquid water on present-day Mars."