Earth's Early Atmosphere Obliterated by Space Rocks

Earth's atmosphere today looks much different compared to its primordial self from four billion years ago when it first formed. One explanation, new research shows, could be tens of thousands of small impacts from space rocks that obliterated this early atmosphere.

Scientists have long wondered the cause behind such dramatic atmospheric losses, and now a team from the Massachusetts Institute of Technology (MIT) may have found the answer.

Around the time the Moon was formed, supposedly a relentless parade of small space rocks, or planetesimals, bombarded Earth, kicking up clouds of gas with enough force to permanently remove portions of the atmosphere and eject them into space. Such impacts may have even peeled off the atmospheres of other planets, such as Venus and Mars.

"[This finding] sets a very different initial condition for what the early Earth's atmosphere was most likely like," researcher Hilke Schlichting said in a statement. "It gives us a new starting point for trying to understand what was the composition of the atmosphere, and what were the conditions for developing life."

Super Smash

You would think that one extremely massive smash would be most effective at wiping out the entire atmosphere - researchers even calculated that it would take an impact the size of Earth itself to do that. But it turns out that thousands of small collisions can do just as much damage.

To find out just what kind of celestial bodies early Earth was getting hit with, the MIT team compared collisions with giant, Mars-sized impactors and smaller ones measuring 25 kilometers or less. The latter would be equivalent in size to space rocks currently whizzing around the asteroid belt - space debris that makes astronomers worry of a future impact event.

It is unlikely, researchers say, that one giant impact tore off piece of our early atmosphere millions of years ago. While such an event could indeed eject a significant fraction, if not all, of the planet's atmosphere, it would also completely melt Earth's interior.

A barrage of thousands of smaller rocks, therefore, is more likely what happened, especially 4.5 billion years ago when space rocks whirled around the solar system, frequently colliding to form the planets, the Moon, and other bodies. The team estimates that such objects collided with Earth at least twice since its formation.

Runaway Process

But if all of these small rocks were zooming around the solar system, shouldn't another nearby planets like Venus display a similar atmosphere?

Atmospheres are composed of noble gases like helium, and compared to Venus, Earth's noble gas budget has been depleted 100-fold.

It seems that due to a phenomenon known as the runaway process, once half of the planet's atmosphere has been lost, it's much easier for space rocks to get rid of what's left.

"Once you manage to get rid of the first half, the second half is even easier," Schlichting explained.

By this calculation, Venus' atmosphere would only have to start out slightly more massive than Earth's in order for it to remain intact while small impactors eroded the first half of Earth's atmosphere.

Origin of Life

"How the Earth lost its primordial atmosphere has been a longstanding problem, and this paper goes a long way toward solving this enigma," said Jay Melosh, a professor of earth, atmospheric, and planetary sciences at Purdue University, who was not involved in the study. "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."

Schlichting and her team found that the same space rocks that ejected gas and stripped Earth of part, or all, of its atmosphere also may have in fact introduced new gases, thereby replenishing the atmosphere it had depleted. In turn, they may have led to the creation of an inhabitable planet.

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