The effect of ambient pressure and impactor geometry on low speed penetration of unconsolidated materials

The dynamic behaviour of projectiles impacting planetary surfaces can be measured to derive mechanical properties of the target material. Several such dynamic penetrometers will be used on Mars, the Moon, Titan, and a comet nucleus. However, solid bodies in the Solar System exhibit a wide range of surface atmospheric and gravitational conditions and previous workers have shown that changes in ambient pressure or gravity can significantly alter penetration dynamics. This presents a challenge for the terrestrial calibration of penetrometers. We have applied a penetration model to low speed impacts in air and vacuum, with the aim of quantifying any differences in a target's measured properties. A 1.05 kg instrumented penetrometer was dropped onto two cohesionless granular materials at speeds of around 2.7 m s(-1). The apparatus was located in a vacuum chamber, allowing tests to be made at low pressures. An initial upper limit for the mean deviatoric stress (a measure of material strength) was found in each case by dividing the gravitational potential energy lost (during the penetrometer's fall and penetration) by the volume penetrated. This value can be reduced using the projectile's recorded deceleration and a penetration model that includes friction and dynamic resistance. Good fits between the recorded and modelled deceleration were obtained for a range of values of dynamic drag coefficient and coefficient of friction. Initial comparison of the air and vacuum drops performed so far suggests behaviour consistent with that described by previous workers, namely that pore pressure aids penetration in loose materials but inhibits penetration in heavily compacted materials, and that these effects are larger for smaller grain sizes.