Ceres and Vesta

Ceres and Vesta are the two most massive residents of the asteroid belt. Vesta is a rocky body, while Ceres is believed to contain large quantities of ice. The profound differences in geology between these two protoplanets that formed and evolved so close to each other form a bridge from the rocky bodies of the inner solar system to the icy bodies, all of which lay beyond in the outer solar system. At present, most of what we now know about Vesta and Ceres comes from ground-based and Earth-orbiting telescopes like NASA's Hubble Space Telescope.

When NASA's Dawn spacecraft sent the first images of the giant asteroid Vesta to the ground, scientists were fascinated by an enormous mound inside a big circular depression at the south pole. In this image, obtained by NASA's Dawn spacecraft from an orbit of about 1,700 miles (2,700 kilometers) above the surface of Vesta, topography in the area surrounding Vesta's south pole area shows impact craters, ridges and grooves. 

Vesta South Pole

Numerous impact craters illustrate the asteroid's violent youth. By counting craters on distinct geological surfaces scientists can deduce relative ages of the asteroid's surface. The asteroid's official name is "4 Vesta" because it was the fourth asteroid discovered. About the length of Arizona, it appears to have a surface of basaltic rock -- frozen lava -- which oozed out of the asteroid's presumably hot interior shortly after its formation 4.5 billion years ago, and has remained largely intact ever since. At the asteroid's south pole is a giant crater - 460 kilometers (285 miles) across and 13 kilometers (8 miles) deep. The massive collision that created this crater gouged out one percent of the asteroid's volume, blasting over one-half million cubic miles of rock into space.

Ceres, Discovered on January 1,1801 by Giuseppe Piazzi of Italy is the first asteroid/dwarf planet discovered.  The object is known by astronomers as "1 Ceres" because it was the very first minor planet discovered. As big across as Texas, Ceres nearly spherical body has a differentiated interior - meaning that, like Earth, it has denser material at the core and lighter minerals near the surface.

Astronomers believe that water ice may be buried under Ceres' crust because its density is less than that of the Earth's crust, and because the dust-covered surface bears spectral evidence of water-bearing minerals. Ceres could even boast frost-covered polar caps. Astronomers estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres' water, unlike Earth’s, is expected to be in the form of water ice located in its mantle.