Why does the Moon have almost no atmosphere?

Its small mass gives weak gravity that cannot hold gas molecules

Why is the Earth geologically active while the Moon is not?

The Earth is large enough to still be hot inside, driving tectonics and volcanism

Suitable temperature (distance from Sun) and atmospheric pressure (held by gravity)

We cannot sample the deep Earth directly, so meteorites provide samples of core-like and mantle-like material

The present is the key to the past (the same processes operate everywhere and at all times)

We usually cannot visit to confirm, and conditions differ between planets

Giant volcanoes such as Olympus Mons

Its relative age: more craters means an older, longer-exposed surface

Radiometric dating of returned rock samples

14questions. Pick an answer and you'll see why right away.

What does a body's surface gravity mainly depend on?
Why does the Moon have almost no atmosphere?
Why is the Earth geologically active while the Moon is not?
Which set of conditions allows liquid water on the Earth's surface but not the Moon's?
Which meteorite type is thought to sample the metallic core of a broken-up rocky body?
Why are meteorites used as evidence for the Earth's deep interior?
Uniformitarianism is best summed up as:
A branching, tree-like network of dry valleys on Mars is best interpreted, by uniformitarianism, as evidence of:
Which is a genuine limitation of interpreting other planets using uniformitarianism?
What are the dark lunar maria made of?
On the Moon, which surface is older: the heavily cratered highlands or the smooth maria?
Which Martian feature records large-scale past volcanism?
What does the density of impact craters tell you about a planetary surface?
To turn crater counts into actual ages in years, geologists must also use: