The New York TimesJun 23, 2020 13:44:41 IST
“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% of meteorites that have been found on Earth.
Their origin, however, remains a mystery.
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 were presented at a virtual meeting of the American Astronomical Society by William Herbst and James Greenwood of Wesleyan University in Connecticut.
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.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 Herbst and 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.
Rather than being representative of other asteroids, the model would instead suggest Earth’s meteorites give us a window into how planets form, Herbst said.
Not everyone is convinced. Harold Connolly, an asteroid specialist at Rowan University in New Jersey, said one issue is that about 15% to 20% of chondrules appear to have experienced multiple heating events, not just one.
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.
Jonathan O'Callaghan c.2020 The New York Times Company
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