Why does the Moon's far side look so different from the near side?

Mostly because lava flooded the near side and almost never the far side. The two faces genuinely look like different worlds. The near side is marked by the familiar dark patches called maria, ancient lava plains covering 31.2 percent of its surface, while the far side is covered in maria for barely 1 percent of its area and is instead a battered, rugged landscape dense with craters. Apollo 8 astronaut William Anders, one of the first humans ever to see it, described it bluntly: it looked like a sand pile his kids had played in, all beat up, with no definition, just bumps and holes. Because it lacks those large dark maria, the far side is actually the brighter, more reflective of the two hemispheres.

Why the lava ended up almost entirely on one side is still debated. A commonly accepted explanation draws on gamma-ray maps made by the Lunar Prospector spacecraft, which found that heat-producing elements are concentrated on the near side, in a region scientists now call the Procellarum KREEP Terrane, the idea being that more heat meant more volcanism. But that can't be the whole story: the far side's South Pole–Aitken basin sits at the Moon's lowest elevations with a notably thin crust, exactly the kind of place basaltic lava should erupt easily, yet it was far less volcanically active than the near side's Oceanus Procellarum.

Another explanation looks at how the young Moon's crust formed. The cooler far side built up a thicker crust, while the near side ended up with a thinner one. That thinner crust meant that when meteoroids struck the near side, they could sometimes punch through it and release basaltic lava to fill the resulting craters as the dark maria we see today, something that rarely happened on the thicker-crusted far side. A rival idea suggests the far side's highlands came from a separate, smaller companion moon that later crashed into the main Moon, though the far side's actual chemical makeup doesn't match that story well.

The far side's crowded craters aren't simply the Earth shielding us from impacts, either: NASA calculates that Earth hides only about 4 of the 41,000 square degrees of sky seen from the Moon, far too little to meaningfully shield either hemisphere, so both sides have likely taken equal numbers of hits. The near side just looks less cratered because its lava flows buried and smoothed over many of its oldest scars. Left un-resurfaced, the far side also holds the Moon's most extreme terrain of all, including its highest peaks and its deepest lows.