Secondary craters on Europa and implications for cratered surfaces

Abstract

For several decades, most planetary researchers have regarded the impact crater populations on solid-surfaced planets and smaller bodies as predominantly reflecting the direct ('primary') impacts of asteroids and comets. Estimates of the relative and absolute ages of geological units on these objects have been based on this assumption. Here we present an analysis of the comparatively sparse crater population on Jupiter's icy moon Europa and suggest that this assumption is incorrect for small craters. We find that 'secondaries' (craters formed by material ejected from large primary impact craters) comprise about 95 per cent of the small craters (diameters less than 1 km) on Europa. We therefore conclude that large primary impacts into a solid surface (for example, ice or rock) produce far more secondaries than previously believed, implying that the small crater populations on the Moon, Mars and other large bodies must be dominated by secondaries. Moreover, our results indicate that there have been few small comets (less than 100 m diameter) passing through the jovian system in recent times, consistent with dynamical simulations.

Automatic Detection of Secondary Craters and Mapping of Planetary Surface Age Based on Lunar Orbital Images

“…In contrast to primary craters, diameters of secondary craters are usually limited by the diameter of their parent craters. T he upper limit of diameters for secondary craters is typically around 5% of the parent crater, as assumed by (Bierhaus et al, 2001(Bierhaus et al, , 2005. T he secondary crater population predicted by these studies is proportional to the primary population observed in the small diameter range (McEwen and Bierhaus, 2006).…”

“…In contrast, secondary craters were formed by impacts of pieces of material ejected during much lar ger primary impacts (Bierhaus et al, 2005).…”

“…Similar to (Bierhaus et al, 2005), our algorithm generates a new uniformly distributed population which has the same density as the detected craters. Then Voronoi tessellation is applied to each population, and distribution parameters are calculated for each iteration of the simulation process.…”

“…In most studies, secondary craters are removed by hand manually. One of the previous attempts to develop a method for estimation of the secondary crater population was made by (Bierhaus et al, 2005).…”

“…T his method combines a Voronoi tessellation, a Monte Carlo simulation of a uniform distribution, and a one-tailed test of clustering which divides the detected craters into two groups based on the probability that t hey exhibit a non-uniform spatial distribution. A Monte Carlo method combined with a hierarchical clustering algorithm has been used by (Bierhaus et al, 2005). Voronoi tessellation has been used to detect non-uniformly distributed craters in the work of (Honda et al, 2014).…”