The planets of the solar system are thought to have formed by condensing out of a hot cloud of gas and dust that surrounded the infant Sun. At first little particles of high-temperature minerals collected into small clumps (called chondrules) and then into larger lumps, reaching about a kilometer in size. Then these in turn accreted into a large number of protoplanets, up to about 1000 kilometers. Finally, today's planets grew as the protoplanets combined.
Planetary geologists want to refine this scenario by comparing it against new evidence. We have pieces of the first particles, preserved in meteorites. And we have examples of the planets, of course. But the intermediate protoplanets are still mysterious, because the planets, as they sorted themselves into today's orbits, cleared out almost all the remaining small bodies.
Computer modeling of this process tells us that many protoplanets were warm enough to cook. These random accretions of primitive material turned into organized bodies with distinct layers. Planetary geologists call this process differentiation.
The basic ingredients of planets are ices, silicate minerals and iron compounds. The level of radioactivity was high in those days, and as protoplanets grew they accumulated heat. At a certain size, the heat buildup was enough to melt and boil off the ices, then to begin melting the minerals together into proper rocks.
Finally, the iron was reduced to the metallic form and sank to the protoplanets' cores, releasing gravitational energy and melting the silicate rocks further. The evidence is that while most meteorites are chondrites, consisting of those primitive chondrules, around 10 percent are differentiated rocks (achondrites) and iron—fragments of protoplanets.
Asteroids or Protoplanets?
The belt between Mars and Jupiter holds tens of thousands of small rocky bodies: the asteroids, or dwarf planets. Spectroscopic observations from spacecraft and telescopes show us that the asteroids are generally ragged chunks of rock and iron metal. The rocks appear to be differentiated, having undergone planetary processes, but the protoplanets are broken—most of them.
The three largest asteroids, however, are not broken, or at least not badly. They are Ceres (approx. 900 km across), Pallas (520 km) and Vesta (500 km). Ceres is a smooth, flattened spheroid. Pallas appears to have a squat egg shape. Vesta has a grapefruit shape. Ceres is rather light for its size, whereas Vesta is about the density of basalt. Pallas is in between. All three of them appear to be intact protoplanets.
Structure of Ceres, Pallas and Vesta
Recent modeling studies suggest that Ceres retained some of its water, and a thin liquid layer may persist to this day beneath a thick icy rind. The water carried off so much heat that the rocky core never differentiated further. Indeed, the rock is the same density as serpentinite, or hydrated basalt. Thus the chemical environment at depth in Ceres could possibly support life.
In 2009, Hubble Space Telescope images of Pallas were analyzed in Science with the conclusion that it, too, was once a water-rich object. Pallas' orbit makes it difficult to visit, so we will probably not learn much more about it.
Vesta is a dry body in which the water boiled off. Radioactive heat then built up enough to produce an iron core, silicate mantle and basaltic outer layer. The wonderful thing is that we have meteorites in hand—the howardite-eucrite-diogenite group—that appear to match Vesta's rocks. The combination of theoretical geophysics, spectroscopic sensing and igneous geochemistry reinforces our theory of planetary formation.
In 2002 Klaus Keil of the University of Hawaii called Vesta "the smallest terrestrial planet." With that, we have a nice collection progressing in size from Earth on down, including Venus, Mars, Mercury and now Vesta, from which to study rocky planets as a species.
New research has found meteorites that appear to represent not just basalt, but the even more evolved rock andesite. Some of the protoplanets, it seems, had crustal rocks like those of our own continents as well as mantles and cores. But unlike Earth, where andesite is made during arc volcanism, no asteroid got as far as plate tectonics—as far as we know. Learn more about that. And basaltic stones from other protoplanets are being found too.
PS: NASA launched the Dawn spacecraft mission in 2007 that visited Vesta starting in 2011 and will visit Ceres in 2015. The program made history in March 2006 by being cancelled, then revived after an appeal by mission scientists. Follow the news at the Dawn site.
Thanks to a fine article in the 7 March 2006 Eos by Thomas McCord, Lucy McFadden, Christopher Russell, Christophe Sotin and Peter Thomas for much of this material.