But Vesta and Ceres attained a stature asteroids didn't. The two objects are considered protoplanets. They acquired enough mass to undergo processes that led to subsurface redistribution of material based on density – a process known as diferentiation and one milestone during the transition from a ball of cosmic rubble to a planet.
The duo's growth, however, was stunted – frozen in time – by gravity from a rapidly bulking Jupiter. Thus, researchers hold that Vesta and Ceres can provide unique insights on an important stage of planet formation that until now has been the province of computer models.
Hubble Space Telescope images of Vesta, as fuzzy as they are, still point to an object with variations in minerals across the surface and hint at interesting features on its surface, notes Carol Raymond, the deputy lead scientist for the Dawn mission.
As the research team pored over images the craft has taken since it arrived, it was clear Vesta's variety has been generated by more than material ejected and spread over the surface by periodic collisions with other objects.
"We have strong structural features that have been preserved over billions of years. We have apparent layering of the surface. The degree to which Vesta appears to have preserved a geologic record is something that's very thrilling," Dr. Raymond says. "It's a small world that's quite unique."
The northern half of Vesta is relatively old and heavily cratered, she explains. But the southern half? It's looking like a planetary geologist's dream, members of the science team suggest.