Introduction to Planetary Science

saturn: the beauty of rings 351
16.4.6 Iapetus and Phoebe
Iapetus has about the same mass and diameter as
Rhea but it is located almost seven times farther
from Saturn than Rhea at an average distance
of 3564 × 10 3 km. Iapetus was discovered by
Jean-Dominique Cassini in 1671, six years after
Christiaan Huygens discovered Titan in 1655.
The bulk density of Iapetus 1160 g/cm 3 is
similar to the densities of the other regular satellites
of Saturn (except Titan), meaning that it
too is composed of a mixture of 90% water ice
and 10% refractory dust particles by volume.
Therefore, Iapetus should have a large albedo
like the other ice satellites of Saturn (e.g.,
Mimas, Enceladus, Tethys, Dione, and Rhea).
Soon after discovering Iapetus, the astronomer
Cassini observed that its albedo changed with
time and sometimes declined to such an extent
that the satellite almost became invisible. The
explanation for the apparent disappearance of
Iapetus is that the leading hemisphere (i.e., the
one that faces in the direction of movement
in its orbit) is very low (5%) because it is
covered by some kind of dark-colored deposit.
The trailing hemisphere (i.e., the backside) of
Iapetus is bright and shiny with an albedo of
50% as expected of a satellite composed of ice.
The bright hemisphere of Iapetus is as densely
cratered as the surface of Rhea and has not
been rejuvenated by cryovolcanic activity. The
leading hemisphere of Iapetus is so dark that
no topographic features are discernible in the
Voyager images.
The explanation for the presence of the darkcolored
deposit on Iapetus involves Phoebe, the
outermost of the regular satellites whose orbit has
a radius of 12 930×10 3 km, placing it about 3.6
times farther from Saturn than Iapetus and nearly
25 times farther than Rhea. The images of Phoebe
that were recorded by the spacecraft Cassini on
its first approach to Saturn on June 11, 2004,
show that Phoebe is an ancient world of ice and
rocks that has been severely battered by impacts.
The plane of Phoebe’s orbit is inclined 150
with respect to the equatorial plane of Saturn.
Consequently, Phoebe revolves in the retrograde
direction when viewed from above the saturnian
system, and its orbit is highly eccentric (0.163).
In addition, Phoebe has a short rotation period
of about 0.4 days (Hartmann, 2005) that does
not match its period of revolution. Therefore,
Phoebe is the only regular satellite of Saturn
that has retrograde revolution, its rotation is not
synchronized with its revolution, and the orbit
is highly eccentric. These unusual properties of
its orbit suggest that Phoebe did not form in
the protosatellite disk of gas and dust but was
captured by Saturn. This conjecture is supported
by the comparatively high bulk density of Phoebe
(16g/cm 3 ; Talcott, 2004), which resembles the
bulk densities of the most massive satellites of
Uranus (i.e., Miranda, Ariel, Umbriel, Titania,
and Oberon), which have bulk densities ranging
from 1.350 to 1680 g/cm 3 .
Most significant for the hypothetical explanation
of the dark deposit on the leading
hemisphere of Iapetus is the low albedo of
Phoebe (6%) caused by the presence of dark
carbonaceous material that covers its surface.
Impacts on the surface of Phoebe may have
dislodged this surface material allowing it to
spiral toward Saturn. Subsequently, Iapetus
ploughed into this cloud of “Phoebe dust”,
which was deposited on its leading hemisphere
thereby decreasing its albedo. However, optical
spectroscopy indicates that the deposit on the
leading hemisphere of Iapetus is reddish in color,
whereas the surface of Phoebe is black. Perhaps
the Phoebe dust was altered in some way to
cause its color to change while it was in transit
in space or after it was deposited on Iapetus. It
is also possible that this hypothesis is incorrect,
in which case we need to consider an alternative
explanation for the dichotomy of the surface of
Iapetus.
Such an alternative is the suggestion that
Iapetus formed in a part of the saturnian protosatellite
disk that was cold enough to cause
methane CH 4 to condense as methane hydrate
CH 4 ·H 2O. When the Phoebe dust impacted on
the leading hemisphere of Iapetus, the volatile
compounds in the ice composed of carbon,
hydrogen, nitrogen, oxygen, and sulfur, reacted
to form large molecules of organic matter resembling
reddish tar similar to the material that
coats the surfaces of some asteroids and comets
(Section 13.1.1). However, this explanation may
also be inadequate and we must await the results
of close encounters of the spacecraft Cassini with
Iapetus before we can explain how the leading
hemisphere of Iapetus came to be covered with