What are the significant differences in topography and geological composition between the 'dark side' (correctly termed the 'far side' or 'back side') of the Moon and the near side we can see? Why?

Okay, this is a fascinating question and one of the biggest curiosities people have about the Moon. Let's talk about it in plain language.

First, we need to correct a common misconception: the far side of the Moon is not called the "Dark Side." It gets sunlight just like the near side, experiencing day and night cycles, with each period of sunlight lasting about 14 Earth days. The reason we never see it from Earth is because the Moon is "tidally locked" by Earth's gravity. This means it takes exactly the same amount of time to rotate once on its axis as it does to orbit Earth once, so it always shows us the same "face." The correct terms are the "Far Side" or the "back side."

Alright, back to the point: the Moon's near side and far side are like two completely different worlds.

I. The Visual Difference: One "Smooth-Skinned Youth," One "Pockmarked Face"

If you could hold the Moon in your hands (which, of course, is impossible), you'd see:

The Near Side (The side we see)
- Features: Large, flat, darker areas. Ancient astronomers thought these were oceans, so they gave them romantic names like "maria" (Latin for "seas"), such as the Sea of Tranquility, Sea of Rains, and Ocean of Storms.
- Appearance: Looks relatively "smooth," with contrasting light and dark areas, like a face with beautiful spots. Of course, there's not a drop of water in the maria; they are vast plains formed by cooled basaltic lava from ancient volcanic eruptions.
The Far Side (The side we don't see)
- Features: Almost no large "maria." Instead, it's covered densely with overlapping impact craters. The entire surface is rugged and pockmarked, with extremely rough, ancient terrain.
- Appearance: Like a weathered, "pockmarked face," recording billions of years of "violent assaults" by asteroids and comets from the solar system. This is home to the Moon's largest and deepest impact basin – the South Pole-Aitken Basin.

Simply put: Near Side = Large plains (maria) + Few highlands and craters; Far Side = Almost entirely highlands and craters.

(A simple comparison diagram: left shows the smooth near side, right shows the rugged far side)

II. Unraveling the Cause: Earth Might Be the "Mastermind"

Why is this the case? Scientists have proposed several hypotheses, but the most widely accepted theory points to a significant influence from our own Earth.

The Core Reason: Different Crust Thickness

The root of all these differences lies in this: The crust on the Moon's far side is much thicker than on the near side.

Near Side Crust: Average thickness about 60 km.
Far Side Crust: Average thickness up to 100 km, even thicker in some places.

Think of the lunar crust like an eggshell. The near side has a relatively thin "shell," while the far side has a particularly thick one.

Why the Difference in Crust Thickness? – Earth's "Baking" Effect

This takes us back over 4 billion years, to when the Moon was just formed.

The Moon's Birth and Tidal Locking: According to the "Giant Impact Hypothesis," the Moon formed from debris ejected after a Mars-sized planet collided with the early Earth. Soon after formation, it became tidally locked by Earth's powerful gravity, forever keeping one face towards us.
"Care" from Earth: At that time, Earth wasn't the blue planet we know today. It was an extremely hot, molten "furnace." Because the Moon's near side always faced Earth, it was continuously "baked" by intense thermal radiation from our planet.
The Result of "Baking":
- Near Side: Under Earth's constant heating, the near side remained significantly hotter than the far side. This kept the rocks on the near side molten or semi-molten for longer, allowing for more thorough differentiation of materials. Lighter elements (like aluminum and calcium) rose to the surface, resulting in a relatively thinner crust.
- Far Side: The far side was effectively in the "shadow" of the Moon itself regarding Earth's heat, receiving none of it. Consequently, it cooled much faster. It solidified early on, forming a thick, hard, ancient crust.
The Later Story:
- In the early solar system, asteroid impacts were extremely frequent. When massive asteroids struck the Moon:
  - Hitting the near side: Because it was "thin-skinned," the impact easily cracked through the crust. Magma from the mantle deep below gushed up, flooding the giant impact basins. This lava cooled to form the vast, flat, dark "maria" we see today.
  - Hitting the far side: Because it was "thick-skinned," asteroids mostly just blasted deep craters into the surface. They couldn't penetrate this thick "armor," so no magma flowed out. Therefore, the far side only bears the scars of impacts (craters) and lacks large maria.

To Summarize

So, the whole story chain is:

Observation: Near side is flat (many maria), far side is rugged (many craters).
Direct Cause: Near side crust is thin, far side crust is thick.
Root Cause: After becoming tidally locked to Earth, the Moon's near side was continuously "baked" by the intense heat of the early Earth, causing its crust to thin; meanwhile, the far side cooled and thickened more rapidly.

It's this "two-faced" nature, indirectly caused by Earth, that makes the Moon such a unique object in our solar system. With China's Chang'e-4 mission successfully landing on the far side for the first time, we are now uncovering more of its mysteries.