Earth, at least as we know it, is a lot different than it once was, at least in its earliest days. Even Earth's atmosphere was once different. But that primordial atmosphere was destroyed, at least twice, and eventually replaced.

So what destroyed Earth's first atmospheres?

Scientists now believe they have the answer: a steady bombardment of space rocks hitting the Earth's atmosphere and surface.

Early theories believe that a large impact affected Earth's first few atmospheres. However, a team of scientists at MIT disproved that theory with simulations. Such a scenario required an impactor as large as Earth itself, which is unlikely. Also, if such a large impact hit Earth, it wouldn't just affect the atmosphere, but also melt everything on the planet, making the Earth's interior mushy. However, we know that there is helium-3 inside the Earth, which disproves this theory.

So what caused the loss of atmosphere? In another simulation, the MIT scientists looked at what would happen if tens of thousands of smaller rocks hit the Earth over a given period of time. Upon impact, these rocks created explosions that release debris and gas. This kicked all the gas out of the atmosphere above the line perpendicular to each rock's trajectory. And because each impact was small, the Earth's interior wasn't affected.

This might sound implausible, but when this happened, about 4.5 billion years ago, there were a lot of these rocks whirling around space. Some collided and formed planets and the moon. If this scenario proves true, other rocks like these hit Earth and destroyed its atmosphere.

"For sure, we did have all these smaller impactors back then," says Hilke Schlichting, an assistant professor in MIT's Department of Earth, Atmospheric and Planetary Sciences. "One small impact cannot get rid of most of the atmosphere, but collectively, they're much more efficient than giant impacts, and could easily eject all the Earth's atmosphere."

Eventually, though, these rocks also helped replace that atmosphere with a new one. Each small rock that hit Earth probably introduced new gases into the atmosphere, gases that eventually made up Earth's new atmosphere. Again, because there were probably tens of thousands of these small rocks hitting Earth at any given time, that's enough to replenish the atmosphere.

So why is this important? Understanding Earth's atmosphere helps us understand how life originally formed here.

"How the Earth lost its primordial atmosphere has been a longstanding problem, and this paper goes a long way toward solving this enigma," Jay Melosh, a professor of earth, atmospheric, and planetary sciences at Purdue University. "Life got started on Earth about this time, and so answering the question about how the atmosphere was lost tells us about what might have kicked off the origin of life."

[Photo Credit: NASA]

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