EXPERIMENTAL OBSERVATION AND COMPUTATIONAL ANALYSIS OF ELECTROSTATICALLY CHARGED IMPACT EJECTA

I have had the great privilege of knowing Pete Schultz as an advisor and mentor and as a colleague and collaborator. Here I will present work that started in my student days but continues to the present. Pete has been interested in lunar magnetic anomalies for a long time. This work grew from that.

In experiments performed with Pete at the NASA Ames Vertical Gun Range, electrostatic charge separation during hypervelocity impact has been characterized for a variety of impactor and target geometries. Early time ejecta are negatively charged and impact-generated plasma, late-time ejecta and materials left in the transient cavity are positively charged. Charge separation can lead to large electrostatic fields with implications for dust motion, especially in low gravity environments. The motion of charged ejecta can create transient magnetic fields with potential implications for paleomagnetism. The experiments demonstrate that total charge separation is a function of impactor kinetic energy with a near linear mass dependence and velocity dependence proportional to v^2.6. The mass dependence, especially, has significant implications for the production of magnetostatic fields during planetary-scale impacts. For example, crater scaling relations can be used to extrapolate from laboratory scale to a 100 m impact crater on the Moon, predicting a ~10¹⁰ V/m electric field and ~10^-4T magnetic field (comparable to Earth’s surface field) lasting for ~0.05 s at the crater rim. If this scaling estimate is true, the electric field is large enough to significantly perturb the motion of micron-sized dust particles even in high gravity environments. In lower gravity environments, lower field strengths from smaller impacts would have a similar effect. Computational studies using the CTH hydrocode have shown that a simple two dimensional model based on electrostatic probe theory can match the experimentally observed charge separation. We are extending the CTH model to three dimensions to refine our estimates of charge separation during planetary impacts and to directly simulate its implications for ejecta transport and paleomagnetism.

Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.