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Relentless blitz of small space rocks erased much of Earth’s primordial atmosphere

The inevitable question arises: What replaced the atmosphere? It’s possible the same impactors that ejected the atmosphere also introduced new gases.

Image credit: NASA

Artist’s concept via NASA

Geochemical evidence suggests that Earth’s atmosphere might have been completely obliterated at least twice since its formation more than 4 billion years ago. The question is … how? This week (December 2, 2014), an international team of researchers from MIT, Hebrew University, and Caltech suggest a possible scenario. They say that a relentless blitz of small space rocks, or planetesimals, might have bombarded Earth around the time the moon was formed. This bombardment from space might have kicked up clouds of gas with enough force to permanently eject small portions of the atmosphere into space. The journal Icarus will publish the team’s results its February 2015 issue.

Tens of thousands of such small impacts, the researchers calculate, could efficiently jettison Earth’s entire primordial atmosphere. Such impacts may have also blasted other planets, and even peeled away the atmospheres of Venus and Mars.

In fact, the researchers found that small planetesimals – minute bodies orbiting the early sun that ultimately came together to form the planets – might be much more effective than giant asteroids in driving atmospheric loss. Based on the researchers’ calculations, it would take a giant impact — almost as massive as the Earth slamming into itself — to disperse most of the atmosphere. But taken together, many small impacts would have the same effect, at a tiny fraction of the mass.

The group examined how much atmosphere was retained and lost following impacts with giant, Mars-sized and larger bodies and with smaller impactors measuring 25 kilometers or less — space rocks equivalent to those whizzing around the asteroid belt today.

The team performed numerical analyses, calculating the force generated by a given impacting mass at a certain velocity, and the resulting loss of atmospheric gases. A collision with an impactor as massive as Mars, the researchers found, would generate a shockwave through the Earth’s interior, setting off significant ground motion — similar to simultaneous giant earthquakes around the planet — whose force would ripple out into the atmosphere, a process that could potentially eject a significant fraction, if not all, of the planet’s atmosphere.

However, if such a giant collision occurred, it should also melt everything within the planet, turning its interior into a homogenous slurry. Given the diversity of noble gases like helium-3 deep inside the Earth today, the researchers concluded that it is unlikely that such a giant, core-melting impact occurred.

Instead, the team calculated the effects of much smaller impactors on Earth’s atmosphere. Such space rocks, upon impact, would generate an explosion of sorts, releasing a plume of debris and gas. The largest of these impactors would be forceful enough to eject all gas from the atmosphere immediately above the impact’s tangent plane — the line perpendicular to the impactor’s trajectory. Only a fraction of this atmosphere would be lost following smaller impacts.

To eject all of Earth’s atmosphere completely, the team estimated, the planet would need to have been bombarded by tens of thousands of small impactors — a scenario that likely did occur 4.5 billion years ago, during a time when the moon was formed. This period was one of galactic chaos, as hundreds of thousands of space rocks whirled around the solar system, frequently colliding to form the planets, the moon, and other bodies.

During the course of the group’s research, an inevitable question arose: What eventually replaced Earth’s atmosphere? Upon further calculations, the team found the same impactors that ejected gas also may have introduced new gases, or volatiles.

The group calculated the amount of volatiles that may be released by a rock of a given composition and mass, and found that a significant portion of the atmosphere may have been replenished by the impact of tens of thousands of space rocks.

Hilke Schlichting, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says understanding the drivers of Earth’s ancient atmosphere may help scientists to identify the early planetary conditions that encouraged life to form. She said:

[This finding] sets a very different initial condition for what the early Earth’s atmosphere was most likely like. 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.

Bottom line: A relentless blitz of small space rocks might have bombarded the early Earth, kicking up clouds of gas with enough force to permanently eject the atmosphere into space.

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