According to a report in National Geographic News, until now, most scientists had believed that the inner solar system bodies had the same composition as primitive meteorites called chondrites. Chondrites are the most common class of meteorites, and, at an estimated 4.5 billion years old, believed to be the oldest. Because the objects chemically resemble the sun, it is widely believed that they represent the basic materials for the entire solar system. But, Earth's chemistry doesn't quite match to this theory.
One telltale signature of chondrites is an abundance of neodymium 142, a by-product of the decay of the rare earth metal samarium. But, in the past several years, researchers noticed that Earth's crust contains too great a ratio of neodymium 142 compared to chondrites.
Now, French researcher Guillaume Caro, from Centre de Recherches Petrographiques et Geochimiques in France, and his colleagues have said that the makeup of Mars and the moon don't correspond either.
Seeking to show that the Earth isn't an oddball, Caro and his team turned to Mars and reviewed old data from Earth's moon.
"We found that Martian and lunar rocks are also characterized by an excess in neodymium 142 compared with chondrites," he said.
In fact, it turns out the three bodies may be more similar to each other than the chondrite-rich asteroids located between Mars and Jupiter.
According to Caro and his team, the difference could come from erosion of planetary crusts in the bodies' formative years. Or the inner planets might have formed long before the rocky bodies of the outer solar system.
"What our results suggest is that the sorting of the elements that make up these planets may have happened at a much earlier stage than had been believed," said Alex Halliday, a study co-author from Oxford University.
"The composition of these worlds is inconsistent with them simply forming out of large 'lumps' of stony meteorites like those we see today in the asteroid belt," he added.