Amalthea's Density Is Less Than That of Water

[thechemist]

This just appeared in Science. Since it is registration based, I copy the abstract :<br /><blockquote><font class="small">In reply to:</font><hr /><p><br />Amalthea's Density Is Less Than That of Water<br />John D. Anderson,1* Torrence V. Johnson,1 Gerald Schubert,2,3 Sami Asmar,1 Robert A. Jacobson,1 Douglas Johnston,1 Eunice L. Lau,1 George Lewis,1 William B. Moore,2 Anthony Taylor,1 Peter C. Thomas,4 Gudrun Weinwurm5<br /><br />Radio Doppler data from the Galileo spacecraft's encounter with Amalthea, one of Jupiter's small inner moons, on 5 November 2002 yield a mass of (2.08 ± 0.15) x 1018 kilograms. Images of Amalthea from two Voyager spacecraft in 1979 and Galileo imaging between November 1996 and June 1997 yield a volume of (2.43 ± 0.22) x 106 cubic kilometers. The satellite thus has a density of 857 ± 99 kilograms per cubic meter. We suggest that Amalthea is porous and composed of water ice, as well as rocky material, and thus formed in a cold region of the solar system, possibly not at its present location near Jupiter.<p><hr /></p></p></blockquote><br /><br />Science, Vol 308, Issue 5726, 1291-1293 , 27 May 2005<br />[DOI: 10.1126/science.1110422] <div class="Discussion_UserSignature"> <em>I feel better than James Brown.</em> </div>

 

[zavvy]

<b>Jupiter's innermost moon just a pile of rubble</b><br /><br />LINK<br /><br />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.<br /><br />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. <br /><br />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.<br /><br />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 92% of the density of solid ice.<br /><br />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. <br /><br />Low pressure interior<br />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." <br /><br />The calculated pressure at the centre of Amalthea is