NLSI-funded scientists have revealed a new theory of the final stages of planet formation based on the study of lunar samples brought back by the Apollo astronauts, meteorites believed to be from Mars, and numerical models.

The team of scientists examined the abundances of highly siderophile, or metal-loving, elements like gold and platinum in the mantles of the Earth, Moon and Mars. Consistent with previous studies, they conclude that the elements were added by a process termed late accretion during a planetary final growth spurt. The new findings address how the last bits of mass (<1% of total mass) were added. The team theorizes that these elements were delivered by a very limited number of large impactors more than 4.5 billion years ago.

The paper, “Stochastic Late Accretion to the Earth, Moon, and Mars,” written by William Bottke and David Nesvorny, Southwest Research Institute (SWRI), Richard J. Walker, University of Maryland, James Day, University of Maryland and Scripps Institution of Oceanography, and Linda Elkins-Tanton, Massachusetts Institute of Technology, was published in the Dec. 10 issue of Science.

The results indicate the largest Earth impactor was between 1,500 and 2,000 miles in diameter, roughly the size of Pluto. The Moon, with a much smaller cross-section than that of Earth, avoided such enormous projectiles and was only hit by bodies 150 to 200 miles across.

“These impactors probably represent the largest objects to hit the Earth since the giant impact that formed our moon,” said William Bottke, principal investigator for SWRI. “While the idea that 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 end stages of planet formation. Our new results provide additional evidence that the effects of large impacts did not end with the Moon-forming event”

“I am thrilled to see the collaboration of three NLSI teams create such a substantial result. Bill Bottke’s work significantly helps explain the surprising abundance of the metal loving elements in the mantle of the Earth. Many scientists think those elements should have been sent to the cores of both the Earth and Moon as a result of the collision of a Mars sized body with the Earth that likely resulted in the formation of our Moon”, stated Yvonne Pendleton, Director of the NASA Lunar Science Institute (NLSI).

“The neat part about these results is that our predicted populations appear consistent with new models of how the building blocks of the planets formed as well as trends found among the largest main belt asteroids and oldest Martian impact basins.” said William Bottke, principal investigator for SWRI.

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The moon is not round, but slightly egg shaped with the large end pointed towards earth.

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