Humans have not set foot on the moon since December 14, 1972, when astronauts Eugene Cernan and Harrison Schmitt of the Apollo 17 mission departed the lunar surface to return home. Thankfully, Cernan and Schmitt, a trained geologist, collected 110 kilograms of lunar material—the largest-ever haul of moon rocks and soil—before heading for Earth.
That material is still yielding new insights into the moon's history, as evidenced by a paper in the July 2 issue of Science claiming the first solid evidence for graphite, the form of carbon commonly used as pencil lead, in a lunar sample.
Andrew Steele, an astrobiologist at the Carnegie Institution of Washington, and his colleagues analyzed an Apollo 17 sample with a technique called Raman spectroscopy, which gives a detailed chemical composition of the material under study.
The researchers found dozens of graphite particles in a small dark patch on the sample—a region just 0.1 square millimeter in area—as well as seven needle-shaped rolls of carbon called graphite whiskers. This is not the first time that carbon has been found on the moon; other samples have yielded traces of the element implanted by the solar wind or locked up in carbide compounds. But discrete, micron-scale pockets of graphite appear to be a unique find.
Terrestrial contamination of the Apollo sample appears highly unlikely, because the deposits exist below the surface of the sample, and graphite whiskers only form in extremely high temperatures that the sample has not likely seen since its formation.
The researchers conclude that the graphite inclusions stem from a meteorite strike, probably during a period of intense impacts about four billion years ago known as the late heavy bombardment (LHB). The moon's craters preserve a valuable record of the LHB, whose effects have largely been erased on Earth by weathering and geologic resurfacing.
The graphite fragments, Steele says, "are a remnant of basically a carbon-rich dust after an impact from a meteorite containing carbon, or the carbon may have condensed from a gas" released by an impact. If the former scenario proves to be the case, the graphite flecks and whiskers may be intact fragments of the meteorite that excavated the giant Serenitatis impact basin near the Apollo 17 landing site.
Paul Spudis, a lunar scientist at the Lunar and Planetary Institute in Houston who did not contribute to the new research, says that the graphite indeed appears to have a violent origin but that it is less clear just which impactor produced it. "I would say that their interpretation of the origin of this stuff is reasonable," Spudis says. "It probably is a remnant of some impactor." But he adds that the impactor may not have been the same one that carved out the massive Serenitatis Basin. He and a colleague wrote a paper in 1981 suggesting that the impact-melted rocks collected during Apollo 17 may stem from multiple impact events.
Whatever the case, it is clear that the scientific resources gleaned from the Apollo program are far from exhausted. The development of ever more sensitive microscopy and chemical-analysis techniques will continue to produce new insights from existing samples into the foreseeable future—good news, considering that no nation appears to be close to returning humans to the lunar surface. "There is a lot still to be learned from these samples," Steele says. "They do represent a treasure trove of data that people will be looking at for generations."