When you look up at the full moon, you can often see the shape of a man's face there. Scientists have long believed that these features are due to a large asteroid impact on the moon's surface. However, now, scientists believe that a plume of magma rising from underneath the moon's surface created these unique characteristics.
In 2012, as part of its Gravity Recovery and Interior Laboratory (GRAIL) mission, NASA sent two spacecraft into orbit around the moon. These spacecrafts measured velocity very carefully and collected data about specific areas on the moon where gravity differs. This allowed scientists to create a detailed high resolution map of the moon's surface.
The map showed that the area typically known as the "man in the moon," also called the Procellarum region, has sharp edges, some at 120 degree angles. As asteroid impacts generally create elliptical edges, scientists determined that this region was not created by an asteroid, as originally thought.
That left scientists with a mystery, so they formed a new theory about where these surface features came from. Shortly after the hot moon formed, its crust cooled down. This caused hot liquid magma from beneath its surface to erupt into a plume. Because the temperature between the crust and the magma was significantly different, the crust cracked and contracted, creating the fractures we see today.
However, a theory wasn't enough. Using a computer simulation, a group of scientists tested this idea and created a model of the moon with this kind of magma activity. The simulation figured out the gravity of that area in this particular scenario. The simulation's gravity measurements matched the data GRAIL collected. This suggests that a plume of magma created the "man in the moon."
"A lot of things in science are really complicated, but I've always loved to answer simple questions," says Maria Zuber, NASA's principal investigator for the GRAIL mission. "How many people have looked up at the moon and wondered what produced the pattern we see - let me tell you, I've wanted to solve that one!"
Of course, no scientific mystery is ever truly solved, and much remains unknown about what lies beneath the moon's surface. Perhaps a future lunar mission could collect more data about the moon's interior.
"It comes to trying to understand the nature of the interior, and how extensive was this concentration of heat-producing elements that would've caused a plume to surface," says Clive Neal, a professor of civil and environmental engineering and earth sciences at the University of Notre Dame. "GRAIL has been a fantastic mission, and this data will be continually used and reinterpreted as we get more data back from the moon."