The composition and origin of the Iapetus dark material

J. F. Bell, D. P. Cruikshank, M. J. Gaffey

Research output: Contribution to journalArticlepeer-review

62 Scopus citations


Telescopic observations, laboratory simulations, and photogeological studies have been conducted to investigate the composition of the dark material on Iapetus and the reasons for its peculiar distribution. The results of our earlier studies (D.P. Cruikshank, J.F. Bell, M.J. Gaffey, R.H. Brown, R. Howell, C. Beermen, and M. Rognstad, 1983, Icarus 53 90-104) are confirmed and extended. Improved telescopic spectra of leading and trailing hemispheres were obtained over the range 0.3-2.6 μm. A mixing model was used to correct the leading-side spectral data for the presence of regions of bright terrain at both poles. The resulting true dark-unit spectrum is much redder in the visible and near-IR than previous uncorrected spectra, but gradually flattens near 2.0 μm. This unusual spectrum is reproduced with an intimate mixture of simulated meteoritic organic polymers (10%) and hydrated silicates (90%) which corresponds to an extension of the compositional trends in known carbonaceous meteorites toward lower formation temperatures. Structures in the dark/bright transition zone visible in the Voyager 2 images are inconsistent with an eruptive origin for the dark material; this zone is probably the region of grazing impacts of dust spiraling in from Phoebe. The dark material is probably a native component of the original icy surface concentrated in a thin, devolatilized regolith produced by the Phoebe-dust bombardment. Similar "ultracarbonaceous" material dominates the surfaces of the D-type asteroids and may be the elusive nonice component in the surfaces of other icy satellites. An Iapetus-like bombardment regime may be the cause of the large hemispheric asymmetry in regolith properties on Callisto.

Original languageEnglish (US)
Pages (from-to)192-207
Number of pages16
Issue number2
StatePublished - Feb 1985
Externally publishedYes

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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