does not exceed 35°. Ida's irregular shape is responsible for the asteroid's very uneven gravitational field. The surface acceleration is lowest at the extremities because of their high rotational speed. It is also low near the "waist" because the mass of the asteroid is concentrated in the two halves, away from this location. Surface features 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. Regolith The surface of Ida is covered in a blanket of pulverized rock, called regolith, about thick. This material is produced in impact events and redistributed across Ida's surface by geological processes. Galileo observed evidence of recent downslope regolith movement. Ida's regolith is composed of the silicate minerals olivine and pyroxene. Its appearance changes over time through a process called space weathering. Because of this process, older regolith appears more red in color compared to freshly exposed material. About 20 large (40–150 m across) ejecta blocks have been identified, embedded in Ida's regolith. Ejecta blocks constitute the largest pieces of the regolith. Because ejecta blocks are expected to break down quickly by impact events, those present on the surface must have been either formed recently or uncovered by an impact event. Most of them are located within the craters Lascaux and Mammoth, but they may not have been produced there. This area attracts debris due to Ida's irregular gravitational field. Some blocks may have been ejected from the young crater Azzurra on the opposite side of the asteroid. Structures Several major structures mark Ida's surface. The asteroid appears to be split into two halves, here referred to as region 1 and region 2, connected by a "waist". This feature may have been filled in by debris, or blasted out of the asteroid by impacts. Region 1 of Ida contains two major structures. One is a prominent ridge named Townsend Dorsum that stretches 150 degrees around Ida's surface. The other structure is a large indentation named Vienna Regio. Ida's region 2 features several sets of grooves, most of which are wide or less and up to long. They are located near,