February 2016 – The History of Our Moon

Heavens Above! is the astronomy section of the Sci@StAnd website, updated each month to highlight a particular phenomenon in the night sky. Last month, we examined the lives of violent galaxies. In this issue, we will discover the origin of our nearest cosmic neighbour, the Moon.

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Fig 1. The Moon looms high in our skies, as seen during a quarter phase. (credit: Blastr)

Everyone is familiar with the Moon. Its colours and phases continually remind us to look upwards towards the night sky. Some of us see a face, others a hare. It has played a role in nearly every religion and culture throughout human history. But how and why is it there?

Tides

In order to understand the history of our moon, it is crucial to understand its features and effects on the Earth. Let us start with the tides.

Standing on the shore over an entire day, one would notice two high tides. If one were to also keep track of the position of the moon, it would be evident that at least one of the high tides is correlated with the appearance of the moon above. Let’s get a wider view:

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Fig 2. The tides are caused by the gravitational influence of the Moon. (credit: Cosmos)

We must keep in mind that the tides are not caused by the Moon orbiting about the Earth, but rather by the Earth revolving underneath. To prove this: it takes the Moon about 28 days to go about the Earth, but it only takes 24 hours for the Earth to complete one rotation. If we are to see two tidal surges per day, then it must be due to the Earth rotating.

The tidal bulge lies beneath the Moon. But why is the other bulge there? Shouldn’t all of the water feel the pull? Well, yes – it does. Newton lends a hand.

Newton explained that although all of the oceans feel a pull, the furthest bodies of water feel the weakest pull. Henceforth, they lag behind. This lag creates the second bulge.

Phases

But what about the phases of the moon? Well this is quite straight forward if we revisit our wider view:

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Fig 3. The geometry of the Moon, Earth, and Sun is crucial in understanding the cause of lunar phases. The diagram is not to scale. (credit: wikimedia/Orion 8)

The part of the Moon which we see as “lit-up” is the side facing the sun. This makes perfect sense. A full moon occurs when the moon and sun are on the opposite sides of the Earth. A “new” or dark moon happens when the moon and sun are on the same side. If they are aligned just right, we enjoy an eclipse.

We should note that there is no so-called “dark side” of the Moon. There is the “other” side of the Moon, but it is not always dark. If it were, then where is the rest of the light when we don’t see a full moon? It’s only an effect of perspective.

What if the Moon rotated? Couldn’t it rotate such that one side is always light – and one side always dark? Yes – it could. But we do not see this. The Moon, as it turns out, is fixed. We always see one side. And here we begin to understand the creation of our Moon.

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Fig 4. These panels demonstrate our current understanding of the formation of the Moon from the so-called “Large Impactor” scenario. (credit: everythingselectric.com)

Moulded from Fire

Leading theories believe that the young Earth was struck by a large object some five billion years ago. This object would have torn apart the Earth’s upper layers, which were then molten. The material stripped from the Earth and the impact object would have been stretched out in an orbit about the Earth, and came together to form the moon very shortly afterwards.

This theory helps to explain why we see only one side of the Moon. As the molten Moon cooled and rounded, the strength of gravity helped to mould it such that one side is actually denser than the other. In other words, the centre of mass of the Moon is closer to the Earth than the true centre of the Moon.

There is no scorching mark left on our Earth because the Earth was hardly developed, and the impact would have left the Earth’s crust nothing more than a sea of molten rock. The Earth then grew to its current size due to smaller collisions.This subsequent growth would have erased any sign of this colossal impact.

What other theories exist to explain our celestial companion?

Binder (1974) explains that the Moon could be created by material ejected from the Earth as it spun, due to a centripetal force. However, the spin of the Earth would have to be many magnitudes greater than it currently is, making this a rather unfavourable theory.

Mitler (1975) theorised that the Moon could have been captured by the Earth, in a similar way to the asteroid-like moons of Mars. However, the physics doesn’t work out. For the Earth to capture such a large object as the Moon, it would need to be substantially more massive, or have a greatly extended atmosphere.

Other theories exist too, but recent agreement among astronomers and astrogeologists have favoured the “large impact” theory, described above. This agreement was the result of a special conference held in 1984 to discuss the origins of the Moon.

But the question arises: how likely is such an impact? How could a Mars-sized object strike the Earth in the vastness of space?

Computer models have shown that large impacts were more common in the early solar system. In 2009, further evidence for the “large impact” scenario emerged from the Carnegie Institute of Washington which showed that lunar rock recovered from the Apollo missions have a striking similarity to those of Earth. While this result disproves many of the alternative theories such as those proposed by Binder or Mitler, it also casts some doubt on the “large impact” scenario due to the exact parameters expected from the computer models.

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Fig 5. Mare Ibrium as imaged by the Apollo 15 astronauts. Note the differences between the “highlands” at the top and the “mare” below. (credit: NASA)

A Modern Moon

But what about the so-called “man on the Moon” or the “hare” or countless other lunar personas recalled by humans throughout history? How did our modern moon come to be?

The lunar surface that we see was shaped by the transformative power of space rocks. Astronomers have theorised that about 4 billion years ago the solar system became a cosmic shooting gallery. In essence, the gravitational instabilities of the gas giants (Jupiter, Saturn, Uranus, and Neptune) caused a disturbance of the asteroid belt, which lies between Mars and Jupiter, and the Kuiper belt, which lies in the outermost regions of our Solar System. This disturbance sent asteroids raining down on the inner planets.

The Earth was almost certainty hit, but our surface recovered due to our tectonic plates. The Moon, however, was eviscerated. The numerous impacts created seas of molten lunar rock. The darker areas on the Moon, known as maria, are remnants of those ancient molten seas.

To understand why we believe that these maria are younger than the rest of the Moon, we have to understand some astrogeology. Our chief assumption must be that there is no favourable place for a planet or moon to be struck. Given enough time, their cratering should be fairly uniform. But there isn’t much crating on the Earth – this is precisely how we know that Earth has a young surface. The tectonic and volcanic activities resurface our planet on timescales much shorter than we are impacted. Smaller planets such as Mars or Mercury have a harder time resurfacing because their volcanic and tectonic mechanisms were stopped long ago when their interiors cooled down. The Earth is much larger than either Mars or Mercury, and therefore has stayed active over time.

On the Moon we see that the maria have far fewer craters than the so-called “lunar highlands”. This means that their surfaces have been reshaped. But not by tectonic or volcanic activity. The Moon cooled off a long time ago. Rather, it is the impacts themselves that have caused the surface to become molten, and spread out over the surface. The cooling of these lava seas produced the maria, contrasted against the much older highlands.

The ancients used to believe that our Moon was home to another civilization, with their own seas like on Earth. Although we know nobody (but robots) lives on the Moon, they were right about the lunar oceans, but just a few million years too late. Since then, we have come very far in our understanding of the Moon and the planets above – and we still have far to go. What mysteries await us on our voyage to the stars?

Next Month: The James Webb Space Telescope: Our New Eyes in the Sky

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