DETERMINING IMPACTOR CONDITIONS FROM OBLIQUE IMPACTS: INSIGHTS FROM EXPERIMENTS

The NASA Ames Vertical Gun Range enables impact experiments from 0.3 km/s to over 7 km/s and has provided critical insights for both designing missions (e.g., Deep Impact, LCROSS) and interpreting results, from pre-Apollo to today. These insights go beyond relating impactor variables to the sizes of craters or providing benchmarks. Instead, high-speed imaging and instruments allow exploring processes and isolating variables. When combined with the planetary record, they can stimulate new computational models. As an example, oblique impact experiments at the AVGR reveal key signatures that can be used to assess the size and nature of the impactor on planetary surfaces as well as the nature of the target (e.g., Schultz, 2007). Two key variables are impactor trajectory and size. During an oblique impact, the trajectory is expressed by a flow field that evolves with time (Anderson et al., 2004; Hermalyn and Schultz, 2011) and is mapped across the surface by the distribution of ejecta, e.g., zone of avoidance and curved uprange rays (Schultz, 2009).

Impactor trajectory is also reflected in the shape and morphology of the central uplift or pit. Laboratory experiments using solid targets reveal that central pits scale directly with impactor diameter regardless of impact angle, whereas crater diameter depends on energy/momentum and target strength. As a result, increasingly oblique impacts result in increasingly large central disruption zones (relative to the crater diameter). This can be understood if the central structure is controlled by the distance at which a critical pressure is reached, regardless of impact angle. Such a strategy accounts for the differences in central uplift shapes/sizes/morphologies on different planets, increasing inner-ring size with decreasing impact angle, and central uplift morphologies on small bodies.

Oblique impacts also can be used to determine impactor size. Laboratory and computational experiments reveal that impactor failure generates distinctive scours downrange and can be used to constrain the lateral dimensions of the impactor (Schultz and Crawford, 2016). Although this strategy only works for oblique impacts, comparisons with similar oblique impact craters on other planets not only allow extrapolations to higher angle trajectories but also testing scaling relations.