Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Secondary craters on Europa and implications for cratered surfaces


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 simulations3,4,5,6.

This is a preview of subscription content, access via your institution

Access options

Buy this article

Prices may be subject to local taxes which are calculated during checkout

Figure 1: A sample region on Europa that demonstrates the extreme ‘clusteredness’ of Europa's small craters.
Figure 2: A comparison of europan and lunar crater size–frequency distributions, demonstrating that lunar secondaries could account for most of the observed small lunar craters.

Similar content being viewed by others


  1. Basaltic Volcanism Study Project. In Basaltic Volcanism on the Terrestrial Planets (eds Kaula, W. M. et al.) Ch. 8 (Pergamon, New York, 1981)

    Google Scholar 

  2. Shoemaker, E. M., Hackman, R. J. & Eggleton, R. E. Interplanetary correlation of geologic time. Adv. Astronaut. Sci. 8, 70–89 (1963)

    Google Scholar 

  3. Zahnle, K., Schenk, P. & Levison, H. Cratering rates in the outer Solar System. Icarus 163, 263–289 (2003)

    Article  ADS  Google Scholar 

  4. Zahnle, K., Dones, L. & Levison, H. Cratering rates on the Galilean satellites. Icarus 136, 202–222 (1998)

    Article  ADS  CAS  Google Scholar 

  5. Levison, H. F. et al. Planetary impact rates from ecliptic comets. Icarus 143, 415–420 (2000)

    Article  ADS  Google Scholar 

  6. Levison, H. F. & Duncan, M. J. From the Kuiper Belt to Jupiter-family comets: the spatial distribution of ecliptic comets. Icarus 127, 13–32 (1997)

    Article  ADS  Google Scholar 

  7. Shoemaker, E. M. Ballistics of the Copernican ray system. Proc. Lunar Planet. Colloq. 2, 7–21 (1960)

    Google Scholar 

  8. Neukum, G., Ivanov, B. A. & Hartmann, H. K. Cratering records in the inner solar system in relation to the lunar reference system. Space Sci. Rev. 96, 55–86 (2001)

    Article  ADS  Google Scholar 

  9. Fielder, G. Ray elements and secondary-impact craters on the Moon. Astrophys. J. 135, 632–637 (1962)

    Article  ADS  Google Scholar 

  10. Roberts, W. A. Secondary craters. Icarus 3, 348–364 (1964)

    Article  ADS  Google Scholar 

  11. Kopal, Z. The nature of secondary craters photographed by Ranger VII. Icarus 5, 201–213 (1966)

    Article  ADS  Google Scholar 

  12. Lucchitta, B. K. Crater clusters and light mantle at the Apollo 17 site—A result of secondary impacts from Tycho. Icarus 30, 80–96 (1977)

    Article  ADS  Google Scholar 

  13. Shoemaker, E. M. Preliminary analysis of the fine structure of the lunar surface. In Ranger VII, Part II, Experimenters' Analyses and Interpretations 75–134 (Tech. Rep. No. 32–700, Jet Propulsion Laboratory, Pasadena, 1965)

    Google Scholar 

  14. Vickery, A. M. Variation in ejecta size with ejection velocity. Geophys. Res. Lett. 14, 726–729 (1987)

    Article  ADS  Google Scholar 

  15. Vickery, A. M. Size-velocity distribution of large ejecta fragments. Icarus 67, 224–236 (1986)

    Article  ADS  Google Scholar 

  16. Bierhaus, E. B., Chapman, C. R. & Merline, W. J. On the clustering of Europa's small craters. Lunar Planet. Sci. Conf. XXXII, abstr. no. 1967 (2001)

  17. Bierhaus, E. B. Discovery that Secondary Craters Dominate Europa's Small Crater Population PhD thesis, Univ. Colorado at Boulder (2004)

    Google Scholar 

  18. Duda, R. O. & Hart, P. E. Pattern Classification and Scene Analysis (Wiley and Sons, New York, 1973)

    MATH  Google Scholar 

  19. Bierhaus, E. B., Chapman, C. R., Merline, W. J., Brooks, S. M. & Asphaug, E. Pwyll secondaries and other small craters on Europa. Icarus 153, 264–276 (2001)

    Article  ADS  Google Scholar 

  20. Duncan, M., Levison, H. & Dones, L. in Comets II (eds Festou, M. C., Keller, H. U. & Weaver, H. A.) 193–204 (Univ. Arizona Press, Tucson, 2004)

    Google Scholar 

  21. Berstein, G. M. et al. The size distribution of trans-Neptunian bodies. Astron. J. 128, 1364–1390 (2004)

    Article  ADS  Google Scholar 

  22. Schenk, P., Chapman, C. R., Zahnle, K. & Moore, J. M. in Jupiter (eds Bagenal, F., Dowling, T. E. & McKinnon, W. B.) 427–456 (Cambridge University Press, Cambridge, 2004)

    Google Scholar 

  23. Kato, M. et al. Ice-on-ice impact experiments. Icarus 113, 423–441 (1995)

    Article  ADS  Google Scholar 

  24. Arakawa, M. et al. Ejection velocity of ice fragments. Icarus 118, 341–354 (1995)

    Article  ADS  Google Scholar 

  25. Kato, M. et al. Shock pressure attenuation in water ice at a pressure below 1 GPa. J. Geophys. Res. 106, 17567–17578 (2001)

    Article  ADS  Google Scholar 

  26. Ivanov, B. A., Neukum, G., Bottke, W. F. & Hartmann, W. K. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 89–101 (Univ. Arizona Press, Tucson, 2002)

    Google Scholar 

  27. Hartmann, W. K. Martian cratering VI. Crater count isochrons and evidence for recent volcanism from Mars Global Surveyor. Meteorit. Planet. Sci. 34, 167–177 (1999)

    Article  ADS  CAS  Google Scholar 

  28. Hauber, E. et al. Discovery of a flank caldera and very young glacial activity at Hecates Tholus, Mars. Nature 434, 356–361 (2005)

    Article  ADS  CAS  Google Scholar 

  29. Murray, J. B. et al. Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator. Nature 434, 353–356 (2005)

    ADS  Google Scholar 

  30. McEwen, A. S. et al. The rayed crater Zunil and interpretations of small impact craters on Mars. Icarus 176, 351–381 (2005)

    Article  ADS  Google Scholar 

Download references


E.B.B. was previously at the University of Colorado at Boulder and Southwest Research Institute, Boulder. NASA's Galileo and JSDAP programs funded the research reported here. We thank B. Ivanov for providing comments that improved the clarity and structure of this manuscript.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Edward B. Bierhaus.

Ethics declarations

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Rights and permissions

Reprints and permissions

About this article

Cite this article

Bierhaus, E., Chapman, C. & Merline, W. Secondary craters on Europa and implications for cratered surfaces. Nature 437, 1125–1127 (2005).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing