Cosmic Rock Fights

Jupiter formed 4.5 billion years ago, and as the planet grew, its powerful gravity pulled in nearby icy and rocky planetesimals, leading them to collide at very high speed. 

The impacts were so strong that the rock and dust from the planetesimals melted on impact, generating tiny molten rock droplets – 0.1-2 millimeters (0.004-0.079 inches) – known as chondrules.  

Some of these chondrules were then incorporated into asteroids as the solar system evolved. Over billions of years, pieces of the asteroids broke off and fell to Earth as meteorites. 

For decades, scientists have wondered how these small spheres came to Earth and why they took on their round shape. 

“Previous formation theories couldn’t explain chondrule characteristics without requiring very specific conditions,” Sin-iti Sirono, author of a new study, said in a statement. “(Our) model requires conditions that naturally occurred in the early solar system when Jupiter was born.” 

The researchers developed a computer program simulating Jupiter’s early formation to track how its gravitational pull would have led to high-speed collisions between rocky and water-rich planetesimals. 

“We compared the characteristics and abundance of simulated chondrules to meteorite data and found that the model spontaneously generated realistic chondrules,” study author Diego Turrini said in the statement.  

The research showed that the features of chondrules, especially their sizes and the rate at which they cooled in space, are determined by the water contained in the impacting planetesimals. 

“When planetesimals collided with each other, water instantly vaporized into expanding steam,”  said Sirono. “This acted like tiny explosions and broke apart the molten silicate rock into the tiny droplets we see in meteorites today.” 

The model also indicated that chondrule formation occurred alongside Jupiter’s rapid accumulation of nebular gas as it grew to its enormous size. 

“As meteorite data tell us that peak chondrule formation took place 1.8 million years after the solar system began, this is also the time at which Jupiter was born,” said Turrini. 

This study allowed researchers to trace the birth order of planets and provided insights into how the solar system formed. 

However, chondrule production from Jupiter’s formation alone is too brief to explain why meteorites showcase chondrules of many different ages. The most likely reason is that other giant planets, like Saturn, also triggered chondrule formation when they were born.