The Earth went through some tough times after its formation 4.5 billion years ago. Geochemical evidence indicates that its atmosphere was obliterated at least twice. One theory is that a large object from space smashed into our planet's surface, but a new study suggests it was actually thousands of small impacts.

Tens of thousands of small impacts, to be more precise. MIT researchers, led by Professor Hilke Schlichting, say that number would have been sufficient to kick up clouds of gas with enough force to eventually jettison Earth's entire primordial atmosphere into space.

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The scientists arrived at their conclusion by calculating the effects of various-sized objects striking the Earth. An impactor as massive as Mars would have generated an immense shockwave through our planet's interior. The result would be global simultaneous earthquakes—whose force would ripple up into the atmosphere, ejecting it into space.

However, a collision that size would also have melted the entire interior of the planet, creating a homogenous slurry. The researchers doubt that this core-melting incident occurred, given the diversity of noble gases deep beneath the surface today.

Instead, the scientists found that a constant bombardment of smaller rocks would have wreaked havoc more efficiently. And the timing would have been right for such a scenario. Around 4.5 billion years ago, when our Moon was being formed, thousands of rocks were zipping around the solar system, frequently colliding with one another.

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During the course of the group's research, however, an inevitable question arose: What eventually replaced Earth's atmosphere?

Upon further calculations, Schlichting and her team found the same impactors that ejected gas also may have introduced new gases, or volatiles.

"When an impact happens, it melts the planetesimal, and its volatiles can go into the atmosphere," Schlichting says. "They not only can deplete, but replenish part of the atmosphere."

Going forward, Schlichting hopes to examine more closely the conditions underlying Earth's early formation, including the interplay between the release of volatiles from small impactors and from Earth's ancient magma ocean.

"We want to connect these geophysical processes to determine what was the most likely composition of the atmosphere at time zero, when the Earth just formed, and hopefully identify conditions for the evolution of life," Schlichting says.