2:45 PM - 3:00 PM
[MIS20-05] Impact cratering on wet sand : Effect of water content and implications for crater formation on habitable bodies
Keywords:Habitable planet, Wet sand, Impact crater, Impact experiment, π-scalling
In order to understand impact phenomena on the surface of habitable bodies, such as the present Earth and past Mars, it is important to clarify the impact phenomena on the surface containing liquid water. Morphological and mineralogical studies of Martian craters suggest that a wet layer with a depth of at least 7 km existed during the Noachian period ( > 3.7 Ga), suggesting the presence of fluid in the sub-surface layer[1]. Previous studies show that the crater formation process on a wet sand layer is different from that on dry sand, and isn’t formed in the gravity-dominated regime[2]. In addition, low-velocity (~m/s) impact experiments with varying water contents have shown that the penetration depth of a projectile can be expressed as the power law of the shear yield strength of a wet sand layer [3]. However, experimental data at impact velocities (~km/s) between protoplanets are insufficient to clarify the effect of water content on the crater formation process on habitable bodies. In this study, we performed high-velocity cratering experiments using targets with various water contents to examine the effect of water content on the crater formation process for wet sand targets. The target was a wet sand consisting of quartz sand with a diameter of 500 µm mixed with liquid water, and the water content was varied from 0 to 13 wt.%. Impact experiments were crrid out using two-stage light gas guns at Kobe University and ISAS/JAXA. The projectile was an aluminum sphere with a diameter of 2 mm, and the impact velocity was 2 km/s or 4 km/s. The target was set at an angle of 30º from the horizontal plane, and the experiment was conducted under conditions near the triple point of H2O where is similar to the surface condition of Mars : H2O among the target grains is in a liquid state. All the experiments were observed by high-speed camera, and the experiments at ISAS were also observed by a high-speed infrared camera to measure the ejecta and plume temperatures.
As a result, the crater depth increased and the crater diameter decreased with increasing the water content. However, both diameter and depth became almost constant when the water content exceeded ~5 wt.%. They agree well with the results of a previous research that the shear yield strength of wet sand increased with water content up to ~5 wt.%, but remained almost constant above 5 wt.% [3]. This suggests that the crater size of wet sand targets might depend on the target shear strength. Applying the -scaling law[4] with this shear yield strength, it is suggested that the crater size of wet sand targets can be explained by the -scaling law in the strength-dominated regime using the shear yield strength.
The ejecta curtain angle did not change with water contents in most of the experiments, and was asymmetric between the upper and lower sides of the slope. This asymmetry is consistent with previous studies[5]. However, at a velocity of 4 km/s, this asymmetry weakened when the water content exceeded ~5 wt.%, and became nearly symmetric above ~9 wt.%. From this result, it is possible that the high impact pressure generated at 4 km/s prevented the penetration of the projectile itself, or deformed the projectile. In addition, the increase in shear yield strength of targets with increasing the water content might also prevent the penetration of projectile fragments after impact.
References: [1] Sun & Milliken (2015) JGR-Planets 120, 2293-2332. [2] Schmidt & Housen (1987) Int J Imp Engin 5, 543-560. [3]Takita & Sumita (2013) Phys. Rev. E88, 022203. [4] Housen & Holsapple (2011) Icarus 211(1), 856-875. [5]Anderson et al. (2004) MAPS 39, 303-320.
As a result, the crater depth increased and the crater diameter decreased with increasing the water content. However, both diameter and depth became almost constant when the water content exceeded ~5 wt.%. They agree well with the results of a previous research that the shear yield strength of wet sand increased with water content up to ~5 wt.%, but remained almost constant above 5 wt.% [3]. This suggests that the crater size of wet sand targets might depend on the target shear strength. Applying the -scaling law[4] with this shear yield strength, it is suggested that the crater size of wet sand targets can be explained by the -scaling law in the strength-dominated regime using the shear yield strength.
The ejecta curtain angle did not change with water contents in most of the experiments, and was asymmetric between the upper and lower sides of the slope. This asymmetry is consistent with previous studies[5]. However, at a velocity of 4 km/s, this asymmetry weakened when the water content exceeded ~5 wt.%, and became nearly symmetric above ~9 wt.%. From this result, it is possible that the high impact pressure generated at 4 km/s prevented the penetration of the projectile itself, or deformed the projectile. In addition, the increase in shear yield strength of targets with increasing the water content might also prevent the penetration of projectile fragments after impact.
References: [1] Sun & Milliken (2015) JGR-Planets 120, 2293-2332. [2] Schmidt & Housen (1987) Int J Imp Engin 5, 543-560. [3]Takita & Sumita (2013) Phys. Rev. E88, 022203. [4] Housen & Holsapple (2011) Icarus 211(1), 856-875. [5]Anderson et al. (2004) MAPS 39, 303-320.