It's common knowledge that the early solar system was a fairly inhospitable place, as massive rocks continuously slammed together for millions of years, eventually creating planets. New research has indicated that during the first 500 million years of Earth’s history, huge asteroids bombarded the planet, causing drastic mixing and reshaping of the surface, which explains why we don’t have samples of those earliest rocks. The research was completed through an international collaboration and the paper was published in Nature. 

Our solar system began to form roughly 4.5 billion years ago. As the dust and gas coalesced into progressively larger rocks that slammed into one another, the rocks that would eventually be known as Earth went through several phases of bombardment that shaped the planet as we see it today. Shortly after the Earth was formed, it collided with a planetary body roughly the size of Mars and generated the debris that would form our moon. Later, it got pelted with asteroids so large, they made the one that wiped out the dinosaurs look like a pebble. 

The Hadean geological eon represents everything that happened prior to four billion years ago. The asteroid collisions near the end of this period did not add a lot of mass to Earth, but were powerful enough to bury or melt some of the earliest rocks on the surface. Additionally, the heat generated from the impacts may have boiled entire oceans, creating a heavy, humid atmosphere multiple times. Previous studies have indicated that water has been on Earth for about 4.3 billion years, and this model does support that claim.

"Prior to approximately four billion years ago, no large region of Earth's surface could have survived untouched by impacts and their effects," lead author Simone Marchi said in a press release. "The new picture of the Hadean Earth emerging from this work has important implications for its habitability.”

So how big were these asteroids? According to this model, as many as four could have been over 600 miles wide while around 3-7 were closer to 300 miles wide. For a size comparison, the asteroid that hit 65 million years ago was around 6 miles wide. Life is believed to have originated within the first billion years of Earth’s history and could have been greatly affected by these impacts. The larger asteroids may have been capable of incinerating life all around the planet, while the smaller ones would have been enough to turn entire oceans into steam. However, these collisions were spaced out and allowed for conditions to settle down.

"During that time, the lag between major collisions was long enough to allow intervals of more clement conditions, at least on a local scale," said Marchi. "Any life emerging during the Hadean eon likely needed to be resistant to high temperatures, and could have survived such a violent period in Earth’s history by thriving in niches deep underground or in the ocean’s crust.”

Size and location of craters by asteroids, color-coded by time of impact. Image Credit: Simone Marchi et al. 2014