New experiments show that the asteroids that hit the Earth and moon more than four billion years ago evaporated into a fog of iron. The findings, published in Nature Geoscience, suggest that the iron fog thrown up by these high-velocity impacts was fast enough to escape the moon’s gravity, but remained gravitationally bound to the more massive Earth. And these results may help explain why the Earth’s and Moon’s chemistry differ.
When and how the Earth’s metal core formed is uncertain. Clues come from known differences in the preferences of certain elements to end up in the silicate mantle or the metallic core. In a mixture of silicate rock and iron metal, some elements, such as gold and platinum, tend to prefer to associate with the metal, while others, such as hafnium, prefer silicate.
As Earth’s iron-rich core formed, it “sucked” the metal-loving elements from the planet’s rocky mantle. However, measurements of the silicate mantle by James Day have previously shown that there are more of them near the Earth’s surface than expected. This is often attributed to a late veneer from asteroids that delivered an extra dose of metal-loving elements to the rocky mantle.
One problem with this photo was that the abundance of metal-loving elements on Earth is 10 to 100 times greater than that measured on the Moon, where the same elements would have been delivered by asteroid collisions. The chemical difference between the Earth and the Moon has been confusing and casts a shadow over the prevailing idea that the Moon formed from the same material 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 article seems to reconcile these differences. The experiment was based on Sandia National Laboratory’s “Z Machine,” a massive electromagnetic cannon — twice as powerful as the world’s total generating capacity — that can launch projectiles into iron targets at ultra-high velocity.
Richard Kraus and colleagues’ impact experiments show that, contrary to expectations, iron vaporizes under the conditions created when an asteroid collides with the Earth or the moon. A cloud of iron fog will have wrapped around the earth after such a collision and fall to the earth as metal rain. These well-mixed droplets will be absorbed into the mantle and release the excess metal-loving chemicals.
However, the same experiments indicate that the speed of the iron raindrops will have been greater than the escape velocity on the moon. The Earth would therefore have captured the metal cores of colliding asteroids, while the Moon would not have. William Anderson of Los Alamos National Laboratory, USA, said: “The Moon may have received, but not retained, a significant portion of its late veneer.”
The results could imply that models for estimating the timescales of Earth’s core formation may be off by as much as a factor of ten, with core formation occurring much earlier in Earth’s history than previously believed.
This article was originally published on The Conversation
Natural Geosciences2014. DOI: 10.1038/ngeo2369 (About DOIs).