Look up at the Moon on a clear night and you’ll see dark patches smeared across its face like spilled ink on parchment. For centuries, astronomers called them “maria”—Latin for seas—because they genuinely believed these were bodies of water. Spoiler: they were spectacularly wrong.
When Ancient Astronomers Mistook Frozen Lava for Ocean Vacations
Galileo peered through his telescope in 1609 and saw what he thought were lunar oceans. The name stuck even after everyone realized the Moon is bone-dry. These “seas” are actually vast plains of solidified basaltic lava, darker than the surrounding highlands because basalt reflects less sunlight than the ancient crustal rock. Mare Imbrium—the Sea of Rains—spans roughly 750 miles across. That’s larger than Texas, and it formed around 3.9 billion years ago when a massive asteroid slammed into the Moon with the energy of maybe a billion nuclear bombs.
The impact melted everything.
Here’s the thing: lunar maria aren’t scattered randomly. They cluster predominantly on the near side of the Moon—the side perpetually facing Earth. The far side, photographed first by the Soviet Luna 3 probe in 1959, shows hardly any dark patches at all. Scientists still debate why this asymmetry exists, though the leading theory involves the Moon’s crust being thinner on the near side, making it easier for magma to breach the surface after those cataclysmic impacts. The far side’s crust averages about 68 kilometers thick; the near side manages only around 33 kilometers. That’s a massive difference when you’re trying to push molten rock upward against gravity.
The Volcanic Floods That Took Milions of Years to Actually Happen
Wait—maybe “flood” is misleading. These lava flows didn’t happen overnight. Mare volcanism stretched across hundreds of millions of years, from roughly 4 billion to 1.2 billion years ago. Individual eruptions could last decades or centuries, slowly filling impact basins like someone pouring syrup into a bowl, except the syrup is 1,200-degree molten rock and the bowl is a crater the size of France. Mare Tranquillitatis—where Apollo 11 landed in 1969—measures about 540 miles in diameter and contains lava flows that cooled in distinct layers, each representing separate volcanic episodes separated by millions of years.
Turns out the Moon wasn’t always the dead, airless rock we see today.
Between 3.8 and 3.1 billion years ago, the Moon experienced its most volcanically active period. Magma welled up through cracks opened by asteroid bombardment during the Late Heavy Bombardment—a period when the inner solar system resembled a cosmic shooting gallery. Some maria, like Mare Orientale, show concentric rings from the initial impact, now partially flooded with darker basaltic rock. The youngest mare rocks, found in regions like Oceanus Procellarum, date to approximately 1.2 billion years old, though controversial findings from China’s Chang’e-5 mission in 2020 suggested some volcanic activity might have persisted until just 2 billion years ago. That would mean the Moon stayed geologically active far longer than anyone expected.
Why Mare Rock Looks Nothing Like Mountain Rock Even Though It’s All Just Moon
The chemical composition tells the real story. Highland material—the bright stuff—is mostly anorthosite, rich in aluminum and calcium. It’s ancient crust that formed when the Moon’s initial magma ocean cooled and lighter minerals floated to the top like scum on soup. The maria, meanwhile, consist of iron-rich basalt that erupted later from the Moon’s mantle. This iron content makes mare soil darker and also slightly magnetic—Apollo astronauts collected samples showing remnant magnetism that suggests the Moon once had a magnetic field, though it’s since vanished.
That’s the kind of detail that makes planetary scientists slightly unhinged with excitement.
The Bizarre Names That Stuck Because Astronomers Apparently Loved Drama
Mare Crisium. Mare Serenitatis. Mare Nectaris. The Sea of Crises, the Sea of Serenity, the Sea of Nectar. Giovanni Battista Riccioli started this naming convention in 1651, assigning emotional or meteorological themes to features he could barely resolve through primitive optics. Some names make poetic sense—Mare Tranquillitatis sounds peaceful. Others feel randomly assigned, like Mare Vaporum (Sea of Vapors) or Mare Spumans (Foaming Sea), which never foamed and contains no vapors. The largest mare structure, Oceanus Procellarum (Ocean of Storms), covers over 1.6 million square miles but isn’t even a proper impact basin. It might have formed from volcanic flooding of a massive tectonic depression, or possibly from multiple overlapping impacts that merged into one gigantic dark patch.
Nobody’s entirely sure, which is cosmically frustrating.
When Modern Robots Finally Touched Down on Those Ancient Lava Fields
Luna 2 crashed into Mare Imbrium in 1959—first human-made object to reach another world. Then came the soft landers: Luna 9 in 1966, touching down in Oceanus Procellarum and transmitting grainy photos proving the surface wouldn’t swallow spacecraft like quicksand, which some scientists genuinely feared. Apollo 11’s Eagle landed in Mare Tranquillitatis on July 20, 1969. Neil Armstrong’s boot prints pressed into regolith—pulverized rock created by billions of years of micrometeorite bombardment—that was once liquid, flowing across the lunar surface like rivers of fire. Those bootprints will last millions of years because there’s no wind or water to erase them.
The maria aren’t just geological curiosities. They’re time capsules preserving evidence of planetary violence from an era when the solar system was still sorting itself out. Every dark patch represents catastrophic impacts followed by slow volcanic healing, the Moon’s way of stiching its wounds with molten rock. And they’re why the Moon looks like it has a face—those maria form the dark features that humans have projected into the “Man in the Moon” for millennia, seeing patterns in ancient lava flows because our brains can’t help finding faces in random shapes.
We’re still finding faces in volcanic scars from 4 billion years ago.
