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The Two Faces of IapetusThe Two Faces of Iapetus

The Brightness Contrast Explored

Colleen Milbury

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 (>10 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 Titan’s 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

• Don’t combine spectra of pure substance

• Use spectra of typical icy bodies, D-type asteroids

•F-normalized Fluxes•Icy=Icy Bodies•Iap=Iapetus•D=D-type asteroids•A-coefficient (percent of material)

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p = geometric albedo

Exogenic Model-Buratti et al. 2001

• Apply to albedo

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

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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

• 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

Inconsistencies