Adrastea is non-spherical and measures 20x16x14 km³ across. What Adrastea is made of and the mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea (~0.86 g/cm³) its mass can be estimated at ~2×10 kg. Amalthea's density implies that moon is composed of water ice with a porosity of 10-15%, and Adrastea may be similar.