When I wrote about the geology of asteroids a few years back, I mentioned the common understanding that a handful of the largest asteroids are actually little planets in the geological sense. (Please, I'm not getting into the Pluto question here.) In particular, the asteroid Vesta is often called "the smallest terrestrial planet." So I was all ears this morning at the AGU meeting when scientists from the Dawn mission presented fresh images and ideas from our first close look at this remarkable object.
We actually have lots of pieces of Vesta in hand: meteorites of the howardite-eucrite-diogenite (HED) group. From them we know that Vesta melted itself an iron core and silicate mantle, just like Earth. The Dawn spacecraft mission shows the signs of many colossal impact events on Vesta that sent those meteorites our way. Images show a big, deep basin at Vesta's south pole, named Rheasilvia, and an older basin almost as big right next to it. Both of these created sets of giant grooves that circle the planet's equator. And the basins themselves are furrowed into weird spirals of debris. The most intriguing theory, presented by Erik Asphaug, is that these are what happen when an impact hits a rapidly rotating world: Coriolis forces govern how the wreckage falls. (See earlier animations from the Planetary Society's Emily Lakadwalla.) What Asphaug told us this morning is that a few other craters in the solar system show these features, and what they have in common is a total absence of impact melt. This favors the "naked floor hypothesis" of Rheasilvia.
You may wonder about Rheasilvia. Vesta is getting its features named after the names of actual Vestal virgins, who in ancient times were the guardians of Rome's luck. Other names come from prominent Roman women. (I forgot to ask whether Sophia Loren is one.)
Vesta NASA/JPL-Caltech/UCLA/MPS/DLR/IDA image