WASHINGTON -- Data from NASA's Cassini mission reveal Saturn's moon
Phoebe has more planet-like qualities than previously thought.
Scientists had their first close-up look at Phoebe when Cassini began
exploring the Saturn system in 2004. Using data from multiple
spacecraft instruments and a computer model of the moon's chemistry,
geophysics and geology, scientists found Phoebe was a so-called
planetesimal, or remnant planetary building block. The findings
appear in the April issue of the Journal Icarus.
"Unlike primitive bodies such as comets, Phoebe appears to have
actively evolved for a time before it stalled out," said Julie
Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion
Laboratory (JPL) in Pasadena, Calif. "Objects like Phoebe are thought
to have condensed very quickly. Hence, they represent building blocks
of planets. They give scientists clues about what conditions were
like around the time of the birth of giant planets and their moons"
Cassini images suggest Phoebe originated in the far-off Kuiper Belt,
the region of ancient, icy, rocky bodies beyond Neptune's orbit. Data
show Phoebe was spherical and hot early in its history, and has
denser rock-rich material concentrated near its center. Its average
density is about the same as Pluto, another object in the Kuiper
Belt. Phoebe likely was captured by Saturn's gravity when it somehow
got close to the giant planet.
Saturn is surrounded by a cloud of irregular moons that circle the
planet in orbits tilted from Saturn's orbit around the sun, the
so-called equatorial plane. Phoebe is the largest of these irregular
moons and also has the distinction of orbiting backward in relation
to the other moons. By comparison, Saturn's large moons appear to
have formed from gas and dust around the planet's equatorial plane
and orbit in that same plane.
"By combining Cassini data with modeling techniques previously applied
to other solar system bodies, we've been able to go back in time and
clarify why Phoebe is so different from the rest of the Saturn
system," said Jonathan Lunine, a co-author on the study and a Cassini
team member at Cornell University.
Analyses suggest that Phoebe was born within the first 3 million years
of the birth of the solar system, which occurred 4.5 billion years
ago. The moon originally may have been porous but appears to have
collapsed in on itself as it warmed up. Phoebe developed a density 40
percent higher than the average inner Saturnian moon.
Objects of Phoebe's size have long been thought to form as
potato-shaped bodies and remain that way over their lifetimes. If
such an object formed early enough in the solar system's history, it
could have harbored the kinds of radioactive material that would
produce substantial heat over a short timescale. This would warm the
interior and reshape the moon.
"From Cassini images and models, we were able to see that Phoebe
started with a nearly spherical shape, rather than an irregular shape
later smoothed into a sphere by impacts," said co-author Peter
Thomas, a Cassini team member at Cornell.
Phoebe likely stayed warm for tens of millions of years before
freezing up. The study suggests the heat also would have enabled the
moon to host liquid water at one time. This could explain the
signature of water-rich material on Phoebe's surface previously
detected by Cassini.
The new study also is consistent with the idea that several hundred
million years after Phoebe cooled, the moon drifted toward the inner
solar system in a solar-system-wide rearrangement. Phoebe was large
enough to survive this turbulence.
More than 60 moons are known to orbit Saturn, varying drastically in
shape, size, surface age and origin. Scientists using both
ground-based observatories and Cassini's cameras continue to search
for others.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. JPL manages the
mission for the agency's Science Mission Directorate in Washington.
