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

[Ron Baalke]

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/ .

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