A quick look at the lunar surface gives the impression of a dead world, a place where, other than the occasional errant comet or asteroid impact, nothing has happened for billions of years. But now planetary geologists have nailed down a location where the moon may still be active. Their work indicates that volcanic eruptions could have taken place within very recent geologic history--possibly only a couple of million years ago.
Most of the moon's surface consists of remnants of ancient volcanic or impact events, covered over by billions of years of space dust. But one region is an anomaly: In the late 1960s, Apollo astronauts happened upon an 8-square-kilometer area called the Ina structure, which is strewn with jagged and relatively dust-free volcanic rocks. "Something that razor sharp shouldn't stay around long," says Peter Schulz of Brown University. That's because, over hundreds of millions of years, bombardment by space debris erodes lunar features, much like water wears down mountains on Earth. Another clue is that the Ina region contains only two impact craters, far less than the moon's average for an area that size.
When Schulz and Brown University colleague Carlé Pieters compared images of the fine-scale structure within Ina, taken by NASA's Clementine spacecraft, to other parts of the moon known to age at specific rates, they estimated that Ina was formed about 2 million years ago. More evidence arguing for recent lunar activity comes from the way Ina's deposits reflect light. As the lunar surface weathers, it reflects different wavelengths of light. Based on those changes, the deposits on Ina's floor are exceptionally young, the team reports today in Nature.
The scientists hypothesize that the moon still periodically burps up volcanic dust and gases to its surface, and that's what happened at Ina. That material blows off any existing deposits, thereby exposing less weathered rock. They say the geology of Ina, which is located at the intersection of two valleys, resembles geologically active areas on Earth, such as Africa's Great Rift Valley. The team also says it has identified at least four other lunar areas that also might be active. The best way to be sure, they add, is to collect soil samples from Ina and those sites whenever lunar landings resume.
It's "convincing" evidence that "shows that the moon is still at least a little active inside," says geologist G. Jeffrey Taylor of the Hawaii Institute of Geophysics and Planetology in Honolulu. But Paul Spudis, a planetary geologist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, says the results aren't totally novel. Scientists have "known for some time that the moon's not totally dead," he notes, because of the existence of moonquakes--the lunar equivalent of earthquakes--discovered by the Apollo missions. Also, the astronauts found some evidence for gas releases. The significance of this research, Spudis says, is that it ties those earlier observations to a known surface feature.