Data from NASA's Dawn mission shows carbon-rich asteroids have been splattering dark material on Vesta's surface for a long period, the space agency reported.
But whereas over time soils on Earth's moon and other asteroids have undergone extensive weathering in the space environment, resulting in the accumulation of tiny metallic particles containing iron that dulls bodies' surfaces, Dawn detected no accumulation of such tiny particles on Vesta.
The protoplanet, or almost-planet, remains bright and pristine, astronomers say.
Scientists estimate about 300 dark asteroids with diameters from 0.6 to 6 miles have likely hit Vesta in the last 3.5 billion years, enough to wrap Vesta in a blanket of mixed material about 3 to 7 feet thick.
The constantly "stirring" of Vesta's outermost layer by impacts means it is always presenting a "young" face, they said.
"Getting up close and familiar with Vesta has reset our thinking about the character of the uppermost soils of airless bodies," Carle Pieters, a Dawn team member based at Brown University, said. "Vesta 'dirt' is very clean, well mixed and highly mobile."
Vesta, with a diameter of 325 miles, is the largest body in the asteroid belt located between the orbits of Mars and Jupiter.