When it comes to space rocks colliding with Earth, two stand out. There is the one that killed the dinosaurs 65 million years ago (goodbye T. rex, hello mammals!) and the one that formed Earth's moon. The asteroid that hurtled toward the Yucatan Peninsula and wiped out the dinosaurs was only 10 kilometers in diameter. On the other hand, the body that formed the Moon was probably about the size of Mars. But between the massive impact that formed the Moon and the relatively scant precursor to the death of the dinosaurs, the Earth must have been hit by other objects.
At the 2023 fall meeting of the American Geophysical Union, scientists discussed what they have discovered when it comes to how our planet was shaped by asteroids that impacted the early Earth, causing everything from Huge melting covers areas of the surface to Ancient tsunamis that tore across the world.
When the impacting object on the Moon collided with Earth, much of the world became a sea of molten rock called a magma ocean (if it hadn't already melted ). After that point, he said, Earth no longer has any significant additions to mass Simone Marchi, a planetary scientist at the Southwest Research Institute who created computer models of the early solar system and its planetary bodies, including Earth. “But this debris is still flying,” he said. This later phase of accretion probably lacked another lunar-scale impact, but likely included incoming large asteroids. Predictions regarding the size and frequency distribution of this space debris indicate that “there should be a large number of objects larger than, say, 1,000 kilometers in diameter,” Marchi said.
Unfortunately, there is little clear evidence in the rock record of these impacts until about 3.5 billion years ago. So scientists like Marchi can look at the Moon to estimate how many objects must have collided with Earth.
Armed with the size and number of impacts, Marchi and his colleagues built a model that describes, as a function of time, the volume of melting that should result from this impact on the Earth's surface. Magma oceans existed in the past, but impacts larger than 100 km in diameter still melted a lot of rock and must have radically changed the early Earth.
Unlike smaller impacts, the volume of melting produced by objects of this size is not localized within the crater, according to the models. Any crater only exists momentarily, as the rock is too fluid to maintain any kind of structure. Marchi compares this to throwing a stone into water. “There's a moment in time where there's a cavity in the water, but then it all collapses and fills up because it's a liquid.”
The volume of melt is much larger than the amount of rock extracted, so Marchi can calculate how much melt may have spilled over and covered parts of the Earth's surface with each impact. The result is a stunning map of melt volume. During the first billion years or so of Earth's history, almost the entire surface had a crust of molten impact at some point. Much of this history is gone because the atmospheric, surface, and tectonic processes of our active planet are constantly modifying much of the rock record.
Until between 3.5 and 2.5 billion years ago, the rock record was sparse. But two places, Australia and South Africa, preserve evidence of impacts in the form of pellets. These tiny glass balls form immediately after the impact that sends the vaporizing rock skyward. As the plume returns to the ground, small droplets begin to condense and rain.
“It is remarkable that we can find these spherical impact layers dating back 3.5 billion years,” Marchi said.
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