[M-IS14] Aqua planetology

[MIS14-P11] Measurements of elastic wave velocity of mafic and ultramafic rocks below the freezing point: Consideration about reflectivity due to ice-water phase transition in Mars

*Haruna Wakabayashi¹, Ikuo Katayama¹ (1.Hiroshima University)

Keywords:Mars, Elastic wave velocity, Aspect ratio

It has been pointed out that a large amount of water existed on the surface of Mars in the past, and it is expected that life may have existed there. However, Mars is currently dry, with an average surface temperature of about -50 degrees Celsius, so water exists mainly as ice in the ice sheets and the deep underground cryosphere. On the other hand, the gravity of Mars is lower than that of the Earth, and rocks are less compressible. Therefore, Mars retains high porosity deep underground, and a phase transition from ice to water is predicted to occur underground from the heat flow rate. Therefore, in this study, we will measure the elastic wave velocity in the phase transition from ice to water in rocks, and verify whether water in the subsurface can be detected by seismic reflection surveys such as the InSight mission on Mars. Diabase, peridotite, and serpentinite which are considered to be the basement rocks of Mars, were used as samples to measure the elastic wave velocity. Elastic wave measurements were performed under dry, wet, and frozen conditions (-55°C). In the measurements, the effect of porosity on elastic waves was examined. In order to examine the effect of porosity on elastic waves, thermal cracks were introduced into the samples. The porosity of the diabase was 0.14% at Intact, 0.55% at 600°C and 1.17% at 800°C. The porosity of the peridotite was 0.32% at Intact, 1.32% at 600°C and 1.70% at 800°C. Elastic wave velocities were calculated using the pulse transmission method by crimping a piezoelectric element with a resonant frequency of 2MHz to the sample. The elastic wave velocities of all rock samples decreased with increasing porosity, and increased as the pores were filled with water, with the highest velocities observed under frozen conditions. The Vp/Vs ratio was also calculated and showed a decreasing trend with increasing porosity in the dry condition, an increasing trend in the wet condition, and constant in the frozen condition. Using these results, the aspect ratio (short axis/major axis) of the voids was estimated from the effective medium theory (Kuster and Toksoz 1974), and the crack with aspect ratio of 0.01 is consistent with the experimental data. Using the results of this velocity measurement, we calculated the acoustic impedance and calculated the reflectivity at 5 km underground, where the ice-water phase transition is predicted to occur in the Martian subsurface. As a result, the reflectivity at the ice-water phase transition was 0.269 for diabase and 0.319 for peridotite. Thus, when the phase transition from ice to water occurs, high seismic wave reflectance is expected, and future seismic surveys on Mars may reveal the existence of water.