 During the early history of the Solar System, the asteroids melted to some degree, allowing elements within them to be partially or completely differentiated by mass.Some of the progenitor bodies may even have undergone periods of explosive volcanism and formed magma oceans.However, because of the relatively small size of the bodies, the period of melting was necessarily brief (compared to the much larger planets), and had generally ended about 4.5 billion years ago, in the first tens of millions of years of formation.