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The Moons of Saturn: Tethys, Rhea, & Iapetus

Joel Schwartz Northwestern Physics & Astronomy May 13 th , 2013. The Moons of Saturn: Tethys, Rhea, & Iapetus. Voyager 1 Image. Saturn. Nov 1980. LPI. 3 May 2013. Moons of Solar System. NASA SSE. 3 May 2013. Discovery. All three satellites were first observed by Giovanni Cassini.

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The Moons of Saturn: Tethys, Rhea, & Iapetus

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  1. Joel Schwartz Northwestern Physics & Astronomy May 13th, 2013 The Moons of Saturn:Tethys, Rhea,& Iapetus Voyager 1 Image. Saturn. Nov 1980. LPI. 3 May 2013.

  2. Moons of Solar System. NASA SSE. 3 May 2013.

  3. Discovery • All three satellites were first observed by Giovanni Cassini. • Iapetus: October 25th, 1671 • Rhea: December 23rd, 1672 • Tethys: March 21st, 1684 • These moons & Dione were originally called Sidera Lodoicea (The Stars of Louis) by Cassini, after Louis XIV.

  4. Orbital Locations Orbital Diagram of Saturn Moons. 2004. U. Cincinnati Physics- Gas Giants. 4 May 2013.

  5. Etymology • Associated with siblings of Kronos, as suggested by John Herschel. • Tethys: Sister of Kronos & wife of Oceanus. Linked to rivers & water. • Rhea: Wife/Sister of Kronos. Referred to as the “Mother of the Gods.” • Iapetus: Brother to Kronos; father of Atlas & Prometheus. Considered the “Father of the Human Race” by the ancient Greeks.

  6. Greek Titans Greek Titans (Various). Wikipedia. 4 May 2013

  7. Previous/Current Space Missions • Pioneer 11: August-September 1979 • Voyager 1: November 1980 (no Iapetus flyby) • Voyager 2: August 1981 • Cassini-Huygens: July 2004 & onward • Initial images by Voyager 1 & 2, then more detailed photography by Cassini-Huygens.

  8. Tethys (Saturn III) • Radius: 531.1km • Density: 985 kg/m^3 • Vis. Geo. Albedo: 0.8 • Semimajor Axis: 294.7k km • Period: 1.888 days (tidally locked) • Inclination: 1.88° Cassini Image. Tethys. National Space Science Data Center. 4 May 2013.

  9. Tethys (cont.) • Heavily cratered, but not to the extent of Rhea. • Largely water-ice with small amounts of rock. • Gravitationally holds Telesto & Calypso. • Quite noticeable is Odysseus Crater, prominent in the Western Hemisphere. Cassini Image. Tethys with Odysseus Crater. NASA SSE. 4 May 2013

  10. Tethys (cont.) • Has a high visual geometric albedo. • Is bombarded by water-ice particles from geysers on Enceladus. • Has the Ithaca Chasma running roughly from the north to south pole. Voyager 2 Image. Tethys with Ithaca Chasma. 26 Aug 1981. NSSDC. 4 May 2013.

  11. Composite Spectroscopy Data A Pair of Pac-Men. 26 Nov 2012. NASA SSE. 4 May 2013.

  12. Rhea (Saturn V) • Radius: 763.8km • Density: 1240 kg/m^3 • Vis. Geo. Albedo: 0.7 • Semimajor Axis: 527.04k km • Period: 4.5175 days (tidally locked) • Inclination: 0.35° Cassini Image. Portrait of a Lady. 10 Mar 2013. NASA SSE. 4 May 2013.

  13. Rhea (cont.) • Cold body with tenuous atmosphere; similar to Tethys. • Density suggests ¾ water-ice, ¼ rock, and homogeneous. • Due to distance from Saturn, Rhea does not receive significant tidal heating effects. Voyager 1 Image. Rhea Mosaic. Nov 1980. NSSDC. 3 May 2013.

  14. Rhea (cont.) • Large amount of craters, which can be subdivided into two regions: • Heavy cratered bright terrain with diameters above 40km. • Polar & equatorial areas with smaller diameter craters. • “Wispy” subsidence fractures also present. Voyager 1 Image. Rhea, Enhanced. 12 Nov 1980. NSSDC. 4 May 2013.

  15. False-Color Imaging • Saturn-facing view of Rhea from 35,000km. • Demonstrates hemispherical color and albedo differences. • Reddish hues near poles identify composition changes (multiple explanations). Cassini Image. False-Color Rhea. 2 Mar 2010. NSSDC. 4 May 2013.

  16. Iapetus (Saturn VIII) • Radius: 734.5km • Density: 1090 kg/m^3 • Vis. Geo. Albedo: 0.05 to 0.5 • Semimajor Axis: 3561.3k km • Period: 79.33 days (tidally locked) • Inclination: 14.72° Voyager 2 Image. Iapetus. Aug 1981. NASA SSE. 3 May 2013.

  17. Iapetus (cont.) • Similar to Tethys, density suggests composition of mostly water-ice. • Overall shape is not ellipsoidal, but rather “walnut.” • Has two recognizable features: ridge at equator & dual albedo. Cassini Image. Iapetus, Equatorial Ridge. NSSDC. 4 May 2013.

  18. Iapetus (cont.) • Although 3rd largest moon of Saturn, Iapetus orbits much further out than Titan. • Inclination is highest of all regular satellites, giving best view of Saturn’s rings. • Pattern of craters follows albedo: heavy in bright region, less elsewhere (Turgis seen here.) Cassini Image. Iapetus, Engelier Crater. NSSDC. 3 May 2013.

  19. Equatorial Ridge • Around equator, Iapetus has a chain of ~20km mountains. • These break up on side facing away from Saturn into the bright Voyager Mountains. • Multiple proposed theories for ridge. Cassini Image. Voyager Mountains. 10 Sep 2007. NASA SSE. 4 May 2013.

  20. Two-Tone Albedo Cassini Image. Global View of Dichotomy. 27 Dec 2004. NASA SSE. 4 May 2013.

  21. Future Mission Prospects • Tethys, Rhea, & Iapetus are all devoid of many life-harboring properties. • Furthermore, there are other bodies in the Solar System which should have preferential scientific importance. • Marginally, Iapetus does have unique albedo & ridge features. • Even still, the prospect for future missions is understandably low.

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