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End of planet formation marked by giant impacts
Posted: 10 December 2010

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New research suggests that massive impactors delivered metal-loving elements to the mantles of the Earth, Moon and Mars during the final phase of planet formation, over 4.5 billion years ago.

It is difficult to decipher the early formation history of the inner Solar System planets since much of the evidence has been erased by impacts and the recycling of the Earth's crust through plate tectonics, for example. But by studying lunar rocks returned from the Moon by apollo astronauts, and meteorites from Mars, and combining that with computer simulations of planet formation, it is possible to build up a picture of this tumultuous time period.

Giant collisions in the inner Solar System 4.5 billion years ago marked the end of the planet forming phase. Image: Lynette Cook for Gemini Observatory/AURA.

In the new study, solar system dynamicists, and geophysical and geochemical modelers joined forces to probe how the inner Solar System planets formed and evolved. They predict that the largest of the impactors colliding with the Earth at the end of the planet-building phase were between 2,400 and 3,200 kilometres in diameter, and for the Moon 240-320 kilometres across.

Scientists already know that the Moon was likely caused by a giant impact of a Mars-sized object colliding with the Earth, an event which inevitably led to the final phase of core formation and global magma oceans on both the Earth and Moon. For this giant impact hypothesis to be correct, however, geologists would expect metal-loving elements such as gold, palladium, platinum and iridium to be bound up with iron in the core, leaving the rocky crusts and mantles of these bodies void of these elements.

However, says Richard J. Walker from the University of Maryland, “The big problem for the modelers is that these metals are not missing at all, but instead are modestly plentiful.”

There is a way around this conundrum, though. Scientists suggest that the iron-loving elements were indeed stripped from the mantle in the giant impact event, but then were later replenished by impacts from other large bodies. To match observations, these late impactors would have to deliver 0.5 percent of the Earth's mass to its mantle.

Computer simulations showed this could be possible if the last stages of planet building were dominated by several giant impactors, comparable in size to Pluto slamming into the Earth. These large planetisimals would have originated by hoovering up smaller rocky bodies left over from the initial planet formation phase, giving rise to this late population of large impactors.

“These impactors are thought to be large enough to produce the observed enrichments in highly siderophile elements, but not so large that their fragmented cores joined with the planet’s core, says William Bottke from the Southwest Research Institute. “They probably represent the largest objects to hit those worlds since the giant impact that formed our Moon.”

Projectiles this large could have contributed to the Earth's axial tilt, as well as created the largest known basins on the Moon and Mars. For example, the largest impactor predicted to strike Mars was 1,500-1,700 kilometres across, large enough to strip away much of the northern hemisphere crust and give rise to Mars' hemispheric dichotomy. On the Moon, the 2,600 kilometre wide South Pole Aitken Basin was produced by the largest lunar-striking bolides.

“Keep in mind that while the idea the Earth-Moon system owes its existence to a single, random event was initially viewed as radical, it is now believed that large impacts were commonplace during the final stages of planet formation,” adds Bottke. “Our new results provide additional evidence that the effects of large impacts did not end with the Moon-forming event.”

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