September 17, 2018

  • Why a rake on the moon messed up our theories of life on Earth

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    Astronaut Harrison Schmitt collected samples with a rake during the Apollo 17 mission NASA

    By Leah Crane

    Not all moon rocks are created equal. Minuscule glass beads brought back from the moon by Apollo astronauts seemed to show that there were far more objects smashing into the moon in the last 500 million years than the rest of the moon’s history. Now it turns out they just didn’t look deep enough, which could spell trouble for some theories about the rise of life on Earth.

    When a rock crashes into the moon or a planet, it vaporises parts of the surface and sends up a spray of debris. As the bits of melted dust and rock fly through the air, some of them cool into tiny beads called impact glass spherules, which can be used to tell when the impact happened.

    When researchers begun to analyse and date the glass bits in samples from NASA’s Apollo missions, they found an odd pattern. There were as many beads from the last 500 million years as from the entire four billion years before that – far more recent glass than they expected.

    Model mission

    Now, David Minton at Purdue University in Indiana and his colleagues have built the most detailed computer model ever of how these spherules are produced.

    They simulated 500,000 landing sites of the kind visited by the Apollo astronauts. When they kept the impact rate the same for the last three billion years, they found glass bead samples taken from the top 10 centimetres of these sites would skew younger, while deeper samples would have a more even number of old and young beads.

    That means that the rate of impacts on the moon likely has not changed much in the last three billion years – it is just that our samples are biased because of new beads from more recent impacts burying the older ones.

    In other words, the Apollo astronauts weren’t digging deep enough to get a true picture of the moon’s history. “They had this little rake tool, and they would rake up a few centimetres,” says Minton. “We’d have to sample a column of a metre or more to see the true impact rate.”

    The history of impacts on the moon has an important bearing on our understanding of Earth’s past. That’s because impacts on Earth tend to be erased by geological activity, which the moon lacks. “The moon preserves its record far better than the Earth does,” says Paul Renne at the Berkeley Geochronology Center in California.

    The time period abound 500 million years ago, when it appeared the moon experienced more impacts, was a particularly important time for life on Earth. There was a dramatic increase and diversification of complex life called the Cambrian explosion, and we are not entirely sure what caused it.

    Some researchers have suggested that the apparent increase in meteorite impacts may have had something to do with it. “If there was this increase in impacts around the same time, it would support the idea that life was stimulated or seeded by organic molecules from elsewhere in the solar system,” says Renne. But if this supposed increase turns out to be a mistake, we will have to look for alternative explanations, he says.

    Journal reference: Geophysical Research Letters, DOI: 10.1029/2018GL077254