The Earth is constantly bombarded by meteorites. But the composition of these space rocks is a little unusual, writes Sarah Kaplan at the Washington Post, differing from those lingering in the asteroid belt between Mars and Jupiter.
So where did earth-bound space rocks come from? Researchers believe most meteorites that rain down on earth today originate from a collision that took place some 466 million years ago when something hit a massive asteroid, triggering a cascade of rocks smashing into one another. Fragments of these rocks began bombarding earth—and still do today.
But until now, scientists weren't sure what meteorites looked like before this initial Ka-Pow. A new paper, published in the journal Nature Astronomy, delves into the difference, and suggests that meteorite composition has dramatically changed overtime.
“Looking at the kinds of meteorites that have fallen to Earth in the last hundred million years doesn't give you a full picture,” Philipp Heck, lead author of the new study and cosmochemist at the University of Chicago, ays in a press release. “It would be like looking outside on a snowy winter day and concluding that every day is snowy, even though it's not snowy in the summer.”
To investigate the rocky space rain, Heck and his colleagues collected samples of micrometeorites more than 466 million years old from rock formations in the St. Petersburg region of Russia. They collected almost 600 pounds of material that contained these micrometeorites and dissolved the rocks in acid, which allowed them to pick out crystals of the precious mineral chromite, which contains hints to the chemcial makeup of ancient meteorites that fell to earth.
"Chrome-spinels, crystals that contain the mineral chromite, remain unchanged even after hundreds of millions of years," Heck says in the release. "Since they were unaltered by time, we could use these spinels to see what the original parent body that produced the micrometeorites was made of."
The researchers found that up to 34 percent of the pre-collision meteorites were a type called achondrites, which make up only 0.45 percent of meteorites today. They also found other meteorites appear to come from a collision involving Vesta, the second largest known asteroid in the solar system, which happened roughly 1 billion years ago.
“Our main finding was that these primitive achondrites and the ungrouped meteorites ... were almost 100 times more abundant than they are today,” Heck tells Kaplan. “That was a big surprise that no one expected.”
The result upends some assumptions about the Solar System. “We knew almost nothing about the meteorite flux to Earth in geological deep time before this study,” Birger Schmitz, researcher at Lund University and co-author of the paper, says in the press release. “The conventional view is that the solar system has been very stable over the past 500 million years. So it is quite surprising that the meteorite flux at 467 million years ago was so different from the present.”
Knowing how collisions in the asteroid belt have impacted the Earth in the past, Heck tells Kaplan, is not only interesting, but it can help researchers understand the behavior of objects in the asteroid belt, which have the potential of showering us with debris.