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Scientists, led by Gary Byerly from Louisiana State University, have found evidence for the oldest impact on Earth, 3.47 billion years ago in the Archean Era, which may testify to a bombardment of the Earth by cosmic projectiles. The research, published today in the journal science, reports the dating of four spherule layers from South Africa and Australia using the decay of radioactive elements found within resistant zircon minerals.
The spherules contained within the layers are thought to have formed as tiny droplets of molten rock condensed from the rock gases produced by vaporisation of the Earth's surface during the impacts. In large impacts the rock gases forms a plume that can escape from the atmosphere, cool and condense into droplets that fall out over the entire surface of the planet. The presence of iridium within the layers suggest their formation by impact since although this element is rare in the Earth's crust it is found in meteorites.
A similar spherule later to those found in the Archean rocks was formed during the impact which caused the extinction of the Dinosaurs 65 million years ago. This K/T spherule layer is, however, much thinner, at only a few centimetres, than those found in the Archean rocks which are around 30 cm thick. On the basis of the size of the layers the researchers suggest that the cosmic projectile was probably twice as large as the K/T impactor at 20 km across.
The rocks in which the layers are found are amongst the oldest on Earth thought to have formed in a shallow sea, only 2 km depth, that covered much of our planet's surface at this time. Only within stable areas of the later formed land masses have rocks of this age survived and those in South Africa and Australia are amongst the least altered by heat and pressure. Most other areas of the Earth's surface from the Archean, however, have been lost by subduction into our planet and it, therefore, seems likely that the massive craters that formed from the impacts will never be found.
The discovery of four impact layers in the 3.47 billion year old rocks suggests that the Earth was being bombarded by asteroids or comets at this time. Scientists have known that between 4.0 and 3.8 billion years ago a great bombardment occurred that also formed the largest of the craters on the Moon. Comets in this great bombardment are thought to have added the water present in the oceans and provided the basic ingredients for life. The discovery of a later bombardment suggests that another wave of cosmic projectiles may have been cast into the inner solar system. Simple organisms in the form of bacteria had already appeared on Earth by this time and this late bombardment is thus likely to have affected early life on Earth.
More info: Science
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