CHONDRULES: DROPS OF FIERY RAIN

GLASSY SPHERES FOUND IN METEORITES ARE FROZEN PIECES OF THE SOLAR NEBULA.

WHEN OUR SOLAR SYSTEM BEGAN TO TAKE SHAPE some 4.6 billion years ago, the Sun and planets as we know them today did not exist. Instead, a large disk of gas and dust known as the solar nebula swirled around a developing Sun. Within this disk, countless small objects collided and stuck together, gradually building larger and larger bodies-including Earth.

Meteorites provide a record of how the solar system developed. Inside certain meteorites, for example, tiny spheres can be found that likely formed when clumps of dust grains drifting in the solar nebula melted and then solidified rapidly, forming small crystals. Later, these glassy beads, known as chondrules, were incorporated into larger bodies-the class of meteorites known as chondrites. Chondrites formed in the first several million years of our solar system's history. Many of these rocks have remained virtually unchanged since that time.

METEORITES UNDER THE MICROSCOPE

Meteorites can tell us about the formation of our vast solar system-but only when we look at them in microscopic detail. In a standard technique used to study meteorites, researchers cut this fragment of the Allende meteorite in half. Then they removed a paper-thin slice of rock, some 30 microns (0.001 inches) thick, mounted this "thin section" on a glass slide and looked at it under a microscope. This technique dates back to the mid-1800s, when geologist Henry Clifton Sorby first looked at rocks, including meteorites, under a microscope.

Microscopic analysis reveals that meteorites are made of many components, reflecting the mix of material in the early solar system. Rounded chondrules, dark dust grains and irregularly shaped white nodules are often packed together densely. Sorby himself studied chondrules, the most abundant of these inclusions: recognizing that these glassy spheres were once molten, he called them "drops of fiery rain."

The thin section of Allende on display can be viewed in ordinary light or in cross-polarized light. The round chondrules (magnified 40 times) consist of crystals of the minerals olivine and pyroxene embedded in glass. Minerals interact with cross-polarized light to produce brilliant colors, helping scientists to identify different minerals. The dark material, or "matrix," between the chondrules is made of dust particles.

A HOT TOPIC

To create glass objects, glassblowers work at temperatures of more than 1,000°C (1,800°F). Chondrules like the ones in this dish, from another piece of the meteorite Bjurböle, contain glass-and must have formed at even higher temperatures followed by rapid cooling.

What caused the quick rise and fall in temperature that led to the formation of chondrules? Experts disagree, suggesting that solar flares, lightning or shock waves might have melted these beads. Evidence is strong, however, that chondrules formed within the solar system's first few million years.

COLLIDING CHONDRULES

In the disk of swirling dust and gas from which our solar system formed, chondrules hit other small particles and stuck together. Sometimes chondrules even ran into one another. These two chondrules from Allende collided more than four billion years ago.

Scientists at the American Museum of Natural History discovered this "compound chondrule"—two chondrules stuck together—in one piece of Allende. The indentation on the larger chondrule indicates that the sphere was still molten and soft when struck by the second body.

A JUMBLE OF CHONDRULES

Dozens of tiny, spherical chondrules jut out from the unpolished surface of the meteorite Chainpur.

LOOK CLOSELY

A single large chondrule protrudes from the surface of Modoc.

A UNIVERSAL PROCESS

Our galaxy is rich in nebulae, star-forming regions of dust and gas. Within these immense clouds, small regions sometimes collapse under the force of gravity, begin spinning around their centers and flatten into disks. Our own solar system began with the formation of a collapsed disk similar to ones in the Orion nebula.

What happens inside these disks of spinning gas and dust? Evidence from meteorites demonstrates that in our solar nebula, small bodies such as chondrules collided with other objects and stuck together to form larger bodies, eventually giving rise to planets. The study of meteorites helps us understand the formation of stars and planets—both in our own solar system and elsewhere in the galaxy.

The Orion nebula (above) is about 1,500 light years away from Earth. Within the larger nebula, small, dense regions (below) are each home to a developing solar system. Scientists think new solar systems form in much the same way that ours did.

A METEORITE SCHOLAR

Henry Clifton Sorby (1826-1908) was an English gentleman fascinated by geology. He pioneered the use of microscopes to study rocks and was the first to examine meteorites this way. Sorby recognized that the glassy chondrules inside meteorites were once molten; he described them as "drops of fiery rain."