Seismic velocity sensitivity to humidity and temperature in a poorly-cemented Mars-analogue sandstone

The Mars Curiosity mission has recorded fluctuations in Mars’ atmospheric humidity that coincide with seasonal variations in seismic velocity from the InSight mission. To assess the impact of humidity on seismic velocity, we conduct a series of laser ultrasonic measurements in a vessel with controlled temperature as low as -7 °C and an initial humidity ranging from 15% to 90%. We use poorly consolidated sandstone as a Mars analogue rock. Results show elastic softening with increasing humidity at room temperature, which can be explained by changes in effective stress resulting from moisture adsorption. At sub-zero temperatures, P-wave velocity trends show no indication of adsorbed water freezing. Numerical models suggest water either remains unfrozen or freezes away from grain contact. We present an empirical formula describing how P-wave velocity varies with water vapor volume mixing ratio and temperature. Estimation based on this formula yields a 2.6% change in P-wave velocity variation, which is in the same order of magnitude as what was observed from InSight. This suggests that seasonal atmospheric variations on Mars may significantly contribute to the near-surface velocity change observed beneath the InSight lander.