Moon Derives From Earth, Space Object, Study Says

John Roach
for National Geographic News
August 11, 2003












"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.


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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