In these days of daily image releases from Saturn, Mars, the moon and other spots in the universe, it's hard to remember just how exciting it was back in the 1950s and 1960s when a few images trickled out to the world at the time. Perhaps one of the biggest early surprises was how jagged and cragged the back side of the moon looked. Where were the smooth lunar "seas" that we saw on the Earth-facing side of the moon?
About 55 years after the first Soviet images of the far side were sent to Earth, a team of researchers led by Penn State University's Arpita Roy may have an explanation.
They say it’s due to the violent way that the moon formed — likely after a Mars-sized object collided with our Earth, creating a sea of debris that gradually coalesced into the moon we see today. The huge crash and gathering together heated up both our planet and the moon, but the moon got cooler first because it was smaller.
Because Earth was still hot — radiating at more than 2,500 degrees Celsius (4,500 degrees Fahrenheit) — and because the moon was very close to the planet, the heat had quite the effect. The far side of the moon cooled down, while the near side remained hot.
The temperature difference played an important role in the formation of the moon's crust.
The crust has high concentrations of aluminum and calcium, which are hard to vaporize. Calcium and aluminum would have been the first elements to "snow out" as the vaporized rock cooled, and they would have remained in the atmosphere on the moon's far side. Eventually, those elements combined with silicates in the moon's mantle to form plagioclase feldspars. The far side's crust had more of these minerals, and thus became thicker.
The seas themselves were formed after huge meteors crashed into the moon's Earth-facing side, rupturing the crust and letting the basaltic lava beneath burst forth. The crust on the far side was too thick for the meteors to penetrate, in most cases, leaving the rugged surface we see today.