Planetary cratering mechanics
Planetary cratering mechanics
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行星陨石坑力学
DOI：
10.1029/93je01330
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发表时间：
1993
影响因子：
--
通讯作者：
T. Ahrens
中科院分区：
文献类型：
--
作者：
J. D. O'keefe;T. Ahrens
这项研究的目的是获得一个定量的了解在广泛的条件下的成坑过程。我们的方法是数值计算撞击诱导流场的演变，并计算陨石坑几何形状（例如深度、直径、唇高）的关键测量值的时间历史，以适应行星重力（0至10^9 cm/s^2）、材料强度（0至2400 kbar）和撞击物半径（0.05至5000 km）的变化。这些结果被用来确定Holsapple和施密特（1987）标度律中的开参数值。我们用四种状态描述碰撞过程：（1）穿透，（2）惯性，（3）终端和（4）弛豫。在侵彻过程中，撞击器侵彻深度线性增长，无量纲时间τ =（Ut/a）5.1，弹坑以较慢的速率增长，直到它被强度或重力阻止。在这种情况下，弹坑深度d和直径D的增加，由射弹半径归一化，由d/a = 1.3（Ut/a）^（0.36）和D/a = 2.0（Ut/a）^（0.36）给出。对于强度主导的陨石坑，在惯性状态结束时停止增长，这发生在τ = 0.33（Y_（eff）/ρU^2）^（−0.78），其中Y_（eff）是有效的行星地壳强度。有效强度可通过断裂和剪切带熔融（例如形成假玄武玻璃岩）从环境强度降低。在引力主导的陨石坑中，当引力超过惯性力时，生长停止，这发生在τ = 0.92（ga/U^2）^（−0.61）。在强度和重力状态下，最大穿透深度分别为d_p/a = 0.84（Y/ρ U^2）^（-0.28）和d_p/a = 1.2（ga/U^2）^（-0.22）。从简单的碗状陨石坑到复杂形状的陨石坑的转变发生在重力开始主导陨石坑形成过程中的力量。发生这种转变的直径由D_t = 9.0 Y/ρg给出，因此当强度与应变率无关时，行星表面的尺度为g^（−1）。这个比例结果与类地行星的陨石坑形状数据一致[Chapman and McKinnon，1986]。我们已经将一些可计算但不可观测的参数（例如最大穿透深度、挖掘深度和最大火山口唇高度）与火山口直径联系起来。例如，最大穿透深度相对于最大弹坑直径为0.6，对于强度主导的弹坑，和0.28，对于重力主导的弹坑。这些数值意味着与大盆地撞击有关的撞击物相对较深地穿透到行星表面。这与早期的假设形成了鲜明对比，早期的假设是从结构数据中错误地推断出相对瞬时的弹坑穿透深度随着直径的增加而减少。同样，最大挖掘深度与最终陨石坑直径的比值是一个常数，对于重力主导的陨石坑为0.05，对于强度主导的陨石坑为0.09。这一结果意味着，当撞击速度低于25公里/秒时，即开始发生显著的蒸发，挖掘出的物质来自不到弹坑直径0.1倍的最大深度。在重力为主的政权，我们发现，表观最终陨石坑直径约为两倍的瞬态陨石坑直径和内环直径小于瞬态陨石坑直径。
The objective of this study was to obtain a quantitative understanding of the cratering process over a broad range of conditions. Our approach was to numerically compute the evolution of impact induced flow fields and calculate the time histories of the key measures of crater geometry (e.g. depth, diameter, lip height) for variations in planetary gravity (0 to 10^9 cm/s^2), material strength (0 to 2400 kbar), and impactor radius (0.05 to 5000 km). These results were used to establish the values of the open parameters in the scaling laws of Holsapple and Schmidt (1987). We describe the impact process in terms of four regimes: (1) penetration, (2) inertial, (3) terminal and (4) relaxation. During the penetration regime, the depth of impactor penetration grows linearly for dimensionless times τ = (Ut/a) 5.1, the crater grows at a slower rate until it is arrested by either strength or gravitational forces. In this regime, the increase of crater depth, d, and diameter, D, normalized by projectile radius is given by d/a = 1.3 (Ut/a)^(0.36) and D/a = 2.0(Ut/a)^(0.36). For strength-dominated craters, growth stops at the end of the inertial regime, which occurs at τ = 0.33 (Y_(eff)/ρU^2)^(−0.78), where Y_(eff) is the effective planetary crustal strength. The effective strength can be reduced from the ambient strength by fracturing and shear band melting (e.g. formation of pseudo-tachylites). In gravity-dominated craters, growth stops when the gravitational forces dominate over the inertial forces, which occurs at τ = 0.92 (ga/U^2)^(−0.61). In the strength and gravity regimes, the maximum depth of penetration is d_p/a = 0.84 (Y/ρ U^2)^(−0.28) and d_p/a = 1.2 (ga/U^2)^(−0.22), respectively. The transition from simple bowl-shaped craters to complex-shaped craters occurs when gravity starts to dominate over strength in the cratering process. The diameter for this transition to occur is given by D_t = 9.0 Y/ρg, and thus scales as g^(−1) for planetary surfaces when strength is not strain-rate dependent. This scaling result agrees with crater-shape data for the terrestrial planets [Chapman and McKinnon, 1986]. We have related some of the calculable, but nonobservable parameters which are of interest (e.g. maximum depth of penetration, depth of excavation, and maximum crater lip height) to the crater diameter. For example, the maximum depth of penetration relative to the maximum crater diameter is 0.6, for strength dominated craters, and 0.28 for gravity dominated craters. These values imply that impactors associated with the large basin impacts penetrated relatively deeply into the planet's surface. This significantly contrasts to earlier hypotheses in which it had been erroneously inferred from structural data that the relative transient crater depth of penetration decreased with increasing diameter. Similarly, the ratio of the maximum depth of excavation relative to the final crater diameter is a constant ≃0.05, for gravity dominated craters, and ≃ 0.09 for strength dominated craters. This result implies that for impact velocities less than 25 km/s, where significant vaporization begins to take place, the excavated material comes from a maximum depth which is less than 0.1 times the crater diameter. In the gravity dominated regime, we find that the apparent final crater diameter is approximately twice the transient crater diameter and that the inner ring diameter is less than the transient crater diameter.