Metis is not a sphere and measures 60×40×34 km across. The composition and mass of Metis are not known, but assuming that its mean density is like that of Amalthea (~0.86 g/cm³), its mass can be estimated as ~7×10 kg. Amalthea's density implies that moon is composed of water ice with a porosity of 10-15%, and Adrastea may be similar.