Sounds good for the broad strokes, but the details remained a mystery. What ingredients were in that primordial mix? Could planets really emerge from dust? And did our solar system arise from slow, quiet coagulation — or from unspeakable violence? As astronomers studied Allende, they quickly realized they had some of the answers in their hands.
The key was in the countless nuggets inside the meteorite, which had once tumbled through the ancient protoplanetary disk, bathed in hot gases and dust. Each nugget provided a time capsule, recording conditions from those first several million years when the sun was still forming.
Allende’s white-colored spheres, known as CAIs (for calcium-aluminum-rich inclusion), stood out first. These spheres were rich in metals that remain solid even at extremely high temperatures. The inclusions were sometimes arranged in delicate crystals, with thin whiskers that protruded outward.
“We don’t have any similar objects on Earth,” says Guy Libourel, a cosmochemist at the Côte d’Azur Observatory in France. Scientists of the 1960s surmised that CAIs were the first solid objects to form in the solar system — the first rocks. As the inner regions of the protoplanetary disk cooled below 3,000 degrees Fahrenheit, they reasoned, those elements condensed out of the hot vapor to form delicate mineral crystals, just as the intricate branches of a snowflake condense from water vapor.
Libourel tested these ideas and published the results in 2006, creating the same crystal structures, whiskers and all, out of hot gas in the lab. “[It’s] a strong hint that CAIs came from the condensation of gas,” Libourel says.
Another supporting fact that CAIs are the oldest rocks? Their age. “They are the oldest dated objects,” says Martin Bizzarro, a planetary scientist at the University of Copenhagen. At 4,567,300,000 years old, “they define the formation age of the solar system.”
But CAIs tell only part of the story. Most of the nuggets inside Allende were darker, rich in glassy silicate minerals that form when molten liquid cools rapidly. Scientists suspect they’re the remains of primordial dust clumps flash-heated and melted by shock waves or collisions.