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Was our two-faced moon in a small collision?

We may finally have an answer to a long-standing cosmic/ cosmetic issue. NASA

As of today, we have a cataclysmic new explanation for one of solar system astronomy’s most long-standing questions: why do the near- and far-sides of the Moon look so different?

This new theory, published in Nature, suggests our moon collided with a smaller object, also a moon of Earth, a few million years after both were formed in a collision that literally tore Earth asunder.

Our two-faced moon

The familiar near-side of our moon (seen on the left, main image) is dominated by the Mare, the great lava-filled impact basins where giant impactors punched through the youthful Moon’s crust, allowing the still-molten rock below to flow out and erase the previous surface details.

Prior to the dawn of the space-age, it was expected the far-side of our moon (seen on the right, main image) would look essentially the same as the one that constantly presents itself to Earth.

So it was something of a shock when the first spacecraft to image the far-side revealed a surface with no great Mare – just a simple, heavily pockmarked rocky surface.

The Moon, it transpired, had two very different faces.

Thin crust vs thick crust – impacts and the lunar pizza

The best explanation for the difference between the features of the Moon’s near- and far-side comes down to the thickness of its crust.

At the time that giant objects were colliding with the Moon, forming the huge impact basins we observe today, its interior was still molten as a result of leftover heat from the Moon’s formation and the decay of deeply buried radioactive isotopes (much as Earth’s interior remains molten to this day – the source of plate tectonics and volcanic eruptions).

The impacts that formed the Mare involved projectiles sufficiently large that they punched through the thin solid crust, creating a hole through which the molten rock below could flow out, filling the basin left by the collision.

This was the genesis of the Mare.

The objects flying into the Moon would, however, have come from all directions – there’s no reason that we could expect one side of the Moon to be pummelled while the other remains untouched.

Why, then, are there no Mare on the lunar far-side? The answer seems to be that, for some reason, the crust on the far-side of the Moon was somewhat thicker than that of the near-side, and so, simply, the giant projectiles did not manage to break through to the molten rock below.

Until recently, the cause of this “geological dichotomy” was unknown – but authors Martin Jutzi and Eirk Asphaug have now put forward an exciting theory they believe may solve the problem.

Collision between the Moon and a companion moon, 4% the lunar mass. Martin Jutzi and Erik Asphaug.

Formation of the Moon – the big splash

It has long been recognised that our moon is something of an oddity. Compared to the moons of the other planets, it is remarkably large, compared to its host.

In addition, although one might expect Earth and its moon to have essentially the same composition (if they formed at the same time, and the same place), studies have shown the Moon is significantly “under-dense” compared to Earth, being depleted in the heaviest elements, such as iron, which make up the bulk of Earth’s deep interior.

The accepted explanation for the peculiar size and density of the Moon is that our satellite was formed as a result of a giant impact, in which the proto-Earth, which had formed as a solitary planet, collided with a Mars-sized object.

The collision shattered our planet, causing its outer layers to “splash” off into space, but leaving the deep interior relatively unscathed.

Such an impact is beautifully rendered (in a movie showing the results of old simulations of the event carried out at the University of Bern.

A significant fraction of the material shed by the proto-Earth remained in orbit around our planet, and over time accreted to form the Moon, which has slowly been receding due to tidal forces ever since.

This “big splash” model does a remarkable job of explaining the differences between Earth and the Moon – Earth is denser because the heavy metals remained trapped within core, while the Moon is under-dense because it’s made of material that was once the crust and the mantle of the proto-Earth.

Solving the dichotomy – the second Moon hypothesis

Jutzi and Asphaug take this well-established idea one step further. What if the collision that resulted in the formation of the Moon also led to the formation of a second, smaller moon?

The pair carried out hugely detailed computer simulations to find out what would happen if our youthful moon was hit by another moon, roughly one third of its size, a few million years after it had formed. What they found was remarkable.

Because both moons were orbiting Earth at the time of the collision, their relative velocity was far lower than that typically observed in such collisions.

Instead of shattering our moon, or leaving a colossal impact basin, the incoming small moon simple smooshed into the Moon’s far-side, thickening the crust on that side of our satellite and simultaneously forcing the hot molten interior of our moon towards the near-side.

The result of the collision is a moon with a far-thicker crust on the far-side than the near-side, and an enhanced magma ocean beneath the thin crust facing Earth.

Over the millions of years after that collision, both Earth and our moon were peppered by an ever-diminishing flux of impactors, ranging in size from metres across to hundreds of kilometres in diameter.

The largest of these impactors to hit the lunar near-side punched through its thin crust, leading to the formation of the Mare.

The far-side was pummelled too, but the combination of its far thicker crust, and far thinner magma ocean, meant very little lava was erupted to the lunar surface, leading to the observed “geological dichotomy”.

While other theories have been proposed over the years to explain the observed differences between the near- and far-sides of the Moon, future sample-and-return missions should provide the evidence needed to test this exciting new theory.

It may well turn out that the origin of our moon was far more dramatic and cataclysmic than anyone had previously thought!

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