It was bound to happen. 355 named and classified moon rocks have already been discovered on Earth. Now it seems scientists have found an Earth rock on the moon! A team of planetary scientists with the Center for Lunar Science and Exploration (CLSE) recently announced their discovery of a possible piece of our planet in a rock brought home by the Apollo 14 astronauts in 1971.
Meteorites slam into the moon just like they do on Earth. Sometimes they even get incorporated into the local rock. Using a new technique for distinguishing between impactors and native rock, the team examined a bits of a 2-gram fragment that had been sawn off a rock officially named 14321 — nicknamed Big Bertha. The minute fragment is composed of quartz, feldspar (a silicate rock related to quartz) and zircon, all common on Earth but rare on the moon.
A chemical analysis revealed that it crystallized on a terrestrial, oxygen-rich planet (not the life-making kind of oxygen but oxygen from water and CO2) from molten rock at a cooler temperature than that in lunar magma. Big Bertha was just one of gazillions of tons of rock blasted out when a small asteroid struck the moon to create the giant Imbrium Basin, excavating material from 20-40 miles (30-70 km) below the surface at the base of the lunar crust. Later, magma welled up from below and flooded the basin which we know today as Mare Imbrium or the Sea of Showers.
Some of that ejected rubble landed 300 miles away to create the hills of the Fra Mauro formation, where the astronauts gathered samples and reported on the local geology. Alan Shepard and Edgar Mitchell hiked more than a mile from their lander to Cone Crater, gathering rocks along the way. That’s when Shepard found Big Bertha. When a crater forms, rocks from the surface fly farthest, with the deep stuff deposited around the crater’s rim. The astronauts tried to get as close to rim as they could to collect the deepest and potentially the oldest samples. They came oh-so-close but never made it — to read more about their difficult journey, click here.
The Imbrium impact of 3.8 billion years ago bit down to the base of the moon’s crust between 18 and 43 miles (30-70 km) deep. But crystals in Shepard’s rock show that it formed much deeper, closer to about 104 miles (167 km) beneath the surface for a body like the moon. All these oddities makes perfect sense if the rock formed first on Earth and then got launched to the moon by an impacting asteroid or comet. Back then, it was easier to get material to the moon because it was three times closer to the planet than it is now. Our little Earth shard slammed into the lunar surface, mixed and welded with other rocks and then got shot back into the air during the Imbrium impact millions of years later to settle into a new home in Fra Mauro.
It wouldn’t surprise me if some of the lunar meteorites we’ve found in the hot deserts and Antarctica are pieces of rubble sent our way from the Imbrium blast. It’s even possible that Earth rocks exist that traveled to the moon during one impact and then returned during a second like a ball in a tennis volley.
While it’s possible that the sample crystallized on the Moon, it would require conditions never hinted at in other lunar samples including having formed at at tremendous depth in the lunar mantle where we would also expect different kinds of rocks. The simplest interpretation is that the sample came from Earth. Further analysis has revealed other cool details about the fragment. It crystallized about 12 miles (20 km) beneath the Earth 4.0-4.1 billion years ago during a wild time in the solar system when impacting asteroids were punching holes hundreds of miles in diameter in the Earth, powerful enough chew into the planet’s crust and send boulders flying into space.
The final impact event to affect this sample occurred about 26 million years ago, when a much smaller asteroid struck the moon, digging out the 1,000-foot-wide (340 meter) Cone Crater. That blast re-excavated the sample from the Imbrium rubble and set it back down on the lunar surface where astronauts collected it almost exactly 48 years ago.
I mean, wow. That’s an incredible story. Wouldn’t it make a great child’s picture book? Let’s hope the team finds more dribs and drabs of Earth in what remains of the 842 pounds (382 kg) of material waiting to be studied.