The moon, thought to be cold and dead, is still alive and kicking—barely. Scientists have found evidence for dozens of burps of volcanic activity, all within the past 100 million years—a mere blip on the geologic timescale. And they think that future eruptions are likely—although probably not within a human lifetime.
For a world thought to have gone cold long ago, the discovery points to a place that still releases internal heat in fits and starts, says Mark Robinson, a planetary scientist at Arizona State University, Tempe, and a co-author on the new study. "The big story is that the moon is warmer than we thought," he says.
Much of the moon's near side is paved over in dark plains of basalt called maria. The activity that created these ancient lava fields peaked about 3 billion years ago and petered out 1 billion years ago. But a strange geologic structure called Ina has intrigued scientists for decades, and in 2006 they found evidence that it was indeed a volcanic vent that was active as recently as 10 million years ago. But Ina remained the anomaly.
Scientists working with NASA's Lunar Reconnaissance Orbiter (LRO), which has been orbiting the moon since 2009, decided to look more closely for possible volcanic structures. The orbiter's camera can capture images with resolutions of between 50 and 200 centimeters per pixel—the sharpest pictures of the lunar surface ever taken.
On the near side of the moon they found 70 structures that appear to be lava flows, ranging in size from 100 meters to 5000 meters across. The researchers think that some of these features are the remnants of low-lying shield volcanoes, which ooze out a soupy lava. " 'Blobs' would be an apt term for them," says Sarah Braden, who recently completed her doctorate at Arizona State and was lead author of the study, which appears online today in Nature Geoscience.
Researchers have found evidence for 70 young volcanic structures on the moon’s near side.
NASA/GSFC/Arizona State University
The team says that the flows must be relatively new because of the sharp contacts between the lava and the underlying rock—distinct boundary lines that moonquakes and micrometeorite impacts would have eroded over time.
The researchers estimated ages for the largest of the flows by counting craters on them—a technique based on the idea that a young surface will be less heavily pocked with craters than an old one. Braden says the youngest flow resides in a tub-shaped depression near the western edge of the Sea of Tranquillity and is just 18 million years old. And if there are flows that young, Robinson says, eruptions are apt to happen again—although not necessarily in the near future. "I doubt if I'll live long enough to see it happen," he says.
Mark Wieczorek, a planetary scientist at the Institute of Earth Physics in Paris, says the result shows the moon has not cooled as quickly as scientists once thought. He says it makes sense that the structures were discovered on the moon's near side, which is heavily enriched in radioactive elements that produce heat and help spur volcanic activity. To this day, the deep near-side crust is a couple of hundred degrees warmer than its counterpart on the far side. And so the LRO team may have detected one of the moon's "last gasps" of volcanic activity before it dies completely, Wieczorek says. "The body is still warm. From time to time, you'll still see a few twitches."
