Humans have not set foot on the moon since Apollo 17 in 1972, but those missions are still producing surprises. An analysis of a collected rock has produced 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 reported in the July 2 Science that they 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. Other samples have yielded traces of the element implanted by the solar wind or locked up in carbide compounds, but discrete pockets of graphite of this relatively large size appear to be a unique find.
The researchers surmise 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. 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 D. Spudis of the Lunar and Planetary Institute in Houston agrees that the graphite “probably is a remnant of some impactor,” but he says that the impactor may not have been the same one that carved out the Serenitatis Basin. He and a colleague hypothesized in 1981 that the impact-melted rocks collected during Apollo 17 may stem from multiple impact events.
Whatever the case, the scientific resources gleaned from the Apollo program are clearly far from exhausted. The development of ever more sensitive microscopy and chemical-analysis techniques will continue to produce new insights from existing samples—good news, considering that no nation appears to be close to returning humans to the lunar surface.