When the Earth Had Two Moons

When I was in school, the planetary system was the gold standard for modeling permanence and regularity. We even had a law for predicting where planets could be found. Of course, whether it was a “law” or a coincidence would depend on observations from other planetary systems around other stars, but in 1977, that was not something to worry about.

Well, the returns are now in, and the universe is far stranger than we can imagine, with large planets migrating around the solar system or spiraling into orbits that take a week to travel around their stars. Planets smashing into each other as dust particles build up into boulders build up into embryos that reach a size called “oligarchs” from which the survivors of the age of bombardment can be called “planets” or “moons” (or “plutinos.”)

Erik Asphaug's “When the Earth had Two Moons” offers the apogee of “Gosh! Wow! Science” with a tour of the Solar System's pre-history. Long before we had our nine planets, there may have been many more planets orbiting the Sun. Some of those planets - lost Jupiters and Saturns spiraled into the Sun. Others collided and left behind one planet as the “lucky shark” surviving in the ocean. The Earth/Moon system is the result of a collision between proto-Earth and a Mars-sized “oligarch,” now called Theia, which struck at just the right angle to deposit a large section of Earth's crust into Earth orbit. This formed the Moon, which has been slowly receding from Earth, so that right now it is at the exact distance to permit full eclipses of the Sun.

When we get reports of Super-Earths, that may simply be an indication that the “Earths” of that system didn't avoid the continuing accretion that could have happened in our system

Asphaug is noted for a pioneering theory of the formation of the Moon. One of the discoveries of the Apollo program was of the Dark Side of the Moon, which had never been observed before. The Moon is tidally locked to the Earth and only ever shows one side to the planet. The Dark Side does not have the familiar “Mares” or dark flat spot we are used to seeing. Instead, it is all mountains and has a thicker crust. Asphaug theorizes that there was a time when the Earth was orbited by two moons. The smaller eventually crashed into what is now the Dark Side, which formed the mountainous terrain we now know to exist.

This is mind-bending stuff. Asphaug's text is accessible to the layman, but I really couldn't figure out how his topics were organized. It seemed that he meandered from one topic to another without any organizing idea.

Nonetheless, this is interesting material and it would be worth buying Asphaug a beer and letting him meander into the night.