Cloudy With a Chance of Iron Rain? How Prehistoric Earth May Have Gotten Its Metal

It’s a well known fact that our solar system, and particularly our planet, has endured a number of disasters from its very beginning as a chunk of rock some 4.6 billion years ago. An early collision with Mars, aptly named for the god of war, may be among the events responsible for first bringing life to Earth. Another collision is responsible for the chunk of earth that broke off to form our moon. Relentless meteors have pummeled its surface ever since, some quite harmless, while others brought about massive extinction events. Earth in its very beginning was hardly a hospitable place to be.

Among the many objects to hit the Earth within its first billion years, many of the meteors were rich in iron. These collisions could have led to it becoming so prevalent across the planet as well as one of the essential minerals needed to support many different forms of life. Iron in these early days would have infiltrated the atmosphere onto Earth’s crust, but also would have melted its mantle at high rates as well. These same meteorites may have also left behind metals such as gold or platinum which easily bond with iron. What the model has not convincingly explained yet, is how the iron is so prevalent that it makes up so much of the mantle of our planet.

Researcher Richard Kraus of the Lawrence Livermore National Laboratory of California, wanted to take the research a step further, in order to find the best way that would measure exactly how iron would behave under such harsh conditions as our planet’s first days, and what would sort of extreme heat would be necessary for iron to vaporize completely.

“We’re never really going to be able to get a situation where we can simulate the actual planetary impact, with objects a thousand kilometres across. It would just be too destructive,” says Kraus. “We’re taking a step back and saying, let’s make a fundamental measure of the entropy of iron.”

In order to investigate further, the team employed the Z machine from the Sandia National Laboratory of Albuquerque, New Mexico, a machine used to accelerate metals to the most extreme speeds with the help of high magnetic fields.

For their project, they shot small iron samples with aluminium plates, each less than a centimeter square and about 1.2 millimeters thick. These plates were accelerated between 30,000 to 40,000 miles per hour. The result was a powerful collision in which shock waves rattled through the iron, causing the pieces to compress and then heat up before they eventually vaporized. The researchers were then able to determine how the properties they found in this lab-made iron rain worked by having them drop upon a window composed of quartz, solid enough to withstand the dropping particles.

Through their experimentation, they soon discovered that it required considerably less pressure for them to vaporize the iron than researchers had once thought – in fact, a full 40 per cent below their original estimation. This realization is painting an entirely new vision of what the early Earth must have looked like. The meteors entering our orbit would typically vaporize upon their impact due to the extreme temperatures and pressure as they accelerated at speeds perhaps faster than those tested under lab conditions. These vanquished meteors would then send a boiling hot plume composed primarily of iron and rock dust into the air. This mixture would afterwards rain down, allowing it to easily and thoroughly blend into the Earth’s mantle.

The way in which iron behaves under pressure also suffices to explain why our moon has significantly less metal across its surface compared to Earth. Many suspect that since it broke off from the Earth, the two bodies should have a similar if not identical composition. Any iron that vaporized from meteor collisions on the moon would instead be able to escape back in space, considering that the moon has relatively low levels of gravity.

The beginning of our planet, may have been little more than chemistry – reactions of not only celestial bodies that smashed into each other, but of what they left behind.

“The reason we’re able to mine gold and make jewellery out of it, and mine palladium and make catalytic converters, is because the silicates have much higher abundances of these elements than one might expect,” said Richard Walker from the University of Maryland. “This is a pretty good way of explaining how they got here and why they’re not located 2900 kilometres below your feet in the core.”

Kraus’ work was published this week in the journal Nature Geoscience.