Phobos: Photometry and origin of dark markings on crater floors

Authors:
  • JGJ. Goguen
    J. Goguen
  • JVJ. Veverka
    J. Veverka
  • PTP. Thomas
    P. Thomas
  • TDT. Duxbury
    T. Duxbury
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Abstract
Viking Orbiter 1 close encounter pictures of Phobos reveal unusual dark patches on the floors of many craters. Photometry of these features indicates that they have a similar normal reflectance but a phase coefficient 30% larger than the average surface. These facts suggest that the "dark" material has a similar composition but a much rougher texture than the average surface of Phobos. By analogy with terrestrial impact, explosion and laboratory craters, the dark patches are interpreted as impact generated melt pools which remain visible on Phobos due to the low value of g and consequent small amount of fallback ejecta and slumping. Since blanketing by ejecta from subsequent impacts and micrometeoroid erosion should eventually obliterate such features, they should be more conspicuous in the fresher craters. Similar features should be visible on small asteroids and other low g bodies. However, they are not prominent on the surface of Deimos—possibly because most of the craters imaged at high resolution on the outer satellite appear to have been blanketed by several meters of fine‐grained material.