New findings from NASA's Dawn spacecraft lay the groundwork for the first geological overview of asteroid (4)Vesta and confirm the existence of not one but two giant impact basins in its southern hemisphere. The findings, published this week in a set of Science papers, will help scientists better understand the early solar system and processes that occurred as it formed and evolved.

The Dawn spacecraft, orbiting asteroid Vesta since July 2011, has already acquired several thousand images of the asteroid's surface, revealing a complex landscape. The images provide many details that help scientists to understand how the surface has evolved since its formation.

The first paper provides an overview of the true complexity of this ancient world. Vesta is not just a ball of rock; its surface is dominated by abundant impact craters of all shapes and sizes, from small fresh craters to giant basins as seen in the southern hemisphere.

The surface of Vesta is complex and varied, with preserved ejecta blankets clinging to some craters, large troughs extending around the equatorial region, enormous mountains, and unevenly distributed enigmatic dark material, but as yet an absence of volcanic features.

"As a volcanologist, and as one who expected to find evidence of volcanism on Vesta based on what we knew from the Howardite-Eucrite-Diogenite (HED) meteorites and prior models of Vesta's formation, the biggest surprise for me was the absence of any evidence of volcanic features. Vesta's surface has been so heavily modified by impact cratering that any evidence of its early volcanic activity has been destroyed," says Dawn mission participating scientist David Williams, co-author on the papers and faculty research associate in Arizona State University's School of Earth and Space Exploration.

The present lack of volcanic features detected on Vesta suggests that volcanism was only active during the short period of rapid cooling of Vesta's interior within the first 100 million years after formation, and that the surface has been eroded by impacts over time.

Like Earth and other terrestrial planets, Vesta has ancient basaltic materials in its crust and a large iron core. It is an irregular asteroid that also has tectonic features, troughs, ridges, cliffs, hills and a giant mountain. But comparisons of the slopes and topography of Vesta show that they are intermediate between planets and small asteroids, underscoring Vesta's unique role as a transitional solar system body.

Prior to the arrival of the Dawn spacecraft, some Vestan surface features had already been resolved using the Hubble Space Telescope and ground-based telescopes. The most prominent of these surface features is an enormous crater Dawn found to be about 500 kilometers (310 miles) in diameter, centered near the south pole, named Rheasilvia after the mother of Romulus and Remus.

Its width is 90 percent the diameter of Vesta and it is estimated that the impact responsible excavated about 1 percent of the volume of Vesta.

The second paper is a geological description of this large impact basin.

"Dawn observations enabled us to recognize that there are actually TWO large basins at the south pole, an older one named "Veneneia" and a younger one named "Rheasilvia"," explains Williams.

The Rheasilvia basin dominates the geology of Vesta, as the basin itself and its impact ejecta cover most of the southern hemisphere. The center of Rheasilvia has a central peak taller than Mt. Everest or Mauna Loa on Earth, similar in height to Olympus Mons on Mars.

This basin appears to have excavated into the mantle of Vesta, exposing material spectrally similar to diogenite meteorites; Vesta's crust is spectrally similar to eucrite and howardite meteorites, thus confirming that Vesta and its family of asteroids are the source of the howardite-eucrite-diogenite (HED) family of basaltic achondrite meteorites.

"For most planets and moons we see the pictures first, then have samples collected later to confirm our geologic interpretations. In the case of Vesta, thanks to the HED meteorites, we have the samples first, and must try to relate them to our emerging geologic picture of Vesta from the Dawn mission," said Williams.