小行星的物理性质与结构演化研究进展

Physical properties and structural evolution of asteroids

小行星上蕴含着丰富的稀有矿物资源,并保存着太阳系形成初期的原始成分,是研究太阳系起源和演化历史的活化石,具有极大的科学研究价值.随着近二十年来小行星探测热潮的兴起,对小行星的研究已从轨道、形状、旋转状态和光谱类型扩展到结构演化机制和外力响应特性.小行星碎石堆结构概念的提出,为所观测到小行星的物理性质和天文现象提供了合理的解释,同时也对小行星探测任务及相关研究提出了诸多挑战.本文从小行星的轨道和物理性质分布出发,分析了小行星结构的演化过程,阐明了大部分等效直径大于300m的小行星应具有碎石堆结构的必然性,并结合颗粒材料的力学性质分析了碎石堆小行星的结构特点和动力学响应特性,对碎石堆小行星的研究现状进行了综述,讨论了其中的科学问题和研究方向.

Asteroids are thought to harbor an abundance of valuable mineral resources, and the original materials that were present in the early solar system. The information contained in these small bodies could provide us with critical clues about how the planets formed as well as life started on the Earth. With the help of observations and the data returned by several successful asteroid missions, the progression of asteroid research has been expanded from understanding the asteroids’ orbits, shapes, rotation states and spectral classes to exploring more details about their internal structures and how they evolve in response to external effects. Among all the possible forms of asteroids’ structures, the rubble-pile structure provides an interesting insight about how asteroids’ structures evolve and a reasonable explanation for some observations. However, the discrete and low-strength characteristics of this structure raise many open questions to understanding its evolution, identifying its physical properties, and designing future asteroid missions. Based on the orbital distribution and the observed physical properties of asteroids, this study attempts to draw a whole picture on how asteroids’ structure evolves and demonstrates the reasons why most asteroids above 300 m size scale should possess rubble-pile structures. From the perspective of the physics of granular mechanics, a review of progress in investigating the structural characteristics and the dynamical evolution mechanisms of rubble-pile asteroids is present. A thorough comparative analysis on the efforts made by different methods shows that, theoretical analyses and laboratory experiments suffer from the limitation of applications and high economic cost, respectively, while numerical modeling techniques can efficiently serve many purposes. Among all the numerical methods, the discrete element method can retain the discrete nature of rubble-pile structures and can be used to analyze both static and dynamic processes. Some basic problems and potential research interests related to the application of discrete element method on rubble-pile asteroids are suggested.