[meteorite-list] Icy Jupiter Moon Throws a Curve Ball at Formation Theories (Amalthea)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue May 31 17:51:19 2005
Message-ID: <200505312150.j4VLogA26252_at_zagami.jpl.nasa.gov>

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov

Carolina Martinez (818) 354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

News Release: 2005-089 May 31, 2005

Icy Jupiter Moon Throws a Curve Ball at Formation Theories

Scientists studying data from NASA's Galileo
spacecraft have found that Jupiter's moon Amalthea is
a pile of icy rubble less dense than water.
Scientists expected moons closer to the planet to be
rocky and not icy. The finding shakes up long-held
theories of how moons form around giant planets.

"I was expecting a body made up mostly of rock. An icy
component in a body orbiting so close to Jupiter was a
surprise," said Dr. John D. Anderson, an astronomer at
NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Anderson is lead author of a paper on the findings
that appears in the current issue of the journal
Science.

"This gives us important information on how Jupiter
formed, and by implication, how the solar system
formed," Anderson said.

Current models imply that temperatures were high at
Amalthea's current position when Jupiter's moons
formed, but this is inconsistent with Amalthea being
icy. The findings suggest that Amalthea formed in a
colder environment. One possibility is that it formed
later than the major moons. Another is that the moon
formed farther from Jupiter, either beyond the orbit
of Jupiter's moon Europa or in the solar nebula at or
beyond Jupiter's position. It would have then been
transported or captured in its current orbit around
Jupiter. Either of these explanations challenges
models of moon formation around giant planets.

"Amalthea is throwing us a curve ball," said Dr.
Torrence Johnson, co-author and project scientist for
the Galileo mission at JPL. "Its density is well
below that of water ice, and even with substantial
porosity, Amalthea probably contains a lot of water
ice, as well as rock." Analysis of density, volume,
shape and internal gravitational stresses lead the
scientists to conclude that Amalthea is not only
porous with internal empty spaces but also contains
substantial water ice.
      
One model for the formation of Jupiter's moons
suggests that moons closer to the planet would be made
of denser material than those farther out. That is
based on a theory that early Jupiter, like a weaker
version of the early Sun, would have emitted enough
heat to prevent volatile, low-density material from
condensing and being incorporated into the closer
moons. Jupiter's four largest moons fit this model,
with the innermost of them, Io, also the densest, made
mainly of rock and iron.

Amalthea is a small red-tinted moon that measures
about 168 miles in length and half that in width. It
orbits about 181,000 kilometers (112,468 miles) from
Jupiter, considerably closer than the Moon orbits
Earth. Galileo passed within about 99 miles of
Amalthea on Nov. 5, 2002. Galileo's flyby of Amalthea
brought the spacecraft closer to Jupiter than at any
other time since it began orbiting the giant planet on
Dec. 7, 1995. After more than 30 close encounters with
Jupiter's four largest moons, the Amalthea flyby was
the last moon flyby for Galileo.

The Galileo spacecraft's 14-year odyssey came to an
end on Sept. 21, 2003. JPL, a division of the
California Institute of Technology in Pasadena,
managed the Galileo mission for NASA.

Additional information about the mission is available
online at: http://galileo.jpl.nasa.gov/ .

- end -
Received on Tue 31 May 2005 05:50:42 PM PDT