Barrage Of Small Asteroids Likely Wiped Out Earth's Atmosphere

Brett Smith for redOrbit.com – Your Universe Online

Previous research has indicated that Earth has lost its atmosphere at least twice since being formed, and a new study has indicated that a bombardment of tens of thousands of space rocks could have been behind the phenomenon.

The bombardment theory in the new study, published in the journal Icarus, refutes a different theory on how the Earth could have had its atmosphere erased – a single, massive impact.

The study team found that thousands of small impacts would have been much more efficient than one giant impact. In fact, a single impact would have had to come from a planetary body almost the same size as Earth – according to the study’s calculations.

Study author Hilke Schlichting, a planetary scientist at MIT, said his team’s work could also help illuminate just how life arose on primordial Earth.

“(This finding) sets a very different initial condition for what the early Earth’s atmosphere was most likely like,” Schlichting said in a statement on Tuesday. “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.”

To reach their conclusion, the team conducted numerical analyses, determining the force produced by an impacting mass at a particular speed and the ensuing loss of atmospheric gases. An impact with something as big as Mars, the scientists discovered, would have produced a shockwave that passed through Earth’s interior, unleashing synchronized massive earthquakes across the planet. The impact force would have then swelled out from the surface, potentially ejecting a considerable part of the planet’s atmosphere.

The research team also concluded that this type of massive impact would have liquefied the planet’s interior. However, evidence based on helium-3 deep inside the Earth suggests that such a liquefaction did not occur.

Next, the team considered the possibility that the Earth was blitzed by tens of thousands of much smaller “planetesimals,” which many scientists say happened about 4.5 million years ago.

“For sure, we did have all these smaller impactors back then,” Schlichting said. “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.”

In calculating the small-impacts scenario, the study researchers came across an interesting conclusion: After one half of a planet’s atmosphere had been lost via these thousands of collisions, it became much easier for additional impacts to drive out the remaining atmosphere.

The researchers also looked into how Earth regained its atmosphere after losing it. They calculated that these small impactors most likely seeded the Earth for the growth of a new atmosphere.

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

“Our numbers are realistic, given what we know about the volatile content of the different rocks we have,” Schlichting said, referring to her team’s conclusion.

Jay Melosh, a professor of planetary sciences at Purdue University, said he was surprised that Schlichting’s team came to this conclusion, refuting the single-impact theory.

“How the Earth lost its primordial atmosphere has been a longstanding problem, and this paper goes a long way toward solving this enigma,” said Melosh, 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.”

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Source: http://www.redorbit.com/news/space/1113291627/barrage-of-small-asteroids-likely-wiped-out-earths-atmosphere-120314/