By William Sager
Every astronomer knows that the Moon has one side facing the Earth, the near side, and another side, that cannot be seen, the far side. This occurs because the Moon’s rotation exactly matches its orbital period. Until the Space Age, nobody had seen the far side. Starting with the Soviet Union’s Luna 3 in 1959, spacecraft began sending back pictures of the far side and today we have high-resolution images thanks to NASA’s Lunar Reconnaissance Orbiter (LRO). Even a casual glance reveals that the near and far sides appear vastly different (Figure 1). The near side contrasts high-albedo anorthosite highlands with dark basaltic lowland plains named mare by early observers who thought they were seas. (Note: anorthosite is an igneous rock consisting mainly of potassium felspar whereas basalt is an igneous rock rich in iron and magnesium). In contrast, the far side is mostly anorthosite with only a few small, scattered and isolated mare. In a recently published paper in the journal Science Advances, a multi-university team led by Matt Jones of Brown University reported having worked out the reason for the Moon’s two faces.
The lunar asymmetry goes deeper than just albedo and elevation. The nearside contains the Procellarum KREEP terrain, a roughly circular area with crust that is high in potassium (K), rare earth elements (REE), and phosphorus (P) as well as elevated thorium and titanium (Figure 2). This KREEP material is thought to have been erupted from a residual magma that accumulated beneath the lunar crust. What is more, the far side crust is on average thicker than that on the near side and this hemisphere contains the largest and oldest impact basin on the Moon, the South Pole-Aitken (SPA) basin (Figure 2), which is nearly antipodal to the KREEP terrain.
For several decades, planetary scientists have hypothesized that the mare and KREEP terrain resulted from lunar mantle convection flow that redistributed a global layer of cumulate material (heavier residual minerals that remained after eruption of lighter magma phases) that caused the mare eruptions to be mostly on one side of the Moon. Jones and colleagues computed mathematical models of the SPA collision that connect this event with KREEP formation. According to these models, SPA was formed by the collision of a large planetoid with the Moon early in its history (~4.3 billion years ago), after the initial formation of anorthosite crust. The impact kinetic energy created an internal heat pulse that drove hemispheric mantle convection over 300-600 million years, with upwelling beneath the SPA and downwelling beneath the KREEP terrain. This flow moved cumulates to the opposite side of the Moon, beneath the near side, causing widespread eruption of mare basalts in that hemisphere, creating the fundamental lunar crustal asymmetry. The model also explains why the KREEP terrain is about 500 million years younger than the SPA basin.
The final piece of this story is the tidal locking. The Moon once rotated more rapidly than it does today. It has been known for decades that tidal friction forces within the Moon, caused by the gravitational attraction of the Earth, would slow the Moon’s rotation. Eventually, the rotation would slow to match the orbital period, with the minimum principle axis of inertia pointing towards Earth. And so, today, as the result of this astrodynamical dance, we see only one side of the Moon.
The article by Jones et al. is open access and can be downloaded at this URL.