Scientists Discover Pluto Kin Is a Member of Saturn Family
Saturn's battered little moon Phoebe is an interloper to the
Saturn system from the deep outer solar system, scientists have
concluded. The new findings appear in the May 5 issue of the
Image right: Saturn's moon Phoebe. Image credit: NASA/JPL/Space Science Institute.
+ Full image and caption
"Phoebe was left behind from the solar nebula, the cloud of
interstellar gas and dust from which the planets formed," said
Dr. Torrence Johnson, Cassini imaging team member at NASA’s Jet
Propulsion Laboratory, Pasadena, Calif. "It did not form at
Saturn. It was captured by Saturn's gravitational field and has
been waiting eons for Cassini to come along."
Cassini flew by Phoebe on its way to Saturn on June 11, 2004.
Little was known about Phoebe at that time. During the
encounter, scientists got the first detailed look at Phoebe,
which allowed them to determine its makeup and mass. With the
new information they have concluded that it has an outer solar
system origin, akin to Pluto and other members of the Kuiper
"Cassini is showing us that Phoebe is quite different from
Saturn's other icy satellites, not just in its orbit but in the
relative proportions of rock and ice. It resembles Pluto in this
regard much more than it does the other Saturnian satellites,"
said Dr. Jonathan Lunine, Cassini interdisciplinary scientist
from the University of Arizona, Tucson.
Phoebe has a density consistent with that of the only Kuiper Belt
objects for which densities are known. Phoebe’s mass, combined
with an accurate volume estimate from images, yields a density of
about 1.6 grams per cubic centimeter (100 pounds per cubic foot),
much lighter than most rocks but heavier than pure ice, which is
about 0.93 grams per cubic centimeter (58 pounds per cubic foot).
This suggests a composition of ice and rock similar to that of
Pluto and Neptune's moon Triton. Whether the dark material on
other moons of Saturn is the same primordial material as on
Phoebe remains to be seen.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL, a
division of the California Institute of Technology in Pasadena,
manages the Cassini mission for NASA's Science Mission
Directorate, Washington, D.C. For Phoebe images and more
information about the Cassini-Huygens mission, visit
Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.