The atmosphere we enjoy on Earth now is drastically different from that which existed in the planet’s early years. Earth’s primordial atmosphere is believed to have been eliminated, although until now it wasn't very clear how this actually happened. A new study reports that around the time the moon formed, heavy bombardment from small objects likely would have caused the early atmosphere to disappear. Hilke Schlichting of MIT is lead author of the paper published in Icarus, the journal of the American Astronomical Society's Division for Planetary Sciences.
Schlichting’s group proposes that after these space rocks, smaller than 25 kilometers across, pierced the atmosphere, they slammed into Earth’s surface and created clouds of gas. Because this happened thousands upon thousands of times, the gas clouds generated enough force to push out chunks of the atmosphere.
“[This finding] sets a very different initial condition for what the early Earth’s atmosphere was most likely like,” Schlichting said in a press release. “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.”
The team’s model suggests that numerous small impacts would collectively have had the same impact as the early Earth colliding with an object that is equal in size. However, if the atmosphere was lost because of an impact that large, the amount of heat that would have been generated would have melted the planet’s interior. The composition of the Earth does not indicate that this is what happened. Instead, constant impacts from smaller objects would have combined to create a force on the atmosphere that is equal, but without causing the melting of the planet’s interior.
“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,” Schlichting explained.
After Earth’s early atmosphere was eliminated, it was eventually replaced. It is likely that when the rocks slammed into the planet’s surface and created gas clouds, they introduced volatiles to the planet. The number of small rocks that would have been capable of pushing out the atmosphere would also have generated enough volatiles to replace the lost atmosphere.
“When an impact happens, it melts the planetesimal, and its volatiles can go into the atmosphere,” Schlichting continues. “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.”
Understanding what factors impacted the primordial atmosphere is important because it could help explain conditions on the planet when life first emerged. Beyond the implications for Earth, a similar experience with persistent small collisions resulting in the loss of an atmosphere could also explain why Mars does not have the thick atmosphere that used to support the existence of water. The atmosphere on Venus could have been saved by starting out with a thicker atmosphere than occurred on early Earth.
“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 concluded.