Microstructural penetrometry of asteroid regolith analogues and Titan’s surface

Abstract

We investigate essential aspects of penetrometer design required to measure particle properties on asteroids using a combination of laboratory analogue regoliths and spaceflight data returned by the Huygens probe from Titan. Penetrometry in granular material is complicated due to multiple and interdependent mechanical processes that occur during penetration. A numerical impact model is developed that simulates the behaviour of a penetrometer and its force sensor in a granular medium. The model is based on the Huygens ACC-E instrument that successfully returned penetrometry data from the surface of Titan. Penetrometry measurements are made in analogue asteroid regoliths using a laboratory copy of ACC-E. The average particle size in the targets ranged from 0.1 to 0.9 of the penetrometer tip diameter. To describe the structure seen in the data a number of metrics are defined to characterise the peaks seen in the returned signal. The significance of the variation of the metrics (such as peak height or width) with particle mass and radius are analysed in terms of penetrometer properties such as impact velocity, elastic properties and data logging parameters.We find the penetrometer can be used to measure average particle radius and mass adequately for a mid-range of particle radii. Electronic noise effects mostly the results from very small and very large particles. For high mass particles there is evidence that particle–particle impacts, within the target are being felt by the tip that make any straight forward interpretations using peak frequency a challenge. Using our numerical model the Huygens penetrometry data from Titan is analysed. A particle diameter, of around half a centimetre, is found to be consistent with the penetrometry data returned by Huygens. Recommendations and lessons learned, regarding data interpretation techniques are made for asteroid penetrometry (or any other extraterrestrial surface) when using this instrument.