A 33-million-year-old layer of Earth's crust is laced with helium-3, which is normally only found in space. Now we might have an explanation for how it got there.
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| Earth as seen by the Apollo 11 astronauts floating above the moon NASA |
A mysterious layer of space dust that coated Earth millions of years ago could be shrapnel from asteroids that smashed into the moon, according to a new analysis of soil from below the sea floor
In the 1990s, geologists found unusually raised levels of helium-3 across Earth’s crust, dating back to around 33 million years ago. This was puzzling because the isotope almost always appears only in objects from space, as a result of bombardment by cosmic rays.
Two large meteor craters dating from around that time – one in Siberia, Russia, and another in Chesapeake Bay, the US – led people to suggest that this dust may have come from a wider meteor shower, but uncertainties in the dating of the craters and the lack of other known craters have made it difficult to prove that meteors were the source. What’s more, most objects larger than a few micrometres would burn up in Earth’s atmosphere and wouldn’t make it to the ground, so alternative explanations are needed.
Now, Jörg Fritz at the Natural History Museum in Berlin, Germany, and his colleagues have found evidence that this dust may have come from one or more impacts on the moon that launched dust into space before it settled over Earth.
“When you have a shower of projectiles over Earth and they also hit the moon, then maybe, as the moon is the largest reservoir of helium-3 after the sun, it would be a good candidate [for the dust],” Fritz told the Goldschmidt geochemistry conference in Lyon, France, on 13 July.
Part of the reason for this line of thinking is that a lunar impact scenario would produce high levels of helium-3 and low levels of the element iridium, which the moon is lacking, whereas meteors – which have more iridium – hitting Earth would deliver high levels of both elements.
To test this idea, the team analysed soil samples from 2 kilometres below the sea floor in the Southern Ocean of Antarctica, because this material is shielded from any iridium that may have been deposited by processes in Earth’s crust. The samples appeared to have very low levels of iridium and high levels of helium-3, consistent with a moon impact scenario.
It is a neat suggestion, says Thierry Adatte at the University of Lausanne in Switzerland, and identifying the composition and age of lunar craters and linking them with soil samples of Earth could strengthen the hypothesis. But these age estimates are very difficult to do and are often wrong by millions of years, he says.
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