SCALING IMPACT MELTING AND CRATER DIMENSIONS - IMPLICATIONS FOR THE LUNAR CRATERING RECORD

The dimensions of large craters formed by impact are controlled to a l arge extent by gravity, whereas the volume of impact melt created duri ng the same event is essentially independent of gravity. This ''differ ential scaling'' fosters size-dependent changes in the dynamics of imp act-crater and basin formation as well as in the final morphologies of the resulting structures. A variety of such effects can be observed i n the lunar cratering record, and some predictions can be made on the basis of calculations of impact melting and crater dimensions. Among t hem are the following: (1) as event magnitude increases, the volume of melt created relative to that of the crater will grow, and more will be retained inside the rim of the crater or basin. (2) The depth of me lting will exceed the depth of excavation at diameters that essentiall y coincide with both the inflection in the depth-diameter trend and th e simple-to-complex transition. (3) The volume of melt will exceed tha t of the transient cavity at a cavity diameter on the order of the dia meter of the Moon; this would arguably correspond to a Moon-melting ev ent. (4) Small lunar craters only rarely display exterior flows of imp act melt because the relatively small volumes of melt created can beco me choked with clasts, increasing the melt's viscosity and chilling it rapidly. Larger craters and basins should suffer little from such a p rocess. (5) Deep melting near the projectile's axis of penetration dur ing larger events will yield a progression in central-structure morpho logy; with growing event magnitude, this sequence should range from si ngle peaks through multiple peaks to peak rings. (6) The minimum depth of origin of central-peak material should coincide with the maximum d epth of melting; the main central peak in a crater the size of Tycho s hould have had a preimpact depth of close to 15 km.