12/2/04ESS 298 Fall The Two Faces of Iapetus The Brightness Contrast Explored Colleen Milbury The Brightness Contrast Explored Colleen Milbury

12/2/04ESS 298 Fall Overview Discovery of Iapetus Pictures Observations Endogenic Models Exogenic Models Summary/Conclusions

12/2/04ESS 298 Fall 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.

12/2/04ESS 298 Fall Pictures of Iapetus

12/2/04ESS 298 Fall 20045

12/2/04ESS 298 Fall Table of Parameters

12/2/04ESS 298 Fall Nice Diagram of Saturn System

12/2/04ESS 298 Fall Observations Order of magnitude contrast Geometric albedo is Centered about apex of motion Low albedo inconsistent with silicates

12/2/04ESS 298 Fall 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

12/2/04ESS 298 Fall Endogenic Model-Smith et al 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)

12/2/04ESS 298 Fall Exogenic Model-Owen et al 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

12/2/04ESS 298 Fall Exogenic Model-Buratti et al Three nights of measurement from Hale telescope on Palomar Mountain Three D-type asteroids and icy Saturn satellites Range is µm

12/2/04ESS 298 Fall Exogenic Model-Buratti et al 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)

12/2/04ESS 298 Fall p = geometric albedo Exogenic Model-Buratti et al Apply to albedo Conclude that dark side Iapetus created by collision of proto- Hyperion and D- type asteroid

12/2/04ESS 298 Fall Exogenic Model-Buratti et al 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

12/2/04ESS 298 Fall 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

12/2/04ESS 298 Fall References Buratti et al. (2001). High Resolution µm Spectra of Iapetus, Hyperion, Rhea, Dione, and D-Type Asteroids. Icarus, 155, Morrison et al. (1974). The Two Faces of Iapetus. Icarus, 24, Owen et al. (2000). Decoding the Domino. Icarus, 149, Smith et al. (1982). A New Look at the Saturn System. Science, Vol. 215, No. 4532, 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 Brightness of Iapetus. Paper presented at IAU Colloq. 28, Cornell University, August Squyres et al. (1984). Voyager Photometry of Iapetus. Icarus, 59,

12/2/04ESS 298 Fall The End!

12/2/04ESS 298 Fall 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