How Deep Do Impacts Probe Planetary Crusts?

The deeper levels of the targets of hypervelocity impacts are revealed in the inverse stratigraphy of ejecta and, in larger craters, in the peaks of central uplifts. However, formulae based on a generalization of observations at terrestrial craters that equate depth of origin (or stratigraphic uplift) to crater diameter may be misleading in several ways. Because sedimentary rocks are generally weaker than crystalline rocks, are more likely to have significant porosity and are more inclined to slip along pre-existing bedding planes, sedimentary rock craters generally collapse more completely than crystalline rock craters; in some cases bedding plane slip allows strata high in the sequence to close over lower rocks during central uplift. Thus the general formulae for depth of excavation based on observations of stratigraphic uplift give values that may be less than the actual depth excavated.

For craters formed in crystalline rocks the depth of excavation in complex craters can be derived from the maximum level of shock metamorphism in central peaks, assuming a specific rate of attenuation of the initial shock compression. The exponent of attenuation is generally taken as approximately -2 and does not vary significantly with crater size as long as the medium remain unchanged. However, the refraction wave that unloads and fragments the compressed target attenuates at a rate of -3 in small craters and even -4 or more in large terrestrial craters as tensile fracturing is inhibited by increased confining pressure. Thus large craters excavate less deeply than small craters relative to the shock imprint, the rocks of the central uplift are more strongly shocked and the proportion of fragmented material to impact melt decreases with size. Hence, by influencing the depth of fragmentation, gravity modulates the size of the window impacts open to planetary interiors.