Which is the moon-like planet?

Concept

A body is called "Moon-like" when its physical surface and environment closely resemble Earth's Moon: a barren, heavily impact-cratered rocky surface, essentially no substantial atmosphere (so no weather, wind erosion or surface water), and extreme day-to-night temperature swings. The phrase "moon-like planet" does not claim the Moon is a planet; it asks which planet most resembles the Moon in appearance and surface conditions.

Application

Checking the planet that best fits this description:

• Mercury has a scorched, rocky surface that is densely covered with impact craters, very much like the Moon's terrain. It has only a thin exosphere rather than a real atmosphere, and its temperature varies enormously between its day and night sides. By every one of the Moon's defining traits, Mercury is the closest match among the given planets, so Mercury is the moon-like planet.

Contrast

• Mars: has a thin but real CO2 atmosphere, polar ice caps and clear evidence of past liquid water, so it is not as bare and airless as the Moon.

• Earth: has a thick atmosphere, abundant liquid water and active plate tectonics, an environment completely unlike the Moon's.

• "More than one of the above" is wrong because these planets differ greatly in atmosphere, water and geological activity and so are not all alike; "None of the above" is wrong because a barren, cratered, airless rocky surface like the Moon's is a genuine planetary profile rather than an impossible one.

Hence Mercury is the moon-like planet.