1. Lockheed Martin, Space Exploration Systems, MS S8110, PO Box 179, Denver, Colorado 80201, USA
2. Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, Colorado 80302, USA

Correspondence to: Edward B. Bierhaus¹ Correspondence and requests for materials should be addressed to E.B.B. (Email: edward.b.bierhaus@lmco.com).

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 comets¹. Estimates of the relative and absolute ages of geological units on these objects have been based on this assumption². 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^{3, 4, 5, 6}.

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