An Earth With Two Moons.

Our Moon has been a constant source of amazement and wonder for the human species. For as long as we’ve been able to observe it from our earthly bounds it has only ever shown us one side and wobbling ever so slightly as if to tease us as to what we couldn’t see. For the longest time we speculated about what could be on the other side of our closest celestial partner with theories ranging from the mundane to the outright fantastical. Of course since 1959 when Luna 3 first photographed the far side of the moon most of that mystery and wonder has since evaporated, but even today it still manages to throw a couple curve balls our way.

One of the most puzzling aspects is the distinct difference in terrain between the near and far sides of the moon. Comparatively the near side of the moon is quite smooth with many “maria” or land seas covering its surface. The far side on the other hand is deeply cratered with a considerably more rough appearance than the side we’re all familiar with. There are many explanations for this with the most accepted being that the near side contained a higher concentration of radioactive elements when it was first formed, and this has been confirmed from data from orbiting craft. There is however a new theory that’s come out and it depicts a story of an Earth that once had two moons:

The moon is thought to have formed when a Mars-sized body slammed into the infant Earth. This threw a cloud of vaporised and molten rock into orbit, which coalesced into the moon.

Simulations have previously shown that additional moons could have formed from the debris cloud, sharing an orbit with the one large moon that survives today. Eventually, gravitational tugs from the sun would destabilise the moonlets, making them crash into the bigger one.

Building off the most accepted theory of the Moon’s formation (the Giant Impact) this new theory about the far side of the moon’s appearance postulates that the impact also created another, smaller Earth bound satellite. Now usually smaller bodies are quickly engulfed by their bigger neighbours but this smaller moon stabilized into an orbit long enough for it to fully form. However millions of years later it impacted with the current moon at a relatively slow pace of about 8,000KM/H (for reference, the International Space Station orbits at around 25,000KM/H). So instead of smashing each other to bits they instead squished together forming a turbulent far side of the moon. Such a hypothesis also explains some discrepancies between mineral concentrations on either side of the moon as such an impact would have pushed the moon’s magma to the other side.

Now whilst this theory would explain some of the phenomena we’re seeing with our celestial sister there’s not a whole bunch of direct evidence to support it. The heavily crated far side of the moon could easily be explained by the tidal locking with Earth, which means any incoming asteroids are far more likely to hit the side facing outwards. This is made all the more difficult by the fact that there has been no landed exploration of the far side of the moon and definitely no sample return missions. Getting some rock samples from the far side of the moon would provide the answers we need to rule out or pursue this theory further.

I always find it amazing how we can think we’ve explored something so thoroughly yet it can still surprise us. The moon is something we’re all so familiar with yet it’s still so foreign when you get up close and it’s origins are as mysterious and intriguing as our own. I love that these ideas could lead to us sending a sample return mission to the far side of the moon and what’s even more exciting is that such a mission would probably lead to many more questions than answers. That’s the beauty of science, it’s a never ending journey of discovery into the origins and mechanics of the universe that surrounds us.

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