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New research presented today at the Lunar and Planetary Science Conference in Houston suggests that giant impacts in our planet’s history may have blown off most of the Earth’s atmosphere. Just how much atmosphere can be removed by impacts was described by Dr Tom Ahrens, from the Californian Institute of Technology.
Ahrens explains that there are two ways to lose an atmosphere. In the first the atmosphere is literally blasted off. An impact releases so much energy that it vapourises huge amounts of rock and generates, a massive plume of hot gas that expands at enormous speeds above the forming crater. The impact plume will blow off the atmosphere above it and some, which is travelling the fastest, can escape from the Earth’s gravity entirely. Even in large impacts, however, the amount of atmosphere removed by the expanding plume is relatively small. The impact of a 150-km diameter asteroid, for example, would remove about 5% of the Earth’s atmosphere. Only if many such impacts occurred could they remove the most of the atmosphere.
The second way the atmosphere can be lost is through the violent shaking generated by a collision. Large impacts generate enormous earthquake waves that travel at many kilometres per second. Those that travel over the surface of the Earth generate winds in the atmosphere and wave after wave can produce make them fast enough to launch them into space. Most of the atmosphere lost due to this blow off is removed from the antipode region of the Earth, the opposite side of the planet to the impact, because it is here that the surface waves meet and become amplified.
Ahrens’ calculations indicate that it takes very large impacts indeed to blast off most of the atmosphere with objects the size of the Moon or Mars needed. Such impacts have occurred in the Earth’s past. The Moon, for example, was probably formed by the impact of an object the size of Mars several tens of millions of years after the Earth had formed. This impact would have removed all of the Earth’s first atmosphere.
The collision of many smaller bodies of 150 km across could also remove a large amount of the Earth’s atmosphere. This probably happened on Earth during the Great Bombardment, 4.4 to 3.8 billion years ago, a period when large numbers of objects collided with Earth. This repeated removal of our planet’s atmosphere changed the very nature of our world and ultimately played a part in shaping the world we now live in.
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