What is Mare made of?

The Moon’s “Seas”: More Than Just Dark Patches

Ever gazed up at the moon and noticed those dark smudges? For centuries, people called them maria – Latin for “seas.” Back then, astronomers figured they were actual oceans up there! Of course, we know better now. These aren’t bodies of water at all, but sprawling plains of solidified lava, covering about 16% of the lunar surface. Mostly on the side that always faces us, these dark, smooth areas are like a lunar history book, telling tales of volcanic eruptions and the Moon’s wild, early days.

So, how did these “seas” even get there? Picture this: billions of years ago, the solar system was a bit of a demolition derby. Asteroids were slamming into everything, including the Moon. These massive impacts carved out huge basins. But here’s the cool part: they also cracked the Moon’s crust, like dropping a bowling ball on a sidewalk. This created pathways for magma – molten rock from deep inside the Moon – to bubble up and fill those impact craters. For ages, volcanoes kept erupting, and lava flowed and flowed, eventually cooling and hardening into the dark plains we see today. It was a messy, fiery process that wrapped up around 3 billion years ago.

What exactly makes up this lunar “seaweed,” then? Well, the maria are mostly basalt, that dark, fine-grained rock you see in volcanic areas here on Earth. But lunar basalt is a bit different. For one thing, it’s bone-dry. No water, no easily vaporized elements. It’s also loaded with iron, magnesium, and titanium – way more than you’d find in Earth basalt.

The main ingredients in this lunar rock soup include:

• Pyroxene: Think of it as a mineral cocktail of calcium, iron, and magnesium.
• Plagioclase Feldspar: A close cousin to other feldspars, but with a calcium and sodium twist.
• Olivine: This one’s a mix of magnesium, iron, silicon, and oxygen.
• Ilmenite: The real star of the show! This iron-titanium oxide (FeTiO3) is what gives the maria that super-dark color. It’s like the lunar version of black food coloring.

That high iron and titanium content is why the maria look so dark, creating a stark contrast with the brighter, more reflective highlands – the lunar “land.”

Now, not all mare basalts are created equal. Scientists classify them based on how much titanium they contain:

• High-Ti Basalts: These are the heavyweights, packing more than 9% titanium dioxide (TiO2).
• Low-Ti Basalts: The middle-of-the-roaders, with a more moderate amount of titanium dioxide.
• Very-Low-Ti (VLT) Basalts: The lightweights, with the least amount of titanium dioxide.

And then there are the oddballs: the KREEP basalts. These are super-rich in potassium (K), rare-earth elements (REE), and phosphorus (P). Basically, they’re the lunar equivalent of a multivitamin.

Oh, and let’s not forget the regolith! Just like the rest of the Moon, the maria are covered in this layer of loose stuff – rock bits, mineral grains, and dust. It’s all thanks to billions of years of tiny meteorites constantly bombarding the surface, grinding the rocks down into smaller and smaller pieces. This layer can be a few meters thick in the maria, but it gets much deeper in the highlands.

Ever wonder why the maria are mostly on the near side of the Moon? It’s a bit of a lunar mystery, but here are a few ideas:

• Thin Crust: The crust on the near side is thinner, making it easier for magma to reach the surface. Think of it like trying to pop a balloon – it’s easier to pop where the rubber is thinnest.
• KREEP Power: Remember those KREEP basalts? Well, a lot of the Moon’s heat-producing elements are concentrated on the near side, in a region called the Procellarum KREEP Terrane. This extra heat might have fueled more volcanic activity.
• Earth’s Pull: Some scientists think that Earth’s gravity might have played a role, somehow making eruptions more likely on the near side.

So, why should we care about these lunar “seas”? Because studying them gives us a peek into the Moon’s past, its ingredients, and how it all came together. By analyzing those basalt rocks – some brought back by the Apollo missions, others found as meteorites – we’ve learned a ton about the Moon’s insides, its volcanic history, and even the early solar system. Plus, this knowledge is crucial for planning future lunar adventures, from picking the best landing spots to figuring out how to use the Moon’s resources.

The lunar maria are way more than just dark spots in the sky. They’re vast, ancient lava plains, packed with iron and titanium, that hold the secrets to the Moon’s fiery youth. And by continuing to explore these fascinating features, we can unlock even more mysteries about our celestial neighbor and the universe around us.