 A body of icy or rocky material in outer space may, if it can build and retain sufficient heat, develop a differentiated interior and alter its surface through volcanic or tectonic activity.The length of time through which a planetary body can maintain surface-altering activity depends on how well it retains heat, and this is governed by its surface area-to-volume ratio.For Vesta (r=263 km), the ratio is so high that astronomers were surprised to find that it "did" differentiate and have brief volcanic activity.