How did iron become scattered throughout Earth's 2,000-mile (3,200km) thick mantle more than four billion years ago? That's a question that has puzzled scientists, with most thinking asteroid impacts were the cause, but now a new theory has been presented - iron rain. By re-analysing the vapourisation point of iron, a study has determined that conditions on early Earth would have caused iron to rain down on the ground. Scientists in New Mexico have a new theory for the origin of Earth's iron. Research suggests iron would have turned into vapour when meteorites hit the young Earth, as shown in the artist's impression above. The vapour would then have drifted up in plumes, condensed and then rained down on the surface of our planet . The research was conducted with the Z machine at Sandia National Laboratories in Albuquerque, New Mexico. The machine is the most powerful producer of pulses of electrical energy on Earth. By studying the shock pressure required to vapourise iron, they found the value was 40 per cent less than previously thought. Astrophysicists say this lower pressure - 507 gigapascals (GPa) compared to 887 GPa - was easily achieved during the latter stages of Earth's formation. 'Because planetary scientists always thought it was difficult to vapourise iron, they never thought of vapourisation as an important process during the formation of Earth and its core,' said principal investigator Dr Rick Kraus. The researchers created a target - consisting of an iron plate 0.008in (5mm) square and 200 microns thick - against which they launched aluminum flyer plates travelling up to 16 miles (25km) per second. At this impact pressure, the powerful shock waves created in the iron cause it to compress, heat up and vapourise. The results showed that the vapourisation threshold for iron is 40 per cent lower than previously thought. 'But with our experiments, we showed that it's very easy to impact-vapourise iron.' The result of this would have been that a solid piece of iron, after impacting Earth perhaps as part of an asteroid, would disperse into an iron vapour due to the force of the impact. Earth is thought to have been pummelled by asteroids during the Late Heavy Bombardment period, 4.1 to 3.8 billion years ago. Forming clouds across the surface, the vapour would then rain down onto the ground and mix with the mantle, which was molten at the time. 'This changes the way we think of planet formation, in that instead of core formation occurring by iron sinking down to the growing Earth's core in large blobs (technically called diapirs), that iron was vapourised, spread out in a plume over the surface of Earth and rained out as small droplets,' said Dr Kraus. 'The small iron droplets mixed easily with the mantle, which changes our interpretation of the geochemical data we use to date the timing of Earth's core formation.' The research, which found iron vapourised at values 40 per cent lower than thought, was conducted with the  Z machine (pictured) at Sandia National Laboratories in Albuquerque, New Mexico. The machine is the most powerful producer of pulses of electrical energy on Earth . In previous theories, it was thought that iron arrived from collisions between Earth and planetesimals - rocks ranging in size from several metres to hundreds of kilometres in diameters. This would have happened in Earth's late formative stages more than four billion years ago, with the iron being sent bullet-like into the region between the core and crust - the mantle. But if that were true, then the moon would also have similar amounts of iron in its mantle, as it underwent the same bombardment - but this does not appear to be the case. This led the scientists to their new conclusion - and they add that the moon's gravity would have been too weak for it to have held onto iron rain-bearing vapour, like Earth. This accounts for the lack of iron seen scattered in the moon's mantle. According to the scientists, the moon's gravity would have been too weak for it to have held onto iron rain-bearing vapour, like Earth, during the Late Heavy Bombardment period (artist's impression of moon shown). This accounts for the lack of iron seen scattered in the moon's mantle .
Scientists in New Mexico have a new theory for origin on Earth's iron . Previously it was unclear how iron was scattered throughout the mantle . But research suggests the vapourisation point of iron is lower than thought . This means it would have turned into vapour when rocks hit early Earth . It would have drifted up in plumes and then rained down on the surface .