How laboratory hypervelocity impact experiments have helped us to understand comet dust samples: a brief review.

In the last two decades, experimental hypervelocity impacts (HVI) using light gas gun (LGG) shots have answered numerous questions about how comet dust can be captured, and have repeatedly provided explanations for phenomena encountered during study of samples returned from comet Wild 2 by the NASA JPL-Caltech Stardust mission. LGG experiments were carried out in several laboratories, especially in NASA, in Japan, and at the University of Kent (Canterbury, UK). Analogue materials were produced for testing and calibration of novel and diverse microanalysis methods in research institutions around the world. Impact tracks on low density silica aerogel and craters on Al alloy foils gave calibration in determination of size and composition for Wild 2 particles, and experimental HVI features revealed how internal grain size and structure of comet dust grains can be interpreted from detailed shapes of impact structures. Firing of analogue mineral materials helped us to understand how specific mineral components are preserved, how crystal structure and composition are altered during capture, and how this may limit interpretation of collected grains. In this review, I explain the range of studies performed so far, and suggest new experiments are needed to help understand: preservation, alteration and loss of subtle internal grain structures; modification of elemental and isotopic signatures in relatively fragile materials (e.g. organic matter); and the size of particles making bulbous aerogel tracks.