CRATER GROWTH MEASURED IN THE LABORATORY

Non-intrusive measurements of crater growth at low impact velocities (<300m/s) indicate that the rate at which a crater shape changes is at odds with the point source assumption of the scaling rules typically used to assess cratering on asteroids and planets. During such subsonic experiments (where the impact velocity is less than or equal to the target sound speed) the projectile does not fail. Instead, the projectile interacts with the target primarily via friction, thereby influencing the cratering process. At low subsonic impact velocities, slow frictional deceleration of the projectile yields a longer interaction time, causing slow dissipation of the curtain velocity as function of distance from the impact. Further, the transient crater formed when ejecta excavation ceases possesses a depth that is large relative to its diameter. At high subsonic impact speeds, rapid frictional deceleration of the projectile causes fast dissipation of the curtain velocity and a transient crater that is small relative to its diameter.

However, most planetary collisions occur as hypervelocity impacts, where the projectile velocity significantly exceeds the sound speed of the target and the projectile fails on impact. Schultz, (In Mercury, Eds. F. Vilas, C. R. Chapman and M. S. Mathews, p. 274-335, 1988) indicates that, much like in the subsonic experiments, the time a projectile interacts with the target could influence the cratering process, but for a different physical reason. Here, shock processes rather than friction control how and when the projectile transfers its energy to the target.

New results for hypervelocity impacts (>1km/s) obtained at the NASA Ames Vertical Gun Range are used to evaluate if and when the time of penetration is important. Easily broken, 1/4" pyrex projectiles are launched at various speeds into targets of fine grained, 220μm glass spheres identical to those employed in the subsonic experiments. Measurements allow the investigation of whether or not the time of projectile penetration controls the rates of crater growth and aspect ratio of the transient crater. We use the same non-intrusive laser sheet technique employed during the subsonic impacts. Changes in a laser profile centered at the impact are monitored in time using two high-speed (2000 f/s) cameras.