Secondary and sesquinary craters on Europa

Abstract We address impact cratering on Io and Europa, with the emphasis on the origin of small craters on Europa as secondary to the primary impacts of comets on Io, Europa, and Ganymede. In passing we also address the origin of secondary craters generated by Zunil, a well-studied impact crater on Mars that is a plausible analog to impact craters on Io. At nominal impact rates, and taking volcanic resurfacing into account, we find that there should be 1.3 impact craters on Io, equally likely to be of any diameter between 100 m and 20 km. The corresponding model age of Europa's surface is between 60 and 100 Ma. This range of ages does not include a factor three uncertainty stemming from the uncertain sizes and numbers of comets. The mass of basaltic impact ejecta from Io to reach Europa is found to meet or exceed the micrometeoroid flux as a source of rock-forming elements to Europa's ice crust. To describe impact ejecta in more detail we adapt models for impact-generated spalls and Grady–Kipp fragments originally developed by Melosh. Our model successfully reproduces the observed size-number distributions of small craters on both Mars and Europa. However, the model predicts that a significant fraction of the 200–500 m diameter craters on Europa are not traditional secondary craters but are instead sesquinary craters caused by impact ejecta from Io that had gone into orbit about Jupiter. This prediction is not supported by observation, which implies that high speed spalls usually break up into smaller fragments that make smaller sesquinary craters. Iogenic basalts are also interesting because they provide stratigraphic horizons on Europa that in principle could be used to track historic motions of the ice, and they provide materials suitable to radiometric dating of Europa's surface.

[1]  Harold F. Levison,et al.  Differential Cratering of Synchronously Rotating Satellites by Ecliptic Comets , 2001 .

[2]  Thomas J. Ahrens,et al.  Impact experiments in low-temperature ice , 1981 .

[3]  David Morrison,et al.  Satellites of Jupiter , 1982 .

[4]  P. Weissman.  The cometary impactor flux at the Earth , 2006, Proceedings of the International Astronomical Union.

[5]  H. Melosh,et al.  Martian Meteorite Launch: High-Speed Ejecta from Small Craters , 2002, Science.

[6]  Kevin R. Housen,et al.  Some recent advances in the scaling of impact and explosion cratering , 1987 .

[7]  Randolph L. Kirk,et al.  The rayed crater Zunil and interpretations of small impact craters on Mars , 2005 .

[8]  Kevin Zahnle,et al.  Cratering Rates in the Outer Solar System , 1999 .

[9]  Erik Asphaug,et al.  Pwyll Secondaries and Other Small Craters on Europa , 2001 .

[10]  A. Vickery,et al.  Size-velocity distribution of large ejecta fragments , 1984 .

[11]  Harold F. Levison,et al.  The Long-Term Dynamical Behavior of Short-Period Comets , 1993 .

[12]  L. Soderblom,et al.  Volcanic eruptions on Io - Implications for surface evolution and mass loss , 1982 .

[13]  Jean-François Molinari,et al.  Characteristic fragment size distributions in dynamic fragmentation , 2006 .

[14]  H. Melosh.  High-velocity solid ejecta fragments from hypervelocity impacts , 1987 .

[15]  K. Zahnle,et al.  Fates of satellite ejecta in the Saturn system , 2005 .

[16]  Michael H. Wong,et al.  Radiation effects on the surfaces of the Galilean satellites , 2004 .

[17]  K. Zahnle,et al.  Cratering rates on the Galilean satellites. , 1998, Icarus.

[18]  Henry B. Garrett,et al.  Energetic Ion and Electron Irradiation of the Icy Galilean Satellites , 2001 .

[19]  Paul M. Schenk,et al.  Ages and interiors: the cratering record of the Galilean satellites , 2007 .

[20]  C. Chapman,et al.  Cratering of planetary satellites. , 1986 .

[21]  J. Veverka,et al.  The physical characteristics of satellite surfaces , 1986 .

[22]  K. Zahnle,et al.  Orbital Evolution of Impact Ejecta from Ganymede , 2002 .

[23]  H. J. Melosh,et al.  Dynamic fragmentation in impacts: Hydrocode simulation of laboratory impacts , 1992 .

[24]  Alfred S. McEwen,et al.  THE IMPORTANCE OF SECONDARY CRATERING TO AGE CONSTRAINTS ON PLANETARY SURFACES , 2006 .

[25]  Clark R. Chapman,et al.  Secondary craters on Europa and implications for cratered surfaces , 2005, Nature.

[26]  H. Melosh.  Impact ejection, spallation, and the origin of meteorites , 1984 .

[27]  Alfred S. McEwen,et al.  The lithosphere and surface of Io , 2004 .

[28]  J. Wisdom,et al.  Symplectic maps for the N-body problem. , 1991 .

[29]  H. Melosh.  Ejection of rock fragments from planetary bodies , 1985 .

[30]  R. Jedicke,et al.  The size–frequency distribution of dormant Jupiter family comets , 2006, astro-ph/0603106.

[31]  K. Zahnle,et al.  Transfer of mass from Io to Europa and beyond due to cometary impacts , 2008 .

[32]  E. M. Shoemaker,et al.  Preliminary analysis of the fine structure of the lunar surface , 1965 .