Is iron rain the reason why Earth and the moon are so different?

March 3, 2015 by Simon Redfern, The Conversation
Artist’s concept of the moon-forming collision. NASA

New experiments show that the asteroids that slammed into Earth and the moon more than 4 billion years ago were vaporised into a mist of iron. The findings, published in Nature Geoscience, suggest that the iron mist thrown up from the high velocity impacts of these asteroids travelled fast enough to escape the moon's gravity, but stayed gravitationally stuck on more massive Earth. And these results may help explain why the chemistry of the Earth and the moon differ.

When and how Earth's metallic core formed is uncertain. Clues come from known differences in the preferences of certain elements incorporated in the silicate mantle or the metal core. In a mixture of silicate rock and iron metal, the atoms of certain elements, such as gold and platinum, tend to prefer to enter the metal, while others, such as hafnium, prefer the silicate.

As Earth's iron-rich core formed it "sucked" the metal-loving elements out of the planet's rocky mantle. However, measurements of the silicate mantle by James Day have previously shown that there are more of them left in the shallower Earth than would be expected. This has often been attributed to a late veneer of asteroids that delivered an extra dose of metal-loving elements to the .

One problem with this picture has been that the abundance of the metal-loving elements on Earth is ten to a hundred times greater than that measured on the moon, which should by this argument have the same veneer. The chemical difference between Earth and the moon has been perplexing, and casts a shadow over the prevalent idea that the moon formed from the same stuff as Earth after an impact from a Mars-sized planet early in the history of the Solar System.

Mighty Earth attracts more metal

The new paper seems to reconcile these differences. The experiment relied on Sandia National Laboratory's "Z-machine": a huge electromagnetic gun – twice as powerful as the world's total generating capacity – that can launch projectiles into iron targets at ultra-high velocity.

The Z machine generates electric currents of up to 20 million amps, to shoot aluminium projectiles at iron targets, replicating the impacts of early asteroids. CC BY

The impact experiments by Richard Kraus and colleagues show that iron vaporises under the conditions created when an crashes into Earth or the moon. A cloud of iron mist will have wrapped around the globe after any such collision, falling to Earth as metal rain. These well-mixed droplets will have become incorporated into the mantle, delivering the excess metal-loving chemicals.

The same experiments, however, indicate that the velocity of the rain droplets will have been greater than the escape velocity on the moon, but below that of Earth. Earth would therefore have captured the metal cores of colliding asteroids, while the moon will have failed to. William Anderson of Los Alamos National Laboratory, US, said: "The may have received, but not retained, a significant portion of the late veneer."

The results could imply that models for estimating the time scales of Earth's core formation could be out by as much as a factor of ten, with the core forming much earlier in Earth's history than previously recognised.

Explore further: Core work: Iron vapor gives clues to formation of Earth and Moon

Related Stories

The Earth and Moon formed later than previously thought

June 7, 2010

The Earth and Moon were created as the result of a giant collision between two planets the size of Mars and Venus. Until now it was thought to have happened when the solar system was 30 million years old or approx. 4,537 ...

A wet Moon

March 26, 2014

The Moon's status as a "dry" rock in space has long been questioned. Competing theories abound as to the source of the H20 in the lunar soil, including delivery of water to the Moon by comets.

Fresh insight into the origins of Planet Earth

June 3, 2010

For the first time, an international team of researchers has incorporated extensive geochemical data on the formation of Earth into a model - with surprising results: more models can be used for the process of Earth’s accretion ...

Recommended for you

Gauging the effects of water scarcity on an irrigated planet

April 20, 2018

Growing global food demand, climate change, and climate policies favoring bioenergy production are expected to increase pressures on water resources around the world. Many analysts predict that water shortages will constrain ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Elliott Shoals
2 / 5 (4) Mar 03, 2015
So many scientific findings make perfect sense in light of God creating the heavens and the earth as He claims. In the beginning the earth was without form. He created the moon and stars and sun on the fourth day. There is no reason to assume that the moon was made from the same materials as earth. The earth was created to be inhabited, and there are trillions of details, every one of them critical, to provide for human life, comfort and even our enjoyment.
jwozniak
3 / 5 (2) Mar 05, 2015
Glad we Catholics emerged from the medieval age a few centuries ago. Or, as Pope Francis puts it: "God is not a magician."

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.