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The effect of a thin weak layer covering a basalt block on the impact cratering process

Authors:	Koji Dohi Masahiko Arakawa Chisato Okamoto Sunao Hasegawa Minami Yasui

Institution:	1. Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan;2. Graduate School of Science, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan;3. Japan Aerospace Exploration Agency, JAXA Space Exploration Center, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan;4. Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara 252-5210, Japan;5. Organization of Advanced Science and Technology, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan

Abstract:	To clarify the effect of a surface regolith layer on the formation of craters in bedrock, we conducted impact-cratering experiments on two-layered targets composed of a basalt block covered with a mortar layer. A nylon projectile was impacted on the targets at velocities of 2 and 4 km s^?1, and we investigated the crater size formed on the basalt. The crater size decreased with increased mortar thickness and decreased projectile mass and impact velocity. The normalized crater volume, π_V, of all the data was successfully scaled by the following exponential equation with a reduction length λ₀: $π_{V} = b_{0} π_{Y}^{- b_{1}} \exp (- λ / λ_{0})$ , where λ is the normalized thickness T/L_p, T and L_p are the mortar thickness and the projectile length, respectively, b₀ and b₁ are fitted parameters obtained for a homogeneous basalt target, 10^?2.7±0.7 and ?1.4 ± 0.3, respectively, and λ₀ is obtained to be 0.38 ± 0.03. This empirical equation showing the effect of the mortar layer was physically explained by an improved non-dimensional scaling parameter, $π_{Y}^{1}$ , defined by $π_{Y}^{1} = Y / (ρ_{t} u_{p}^{2})$ , where u_p was the particle velocity of the mortar layer at the boundary between the mortar and the basalt. We performed the impact experiments to obtain the attenuation rate of the particle velocity in the mortar layer and derived the empirical equation of $\frac{u_{p}}{v_{i}} = 0.50 \exp (- \frac{λ}{1.03})$ , where v_i is the impact velocity of the projectile. We propose a simple model for the crater formation on the basalt block that the surface mortar layer with the impact velocity of u_p collides on the surface of the basalt block, and we confirmed that this model could reproduce our empirical equation showing the effect of the surface layer on the crater volume of basalt.

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