In the Museum: The Rock

WITHIN NASA IT IS KNOWN AS sample 70215, 84. A geologist would classify it as a dense, aphanitic basalt. To visitors, however, it is one of the most popular objects in the National Air and Space Museum. “It” is the lunar Touch Rock, one of only three samples returned by the Apollo astronauts that the public is allowed to feel with their bare hands. On any given day, thousands of people wait patiently in line for a chance to examine this piece of the moon. Yet despite its popularity and prominent location in the Milestones of Flight gallery, near such artifacts as the Spirit of St. Louis and the Wright Flyer, few people know its history.

“So that’s a moonrock, huh?” said a recent visitor. “I have a couple of those in my back yard.” Depending on where he lives, he may—or at least he may have something like it. Basalts make up entire islands on Earth, such as Hawaii, and cover vast areas, including parts of the northwestern United States. On the moon, basalts are so prevalent that they can be seen as the dark patches, or maria, that make up what appears to be “the man in the moon.”

Between December 11 and 14, 1972, Apollo 17 astronauts Eugene Cernan and Harrison Schmitt explored the edges of one of these patches, Mare Serenitatis, in a valley known as Taurus-Littrow. It was during their extra-vehicular activity—in this case a moonwalk—that Schmitt, a geologist by training, saw a rock that stood out from the others. All rocks cool from some original melt. If they cool slowly, they can develop large interlocking grains, or crystals. Such rocks were common in Taurus-Littrow. This rock, however, looked out of place because it had no obvious grains. And it was very large. Instead of collecting it right away, Schmitt stood it on its side so he could come back to it later in case he and Cernan were not able to find something smaller. As their final EVA was winding down and they were returning to the lunar module for the last time, they came back and collected the rock that Schmitt had set on its side, sample 70215. The number identifies the mission, the rock’s location, and the bag the rock was placed in for return to Earth. At 17.7 pounds, sample 70215 was the largest rock that the Apollo 17 crew brought back.

The Apollo lunar samples have been studied more than any other rocks. Sample 70215 was found to belong to a group of moonrocks that contain large amounts of titanium and other elements so uncommon that they are considered rare Earth elements. Like all of the other lunar samples, 70215 does not contain water or volatile elements. Its chemistry can be explained by a widely accepted hypothesis that the moon formed when a Mars-sized object collided with the early Earth. Part of the object was instantly vaporized (along with parts of Earth) and thrown into orbit. The extreme heat from the collision drove away any water. Eventually the vaporized material coalesced into the moon, but it remained hot long enough for minerals to collect in a variety of ways. Basaltic materials sank into the interior to form the moon’s mantle. Subsequent impacts carved away portions of the moon’s crust, providing a pathway for basalts to erupt onto the surface. The impact that formed the Serenitatis Basin provided one such pathway, allowing basalts to flow into the neighboring Taurus-Littrow Valley. The dense, fine-grained nature of 70215 suggests that it cooled quickly, probably near the surface of one of these basalt flows about 3.7 billion years ago. Although fairly young by lunar standards, 70215 is the oldest thing many people will ever touch.

NASA, which carefully manages all 841.5 pounds of rock and soil the Apollo astronauts returned with, decided to donate three pieces of 70215 for public display. Daughter sample 84 was loaned to the National Air and Space Museum in 1976, and the other two pieces are on display in Mexico City and at NASA’s Johnson Space Center in Houston, Texas. Since 70215 is so large, there is still plenty left over for scientists to study. And its dense nature has allowed it to stand up to almost continuous handling; no pieces have ever come loose or broken off. Even so, it is under constant surveillance during the hours that the Museum is open to the public.

The security officers responsible for guarding it probably know better than anyone what a piece of the moon actually means to some visitors. “People from all over the world come to see it,” says one Museum security officer. “I’ve seen people kneel down and pray at it.” On a busy day last summer, one teenager said: “It didn’t feel like cheese.” Others held back tears or even blessed themselves after seeing it. The rock’s popularity may best be explained by the reaction of an older couple, who walked away from it shaking their heads in disbelief. Didn’t they think it was real? “I cannot believe that we did something that incredible, that fantastic,” said the man. “It almost seems like a dream now, but here’s proof. My God, what we can do when we put our minds to it.”

—Bob Craddock is a planetary geologist at the National Air and Space Museum’s Center for Earth and Planetary Studies.

 

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On the Mall

“Voyage: A Journey Through Our Solar System” opened on the National Mall on October 17. The permanent exhibit features a one-ten-billionth scale model of the solar system, which consists of 13 stainless steel stations set up along a 650-yard stretch of sidewalk that runs between the National Air and Space Museum and the Smithsonian Castle on Jefferson Drive. The stations are mounted with sculptures of the sun (right), planets, and asteroids.