Original: The bulk, composition and mass of Adrastea are not known, but assuming that its mean density is like that of Amalthea,(around 0.86 g/cm³) its mass can be estimated at about 2 kg.
Simplified: 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 ~2×1015 kg.