YOU don't have to visit the moon to hold a chunk of it in your hand. Every day around 160 tonnes of rubble from space rains down on Earth, and some of it comes from the moon. All you need to find a piece of moon rock is keen eyesight, patience and an expanse of ice or desert against which a dark little chunk of our neighbour will stand out.
In this issue's Feedback you'll find details of a competition to win just such a piece of the moon, worth more than £1000. The prize is part of a lunar meteorite found by French collector Luc Labenne, who has been scouring deserts for meteorites since 1997.
Labenne found the rock, known as Dhofar 458, on 3 April 2001 in the gravel plains of southern Oman's Dhofar (or Zufar) region, and it was authenticated as a lunar meteorite by researchers at the University of California, Los Angeles. Before handing our piece over we wanted to use this opportunity to find out first-hand just how tricky it can be to distinguish a fragment of moon rock from a common or garden meteorite. For this we turned to researchers at the Open University (OU) in Milton Keynes, UK.
Meteorite hunters like Labenne follow an approach pioneered in the 1930s by the American collector Harvey Nininger, working on the Great Plains that stretch west from the Mississippi river to the Rocky mountains. Nininger taught local people to seek out black stones on the pale ground, and thanks to their efforts he bagged more than 200 meteorites over three decades.
Another rich source was identified in 1969 by the Japanese Antarctic Research Expedition, which found meteorites on a blue ice field near the Yamato mountains in East Antarctica. Other deserts became a focus of interest in the early 1990s, including the one that yielded our moon rock.
Lunar meteorites are exceedingly rare: only around 60 have been identified. And they don't just interest collectors. Space scientists are also keen to get hold of them, because they hold clues to what the rock is like on parts of the moon beyond the areas explored by the Apollo and robotic landers.
The vast majority of the tens of thousands of meteorites that have been studied are not from the moon. Around 6 out of 7 of them are of a type known as chondrites, characterised by stony spheres called chondrules which grew from the vast cloud of dust and gas that gave birth to the solar system. Some 4.5 billion years ago, budding planets and asteroids formed by sweeping material up from this cloud. Rocky remnants of these asteroids are still orbiting the sun, mostly between Mars and Jupiter. As the solar system developed, chance collisions between the asteroids caused them to spiral into chaotic orbits, later to crash into planets and their moons.
Occasionally, collisions happened on a far more dramatic scale, and one of them is now believed to have given birth to the moon. The consensus is that a stray planet as large as Mars crashed into the infant Earth, flinging globs of molten rock into space that coalesced to form what became our companion.
Small asteroids continue to smash into the moon's surface every day because there is no atmosphere to destroy them by frictional heating. If the impact is hard enough, the ejected material escapes the moon's weak gravity and can land on Earth after tens of thousands or perhaps millions of years.
Still, these events are rare, and finding a lunar meteorite is not easy. Because the moon was born from the Earth, their rocks share many features. "It is more difficult to tell a lunar rock from a terrestrial one than any other," says Colin Pillinger, a space scientist at the OU who worked on samples collected by the Apollo 11 astronauts.
What, then, does it take to find out if a meteorite has come from the moon? The traditional way to study any rock is to slice off a section with a diamond saw to reveal the mineral grains. This costs 1 to 2 grams of material, but as our rock weighs just 1.4 grams there would not have been much of it left if Pillinger's team had done that.
Other important clues can come from the look of the stone and the way it has weathered. "An experienced collector can tell a Mars from a moon meteorite," says Richard Greenwood, at the OU's Planetary and Space Sciences Research Institute, so this was the OU team's first line of attack.
It got off to a bad start. Planetary scientist Ian Franchi, who has hunted meteorites in Morocco, took one look and decided that our rock looked a little dark to come from the moon. Rocks from the lunar highlands contain a large proportion of light-coloured feldspar, a common mineral both on Earth and in space rocks. It was a heart-stopping moment.
Fortunately, we had a useful benchmark to check against. Our prize is from one of 13 known fragments originating from a single parent meteorite. The largest of these fragments, known as Dhofar 026, has been studied closely by the Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences in Moscow. Using an electron microscope, Greenwood compared a little piece of our prize, which his colleague Michelle Higgins had mounted in epoxy resin and polished, with its larger sibling. He and another colleague, Diane Johnson, took a series of images and conducted chemical analyses. We waited tensely for Greenwood's verdict.
After looking at the images, he reported: "The texture present is virtually identical to meteorite Dhofar 026." The initially off-putting colour can be explained by the fact that many minerals change structure when they experience a massive, instantaneous rise in pressure, as happened in the impact that gave rise to our meteorite and its siblings. "This meteorite is both strongly shocked and partially melted, which is probably why it appears so dark," Greenwood says.
The test came at a cost, though. Because of a fracture in the rock, it is now in two pieces.
The results from the chemical analysis proved encouraging too. We have learned from samples brought back from the moon that lunar rock contains very little sodium compared with equivalent materials on Earth. The heat generated by the impact that formed the moon left the ejected rock so hot that it lost most of its volatile materials, including water and sodium.
Greenwood and Johnson's electron microscopy showed that the feldspar in our piece of moon rock had a similar composition to Dhofar 026 and other lunar rocks. Given the similarity of textures, the conclusion was clear: it looked lunar.
We wanted to run one more investigation of our rock, however. For this we enlisted the help of geochemist Andy Tindle, who is in charge of the OU's electron microprobe, an instrument that provides particularly accurate chemical analyses. His analysis not only confirmed that the feldspar was very sodium-poor, it also showed that two other key minerals, olivine and pyroxene, were a good match for those in Dhofar 026 and other lunar samples. "We can be very confident that your sample is lunar," Greenwood concludes.
What could be a more fitting prize to mark the anniversary of the Apollo 11 adventure?