杏吧原创

JUPITER鈥橲 innermost moon, Amalthea, is less dense than water. The finding has deepened the mystery of how and where the moon formed.

In 2002 the Galileo spacecraft flew to within 244 kilometres of the 250-kilometre-long potato-shaped moon. But the probe lost its two-way radio link to Earth during the flyby. Initial results from a one-way link indicated that Amalthea is mainly rubble with a density close to that of water ice. Now John Anderson of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, and his team have rigorously reanalysed the data and shown that the moon is a porous mass of rock and water ice, with a density about 85 per cent that of water (Science, vol 308, p 1291).

But the origin of Amalthea鈥檚 ice is 鈥渟omewhat of a mystery鈥�, says Anderson. The moon orbits about 110,000 kilometres from Jupiter, and models suggest that temperatures there were too high for ice at the time the gas giant鈥檚 inner satellites were born. It is possible that Amalthea formed later than the other moons, when the region was cooler, or it formed further out and migrated inwards.