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Icy Satellites of Saturn: Impact Cratering and Age Determination

  • Luke Dones
  • Clark R. Chapman
  • William B. McKinnon
  • H. Jay Melosh
  • Michelle R. Kirchoff
  • Gerhard Neukum
  • Kevin J. Zahnle

Abstract

Saturn is the first giant planet to be visited by an orbiting spacecraft that can transmit large amounts of data to Earth. Crater counts on satellites from Phoebe inward to the regular satellites and ring moons are providing unprecedented insights into the origin and time histories of the impacting populations. Many Voyager-era scientists concluded that the satellites had been struck by at least two populations of impactors. In this view, the Population I im-pactors, which were generally judged to be “comets” orbiting the Sun, formed most of the larger and older craters, while Population II impactors, interpreted as Saturn-orbiting ejecta from impacts on satellites, produced most of the smaller and younger craters. Voyager data also implied that all of the “ring moons,” and probably some of the mid-sized classical moons, had been catastrophically disrupted and reac-creted since they formed. We examine models of the primary impactor populations in the Saturn system. At the present time, “ecliptic comets,” which likely originate in the Kuiper Belt/Scattered Disk, are predicted to dominate impacts on the regular satellites and ring moons, but the models require extrapolations in size (from the observed Kuiper Belt Objects to the much smaller bodies that produce the craters) or in distance (from the known active Jupiter family comets to 9.5 AU). Phoebe, Iapetus, and perhaps even moons closer to Saturn have been struck by irregular satellites as well. We describe the Nice model, which provides a plausible mechanism by which the entire Solar System might have experienced an era of heavy bombardment long after the planets formed. We then discuss the three cratering chronologies, including one based upon the Nice model, that have been used to infer surface ages from crater densities on the saturnian satellites. After reviewing scaling relations between the properties of impactors and the craters they produce, we provide model estimates of the present-day rate at which comets impact, and catastrophically disrupt, the satur-nian moons. Finally, we present crater counts on the satellites from two different groups. Many of the heavily cratered terrains appear to be nearly saturated, so it is difficult to infer the provenance of the impactors from crater counts alone. More large craters have been found on Iapetus than on any other satellite. Enceladus displays an enormous range of surface ages, ranging from the old mid-latitude plains to the extremely young South Polar Terrain. Cassini images provide some evidence for the reality of “Population II”. Most of the observed craters may have formed in one or more “cataclysms,” but more work is needed to determine the roles of heliocentric and planetocentric bodies in creating the craters.

Keywords

Impact Rate Giant Planet Astronomical Unit Kuiper Belt Outer Solar System 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

We thank Nico Schmedemann, Amy Barr, Beau Bierhaus, Bill Bottke, Hal Levison, Alessandro Morbidelli, David Nesvorný, and David Vokrouhlický for discussions, and the editors for their patience. This research was supported by grants from the Cassini Data Analysis Program to LD, MRK, and WBM.

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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Luke Dones
    • 1
  • Clark R. Chapman
    • 1
  • William B. McKinnon
    • 2
  • H. Jay Melosh
    • 3
    • 4
  • Michelle R. Kirchoff
    • 5
    • 6
  • Gerhard Neukum
    • 7
  • Kevin J. Zahnle
    • 8
  1. 1.Southwest Research InstituteBoulderUSA
  2. 2.Washington University in Saint LouisSt. LouisUSA
  3. 3.University of ArizonaTucsonUSA
  4. 4.Now at Purdue UniversityWest Lafayette
  5. 5.Lunar and Planetary InstituteHoustonUSA
  6. 6.Now at Southwest Research InstituteBoulderUSA
  7. 7.Freie UniversitätBerlinGermany
  8. 8.NASA Ames Research CenterMoffett FieldUSA

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