The anatomy of fresh complex craters on the mid‐sized icy moons of Saturn and self‐secondary cratering at the rayed crater Inktomi (Rhea)

Abstract

AbstractCassini mapping of Saturn’s mid‐sized icy moons of well‐preserved complex craters in the 45–95 km size range provides insight into cratering processes at lower surface gravity and on icy targets. These craters are characterized by steep rim scarps, rugged hummocky floor deposits of curvilinear ridges and scarps, and rugged conical central peaks. Ponded impact melt or related deposits are not observed on any floor or ejecta units of any of these complex craters, indicating that melt production may be much lower than predicted. Mantling ejecta units drape over pre‐existing topography, grading into concentric zones of secondary craters at ~1 crater diameter from the rim, demonstrating that secondaries occur on mid‐sized icy planetary bodies. The largest and youngest bright‐ray system is the 49 km central peak crater Inktomi, target of a mapping campaign down to 34 m pixel scales. In addition to classical secondary craters up to 3 km, several hundred small craters <1 km in size form an unusual densely spaced cluster across the eastern floor and ejecta deposit. The terrain‐indiscriminate distribution of these eastern floor craters indicates they are not explosive volatile release pits, such as mapped on Vesta or Mars, but self‐secondary craters formed by the fallback of ejected blocks back into the crater itself during crater formation. Self‐secondaries likely formed by an irregular and stochastic but poorly understood impact process, such as spallation influenced by irregular surface topography. Self‐secondary craters of this type could influence the interpretation of crater counts on large fresh impact craters and basins elsewhere.