Look up at the full moon tonight. You’ll see it—that "Man in the Moon" face or the "Rabbit" shape. Those large, dark, blotchy patches aren't shadows from mountains, and they definitely aren't oceans. Early astronomers like Giambattista Riccioli actually thought they were water, which is why we call them lunar maria. "Maria" is just Latin for "seas."
They were wrong about the water, obviously. But the name stuck.
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Basically, when you’re looking at a lunar mare (the singular form), you’re looking at a massive scar from the Moon’s violent childhood. These are gigantic plains of solidified basaltic lava. They cover about 16% of the lunar surface, mostly concentrated on the side we see from Earth. If you’ve ever wondered why the Moon looks so lopsided—dark on the front, light and cratered on the back—you’ve stumbled into one of the most interesting mysteries in planetary science.
The Volcanic Origin of Lunar Maria
Let's get the "how" out of the way. About 3 to 4 billion years ago, the Moon was a chaotic mess. It was getting hammered by massive asteroids. We’re talking rocks the size of small states. These impacts didn't just leave craters; they cracked the Moon's crust wide open.
Because the Moon’s interior was still hot and molten back then, magma seeped up through these fissures. It was a slow-motion flood. Think of a basement flooding with muddy water after a pipe bursts, except the water is liquid rock and the basement is a thousand-mile-wide impact basin. This low-viscosity basaltic lava flowed into the low-lying basins, filling them up and eventually cooling into the smooth, dark rock we see today.
Most of this happened during the Imbrian period. It wasn't a single "pop" and then done. This was a series of volcanic episodes spanning hundreds of millions of years. Scientists like Paul Spudis have noted that the composition of this basalt is rich in iron, which is why it reflects less light than the surrounding "highlands." The highlands are mostly anorthosite, a lighter-colored rock. That contrast is exactly why the lunar maria stand out so vividly against the rugged, crater-filled terrain.
Why the "Far Side" is So Different
If you look at photos from the Soviet Luna 3 mission or NASA’s Apollo orbits, the back of the moon looks weird. It's almost entirely light-colored and covered in craters. There are hardly any maria there.
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Why?
For a long time, we didn't really know. But the prevailing theory now involves the "crustal thickness" model. Earth’s gravity locked the Moon into a synchronous orbit very early on. Because the Moon was much closer to a hot, early Earth, the "near side" stayed warmer for longer. This prevented the crust from thickening as quickly as it did on the far side.
When the big asteroids hit, they could easily punch through the thin crust on the near side to reach the magma beneath. On the far side, the crust was just too thick. The impacts happened, sure, but the lava couldn't find a way out. It’s like trying to poke a straw through a brick versus poking it through a piece of cardboard.
The Most Famous Maria You Can See
You don't need a massive telescope to find these. A pair of cheap binoculars—or even just decent eyesight—will do.
- Mare Tranquillitatis (Sea of Tranquility): This is the big one. It’s where Neil Armstrong and Buzz Aldrin landed the Apollo 11 Lunar Module. It’s relatively flat, which is exactly why NASA picked it. They didn't want to tip the lander over on a mountain.
- Mare Imbrium (Sea of Rains): One of the largest maria in the solar system. It’s roughly 700 miles across. If you see a giant circular dark patch near the top-left of the Moon, that's it.
- Mare Serenitatis (Sea of Serenity): Located right next to Tranquillitatis. It’s famous for having very distinct "wrinkle ridges," which are basically folds in the rock where the lava cooled and contracted.
Honestly, the names are way more poetic than the reality. "Sea of Rains" sounds like a vacation spot. In reality, it’s a dry, vacuum-sealed wasteland of volcanic glass and dust.
The "KREEP" Factor
There is a specific chemical signature found in many lunar maria regions known as KREEP. No, it’s not a typo. It stands for Potassium (K), Rare Earth Elements (REE), and Phosphorus (P).
These elements don't like to be part of standard crystal structures when rock cools, so they stay in the liquid magma until the very end. The fact that we find so much KREEP on the near side tells us that this side of the Moon had a very different thermal history. It was chemically "enriched." This heat-producing radioactive material likely helped keep the magma liquid for longer, fueling the eruptions that created the maria we see today.
Misconceptions About the Age of the Maria
A lot of people think the maria are the "oldest" parts of the moon because they look so established. Actually, it's the opposite.
The bright, cratered highlands are the original crust. They are the survivors of the "Late Heavy Bombardment." The maria are younger. We know this because they have far fewer craters. If the maria were as old as the highlands, they’d be just as chewed up by impacts. Instead, the lava flows acted like a "reset" button, paving over the older craters and starting a new geological clock.
That said, "younger" is relative. We’re still talking about rocks that are 3 billion years old. To put that in perspective, the dinosaurs only died out about 66 million years ago. These lunar plains haven't changed much since long before the first multi-celled organisms appeared on Earth.
Why Should You Care?
Understanding what a lunar maria is actually helps us understand Earth. Because Earth is geologically active—thanks to plate tectonics and erosion—we’ve wiped away almost all evidence of our early history. The Moon is like a fossil. It’s a time capsule.
By studying the basalt in the Sea of Tranquility, we learn about the composition of the solar system during its most formative, violent years. We learn about the volcanic processes that happen on small, airless bodies.
Also, if humans ever build a permanent base on the Moon, it’s going to be in a mare. The flat terrain is better for landing, and the basaltic soil (regolith) is rich in iron and magnesium, which could potentially be mined.
Next Steps for Moon Watching
To see the maria for yourself, don't look during a Full Moon. The light is too direct and washes out the contrast. Instead, look during a First Quarter or Last Quarter phase. Look along the "terminator"—the line between light and dark. The long shadows will highlight the edges of the maria, the wrinkle ridges, and the craters that have been partially flooded by lava.
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Download an app like "Lunascope" or use a basic moon map to identify Mare Imbrium and Mare Crisium. Once you spot the difference between the dark basalt and the white highlands, you'll never look at the Moon the same way again. It stops being a glowing disk and starts being a complex, scarred world with a story to tell.