PLANETESIMALS: BUILDING BLOCKS OF THE SOLAR SYSTEM

IN THE EARLY SOLAR SYSTEM, ORBITING BODIES OFTEN COLLIDED POWERFULLY—AS CERTAIN METEORITES REVEAL.

THE SMALL PARTICLES DRIFTING IN ORBIT AROUND THE DEVELOPING SUN were initially no bigger than grains of sand. But over time, the particles clustered together, building up into larger and larger bodies, known as planetesimals. While these objects were still relatively small, the growth process was fairly slow and steady. But once planetesimals reached the size of the island of Manhattan, the solar system became a more chaotic and violent place, with planetesimals repeatedly crashing into each other as they orbited the Sun.

These forceful encounters could shatter planetesimals or compact them into one larger body. In some of the most powerful collisions, the energy from the impact heated the planetesimals so much that certain regions began to melt. Evidence of all these processes can be seen in meteorites.

A CHAOTIC SOLAR SYSTEM

The solar system today is home to large planets revolving steadily around the Sun in fixed orbits. The asteroid belt consists of hundreds of thousands of asteroids spread out so thinly that they rarely come in contact with each other. But billions of years ago, as the planets, moons and asteroids were forming, the solar system was more densely packed. Countless planetesimals circulated around the Sun in irregular orbits. These bodies often crashed into each other with dramatic results.

The meteorite Sierra Colorada has two distinctly colored regions, representing fragments of two different bodies. The fragments fused after two planetesimals collided in the early solar system.

MELTING METEORITES

The most forceful collisions in the early solar system contained enough energy to melt portions of the planetesimals. The meteorite Chico shows evidence of melting on the lighter-colored half. The meandering boundary of this region indicates that the material was once partially liquid. During melting, the metal in the matrix began to separate from the rocky minerals. The darker portion did not melt and the rock and metal are still completely mixed.

A collision also caused melting in Rose City. Here, metal separated from rock throughout the meteorite.

THE HEAT OF IMPACT

The parent body of Portales Valley was hit at least twice by other planetesimals. The first impact caused the metal present in the matrix to melt. A second impact caused the molten metal to flow through veins in the shattered rock, creating the pattern seen here.

PULVERIZED ROCK

The force of an impact between planetesimals was often strong enough to pulverize rock. The meteorites Paragould and Naryilco (right) illustrate two such events. In each, the dark portions of the meteorites are tiny pieces of the same rock that were shattered and heated by impact.

LOOK CLOSELY

The dark clump in the middle of this fragment of Allende predates the surrounding material. This fragment of a primitive, carbon-rich chondrite was probably floating freely in orbit around the Sun billions of years ago when it encountered the Allende parent body and was incorporated into it.