The%20Two%20Faces%20of%20Iapetus

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The Two Faces of Iapetus

• The Brightness Contrast Explored
• Colleen Milbury

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Overview

• Discovery of Iapetus
• Pictures
• Observations
• Endogenic Models
• Exogenic Models
• Summary/Conclusions

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Discovery of Iapetus

• In Greek mythology Iapetus was a Titan, the son
  of Uranus, the father of Prometheus and Atlas and
  an ancestor of the human race.
• Discovered by Giovanni Cassini in 1671
• He said the following about Iapetus in 1673
• One part of Iapetus surface is not so
  capable of reflecting the light to us the light
  of the Sun which maketh it visible, as the other
  part is
• Cassini was able to observe Iapetus during
  western elongation, he could not detect it during
  eastern elongation.

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Pictures of Iapetus
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Table of Parameters
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Nice Diagram of Saturn System
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Observations

• Order of magnitude contrast
• Geometric albedo is 0.04-0.5
• Centered about apex of motion
• Low albedo inconsistent with silicates

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More Observations

• Low albedo (dark) material is Red (albedo
  increases at longer ?s)
• -Does not match C-type (carbonaceous) asteroid
  similar to Phoebe, only redder D-type asteroids
• Dark floored craters imply dark material is
  younger
• Absence of bright craters imply
• -deposit is thick (gt10 km), or
• -bright material darkened/buried with 100
  million year period

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Endogenic Model-Smith et al. 1982

• Site symmetry, Phoebe exogenic example
• Favor endogenic model
• Dark floored craters as strong endogenic evidence
• Site examples of other hemispheric alignments in
  solar system (Mars, Moon, and Io)

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Exogenic Model-Owen et al. 2000

• Use mixing models of pure substances
• Satisfied 1,3, and 4 with amphorus carbon and
  water-ice
• Turned to organic compounds to satisfy 2 and stay
  consistent with 4
• Conclude that component comes from Titans
  nitrogen rich atmosphere
• Titan impact that could have sprayed particles
  onto Hyperion and Iapetus and even produced
  Hyperion

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Exogenic Model-Buratti et al. 2001

• Three nights of measurement from Hale telescope
  on Palomar Mountain
• Three D-type asteroids and icy Saturn satellites
• Range is 0.33-0.92 µm

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Exogenic Model-Buratti et al. 2001

• F-normalized Fluxes
• IcyIcy Bodies
• IapIapetus
• DD-type asteroids
• A-coefficient (percent of material)

• Dont combine spectra of pure substance
• Use spectra of typical icy bodies, D-type
  asteroids

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Exogenic Model-Buratti et al. 2001

• Apply to albedo
• Conclude that dark side Iapetus created by
  collision of proto-Hyperion and D-type asteroid

p geometric albedo
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Exogenic Model-Buratti et al. 2001

• Hyperion - irregular shape and chaotic rotation
  imply it is remnant of larger body
• Also note discovery of 5 new satellites that
  appear to be created from this impact
• Most particles are accreted by Titan

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Summary and Conclusions

• Iapetus is somewhat of a rarity in the solar
  system
• Probably not of endogenic origin
• Probably not from Phoebe material
• Strong evidence of some type of exogenic origin
• Buratti et al. most likely

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References

• Buratti et al. (2001). High Resolution 0.33-0.92
  µm Spectra of Iapetus, Hyperion, Rhea, Dione, and
  D-Type Asteroids. Icarus, 155, 375-381.
• Morrison et al. (1974). The Two Faces of Iapetus.
  Icarus, 24, 157-171.
• Owen et al. (2000). Decoding the Domino. Icarus,
  149, 160-172.
• Smith et al. (1982). A New Look at the Saturn
  System. Science, Vol. 215, No. 4532, 504-537.
• Satellites. Edited by Joseph A. Burns, Mildred
  Shapley Matthews Tucson University of Arizona
  Press, c1986.
• Saturn. Edited by Tom Gehrels, Mildred Shapley
  Matthews. Tucson, Ariz. University of Arizona
  Press, c1984
• Soter, S. 1974. Brightness of Iapetus. Paper
  presented at IAU Colloq. 28, Cornell University,
  August 1974.
• Squyres et al. (1984). Voyager Photometry of
  Iapetus. Icarus, 59, 426-435.

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The End!
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The Facts
Inconsistencies

• Phoebe exogenic models (Soter 1974) - fact 4
• Endogenic models - fact 3
• Superficial layer of ice covering underlying dark
  surface - fact 5

Saturn book p. 853