 Ida 's surface appears heavily cratered and mostly gray , although minor color variations mark newly formed or uncovered areas . Besides craters , other features are evident , such as grooves , ridges , and protrusions . Ida is covered by a thick layer of regolith , loose debris that obscures the solid rock beneath . The largest , boulder @-@ sized , debris fragments are called ejecta blocks , several of which have been observed on the surface . 
