Planetary Scientists Shed New Light on Earth’s Core Formation
Violent collisions between the infant Earth and other objects in our Solar System generated significant amounts of iron vapor, says a group of researchers led by Dr Richard Kraus of Harvard University and the Lawrence Livermore National Laboratory.
Earth. Image credit: Kelvin Song / CC BY-SA 3.0.
Dr Kraus and his colleagues used one of the world’s most powerful radiation sources, the Sandia National Laboratories Z-machine, to recreate conditions that led to the formation of our planet.
They developed a new technique to determine the critical impact conditions needed to vaporize the iron.
They subjected iron samples to high shock pressures in the machine, slamming aluminum plates into iron samples at extremely high speeds.
The results, published in the journal Nature Geoscience, show that iron vaporizes easily during impact events, which forces planetary scientists to change how they think about the growth of planets and evolution of our Solar System.
Dr Kraus said: “the results may shift how planetary scientists think about the processes and timing of Earth’s core formation.”
For scientists, one of the most important and complex research areas is predicting how planets form and evolve to their current state.
Generally speaking, planets form by a series of impacts, with the speed of the impacts being slow at first, a few km per hour, but then faster as the planets grow larger, up to 160,000 km per hour.
At the end stages of formation, when the impact speeds are high and the material conditions are extreme, scientists don’t have great models for how to describe what happens to the colliding bodies.
“One major problem is how we model iron during impact events, as it is a major component of planets and its behavior is critical to how we understand planet formation,” Dr Kraus said.
“In particular, it is the fraction of that iron that is vaporized on impact that is not well understood.”
He added: “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. This means that the iron can mix much more easily with Earth’s mantle.”
“The timing of Earth’s core formation can only be determined via chemical signatures in Earth’s mantle, a technique that requires assumptions about how well the iron is mixed,” he noted.
“This new information actually changes our estimates for the timing of when Earth’s core was formed.”
The findings may also explain why the Moon lacks iron-rich material despite being exposed to similarly violent collisions.
The scientists suggest the Moon’s reduced gravity could have prevented it from retaining most of the vaporized iron.
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Richard G. Kraus et al. Impact vaporization of planetesimal cores in the late stages of planet formation. Nature Geoscience, published online March 02, 2015; doi: 10.1038/ngeo2369
