撞击过程和内太阳系撞击历史

Impact cratering and impact history of the inner Solar System

在太阳系的形成和演化过程中,发生在天体物质间的撞击作用是最重要的地质过程之一。撞击构造是地外天体表面最常见的地貌单元,大部分天体的地貌演化主要受撞击作用控制。撞击过程产生的温度、压力和应变速率比岩石圈内的其他地质过程高多个数量级,形成广泛分布的撞击产物,如气化物、熔融物、冲击变质和变形等。虽然撞击过程转瞬即逝,撞击作用向天体注入能量并改变其内、外结构,对天体的圈层系统产生长远影响。持续撞击在天体表面累积了大量的撞击坑,撞击坑的空间分布反映了受外来撞击的历史。内太阳系在~3.8 Ga前的撞击频率更高,但是大量撞击盆地是否灾变式的密集形成仍在持续争议;~3.8 Ga以来的撞击频率趋于稳定,但是缺乏具有明确事件指代性的标定样品。在同一天体上,撞击坑的空间密度指示了相应地质单元的形成时间,因此撞击坑统计常被用于估算地外天体表面地质单元的相对年龄。基于月球软着陆探测任务返回的样品,前人已约束了不同直径的月球撞击坑的形成频率,进而建立了使用撞击坑统计估算月球表面地质单元的绝对模式年龄的方法。另外,内太阳系天体可能经历了相似的撞击历史,因此地-月系统的撞击频率已被缩放至其他类地行星。撞击坑统计是探索太阳系天体的撞击历史、遥估地外天体表面的相对和绝对年龄的主要方法,也是行星地质研究的基本工具。该方法的整体可靠性已得到大量实验的验证。同时,该方法在理论基础和技术细节上还存在大量的不确定性。修正该方法是完善太阳系撞击历史的重要研究内容,也是未来采样返回探测任务的重要科学目标。

Collisions between celestial materials are a fundamental process in the formation and evolution of our Solar System. Impact structures are the most common landform on Solar System bodies that have solid surfaces. On most planetary bodies, impact cratering has long been the dominant process modifying surface topography. Compared with the other geological processes that occur in lithospheres of planetary bodies, impact cratering features temperatures, pressure, and strain rate that are higher by several orders of magnitude, resulting in widespread vaporization, melting, metamorphism, and deformation. Impact cratering is a transient process, but it changes the interior and exterior structures of planetary bodies by injecting energy and can profoundly affect the evolution of planetary systems. Due to continuous impact cratering, impact craters have accumulated on planetary bodies since their birth, so the spatial density of impact craters reflects the impact history. In the inner Solar System, the impact flux was much higher at ~3.8 Ga, but it has long been debated whether many impact basins were formed catastrophically within a short time. The impact flux has been more-or-less constant since ~3.8 Ga, but calibration samples that have unambiguous geological affiliations are lacking. On a given planetary body, the spatial density of impact craters can be used to evaluate relative ages of different geological units. Samples recovered from the Moon have been used to reconstruct the formation rates of different-sized impact craters, so crater statistics can be used to estimate absolute model ages for lunar surfaces. Meanwhile, the Moon and terrestrial planets may have experienced the same impact flux, so formation rates of lunar craters can be translated to the other bodies based on assumed ratios of impact flux and cratering scaling laws. As a basic technique of planetary geology, crater statistics has been a major method in the study of impact history and remote estimation of relative and absolute model ages for planetary surfaces. While the reliability of this technique has been verified in many trial tests, both the theoretical basis and technical details of this technique have uncertainties. Calibrating this technique is an important target in both sample return missions and planetary science.