Planetary scientists can usually tell a great deal about a world’s history by looking at its surface. This is particularly true of the Moon. Of course, we’re all familiar with the so-called “near side”—it’s what we see from here on Earth. It has large plains called “maria” (Latin for “oceans”). These are wide, dark-colored old lava flows that flooded the surface between 1 and 3 billion years ago. There are also many craters, which were “excavated” by objects plowing into the surface. In fact, the entire Moon is pockmarked with craters from that period.
The lunar far side (the one that always faces away from us), looks much different. There are no maria, but we can see a LOT more craters over there. It’s almost like each side of the Moon has its own history. The Moon is tidally locked to Earth (meaning the far side always faces outward from Earth). So, we had no idea how different they were until the Space Age allowed orbiters to study the entire lunar surface.
Explaining the Moon’s Two Sides
Most of the Moon’s craters formed during an event called the “Late Heavy Bombardment”. That happened some 3.8 billion years ago, although objects do still collide with the Moon today.
Still, there are some mysteries about the Moon that need solving. One of them involves a strange mix of chemical elements known as KREEP, and what their existence on the lunar near side tells us about its history.
Could KREEP be related to the difference between the two sides of the Moon? That’s a question that planetary scientists have been working to answer for many years. The visual evidence tells them that something happened. But, to get a true understanding of the event that caused the Moon to look so different, they looked for reasons that might explain the “two-sided dichotomy”.
A Heck of an Ancient Impact
Lately, scientists have pursued an idea that may tell the tale. It all starts with a feature near the lunar south pole called the “South Pole-Aitkin” (SPA) basin. It’s an impact basin that measures about 2500 kilometers across and over eight kilometers deep. It probably formed about four billion years ago. The most likely explanation is that a slow-moving object plowed into the Moon. The force of that impact melted a lot of rock and launched a plume of heat through the Moon.
As it traveled through the Moon, the plume would have carried such elements as thorium (which produces heat), rare earth metals, phosphorus, and other metals. That “splash” of heat would have spurred volcanic activity on the near side, creating the maria. In fact, there’s evidence for this idea in the chemical composition in a near side region called Oceanus Procellarum. It has a high concentration of these materials, often referred to as KREEP (K for potassium, REE is for rare earth elements, P is for potassium).
KREEP Tells the Tale
For a long time, scientists wondered how Procellarum could be rich with KREEP materials when the rest of the Moon isn’t. The impact that created the South Pole-Aitkin region would have been the trigger. In addition, the far side didn’t get “resurfaced” by volcanism since all the heat went to the near side. That could explain why the far side appears so much more cratered—its impacts weren’t obliterated by volcanic flows.
This hypothesis about the South Pole-Aitkin impact and its effect on the near side of the Moon is the latest attempt to tell the tale of the Moon’s two faces. It has been the subject of intense computer modeling by scientists at Brown University, Purdue, the Lunar and Planetary Science Laboratory, Stanford University, and NASA-JPL. You can read more about their work at this site.