Vesta is the most geologically diverse of the large asteroids and the only known one with distinctive light and dark areas much like the face of our Moon. Hubble images have revealed a diverse world with ancient lava flows and a gigantic impact basin that is so deep, it exposes the asteroid’s subsurface, or mantle.
Vesta’s surface shows a geology similar to that of terrestrial worlds such as Earth and Mars. Ground-based spectroscopy of Vesta indicates regions that are basaltic, which means lava flows once occurred on its surface. This is surprising evidence that the asteroid once had a molten interior, like Earth does. This contradicts conventional ideas that asteroids are essentially cold, rocky fragments left behind from the early days of planetary formation.
One possibility is that Vesta agglomerated from smaller material that includes radioactive debris (such as the the isotope Aluminum-26) that was incorporated into the core. This radioactive “shrapnel” probably came from a nearby supernova explosion. This hot isotope may have melted the core, causing the asteroid to differentiate: heavier, dense material sank to the center while lighter rock rose to the surface. This is a common structure for the terrestrial planets.
After Vesta’s formation, molten rock flowed onto the asteroid’s surface. This happened more than four billion years ago. The surface has remained unchanged since then, except for occasional meteoroid impacts.
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