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.
    
The researchers found that small planetesimals may be much more effective than giant impactors in driving atmospheric loss.
    
Based on their 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.
    
"(This finding) sets a very different initial condition for what the early Earth's atmosphere was most likely like," Hilke Schlichting, an assistant professor in the Massachusetts Institute of Technology (MIT)'s Department of Earth, Atmospheric and Planetary Sciences, said.     

"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," said Schlichting.
    
The group at MIT and the Hebrew University of Jerusalem examined how much atmosphere was retained and lost following impacts with giant, Mars-sized and larger bodies and with smaller impactors measuring 25 kilometres or less.
    
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," researchers said.
    
To completely eject all of Earth's atmosphere, 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.
    
"For sure, we did have all these smaller impactors back then. 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 said.
    
The research was published in the journal Icarus.