Jupiter Moon Throws Curve Ball

The little inner moon of Jupiter, Amalthea, isn’t dense enough.  A press release from Jet Propulsion Laboratory says that data from the Galileo spacecraft “shakes up long-held theories of how moons form around giant planets.”  Density of moons is supposed to decrease with radius around Jupiter, meaning that Amalthea should be the most solid.  Instead, it appears to be a loose rubble pile less dense than water.  An alternative theory, that it formed farther out and migrated or was captured, is also implausible.  “Amalthea is throwing us a curve ball,” one scientist said.  The original paper was published in Science May 27.¹

¹Anderson et al., “Amalthea’s Density Is Less than That of Water,” Science, Vol 308, Issue 5726, 1291-1293, 27 May 2005, [DOI: 10.1126/science.1110422].

Planetary science is not exactly batting a thousand these days, even without curve balls.  Looks like she didn’t accept the date when sports dropout McFly proposed, “Amalthea, you are my density… er, I mean, my destiny.”