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Moon Derives From Earth, Space Object, Study Says

John Roach
for National Geographic News
August 11, 2003
 
Science has a better understanding of how and when the moon came to be thanks to a team of German geochemists who compared ratios of trace elements in rocks from the Earth, the moon, Mars, and meteorites.

Most astronomers subscribe to the so-called "cratering" theory to explain the moon's creation: Billions of years ago, a Mars-sized body slammed into Earth, projecting a mixture of rocky debris into space, some of which lumped together to form the moon. The remaining debris rained back down on Earth.


But until recently, scientists debated how much lunar material originally came from the Earth and how much came from the celestial body that impacted it. Estimates ranged from 1 to 90 percent.

The rock comparison work of Carsten Münker, a geochemist at Germany's University of Münster, and his colleagues places the upper limit of impactor material at 65 percent. Their work also dates the moon to be at least 4.53 billion years old.

Jay Melosh, a geophysicist at the University of Arizona in Tucson and an expert on the impact-origin theory of the moon, said the limit on the amount of impactor material in the moon proposed by Münker and colleagues is reasonable.

"It will generate more smiles among the cratering community than gasps of horror," he said. Melosh said the German research team's estimates syncs up with that of another researcher team that has yet to be published.

The research by Münker and colleagues appeared in the July 4 issue of Science, the research journal of the American Association for the Advancement of Science, based in Washington, D.C.

Constraints

The German geochemists' findings are based on an analysis of the ratio of the elements niobium to tantalum. Niobium is a metal commonly found in alloy steels. Tantalum is a metal resistant to acid corrosion commonly used in chemical, dental, and surgical instruments.

Both niobium and tantalum are nearly geochemical twins in magmatic systems like Earth's core, where the elements are barely separated from each other, said Münker. As such, scientists thought that Earth and moon shared the same niobium to tantalum ratio.

Recent studies have shown that niobium is attracted to iron at high pressures. Tantalum is not. When the Earth's iron core separated from our planet's rocky mantle, high pressure caused by Earth's gravity caused the two metals to separate, with the niobium going to the iron core.

Analysis of rock samples by scientists lead them to believe that the only instance when pressures would be large enough to force such a separation between the elements niobium and tantalum is at the time when the Earth formed. Other celestial bodies, such as the moon, Mars, and meteorites are too small to generate such high gravitational pressure.

"Since so far this is only known for Earth, both elements are a good tracer to estimate the fraction of terrestrial material in the moon," said Münker.

Analyzing Earth and moon rock samples, the German researcher team found that the moon's silicate mantle has a niobium to tantalum ratio of about 17, whereas the Earth's silicate mantle has a slightly lower ratio of 14. The researchers presume that much of the Earth's niobium is incorporated in its iron core, leaving less niobium in the mantle.

The silicate portions of Mars and other meteorites have a niobium to tantalum ratio of 20. Because the impactor is thought to have been Mars-sized (or about one-half the size of Earth), the scientists believe that the impactor also would have had a niobium to tantalum ratio of 20.

Calculations

According to the researchers, the difference between the niobium to tantalum ratios of the Earth and moon are a result of the mixing of the Earth's mantle material with the impactor. Based on the ratios of niobium to tantalum in the two bodies, the researchers can estimate the percentage of impactor material in the moon.

According to their calculations, the amount of impactor material in the moon is 50 percent, plus or minus 15 percent. Hence, 65 percent becomes the upper limit of impactor material found in the moon today, Münker said.

Scientists can also draw conclusions about the time at which the moon formed, based on the known ratios of niobium and tantalum found on Earth that were determined at the time our planet's core and mantle separated.

The German researchers say the moon had formed by the time Earth's core and mantle separated, based on tantalum ratios in rock samples they studied.

According to radioisotope dating of moon rock samples, the lunar body's core and mantle separated by 4.53 billion years ago. That suggests the same figure serves as the moon's minimum age.

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