Earth: Huge impacts produced heavy elements

Explanation of hitherto mysterious occurrence of siderophilic elements discovered in the Earth's mantle

Meteorite impact © Don Davis / NASA / JPL
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The earth may have heavy elements such as palladium, gold or osmium in the earth's mantle, possibly due to a series of catastrophes: the impact of huge, pluto-sized boulders nearly 4.5 billion years ago. As researchers now show in Science, these impacts could have replenished Palladium and Co. after the other heavy elements had already sunk into the Earth's core.

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Huge impacts and collisions of cosmic rocks were not uncommon in the early days of the solar system. Also, the moon probably formed around 4.5 billion years ago from the wreckage of a collision of the early Earth with a Mars-sized planetoid. Only after this catastrophe so the common theory, set inside the two celestial bodies, the core formation. Heavy elements such as iron or nickel sank inside, while lighter elements and connections stayed higher up.

Riddle about heavy elements in the mantle

But this is exactly where the discrepancies between theory and reality begin: For a certain group of heavy elements, the so-called siderophiles, which also include cobalt, osmium, palladium, manganese or gold, have by no means sunk into the earth's core but are still found today often in the mantle and in the earth's crust. "The big problem for the modelers is that these metals are not missing, but instead are relatively common, " explains Richard J. Walker of the University of Maryland. Until now, it was completely unclear where these elements came from and why they did not also land in the core.

Meteorite impacts as element suppliers?

Now, however, a research team from the NASA Lunar Science Institute (NLSI) led by William Bottke of the Southwest Research Institute is providing a possible explanation for this for the first time. Thus, the elements in question may have reached the Earth only after the Earth's interior was differentiated - by the impact of meteorites carrying the elements. However, in order to explain the amount of palladium and Co. present in the earth's mantle, the impacts would have to have totaled 0.5 percent of the total earth mass. This corresponds to at least a third of the lunar mass. display

Impactors of Pluto size

That sounds immense, but it is conceivable, as Bottke and his colleagues now use numerical models. The required quantities can be reproduced if the impacts were made by objects of at least 2, 400 to 3, 200 kilometers in size as large as the dwarf planet Pluto. Few inclusions within the first ten million years after the formation of the moon could have sufficed to enrich the mantle with the siderophilic elements.

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"These impactors were large enough to produce the observed enrichments with highly siderophilic elements, but not so large that their fragmented nuclei would be fused to the planetary core, " explains Bottke, "They probably represent the largest objects that hit these worlds since the giant collision that shaped our moon."

Some of these inclusions could even have changed the inclination of the Earth's axis by up to ten degrees. Others, smaller impacts, hit the moon and carry water down to its mantle.

(Southwest Research Institute, 10.12.2010 - NPO)