3D Numerical Modelling of Impact Cratering in the Solar System

小行星和彗星与固体行星表面的猛烈碰撞是太阳系中一个基本而普遍的过程。在我们太阳系的早期历史中，小的岩石粒子相互碰撞和吸积，形成越来越大的天体，直到它们成长为我们今天看到的行星。对于地球来说，最后一次行星级别的碰撞是一次最终形成月球的巨大碰撞。在太阳系历史上，一次灾难性的撞击事件导致了地球上的大规模灭绝，包括恐龙的灭绝。今天，小行星或彗星的撞击对人类构成了真正的威胁，尽管人们对此知之甚少。撞击坑也被用来研究太阳系。对固体行星表面撞击坑数量的计数被用来区分古老和年轻的地形，而大型陨石坑的形状为了解行星的近地表结构提供了线索。撞击陨石坑的灾难性潜力及其深远的后果，使得我们必须准确地了解撞击过程是如何运作的。其核心是将撞击坑的大小与撞击物体的属性(例如，大小和速度)以及目标体上的条件(例如，重力)联系起来的基本方程。换句话说，这些方程给出了基本问题的答案：如果给定的撞击器击中给定的目标表面，陨石坑将有多大？目前，我们不能很好地回答这个问题，因为方程没有适当地包括所有重要的冲击器和靶材料的性质。这项建议的第一个目标是利用计算机模型更好地约束撞击事件中所有重要变量之间的关系。由于这些方程是估计撞击后果的基本工具，这项工作将促进行星科学许多领域的理解。在这些撞击方程中，缺少的主要部分是了解弹坑的生长如何受靶体中的孔隙空间的影响，以及冲击器撞击靶体的角度。确定撞击角度的影响很重要，因为经过充分研究的垂直撞击发生的可能性比与垂直夹角大于30度的撞击要小得多，后者所知的要少得多。在实验室撞击中观察到撞击角度影响陨石坑的大小和形状，但在较大撞击中的影响只能通过数值模拟来确定。孔隙度是小行星、彗星和大多数太阳系天体近地表的重要属性。从实验室实验中得知，具有高孔隙率的目标岩石减少了在生长过程中从陨石坑中排出的物质的体积，但这些影响尚未得到适当的量化。特别是，撞击过程中的多孔压实效应可能会对太阳系的演化产生重要影响。行星是通过小行星的碰撞而生长的。因此，量化这种碰撞中的能量和动量转移对于理解太阳系的热、化学和物理演化至关重要。在大多数以前的工作和模型中，行星增长过程中的碰撞被假定为完全低速和/或非多孔性行星体之间的碰撞。然而，最近的研究表明，早期的行星体孔率非常高(高达80%)，行星胚胎的生长将使行星体间的相对速度达到&gt；1公里/S。实验和我们自己的建模工作表明，目标孔隙率极大地影响碰撞的后果：增加热量和减少抛射质量。因此，这项工作的第二个目标是使用数值撞击模拟来量化行星小孔隙率对早期行星小碰撞期间的加热、压缩和抛射的影响，并评估这对太阳系演化的影响。

The violent collision of asteroids and comets with solid planetary surfaces is a fundamental and ubiquitous process in the solar system. In the early history of our Solar System, small rocky particles collided and accreted to form larger and larger bodies until they grew into the planets we see today. For the Earth, the last of these planetary-scale collisions was a giant impact that ultimately formed the Moon. More recently in solar system history, a catastrophic impact event caused a mass extinction on Earth, including the demise of the dinosaurs. Today, the impact of an asteroid or comet poses a real, if poorly understood threat to humanity. Impact craters are also used to study the solar system. Counts of the number of impact craters on a solid planetary surface are used to distinguish older from younger terrains, and the shapes of large craters provide clues to the near-subsurface structure of a planet. The catastrophic potential of impact cratering, and its far-reaching consequences, make it imperative to understand exactly how the impact process works. Central to this are fundamental equations that relate the size of an impact crater to properties of the impacting object (for example, size and velocity) and conditions on the target body (for example, gravity). In other words, equations that give the answer to the fundamental question: how large will the crater be if a given impactor strikes a given target surface? At present we cannot adequately answer this question, because the equations do not properly include all the important impactor and target-material properties. The first aim of this proposal is to better constrain the relationship between all the important variables in an impact event using computer models. As these equations are fundamental tools for estimating the consequences of impact, this work will advance understanding in many areas of planetary science. The major missing pieces in these impact equations are an understanding of how the growth of the crater is affected by pore space in the target, and the angle at which the impactor strikes the target. The effect of impact angle is important to establish because well-studied vertical impacts are much less likely to occur than impacts at an angle greater than 30 degrees to the vertical, about which far less is known. Impact angle is observed to affect the size and shape of the crater in laboratory impacts, but quantifying the effect in larger impacts can only be established through numerical modelling. Porosity is an important property of asteroids, comets and the near-surface of most Solar System bodies. It is known from laboratory experiments that target rocks with a high porosity reduce the volume of material expelled from the crater during growth, but these effects have not yet been properly quantified. The effect of porous compaction during impact, in particular, may have important implications for the evolution of the solar system. Planets grow by the collision of planetesimals. Quantifying the transfer of energy and momentum in such collisions is therefore of vital importance for understanding the thermal, chemical and physical evolution of the solar system. In most previous work and models, collisions during planetary growth were assumed to be exclusively low-velocity and/or between non-porous planetesimals. However, recent work suggests that early planetesimals had very high porosities (up to 80%), and the growth of planetary embryos would have stirred relative velocities between planetesimals to >1 km/s. Experiments and our own modelling work show that target porosity dramatically affects the consequences of impact: increasing heating and reducing ejected mass. A second aim of this work is therefore to use numerical impact simulations to quantify the effect of planetesimal porosity on heating, compaction and ejection during early planetesimal collisions and assess the implications of this for solar system evolution.