Rheasilvia

Rheasilvia is the largest impact crater on the asteroid Vesta. It is 505 km in diameter, which is 90% the diameter of Vesta itself, and is 95% the mean diameter of Vesta, 529 km. However, the mean is affected by the crater itself. It is 89% the mean equatorial diameter of 569 km, making it one of the largest craters in the Solar System, and at 75°S latitude, covers most of the southern hemisphere. The peak in the center of the crater is 200 km in diameter, and rises 22.5 km from its base, making it one of the tallest mountains known in the Solar System.

Observation and naming
Rheasilvia was discovered in Hubble Space Telescope images in 1997, but was not named until the arrival of the Dawn spacecraft in 2011. It is named after Rhea Silvia, a mythological vestal virgin and mother of the founders of Rome, Romulus and Remus. The name Rheasilvia was officially approved by the International Astronomical Union (IAU) on 30 September 2011.

Characteristics
The crater partially obscures an earlier crater, named Veneneia, that at 395 km is almost as large.

Rheasilvia has an escarpment along part of its perimeter which rises 4–12 km above the surrounding terrain. The crater floor lies about 13 km below the surrounding surface. This basin consists of undulating terrain and a central mound almost 200 km in diameter, which rises 20-25 km from its base, one of the tallest known mountains in the Solar System, and possibly formed due to a planetary scale impact.

Spectroscopic analyses of Hubble images have shown that this crater has penetrated deep through several distinct layers of the crust, and possibly into the mantle, as indicated by spectral signatures of olivine.

Vesta has a series of troughs in an equatorial region concentric to Rheasilvia. These are thought to be large-scale fractures resulting from the impact. The largest is Divalia Fossae, approx. 22 km wide and 465 km long.

It is estimated that the impact responsible excavated about 1% of the volume of Vesta, and it is likely that the Vesta family and V-type asteroids are the products of this collision. If this is the case, then the fact that 10-km fragments have survived bombardment until the present indicates that the crater is at most about 1 billion years old. It would also be the origin of the HED meteorites. Known V-type asteroids account for 6% of the ejected volume, with the rest of the fragments presumably either too small to observe, or removed from the asteroid belt by approaching the 3:1 Kirkwood gap, by the Yarkovsky effect, or (in the case of small fragments) by radiation pressure.