Joel Schwartz
Northwestern Physics & Astronomy
May 13th, 2013
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.
 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.
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Orbital Locations
Orbital Diagram of Saturn Moons. 2004. U. Cincinnati Physics- Gas Giants. 4 May 2013.
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.
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Greek Titans
Greek Titans (Various). Wikipedia. 4 May 2013
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 CassiniHuygens.
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Tethys (Saturn III)
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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.
Tethys (cont.)
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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
Tethys (cont.)
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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.
Composite Spectroscopy Data
A Pair of Pac-Men. 26 Nov 2012. NASA SSE. 4 May 2013.
Rhea (Saturn V)
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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.
Rhea (cont.)
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Voyager 1 Image. Rhea Mosaic. Nov 1980. NSSDC. 3 May 2013.
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.
Rhea (cont.)
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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.
False-Color Imaging
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Cassini Image. False-Color Rhea. 2 Mar 2010. NSSDC. 4 May 2013.
Saturn-facing view of
Rhea from 35,000km.
Demonstrates
hemispherical color and
albedo differences.
Reddish hues near poles
identify composition
changes (multiple
explanations).
Iapetus (Saturn VIII)
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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.
Iapetus (cont.)
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Cassini Image. Iapetus, Equatorial Ridge. NSSDC. 4 May 2013.
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.
Iapetus (cont.)
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Cassini Image. Iapetus, Engelier Crater. NSSDC. 3 May 2013.
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.)
Equatorial Ridge
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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.
Two-Tone Albedo
Cassini Image. Global View of Dichotomy. 27 Dec 2004. NASA SSE. 4 May 2013.
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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