“Droplets of fiery rain.” That’s how Henry Clifton Sorby, a 19th-century British mineralogist, described the tiny spheres called chondrules found within meteorites. Chondrules are such dominant features of these meteorites that they are called chondrites, and they account for 86 percent of meteorites that have been found on Earth.
Their origin, however, remains a mystery.
“There’s nothing that predicts them,” said Rhian Jones of the University of Manchester in England, an expert in chondrites.
Now some scientists think they have a new answer to this rocky enigma: The chondrites may have formed in an unusual event during a narrow window of time in the early solar system. The findings, presented at a virtual meeting of the American Astronomical Society this month by William Herbst and James Greenwood of Wesleyan University in Connecticut, paint a strange picture of what parts of the solar system were like before the planets we know began to form.
Multiple hypotheses have been put forward for the origins of chondrites. Some agree their formation must have occurred early in the solar system’s history, about 4.6 billion years ago. But each of these hypotheses has its downsides. To form, chondrules must be heated and then cooled rapidly, a scenario that is difficult to explain. So the researchers came up with a model for an idea they thought might work, then simulated the conditions with rocks in a lab furnace to test their hypothesis.
Their results suggest a crowded landscape in the early inner solar system, with a nascent sun surrounded by thousands of planetesimals, the rocky building blocks of planets, each tens of miles or so in size. On some of their surfaces were oceans of lava that reached temperatures of more than 3,000 degrees Fahrenheit. When even smaller rocky bodies — asteroids — got close to these young worlds, they would have been briefly heated by this lava. That rapidly melted portions of these objects, which then passed by and cooled to form chondrules.
It has long been assumed that meteorites that make it to Earth are similar to most of the space rocks that travel around the solar system in areas like the asteroid belt between Mars and Jupiter. But if Dr. Herbst and Dr. Greenwood are correct, the implications are significant. Because flybys over worlds with lava oceans might not have been that common, chondrites might actually be quite rare.
“The meteorite record is a record of those materials that have the tensile strength to survive to the surface of Earth,” said Dr. Herbst. “So what fraction of the primitive material out there has that tensile strength? Our model makes a prediction. It’ll be small.”
Rather than being representative of other asteroids, the model would instead suggest Earth’s meteorites give us a window into how planets form, Dr. Herbst said. They would tell us more about the nature of planetesimals in the early solar system, with chondrites being the scorched leftovers of the process that formed worlds such as Earth.
Not everyone is convinced. Harold Connolly, an asteroid specialist at Rowan University in New Jersey, said one issue is that about 15 to 20 percent of chondrules appear to have experienced multiple heating events, not just one.
“You can’t just have one flyby,” he said. “It’s got to be something that is cyclical.”
But without a clear way to explain chondrules at the moment, every hypothesis is on the table.
“Chondrule formation is just a really thorny problem,” Dr. Jones said. “So anytime somebody comes up with something completely different it’s very welcome, because we don’t have the answer.”
There may be a way to test the hypothesis soon. Two space missions are aiming to return samples of near-Earth asteroids to our planet in the coming years — Japan’s Hayabusa2 is on its way home to Earth this year after collecting samples from the asteroid Ryugu, and NASA’s Osiris-Rex will attempt to collect samples at Bennu, another near-Earth body, in October, arriving back at Earth in 2023.
“Most people would be absolutely stunned if there weren’t any chondrules in those materials,” Dr. Jones said.
But if those samples do not contain evidence for chondrules, it could add weight to Dr. Herbst and Dr. Greenwood’s model, and provides a fascinating window into conditions in the very early solar system.