Silica-Rich Materials In Earth's Crust May Have Rained Down From A Steamy Atmosphere

Over 90 percent of Earth’s crust — the thin outer layer of the planet — consists of silicate minerals. Geologists have long believed that these minerals were formed by volcanic activity, but in a new study, researchers from McGill University in Canada argue that the silica-rich minerals may have been deposited in the crust of a young Earth by “silicate rain.”

The study, published in the latest edition of the journal Earth and Planetary Science Letters, calls this mechanism of deposition of silicate minerals “aerial metasomatism.”

Read: Scientists Discover Parts Of Earth's Original Crust

According to the widely accepted “giant impact hypothesis,” proposed in the 1970s, the moon formed about 4.5 billion years ago when a Mars-sized planet slammed into early Earth. This theory suggests that most of what became the moon came from this “giant impactor,” which may very well have had a composition similar to that of Earth’s.

In the wake of that impact, Earth’s surface gradually cooled until it was more or less solid. However, Earth’s atmosphere remained turbulent for much longer, and our planet may have been shrouded in a cloud of hot steam for up to a million years.

According to the authors of the study, this high-temperature atmosphere dissolved rocks on the Earth’s surface, and carried the dissolved minerals to the upper atmosphere.

“Then these silicate materials that were dissolved at the surface would start to separate out and fall back to Earth in what we call a silicate rain,” Don Baker, an Earth scientist at McGill University, said in a statement released Friday. “Our experiment shows the chemistry of this process.”

In their experiment, Baker and colleague Kassandra Sofonio melted a mixture of silicate materials and water at a temperature of about 1,550 degrees Celsius (2,822 degrees Fahrenheit), and then ground it to a powder. The powder, along with water, was then enclosed in gold palladium capsules, placed in a pressure vessel and heated to about 727 degrees Celsius and subjected to 100 times the surface pressure of Earth — conditions simulating the young Earth’s atmosphere.

After carrying out a series of such experiments and analyzing the material that had been dissolved in the high temperature steam, they found that the dissolved silicate material were surprisingly similar to that found in Earth’s crust.

“This time in early Earth's history is still really exciting,” Baker said. “A lot of people think that life started very soon after these events that we're talking about. This is setting up the stages for the Earth being ready to support life.”

The findings of the study are still, to an extent, speculative, as they are dependent on the giant impact hypothesis being true. And this, if some recent computer simulations are to be believed, is far from certain.

The simulations suggest that the moon may have formed when several small "moonlets" — created not by a single impact with one giant body, but due to multiple impacts with a series of smaller ones — coalesced into a single object. This removes the need for a giant impactor, and, by extension, the premise the latest study is based on.