Its surface exhibits multiple kilometer scale ridges and concavities. The flat facets and concavities give Gaspra a very angular appearance. An occlusion mesh process generates a silhouette based geometry that removes internal features and concavities. They are covered by a membrane that bursts, uncovering a network of concavities. It uses a stack to detect and remove concavities in the boundary efficiently. Concavities, troughs, and tubes observed on grain surfaces are the result of broken vesicle walls.