The Moon's dry, crater-riddled surface stands in stark contrast to the watery, nearly craterless Earth. Why such a difference?

• The Moon originated from hot vapor and molten debris. When this matter coalesced, water vapor and other gases were lost to space, leaving a dry and airless world.
• Most ancient Earth craters have been destroyed by plate tectonics and erosion. The Moon has experienced neither plate tectonics nor significant erosion to alter its surface.

The Moon — Vital Statistics

Size: 3,476 km (2,160 mi) in diameter.

Atmosphere: With virtually no atmosphere, the Moon has no wind or water to alter its landscape. Also, there is no protection from meteoroids and harsh solar radiation.

Surface: Craters of all sizes dominate the lunar landscape. the pockmarked surface has been shaped by 4.4 billion years of meteoroid bombardment.

Water: The Moon is bone dry.

The Earth — Vital Statistics

Size: 12,742 km (7,919 mi) in diameter.

Atmosphere: Earth’s atmosphere is composed mostly of nitrogen and oxygen. This protective blanket slows down small meteoroids and most harmful solar radiation.

Surface: Continents, ocean basins, and other products of plate tectonics dominate the surface. Meteorite craters have been largely obliterated.

Water: Oceans cover two-thirds of the planet's surface.