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Bowling with asteroids

 By Eniday Staff

Here’s a new word: terrestroid. All it means is a mineral object, more precisely a piece of rock from Earth. Take a cobble from the streets of Rome, or a stone from the banks of a stream, and you’ve got your terrestroid…

There are at least a few thousand billion of them orbiting Earth. Nothing out of the ordinary, then. Until one of these terrestroids ends up on the Moon and happens to be taken to the NASA’s laboratories. Strange? Hardly. It’s just what happened on 6 February 1971, when Alan Shepard gathered a little, now famous, collection of rocks and stones from the lunar surface, during the Apollo 14 mission. Shepard died in 1998, but if he was still alive today, he would marvel at what the researchers at the Lunar and Planetary Institute in Houston have discovered. The largest rock he collected, twenty centimetres long and weighing in at nine kilos, turned out to be encrusted with crystals that formed on Earth around four billion years ago. This makes it one of the oldest Earth rocks ever found.

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The terrestroid collected by Alan Shepard during the Apollo 14 mission (NASA)

 A ticket for the Moon

According to the geologists at the Lunar and Planetary Institute, this is how it happened. Those crystals will have been part of a small pebble dwelling peacefully underground on our planet. But these were turbulent times. Happily living out its days as an out-of-the-way pebble was not going to be easy, as shown when an asteroid hit Earth with enough violence to blast the poor little thing beyond the stratosphere. It travelled so fast that it escaped Earth’s gravity and landed on the Moon, then three times closer to Earth than it is now. A second asteroid then smashed into the Moon, creating a huge lunar ‘sea’, or mare, known as Mare Imbrium, second only to Oceanus Procellarum in size. In the midst of this disaster, the pebble from Earth must have been catapulted five hundred kilometres, to the point where the Apollo 14 lunar module would land billions of years later. During this abrupt change of address, it was incorporated into a bigger matrix, the large rock brought back by Shepard.

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The Imbrium sea on the Moon (NASA)

Gifts exchange

It’s almost science fiction, and yet it’s all perfectly plausible. In that period, getting hit by asteroids was par for the course for the Moon and Earth. We’ve found hundreds of meteorites that we know for certain come from the Moon and even Mars, so it’s just as plausible that the reverse happened, and pieces of Earth ended up leaving the planet. It’s even more likely if you factor in the relatively small distance between them in those far-off times. Finding traces of material from Earth in the rocks brought back by American astronauts is like a dream to the researchers who study them. It was in 2012 that they finally identified traces of minerals that were foreign to the Moon in those rocks. They came not from Earth, but from asteroids smashing like bowling balls into our pale satellite. Specifically, the researchers focused their attention on one small fragment, weighing less than two grams, found within the structure of the large rock Shepard brought to Earth. These kinds of minerals have not been found in any other rock sample taken from the Moon. There have been hundreds of studies on these materials, but all using technology from the end of the last century. Now, ion probes can shoot an atomic beam out onto the sample, extracting a handful of ionised atoms from it, which can then be identified and quantified using a mass spectrometer. It is therefore possible to highlight details that would otherwise be imperceptible. Thanks to this method, they’ve managed to discern a light excess of cerium ions, within the crystals hidden in the lunar rock. This shows that the rock was exposed to an oxidising environment, which cannot, almost by definition, have been the Moon. And that’s not all. Very high pressure is needed to form crystals like the ones in the traces, with their high concentration of titanium. On the Moon, pressure like that only exists at least 167 kilometres underground, which is slightly too deep for our crystal to have possibly ended up on the surface. On Earth, on the other hand, gravity means there are similar pressure levels a few kilometres below the surface, within reach of the asteroids that flew in like bowling balls all that time ago.

READ MORE: Drilling on Mars by Sandeep Ravindran

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Eniday Staff