Impact-generated seismic signals on Mars

NASA计划于2018年发射的“洞察号”任务将在火星上放置一个地震仪，以探测陨石撞击和火星地震产生的地震波。地震信号，微小的地面运动，被记录为地震仪，将被用来确定今天火星表面的活跃程度和目前陨石撞击的速度。地震图所包含的信息远远超过其成因、陨石撞击或地震；它们也记录了行星的结构。深层岩石的化学性质和温度会影响地震波穿过岩石的速度，因此测量地震能量到达的时间可以像医学断层扫描中的x射线一样用于绘制行星的内部结构。火星和地球一样，可能有分层结构（例如，地壳、地幔、地核），这些边界也将被地震探测到，而不是像一个遥远的结构地质学工具。要实现洞察号的科学目标，关键是要很好地理解任务期间火星上的小陨石撞击所产生的地震信号。然而，人们对陨石撞击产生的地震波的特征知之甚少，这在很大程度上是因为在地球上形成陨石坑的撞击很少；小行星被地球厚厚的大气层挡住了。当小行星撞击行星表面时，它的大部分能量被加热、破裂和移动表面的岩石，形成一个陨石坑。一小部分能量表现为地震波，但确切的比例——撞击的地震效率——并不为人所知，很可能取决于小行星和表面的特性。了解地震效率，以及从撞击地点传播很远的地震波的其他特征，对于解释“洞察号”地震仪记录的数据，以及区分撞击产生的地震信号和火星地壳断层产生的地震信号至关重要。陨石撞击和它们产生的地震波可以在实验室和电脑上复制。计算机模拟的一个优点是，可以研究在实验室中无法实现的真实撞击条件，例如超过7km/s的撞击速度。但是计算机模型必须根据实验数据进行校准。该项目将采用一个已经经过实验验证的数值模型来调查和量化撞击产生的地震波。这些模型将建立撞击产生的地震波的效率、特征和振幅衰减，用于一系列小行星和表面特性，包括小行星的大小、速度和组成，以及“洞察号”着陆点火星表面的强度、孔隙度和组成。除了帮助科学家解释“洞察号”记录的地震信号外，该项目的结果还将提供信息：了解地面震动对地球的潜在影响；太阳系中小天体的表面如何被撞击引起的地面震动所改变；以及未来对阿波罗任务期间获得的月球地震记录的重新分析。

InSight, a NASA mission due for launch in 2018, will place a seismometer on Mars to detect seismic waves produced by meteorite impacts and marsquakes. The seismic signals, tiny ground movements, recorded as seismograms will be used to determine how active the surface of Mars is today and the present rate of meteorite strikes. Seismograms contain much more information than their cause, meteorite impact or marsquake; they document planetary structure as well. The chemistry and temperature of rocks at depth affects the speed at which seismic waves travel through them so measuring arrival-times of seismic energy can be used like X-rays in medical tomography, to map the internal structure of a planet. Mars, like Earth, likely has a layered structure (e.g., crust, mantle, core), and these boundaries too will be detected seismically, rather like a remote structural geology tool. Key to achieving InSight's science goals is a good understanding of the seismic signals produced by small meteorite impacts on Mars that will occur during the mission. However, the characteristics of seismic waves generated by meteorite impacts are poorly understood, in large part because crater-forming impacts on Earth are rare; small asteroids are blocked by Earth's thick atmosphere. When an asteroid strikes a planetary surface, most of its energy is expended heating, fracturing and moving the surface rocks to form a crater. A tiny proportion of the energy manifests as seismic waves, but the exact proportion-the seismic efficiency of an impact-is not well known and is likely to depend on properties of the asteroid and surface. Knowledge of the seismic efficiency, as well as other characteristics of the seismic waves that travel large distances from the impact site, is crucial for interpreting data recorded by the InSight seismometer and distinguishing seismic signals generated by impacts from those generated by faulting in Mars' crust. Meteorite impacts and the seismic waves they produce can be replicated in the laboratory and using a computer. An advantage of computer simulation is that realistic impact conditions that are not practical to achieve in the laboratory, such as impact speeds exceeding 7km/s, can be investigated. But computer models must be calibrated against experimental data. This project will employ a numerical model that is already well tested against experiment to investigate and quantify seismic waves generated by impacts. The models will establish the efficiency, character and amplitude decay of impact-generated seismic waves for a range of asteroid and surface properties, including asteroid size, speed and composition, as well as the strength, porosity and composition of the surface of Mars at the InSight landing site. As well as helping scientists to interpret the seismic signals recorded by InSight, results from this project will also inform: understanding of the ground shaking hazard potential of impacts on Earth; how the surfaces of small bodies in the Solar System are modified by impact-induced ground shaking; and future reanalysis of lunar seismic records acquired during the Apollo missions.