Despite its otherworldly appearance the moon’s origin has deep ties to Earth. Bob King
As the moon parades across the southern sky this week, passing above Mars this evening, let’s pause to consider its origin. How did that big, bright orb form in the first place? Venus and Mercury have no moons, Mars two, Jupiter 79, Saturn 62, Uranus 27 and Neptune 14. We’re the only planet with a single satellite.
Models of the Earth’s and moon’s cores. NASA/JPL-Caltech
We find clues to the origin of the moon by comparing it to Earth. Let’s start with their cores. Earth has a liquid outer core that sheathes a solid inner core. Both are made primarily of iron and nickel and together span about 4,000 miles or half the planet’s diameter. While the moon also has a solid inner core and a semi-molten outer core, combined they’re only 410 miles across or just under 20 percent of the moon’s diameter.
That’s a big difference, enough to say that the moon is iron-poor relative to Earth — a key clue to its origin as we’ll see in a minute. The moon also has orbits the Earth faster compared to many other moons in ...