“Burning rain drops”. This is how Henry Clifton Sorby, a 19th century British mineralogist, described the small spheres called chondrules found within meteorites. Chondrules are such dominant characteristics of these meteorites that they are called chondrites, and they account for 86 percent of the meteorites found on Earth.
Its origin, however, remains a mystery.
“There is nothing that predicts them,” said Rhian Jones of the University of Manchester in England, an expert on chondrites.
Now some scientists think they have a new answer to this rocky conundrum: 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 about the origins of chondrites. Some agree that its formation must have occurred early in the history of the solar system, about 4.6 billion years ago. But each of these hypotheses has its disadvantages. To form, chondrules must heat up and then cool down quickly, 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 laboratory oven to test their hypothesis.
Their results suggest a crowded landscape in the early inner solar system, with a rising sun surrounded by thousands of planetesimals, the rocky building blocks of the planets, each tens of miles in size. On some of its surfaces were oceans of lava that reached temperatures of over 3,000 degrees Fahrenheit. When even the smallest rocky bodies, asteroids, approached these young worlds, this lava would have briefly warmed them up. That quickly melted away portions of these objects, which then passed and cooled to form chondrules.
Meteorites reaching Earth have long been assumed to be similar to most 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 right, the implications are significant. Because overflights over worlds with lava oceans may not have been as common, chondrites may be quite rare.
“The meteorite record is a record of those materials that have the tensile strength to survive on Earth’s surface,” said Dr. Herbst. “So what fraction of the primitive material has that tensile strength? Our model makes a prediction. It will be small.
Rather than being representative of other asteroids, the model suggests that Earth’s meteorites give us a window on how planets form, Dr. Herbst said. They would tell us more about the nature of planetesimals in the early solar system, and chondrites are the burnt-out leftovers from the process that formed worlds like Earth.
Not everyone is convinced. Harold Connolly, an asteroid specialist at Rowan University in New Jersey, said one problem is that about 15 to 20 percent of chondrules appear to have experienced multiple warming events, not just one.
“You cannot have a single flyby,” he said. “It has to be somewhat cyclical.”
But without a clear way to explain chondrules right now, every hypothesis is on the table.
“Chondral formation is a really thorny problem,” said Dr. Jones. “So whenever someone comes up with something completely different, they are very welcome, because we don’t have the answer.”
There may be a way to test the hypothesis soon. Two space missions aim to return samples from near-Earth asteroids to our planet in the coming years: Japan’s Hayabusa2 will return to its home this year after collecting samples from the asteroid Ryugu, and NASA’s Osiris-Rex will attempt to collect samples. . in Bennu, another near-Earth body, in October, that returned to Earth in 2023.
“Most people would be absolutely stunned if there were no chondrules in those materials,” said Dr. Jones.
But if those samples don’t contain evidence of chondrules, it could add weight to Dr. Herbst and Dr. Greenwood’s model, and provide a fascinating window into conditions in the very early solar system.