Early Earth Might Have Had Two Moons, Which Collided To Form One

Imagine that the full moon night had two moons instead of one, making it doubly romantic! If you could somehow magically survive in the volcanic early Earth, then you would have been able to play the perfect Romeo. Scientists think that the early Earth had two moons, which collided and fused to make the one that we see today.

This startling new revelation comes from the fact that the two sides of the moon the one facing us and the dark side have very different landscapes. The dark side is so called because we just cannot see it, even though it is just as lit as the visible side.

Lunar Apocalypse?

The tell-tale marks, or lack of them

The visible side has flat expansive plains, called maria’ (meaning sea’ in Latin), while the far side is mountainous, with mountains as high as 2 km. Simulations, trying to explain this discrepancy, now point to the earlier presence of a second moon, which might have slammed into the larger one, producing the maria and giving the features of the moon we see today. This is further backed up by known observations that the lunar crust on the near side has more phosphorus and radioactive elements of potassium (the radioactive isotope), uranium and thorium. On the far side, as far as we know, they do not occur or are deeply embedded in the surface. This can be explained by noting that impact could have melted a part of the near side, allowing for hidden elements to rise to the surface. The companion body is calculated to have been just 4% as massive as the bigger moon and about 750 miles across.

The Simulation done, which shows the collision (Credit: Asphaug and Martin Jutzi, Bern University, Switzerland)

The stability of the companion body: why it crashed into the bigger moon

There is also another explanation that is required: why the companion moon crashed into its larger partner. Asked to a physicist, this implies a stability problem and the underlying question involved asking about why instability crept into the system. There are points in the space around any celestial body orbiting another celestial body, where the gravitational attractions of the two bodies cancel each other out. These points are stable points and are known as Lagrangian points’. There are two Lagrangian points in the current Earth-Moon system, one just in front of the moon and one just behind it. It is conjectured (and can be calculated via lengthy calculations using Newton’s gravity law) that the early Earth-two Moon system also had Lagrangian points just in front and behind the bigger moon. The companion moon would’ve nested itself in and around that Lagrangian point. When the moon moved away from the Earth (which is still happening due to the presence of tidal forces), the stable Lagrangian points also shifted. This led to the destabilizing of the orbit of the smaller moon, which then crashed into the nearby large body.

Explaining the dark side of the moon

Even though slightly off-topic, it is worth mentioning that the equality of the periods of orbital and axial spins of the moon is due to the energy loss mechanism due to tides. The most efficient way to distribute energy in the Earth-Moon system, given that the energy dissipation from the entire system is miniscule, is by having the period of the moon around the Earth and on its own axis as identically same. This, remember, leads to the dark side phenomenon. The enigmatic dark side, which has even inspired great music (Pink Floyd, anyone?),  is closely related to tides.

Asphaug and Martin Jutzi of University of Bern, Switzerland, who are responsible for coming up with the calculations and simulations, published their findings in the August 4 issue of Nature.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.