[meteorite-list] Jupiter's Innermost Moon Just a Pile of Rubble (Almathea)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu May 26 17:16:56 2005
Message-ID: <200505262116.j4QLGJ124780_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/article.ns?id=dn7433

Jupiter's innermost moon just a pile of rubble
Jeff Hecht
New Scientist
May 26, 2005

Jupiter's innermost moon Amalthea is a mass of icy rubble that could not
have formed as close to the planet as its present orbit. A new analysis
does not pinpoint its true origin, but does indicate that the porous
hunk of ice and rock is near its maximum possible size.

The new analysis is of data from the Galileo spacecraft, which sped past
Amalthea at a distance of only 244 kilometres on 5 November 2002 on its
way to a death-plunge into the Jovian atmosphere.

Astronomers had hoped to measure the moon's mass and density, but the
ageing spacecraft lost its two-way radio link to Earth during the
200-second flyby. Initial analysis of the little data retrieved
indicated only that Amalthea appeared less dense than water.

Now astronomers have gone back through the data to estimate Amalthea's
mass at just over two trillion tonnes (2.08 x 1015). Dividing that
figure by the satellite's measurement of the moon's volume - 2.4 million
cubic kilometres - gives a density of about 850 kilograms per cubic
metre. This is lighter than solid ice.

But there is an uncertainty of 11% in this density calculation, which is
large enough that Amalthea might actually be as heavy as ice. But John
Anderson of the Jet Propulsion Laboratory, in California, US, is
convinced that Amalthea is a rubble pile with voids between chunks of
ice and rock. Larger bodies that have melted tend to be spherical, but
Amalthea is elongated, with three axes measuring 250, 146 and 128
kilometres.
          
Low pressure interior

Astronomers have spotted several other porous bodies recently. That had
not been expected, but Anderson told New Scientist: "Now we understand
that the pressures are so low in the interior that they can maintain
their porosity."

The calculated pressure at the centre of Amalthea is just less than the
strength of natural ice. But if a body was any larger, the internal
pressure would be enough to flatten ice chunks at their cores, squeezing
out any voids.

And while Amalthea's circular orbit of just 110,000 kilometres above
Jupiter's surface makes it look old, ice could not have survived the
heat at that distance as Jupiter's four largest moons formed, so
Amalthea must have formed elsewhere.

It might have hailed from the asteroid belt, the Kuiper belt, or from a
more distant orbit around Jupiter. Anderson suggests Amalthea may have
collided with another object close to Jupiter, shattering into the
fragments that later collected to form the rubble-pile moon.

Journal reference: Science (vol 308, p 1291)
 
Received on Thu 26 May 2005 05:16:18 PM PDT