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When an iron rain fell on Earth

Now new research conducted by the California-based Lawrence Livermore National Laboratory (LLNL) gets to the core of the problem, as it were.

Over four billion years ago, when Earth was in its infancy, violent collisions between our nascent planet and other objects whizzing around the early solar system generated significant amounts of iron vapor, according to the new study by LLNL scientist Richard Kraus and colleagues.

Earth started to form over 4.6 billion years ago from the same cloud of gas and interstellar dust that went to make up the Sun and other planets of the solar system.

That process was a continuation of one that started — at least in terms of our galaxy, the Milky Way — about 13.6 billion years ago, just a few million years after the Big Bang.

As our solar system began to gel, the Sun formed within a cloud of dust and gas. The gases mostly consisted of hydrogen and helium. The massive gas cloud contracted in on itself due to its own intense gravitational forces. This contraction sparked a fusion reaction within the entity that would become the Sun, causing it to generate light, heat and other radiation.

As part of the same process, clouds of gas and dust, left over from the Sun’s formation began their own process of coming together to form lumps of matter called planetesimals. These planetesimals were effectively the embryos of what would become the planets of the solar system we see today.

This artist s conception shows a newly formed star surrounded by a swirling protoplanetary disk of d...

This artist’s conception shows a newly formed star surrounded by a swirling protoplanetary disk of dust and gas. Debris coalesces to create rocky ‘planetesimals’ that collide and grow to eventually form planets.
NASA/University of Copenhagen/Lars Buchhave

During its planetesimal phase, which lasted from approximately 4.6 billion to 4.4 billion years ago, Earth experienced a period of catastrophic and intense formation, known as the pre-Cambrian Period. It was only later, between 3.8 to 4.1 billion years ago that Earth would become a planet with an atmosphere although its atmosphere back then was radically different from today’s atmospheric blanket comprising mostly nitrogen and oxygen.

At that chaotic period when the embryonic Earth was under bombardment from other objects in the solar system, one of the chemicals that resulted from such impacts was iron vapor.

Today we think of iron mostly as a solid metal, although, in the high temperatures of furnaces, liquid iron is of course used to make steel. But iron can also exist as a vapor. It requires a lot of energy for it to do so but from their study the LLNL researchers established that these cataclysmic cosmic impacts to the early Earth provided ideal conditions for iron to vaporize as a consequence.

Their results could force planetary scientists to rethink theories on the growth of planets and the evolution of our solar system.

Predicting how planets form and evolve to what we see today is a vital and complex research area for planetary scientists. Planets usually come into being as a result of a series of impacts. These impacts are painfully slow at first — just a few miles per hour — but as planets increase in size and their gravitational pull increases, so the speed of such impacts ramps up with some impacts occurring at speeds of up to 100,000 miles per hour.

For the final stages of planetary formation, when impact speeds are at the high end of the scale, coupled with extremes of temperature and pressure, planetary scientists don’t yet have great models to illustrate what happens to the colliding bodies.

“One major problem is how we model iron during impact events, as it’s a major component of planets and its behavior is critical to how we understand planet formation,” Kraus said. “In particular, it’s the fraction of that iron that’s vaporized on impact that isn’t well understood.”

To try and replicate what was going on during Earth’s formative years, the LLNL team used the Sandia National Laboratory’s Z-Machine.

Sandia’s Z machine is the world’s most powerful and efficient laboratory radiation source. It uses high magnetic fields associated with high electrical currents to produce high temperatures, high pressures, and powerful X-rays. The Z-machine is unique in that it has the ability to create conditions found nowhere else on Earth.

The Sandia Z Machine was used to develop a new shock-wave technique to measure an important material...

The Sandia Z Machine was used to develop a new shock-wave technique to measure an important material property
Randy Montoya

LLNL scientists developed a new shock-wave technique to measure an important material property, namely the entropy gain during shock compression. Entropy, in thermodynamics, usually refers to the premise that everything in the universe eventually moves from order to disorder, for example ice melting to water. Entropy is the measurement of such a change.

By using entropy the LLNL team determined the critical impact conditions necessary to vaporize the iron within objects that collided with the still-forming Earth.

Their research revealed that iron vaporizes at significantly lower impact speeds than previously thought possible. What this means, therefore, is that more iron was vaporized during Earth’s period of formation.

An iron rain on Earth

“This causes a shift in how we think about processes like the formation of Earth’s iron core,” Kraus explained, adding, “Rather than the iron in the colliding objects sinking down directly to the Earth’s growing core, the iron is vaporized and spread over the surface within a vapor plume. After cooling, the vapor would have condensed into an iron rain that mixed into the Earth’s still-molten mantle.”

According to Kraus, determining when Earth’s core formed can only be calculated by means of the chemical signatures in Earth’s mantle. The mantle is one of Earth’s three main layers and is sandwiched between the central core and the thin outermost layer, the crust.

That technique requires assumptions to be made about how well the iron is mixed. “This new information actually changes our estimates for the timing of when Earth’s core was formed,” Kraus added.

Details of the latest LLNL research were published online, March 2, in the journal Nature Geoscience.

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