Saturday Brain Boost: The Theory of Earth’s Two Moons

Some scientists believe that early in the Moon’s history, there was not just the large Moon we know today, but a second, smaller companion moon. According to this theory, the two moons eventually collided, and that collision shaped many of the features we now see on the Moon’s far side—mountainous highlands and thicker crust, in contrast to the smoother near side.

The idea arises from research into the striking differences between the near side of the Moon (which always faces Earth) and the far side. The nearside has large dark basalt plains called maria, relatively flat and smooth regions formed by ancient volcanic activity. In contrast, the far side is rugged, heavily cratered, and has a much thicker crust. Geologists and planetary scientists have long sought to explain why these differences are so pronounced.

One model, supported by computer simulations, suggests that a smaller moon—perhaps formed in the same giant-impact event as the main moon—orbited alongside the larger one for a while. Over time, gravitational interactions (including Earth’s pull and tidal forces) could have destabilized the smaller moon’s orbit. Eventually, it would have collided with the larger moon, merging into it in a relatively low-velocity collision. That merger would have deposited extra rocky material on the far side, thickening the crust there and contributing to the uneven terrain.

Studies by researchers such as those reported in Space.com and Nature provide evidence for this companion-moon scenario. The computer models show that if the collision was gentle enough, it wouldn’t leave a gigantic crater but instead spread material over the far side, building up the crust there. Those same models help explain why there is less maria (flooded basalt) on the far side—it needed thinner crust to allow lava to flow and form the smooth plains.

It’s important to note that this theory is still not proven, and not all scientists agree. Some propose that a large asteroid or dwarf planet struck the Moon, changing its structure without the need for a full second moon. Others suggest that internal lunar dynamics, differences in heating by Earth, or how the Moon formed in the first place might also account for the asymmetries.

What is less contentious: the Moon has a very uneven crust, and its two faces are different in appearance and geology. Satellite missions, gravity measurements, and lunar samples confirm there are variations in crust thickness, composition, and surface features. The “companion moon” idea is one plausible explanation among several, backed by simulations, geology, and lunar observations.

What makes this cosmic theory so compelling is how it might tie together several lunar puzzles. Why is the far side more rugged? Why is there less volcanic basalt there? Why is the crust thicker on one side? The merging-moon model offers a consistent way to account for those facts.

So while we can’t say with certainty that Earth definitely had a second moon, we do know enough to say it’s a serious possibility—one that makes the Moon’s strange near-side / far-side difference less mysterious.