We all look up to the Moon as one of the greatest works of art in the night sky. It inspires thoughts that are greater than most of our day-to-day concerns. And the interest arises at all ages: from the 2 year old who squeals with glee at spotting the Moon to the grandparent whose sight of it may inspire somehow a hope for a better future. We have even been there, and walked on that alien land. So where did it come from?
The origin of the Moon is a topic of active debate. We used to be taught that the Moon came about as a result of an impact with a large asteroid. The story goes that an impactor striking Earth at a glancing blow can drag off enough of its outer mantle to explain its size and stable orbit. The story goes on to say that if this all happened very early on in Earth’s history, when it was still in a molten state, then Earth could recover from this blow to retain again its mostly spherical shape that is has today.
If true, then the composition of the Moon should be more or less equal that of the impactor. To test this hypothesis, lunar samples physically collected by humans and brought back to Earth were studied. When their compositions were measured and compared with that of Earth, it turns out that the Moon and Earth have similar compositions (apart from a lunar iron deficiency and a lack of lunar water). Uh oh - this would seem to rule out the major impactor story. So how did the Moon form?
Two new models to form the Moon are proposed already in 2017. In the first model, Earth sustained multiple hits from small impactors. In this scenario, each impact would result in Earth material being flung out into rings similar in concept to the rings we know and love around Saturn. Unlike on Saturn, the rings around Earth would have joined up to form larger and larger-sized masses until they accumulated into what we now call the Moon. This process would take about 100 million years, consistent with some measurements of the relative difference in age between the Moon and Earth (but not all).
In Part Two we will review the second formation model proposed to explain this great enduring sculpture.