Determination of physical properties of planetary sub-surface layers by artificial impacts and penetrometry

In recent years the exploration of planetary bodies by surface probes has entered a new phase of interest. The mechanical properties of the near-surface layers of cometary and planetary bodies, including their strength, texture and layering, are important parameters needed both for a proper physical understanding of these bodies and for the design of lander missions. The strength properties of such a surface can be determined either by measuring the penetration resistance encountered by a slowly penetrating tip (quasi-static penetrometry), or by impacting the surface with an artificial body whose mechanical properties and impact velocity are known. At low speeds (up to about 300 m s −1), it is often feasible to measure the force or deceleration during penetration (‘impact’ or ‘dynamic’ penetrometry). If the event is better described in terms of hypervelocity impact cratering than penetration, analysis of the resulting crater is performed instead. Other uses of artificial impacts include the generation of seismic waves to aid the calibration of seismometers and the formation of a crater and / or ejecta for analysis or sampling of the target body. Such methods feature in many recent, current and forthcoming planetary missions. Examples include: the Surface Science Package of the Huygens probe, the anchoring harpoon of the Rosetta Lander, various Mars penetrators and landers, NASA's Deep Impact mission and ESA's planned Mercury cornerstone mission BepiColombo. A short overview of missions (past, present, future) featuring penetrometry experiments or artificial impacts is given. The theory of impact penetration and methods to interpret deceleration profiles in terms of material strength are discussed and applied to data sets obtained from test shots performed with the Rosetta Lander anchoring harpoon.