[meteorite-list] Cassini Finds Saturn Moon Phoebe has Planet-Like Qualities

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 26 Apr 2012 14:53:28 -0700 (PDT)
Message-ID: <201204262153.q3QLrSNZ011168_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2012-119

Cassini Finds Saturn Moon has Planet-Like Qualities
Jet Propulsion Laboratory
April 26, 2012

PASADENA, Calif. -- 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 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
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. Saturn's large moons appear to have formed from gas and
dust orbiting in the planet's equatorial plane. These moons currently
orbit Saturn 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 it 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, Ithaca, N.Y.

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 may originally 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 remained 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 the shape seen in Cassini images and modeling the likely cratering
history, we were able to see that Phoebe started with a nearly spherical
shape, rather than being 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. The
California Institute of Technology in Pasadena manages JPL for NASA.

For more information on the Cassini mission, visit:
http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook at jpl.nasa.gov

Dwayne Brown 202-358-1726
NASA Headquarters, Washington
                                                               
dwayne.c.brown at nasa.gov

2012-119
Received on Thu 26 Apr 2012 05:53:28 PM PDT


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