 Amalthea's irregular shape and large size led in the past to a conclusion that it is a fairly strong, rigid body, where it was argued that a body composed of ices or other weak materials would have been pulled into a more spherical shape by its own gravity. However, on 5 November 2002, the Galileo orbiter made a targeted flyby that came within 160 km of Amalthea and the deflection of its orbit was used to compute the moon's mass (its volume had been calculated previously — to within 10 % or so — from a careful analysis of all extant images). In the end, Amalthea's density was found to be as low as 0.86 g / cm3, so it must be either a relatively icy body or very porous "rubble pile" or, more likely, something in between. Recent measurements of infrared spectra from the Subaru telescope suggest that the moon indeed contains hydrous minerals (or organic materials), indicating that it cannot have formed in its current position, since the hot primordial Jupiter would have melted it. It is therefore likely to have formed farther from the planet or to be a captured Solar System body. Unfortunately, no images were taken during this flyby (Galileo's cameras had been deactivated due to radiation damage in January 2002), and the resolution of other available images is generally low.
