Meteorites Hold Clues to How the Solar System Formed

Analyses of chondrites have provided clues to the puzzle of why different planets in the solar system have different chemical compositions. All the planets formed from the same solar nebula, a swirling cloud of gas and dust. So we might expect that the planets would have the same composition. But they don't. The inner planets—Mercury, Venus, Earth, and Mars—are made of rocky material. The outer planets—Jupiter, Saturn, Uranus, Neptune, and Pluto—consist chiefly of gases. Some process must have been responsible for the separation of the elements as the hot nebula began to cool and matter began to clump together to form the planets.

In the late 1960's, I and a number of other scientists began exploring the theory that differing temperatures of condensation (the process by which a gas changes to a solid) could account for the differences. In other words, we thought that some elements in the solar nebula must have condensed and formed mineral grains while temperatures were still so high that other elements remained in a gaseous state. These first grains became the building materials for some of the larger bodies called protoplanets that collided, broke up, and recombined to form the planets and asteroids. Grains that condensed later became the building blocks for other protoplanets. I had calculated, for example, that as the solar nebula began to cool, aluminum, calcium, and titanium would have been among the first elements to form mineral grains. But we had no way to simulate the condensation of the solar nebula in the laboratory.

We thought that carbonaceous (carbon-containing) chondrites, the oldest objects in the solar system, might hold some evidence that would confirm our theories. However, this type of meteorite was very hard to come by. Carbonaceous chondrites are still very rare today. Twenty years ago, researchers could get only small samples—pieces about the size of a fingernail—from museums.

Then, in February 1969, the Allende meteorite, a carbonaceous chondrite weighing nearly 2 tons, landed in Mexico. This enabled scientists to remove and study large slices of this kind of meteorite. We found that the Allende meteorite contained irregularly shaped particles. When scientists analyzed these particles, they found that they consist of aluminum, calcium, and titanium, in just the mineralogical forms predicted by my calculations. This finding indicated that elements had indeed condensed from the solar nebula at different temperatures and so verified the theory of condensation, accounting for the different compositions of the planets.