A Moon Rock's 3.9-Billion-Year Journey to President Joe Biden's Office

NASA loaned the moon rock specimen 76015,143 that was put on display in the Oval Office this week. Its quiet presence belies a violent history. NASA 

I was thrilled the other day to see a moon rock in President Biden's office. At the request of the Biden Administration, NASA loaned the moon rock in a special display case which now occupies a book shelf in the Oval Office. For the incoming administration it's a "symbolic recognition of earlier generations' ambitions and accomplishments, and support for America's current moon to Mars exploration approach," according to a NASA press release. 

In a different Apollo moon rock the dark spots are glass-lined impact craters called zap pits surrounded by lighter rock chipped off during impact. NASA

Apollo 17 moonwalkers Harrison Schmidt and Eugene Cernan chipped the sample from a large boulder at the base of a blocky mountain called the North Massif in the Taurus-Littrow Valley on Dec. 13, 1972. Designated as lunar sample 76015,143 it weighs 11.7 ounces (332 grams). Micrometeorites drilled tiny craters called zap pits on the rock's exterior. They accomplish this feat because the moon lacks an atmosphere, offering no resistance to sand-sized meteoroids traveling at 45,000 mph (20 km/sec). 

In this close-up of the moon rock 76015,143 you can clearly see vesicles on the sawn face at right. Some are visible on the crusted side, too. NASA

The sawn side reveals small cavities called vesicles that formed when hot gases bubbled up through the rock when it was molten. Scientists classify 76015,143 as a vesicular impact melt breccia. Quite a mouthful, but boiled down, it tells us the rock formed during a titanic impact that shattered and melted the local lunar rock and flung it far and wide.

This is the complete, 6.2-pound sample of 76015 before it was slabbed into a number of different pieces, one of which became the display specimen 76015,143. It's riddled with vesicles and zap pits. Courtesy Mike Gentry / NASA Johnson

Rocks hold their histories close, but chemists and physicists have ways of wooing their remarkable stories. Prior to arriving in Biden's office, the specimen had been at NASA's Lunar Sample Laboratory Facility at Johnson Space Center in Houston. It's just one piece of the complete specimen, called 76015, that weighs 6.2 pounds (2.8 kg). The astronauts removed 76015 from a huge, broken boulder called Tracy's Rock, named for Gene Cernan's daughter who was 9 years old at the time. 

Sample 76015 at the LRL is cut into two with a special saw in 1976. Sample portion 18 is on the left. Portion 19, the parent of the presidential display piece,143, is on the right. Courtesy Charles Meyer / NASA Johnson

Tracy's Rock, actually five large, related boulders called blocks, is located at Station 6, a sampling waypoint visited by the astronauts in the lunar rover. Biden's chunk came from the top of Block 5 and was cut into slabs for scientific study in 1976 at the former Lunar Receiving Laboratory (LRL), also located in Houston. 

Apollo 17 astronaut Harrison Schmitt gathers samples from Tracy's Rock. You're seeing the two largest pieces, Blocks 1 (left) and 2, which appear to touch from this angle. Blocks 4 and 5 are hidden by Block 2 (center) in this image. The lunar rover is at right. NASA

As Cernan and Schmitt collected rocks from the site that December day, they noticed the tracks left by the monster rock leading upslope of the North Massif and joked about how terrifying it would have been to see the beast come bounding by. Of course, that danger had long passed. Scientists later determined that the boulder had rolled, bounced and slid down the mountain about 22 million years ago and broke into pieces when it came to a thudding halt on the valley floor. 

This is the same scene but taken from a different angle showing all five pieces of Tracy's Rock. The Oval Office sample came from Boulder 5 which is partially hidden by 4. NASA

In these before and after photos you can see the 76015 sample "in situ" where Harrison Schmidt collected it from Block 5. NASA

We know when it began its tumble thanks to cosmic rays. Cosmic rays are high-speed particles shot into space by the sun and other sources in the galaxy. When they bombard exposed rock, the particles alter the atomic makeup of the elements composing the rock's minerals. Scientists compared samples gathered from parts in the boulder shielded from the radiation to more recently exposed surfaces to determine how long ago it had taken a tumble.

NASA's Lunar Reconnaissance Orbiter photographed the fragments of Tracy's Rock (labeled with block numbers) and the track it made in the lunar soil as it bounced down the North Massif 22 million years ago. NASA / GSFC / Arizona State University

What caused the boulder to roll? That we don't know, but it's possible that a moonquake or a significant meteorite impact in the region could have sent it sliding. Perhaps the 500-degee-plus temperature difference between day and night ultimately gave it the nudge. Besides quakes caused by Earth's incessant tug and internal cooling, the moon also experiences thermal quakes. These occur when the frigid crust first warms under the morning sun after the two-week-long bitter cold night. 

An artist's view of the formation of Imbrium Basin when the moon got smacked by a large asteroid 3.9 billion years ago. LPI / Leanne Woolley / David A. Kring

How the boulder arrived on top of the massif takes us back to the creation of the Imbrium Impact Basin, an enormous hole in the moon's crust 712 miles (1,145 km) in diameter punched out by a 155-mile-wide (250 km) asteroid. The catastrophic impact occurred 3.9 billion years ago, which also happens to be the age of the presidential moon rock. Later, the basin filled with lava that gushed from the mantle through cracks in the crust to create today's Mare Imbrium — the Sea of Showers. It's one of the "eyes" of the Man in the Moon and easily visible without optical aid around the time of full moon. 

In this shaded relief map of Mare Imbrium you can clearly see the lava-filled basin's circular outline. Huge amounts of melted, shattered rock were thrown far and wide during the impact including the boulder from which the President's moon rock originated. James Stuby / NASA / Arizona State University

Before the impact, 76015 was part of the ancient crust of the moon composed of a pale, aluminum-rich rock called anorthosite, but the impact heated and melted this material and ejected gobs of if far and wide across the moon. One of those pieces, the future 76015, came to a thud hundreds of miles to the east about the same time the first microscopic life wriggled into being on Earth. 

This image shows an imagined trajectory for Tracy's Rock. The impact launched it hundreds of miles from what became Mare Imbrium. NASA / LRO with trajectory added by the author

There the boulder sat for some 3,878,000,000 years until an environmental disturbance sent it rolling from its hilltop perch. Sundering into five pieces when it finally came to rest, the traveler passed 22 million years in silence beneath the blue globe of Earth where life became steadily more complex and interesting. In the late 20th century two of those lifeforms came to pay a visit, chipped off samples and put them in a bag to bring home. After fleshing out its secrets in the lab, a fragment now resides on a shelf in the Oval Office, inspiring our next venture to the moon. 

I can't think of a more satisfying way of coming full circle.