Long-held secrets continue to be unlocked from the Moon. Researchers taking a new look at a rock brought back by the Apollo 17 mission have discovered graphite in the form of tiny whiskers within the lunar sample. Just like the recent finding of water on the Moon, it was previously thought that any carbon present in the Apollo rocks came from terrestrial contamination from the way the lunar samples were collected, processed or stored. Andrew Steele, who led a team from the Carnegie Institution’s Geophysical Laboratory said the graphite could have come from carbonaceous impactors that struck both the Moon and Earth during the Late Heavy Bombardment, approximately 4.1 to 3.8 billion years ago, and if so, could provide a new and important source of information about this period in the solar system’s early history.
“We were really surprised at the discovery of graphite and graphite whiskers,” Steele said. “We were not expecting to see anything like this.”
The tiny graphite whiskers or needles were found in multiple spots within a specific area of lunar sample 722255 from the Mare Serenitatis impact crater in the Taurus-Littrow region, indicating that the minerals are in fact from the Moon and not just contamination.
Steele told Universe Today that he and his team don’t think the graphite originated on the Moon, but haven’t ruled it out completely.
“Our initial thought is that it is from the impactor, as we find it in a very fine grained impact melt breccias,” he said in an email. “I am currently looking in more pristine lunar rocks, i.e. lavas that do not contain evidence of meteorite material, for carbon phases.”
He added that the graphite may have come from the impactor itself, or it may have formed from the condensation of carbon-rich gas released during the impact.
The team used Raman imaging spectroscopy (CRIS) on a thin section of a freshly fractured surface of the rock. This identifies minerals and carbon species and their spatial relationship to each other beneath the surface of a sample. Steele said even though this rock has been on Earth since 1972, new techniques and instruments allowed for the new discovery.
“The analytical spot size is smaller and so we can look at smaller phases,” he said. “The sensitivity is better in the newer instruments and we can use spatially resolved methods that are much more sensitive than in the Apollo era.”
Impact breccias are made up of a jumble of smaller fragments that formed when the moon was struck by an asteroid or other object.
Other previous spectroscopy of the Moon’s surface has also found trace amounts of carbon, but it was thought to have come from the solar wind. However, Steele said he and his team have also ruled that out as the source.
“Several lines of reasoning confirm that the observed graphite and graphite whiskers (GW) are indigenous to the sample,” said the team in their paper. “In particular, all known GW synthesis methods involve deposition from a carbon-containing gas at relatively high temperatures ranging from 1273 to ~3900 K. Thus, the GWs identified in 72255 cannot have been synthesized as a result of sample handling and preparation. Moreover, they could not have been implanted by solar wind, because this carbon is typically too small to identify structurally at the magnifications used. The crystalline graphite grains detected here are likely either intact remnants of graphite and GWs from the Serentatis impactor, or they could have formed from condensation of carbon-rich gas released during impact.”
Steele said their findings indicate that impacts may be another process by which GWs can form in our solar system. Additionally, it appears carbonaceous material from impacts at the time of the Late Heavy Bombardment (LHB), and at a time when life may have been emerging on Earth, does survive on the Moon.
“The Solar System was chaotic with countless colliding objects 3.8 billion years ago,” Steele said in a press release. “Volatiles—compounds like water and elements like carbon were vaporized under that heat and shock. These materials were critical to the creation of life on Earth.”
While the impacts to Earth during that period have since been erased, craters on the Moon are still pristine, so the Moon potentially holds a record of the meteoritic carbon input to the Earth-Moon system, when life was just beginning to emerge on Earth.
The research is published in the July 2, 2010, issue of Science.
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