For two nights before holi we were away from any city. Holi is always on the first full moon day after the spring solstice, so, being away from cities, we had very clear views of the moon at any time between sunset and sunrise. I took a few photos of the last days of its waxing phase. Later when I looked at them, I remembered that I’d heard about how much more is known about the moon today than when I was in school. But I hadn’t ever found out what. So I looked. Apart from a Wikipedia article, there aren’t any books or popsci articles, but there’s a lot of scientific literature. Most of it was beyond me, but I did find a few that referred me to more and more recent papers until I landed on one that gave a nice and brief account of the current state of our knowledge.
Why don’t people photograph the moon more often? It is the only other planet whose landscape we can see with the naked eye. Most good zoom lenses allow us to take good photos of sunrise over lunar mountains, and the changing soil across its landscape. I find it fascinating to live on the only double planet in our solar system, constantly gazing up at a companion whose diameter is a bit more than a fourth of the Earth. There’s a mystery about it. Since the Earth’s diameter is (more accurately) three and two thirds as large as the moon’s, its volume should be about fifty times larger. If the mass of a planet is in proportion to its volume, then why does every source say that the earth’s mass is 85 times that of the moon’s? Does this mean that the moon is not made of the same material as the earth?
The moon rocks that the Luna and Apollo missions brought back fifty years ago showed us that lunar minerals are very similar to that of the earth. But that’s just a comparison of the crust. Almost 15 percent of the earth’s volume is in its iron core. The moon’s core is much smaller, perhaps less than a percent of its volume. This clue, along with detailed chemical analysis of moon rocks, and the ability to simulate numerically many alternative hypotheses about the early solar system, have led to tremendous advance in the understanding of the origin of our double planet. This dates the assembly of the earth-moon system at four and a half billion years, a scant 50 to 60 million years after the explosion of a nearby supernova compressed a cloud of gas and dust into the star we call the sun. When you look back this far, the story of the earth is the story of the sun and the moon.
The part of the story that is no longer in dispute is that two planets, the proto-Earth, a little smaller than today’s Earth, and the other, now named Theia, about the size of Mars, had a glancing collision at a relative speed of more than twice the speed of sound in rock. The collision threw off some debris. But the larger planet swallowed much of the mass of the smaller, especially most of the core, and became the earth, leaving a little iron, and a lot of rock, to become the moon. A large fraction of the energy of the collision went into melting rock, leaving too little energy for the two planets to escape each others’ gravity. As a result they became a double planet perpetually circling their common center. The explanation bears up against the test of detailed simulations.
Controversy has not yet completely settled on the immediately preceding history of our planetary system. Did this era contain a few dozen planetary “oligarchs” which had swept up all the material of the nebula from which the sun condensed? Numerical simulations show that multiple collisions between them could create a system where four or five rocky planets circle the sun inward of Jupiter, but none of them look very much like our solar system. This scenario also involves different mechanisms for the formation of the outer giant planets and the rocky inner planets. Nowadays the preferred scenario is that the solar system condensed into pebble-sized rocks which formed both the nuclei of the outer planets and five inner proto-planets, and only one major collision happened later to form our double planet. But this keeps open the question of why there was a collision. Perhaps the Mars Sample Return mission will tilt the balance of opinion in this remaining controversy. Perhaps we will find a simpler explanation of this older epoch in the assembly of the planets.