“Droplets of fiery rain.” This is how Henry Clifton Sorby, a British mineralogist of the 19th century, described the tiny spheres called chondrons found in meteorites. Chondrums are so dominant features of these meteorites that they are called chondrites, and they make up 86 percent of the meteorites found on Earth.
However, their origin remains a mystery.
“There is nothing she predicts,” said Rhian Jones, a chondrite expert at Manchester University in England.
Now some scientists believe they have a new answer to this rocky riddle: the chondrites may have formed in an unusual event during a narrow window in the early solar system. The ResultsWilliam Herbst and James Greenwood of Wesleyan University in Connecticut, at a virtual meeting of the American Astronomical Society this month, presented a strange picture of what parts of the solar system looked like before the planets we knew formed.
Several hypotheses have been put forward for the development of chondrites. Some agree that their formation must have taken place early in the history of the solar system about 4.6 billion years ago. But each of these hypotheses has its downsides. To form, chondrons have to be heated and then cooled quickly, a scenario that is difficult to explain. The researchers developed a model for an idea they thought could work, and then simulated the conditions with stones in a laboratory furnace to test their hypothesis.
Their results suggest a crowded landscape in the early inner solar system, with an emerging sun surrounded by thousands of planetesimals, the rocky building blocks of planets, each about ten miles tall. There were lava oceans on some of their surfaces that reached temperatures of more than 3,000 degrees Fahrenheit. If even smaller rock bodies – asteroids – approached these young worlds, they would have been briefly heated by this lava. This quickly melted parts of these objects, which then passed and cooled to form chondrons.
It has long been believed that meteorites that make it to Earth are similar to most space rocks that move around the solar system in areas such as the asteroid belt between Mars and Jupiter. But when Dr. Herbst and Dr. Greenwood are correct, the impact is significant. Because fly-bys over lava sea worlds 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 to the surface of the earth,” said Dr. Autumn. “What proportion of the primitive material out there has this tensile strength? Our model makes a prediction. It’ll be small. “
Instead of being representative of other asteroids, the model would instead suggest that Earth’s meteorites give us an insight into the formation of planets, said Dr. Autumn. They would tell us more about the nature of planetesimals in the early solar system, where chondrites are the burned remains of the process that made worlds like Earth.
Not everyone is convinced. Harold Connolly, an asteroid specialist at Rowan University in New Jersey, said the problem was that about 15 to 20 percent of the chondrals had experienced multiple warming events, not just one.
“You can’t just have a flyby,” he said. “It has to be something that is cyclical.”
But without a clear way to explain chondrums, every hypothesis is currently on the table.
“The formation of chondrons is just a really delicate problem,” said Dr. Jones. “Whenever someone comes up with something completely different, it is very welcome since we have no answer.”
There may be a way to test the hypothesis soon. Two space missions aim to bring samples of near-Earth asteroids back to our planet in the coming years – Japan’s Hayabusa2 is on his way home to Earth this year after collecting samples from the asteroid Ryugu, and Osiris-Rex from NASA will try to collect samples in Bennu, another near-earth body, that will be back on Earth in 2023.
“Most people would be absolutely stunned if these materials didn’t contain chondrons,” said Dr. Jones.
However, if these samples contain no evidence of chondrons, this could be Dr. Herbst and Dr. Strain greenwood and provide a fascinating window into the conditions of the very early solar system.