A Potential Water Source for Subterranean Life on Mars

In 2018, the SEIS (Seismic Experiment for the Interior Structure) instrument was deployed by NASA’s InSight lander onto the surface of Mars. After deployment, the SEIS data revealed discontinuities in the Martian crust at depths of approximately 10 km and 20 km. These discontinuities have been previously interpreted as changes in porosity or geochemistry. However, Ikuo Katayama and Yuya Akamatsu, have proposed a new model explaining these discontinuities as transitions from dry to water-filled cracks. This model could explain seismic velocity increases at the discontinuities without changes in porosity or composition. Their model suggests the presence of liquid water in the present-day Martian crust, potentially indicating subsurface habitats for life.

The SEIS instrument is a seismometer that utilizes naturally occurring seismic waves from Marsquakes or meteorite impacts to probe the planet's interior. When a Marsquake or impact event happens, SEIS detects the energy released in the form of P-waves, S-waves, and surface waves, which are then used to construct a detailed image of Mars's subsurface. By analyzing the behavior of P-waves and S-waves, geophysicists can tease out information about the rocks comprising Mars, such as their density and potential variations in composition. S-waves are unable to propagate through water and travel at a slower velocity compared to P-waves. As a result, the presence or absence of S-waves provide critical information about the phase composition of the subsurface. Additionally, P-waves travel fast through high density materials and slow through low density materials. Thus, their speed can reveal the density of the material they pass through and indicate any variations in density along their path.

Cross section of the SEIS seismometer Courtesy: David Ducros

The authors combined the SEIS data with experimental data to ground truth their results. Experiments were carried out using diabase (dolerite) samples from Sweden as the analog for Martian crust rock. Diabase is a basaltic, intrusive, mafic rock that is often found in shallow dikes and sills here on Earth. Due to its similar mineralogy and geochemistry it has thermal and mechanical properties that are comparable to those of Martian basalts, making it a perfect candidate for analog experiments. The group created thermal cracks in the samples and measured seismic velocities under different conditions such as dry, water saturated, and frozen. They used a pulse transmission method via piezoelectric transducers which essentially use electrical energy as a wave source. Then, they systematically examined how seismic velocities changed with increasing porosity under different crack-filling phases. To interpret these results, they applied the effective medium theory. This model allowed them to relate the effect of bulk and sheer resistance to porosity and crack aspect ratio.

Relationship between seismic velocity and porosity of thermally cracked diabase, with curves and diamonds representing theoretical models and experimental observation, respectively. Katayama, I., & Akamatsu, Y. (2024). Figure 1

The experiment revealed significant differences in seismic velocity among the different crack-filling phases. Dry and water-saturated cracks significantly reduce seismic velocity, while ice-filled cracks had little effect due to their relatively stiff moduli. The results support the interpretation made by the group that the boundary at 10 km and 20 km may be from variation in water saturation. These findings then serve as evidence for the presence of liquid water beneath the surface of Mars. While many geomorphology studies have shown that ancient Mars likely had water present on it’s surface, this model suggests the presence of water on present-day Mars. With water being the essential for life as we know it-serving as a solvent for biochemical reactions and a medium for nutrient transport-this discovery could have serious implications for astrobiology.

References:

Katayama, I., & Akamatsu, Y. (2024). Supplemental Material: Seismic Discontinuity in the Martian Crust Possibly Caused by Water-Filled Cracks. https://doi.org/10.1130/geol.s.26980444.v1