Impact-generated seismic signals on Mars

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.