Simplify the following text for a Grade 6 native English-speaking person.
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\/cm Â , 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\/cm Â ) its mass can be estimated at ~ 2 Ã -- 1015 kg .