The importance of secondary cratering to age constraints on planetary surfaces

Alfred S. McEwen, Edward B. Bierhaus

Research output: Contribution to journalReview articlepeer-review

237 Scopus citations


Small craters (less than one kilometer diameter) can be primary craters produced by impact of interplanetary debris, or they can be secondary craters produced by fallback of high-velocity ejecta blocks from much larger but infrequent primary impacts. The prevalent assumption over recent decades has been that primaries are most abundant, so most small craters are independent random events and can be used for dating. However, recent results from Europa and Mars support the early theory that distant secondaries globally dominate the number of small lunar craters; this would invalidate part of production functions that have been widely used for age dating. Crater excavation results in higher mean ejection velocities for smaller fragments, resulting in a steeper size-frequency distribution for secondary craters than is produced by the same size-frequency distribution of interplanetary debris. This review also discusses how small craters can sometimes be used to derive meaningful age constraints.

Original languageEnglish (US)
Pages (from-to)535-567
Number of pages33
JournalAnnual Review of Earth and Planetary Sciences
StatePublished - 2006


  • Asteroids
  • Chronology
  • Europa
  • Impact processes
  • Mars
  • Moon

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Earth and Planetary Sciences (miscellaneous)
  • Space and Planetary Science


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