35: The Man in the Moon
We last left our story on the Moon 3.8 billion years ago, early March on our imaginary Earth Calendar. Our poor Moon has been bombarded by asteroids since Day 1, making countless craters on its’ pale surface. Craters from these early, violent days are some of the largest in the Solar System, titanic pits the size of Alaska, Egypt, and even the Arctic Ocean.
But 3.8 billion years ago, things are changing on the Moon. Two things in particular:
First, the age of giant, nation-sized craters is coming to a close. Asteroids will gradually become smaller as the Solar System calms down.
On the other hand, the Moon itself is heating up. The old white craters are now filling in with seas of angry red lava. As the lava cools from red to black, we can finally see our old friend, the Man in the Moon.
Today, we’ll take a closer look at these volcanos. Why are they erupting now? How long will they last? What new rocks are they making?
To answer these questions, we need a full picture of our Moon. Until now, we’ve only looked at our side, the side constantly facing Earth, the side with the Man looking down.
But to understand how that Man got there, we must take a peek just around the corner.
Part 1: The Far Side
The year is 1959. Dwight Eisenhower is the US president, Nikita Khrushchev is the Soviet premier, and Elvis Presley is the King of the World.
The Space Race has just begun between the United States and the Soviet Union, and the Soviets are in the lead. Just two years before, the Soviets started the race by launching Sputnik, the first satellite. Now, they set their sights higher. 1959 was the year of the moon.
In January, the Soviets launched Luna 1, the first manmade object shot beyond Earth’s orbit. That was actually a mistake- the goal was to hit the moon, not shoot past it. But! In September, Luna 2 succeeded, crashing straight into the Moon’s face.
One month later, Luna 3 was sent into the void, roughly the size and shape of a large garbage can. This was the most ambitious mission yet. Luna 3 would fly around the far side of the moon- a place no human had ever seen before.
As Luna passed over this unknown land, a camera inside took 29 pictures and processed them on actual physical film, like a Polaroid in space but much tougher. The film would never make it back to Earth’s surface, but the Soviets knew this. Instead, inside Luna 3 was a scanner, a tiny beam of light passing over the film that could sense how dark or bright each portion was. That brightness data became an electric signal sent back to Earth- the brighter the spot on the photo, the stronger the signal sent down.
In short, the first mission to the Moon’s far side was a flying fax machine. If you don’t know what a fax is, look it up or ask your parents.
Piece by piece, the Soviets watched the bright and dark colors assemble into pictures. The shots were black and white and grainy, but even this first glimpse showed a very different side of the moon.
As a review from last episode, our side of the moon is divided into pale highlands and dark flat plains called maria (MA-ree-ah). Many maria sit inside gigantic ancient craters, hundreds of miles wide, like vast puddles of frozen ink. These patterns make the Man in the Moon, or a rabbit, or any other shape you see up there.
If that’s our side of the moon, what could possibly be on the far side, unseen by humanity? What did Luna 3 see in 1959? Was it a single, massive crater, larger than all the others combined? Was it a hidden city, with aliens just around the corner from us waiting to strike? Was it a vast plain of Manchego cheese?
None of the above. The far side of the moon is very different from the near side because, it’s extremely dull. It’s all pale highlands- lots and lots of empty craters, with only a few small blobs of dark maria. Even the most imaginative child would have a hard time making any image here.
And yet, the fact that there is this difference, and that our side of the moon is by far more interesting and exciting, is worth investigating.
Part 2: Will It Float?
To recap: the far side of the moon has lots of craters, but is plain pale gray, like an old dusty golf ball. The near side of the moon also has many craters, but the largest ones are filled in with dark volcanic rock- the Man in the Moon. What happened here, why is there such a big difference?
The answer starts way back in Episode 9. Even if you’re binging this show, that’s a while back, so here’s a recap.
The Moon’s first surface was a vast, deep ocean of molten magma. As this ocean cooled, crystals began to form, just like ice in freezing water. Unlike water, though, magma has many different ingredients, many different elements: silicon, iron, calcium, etc. As magma freezes, different crystals pop out at different temperatures. Some sink to the bottom, while others float to the top.
Let’s take this idea to the ancient moon. As the magma ocean cools, we’re going to play a game, made famous on TV by David Letterman: Will It Float? But instead of throwing objects into water, we’ll look at three crystals and see how they act inside magma. It might sound silly, but this simple game will have huge effects on the Moon’s surface.
Candidate 1: The first crystal to appear is our old green friend from Season 1: you know it, you love it, it’s olivine. As a reminder, olivine is rare on Earth’s surface, but very common in Earth’s mantle, the layer just below us. With that in mind, Will Olivine Float?
No chance. Green olivine is dense, and sank deep inside the magma ocean, just like Earth.
Candidate 2: This next crystal is a dark horse- it’s usually jet black. We’ve only discussed it twice on the show, so let’s give it a little more time. This crystal is called pyroxene (peer-ucks-een), that’s P-Y-R-O-X-E-N-E, a great Scrabble word. Pyroxene literally means “fire stranger” in Ancient Greek, and there’s a reason for that. Like green olivine, dark pyroxenes are common in Earth’s mantle below our feet, but they also show up on Earth’s surface, through volcanic eruptions.
Lava flows with lots of pyroxene will cool into dark black volcanic rocks, and maybe that rings a bell. The mineral pyroxene may be a stranger on our show, but dark pyroxene-rich rocks are an old friend: basalt. We’ve talked about basalt since Episode 2, and we just met Earth’s oldest basalts in Episode 31, in Quebec. If there are only three things to remember about basalt, here they are: 1: it’s volcanic, 2: it’s black (thanks to pyroxene crystals), and 3: it’s usually found on the deep seafloor. But once in a while basalt bursts onto land: the Giant’s Causeway in Northern Ireland, the Palisades of New York State, and most of Iceland.
To recap: on Earth’s surface, dark pyroxene is usually found in basalt, usually in the deep ocean. Pyroxene is also very common in the mantle below our feet. So, place your bets! In the Moon’s magma ocean, Will Pyroxene Float?
The answer is: No. When scientists throw pyroxene into magma, like Norm Bowen did in Episode 9, pyroxene sinks like a stone. But pyroxene can break free to the surface- we see it all the time in black volcanic basalts. The same will be true on the ancient moon, but for now, in the beginning, dark pyroxene is lurking deep below, waiting to escape.
Candidate 3: Our final contestant is an irregular guest on the show- not as frequent as green olivine, but more than dark pyroxene. This mineral is pale plagioclase (pleh-jo-clays), or “plag” (pleh-j) for short. Plag comes in many shades of gray but is usually much lighter than pyroxene. Where can we find it? On Earth, plag is rare in the deep mantle, but it’s very common on Earth’s surface, in the oceans and on land.
In short, plag is pale, abundant in surface rocks, and our only remaining contestant. With that in mind, looking at the Moon’s magma ocean: Will Plag Float?
Yes! As the magma ocean cooled ever further, plag crystals rose to the top like a trillion tiny corks. The Moon’s cratered highlands are filled with pale plagioclase, especially on the far side.
But on our side of the Moon, 3.8 billion years ago, dark pyroxene made a jailbreak to the surface from deep in the Moon’s mantle. This escape coated the old craters with black basalt, making the Man in the Moon. But how did this happen? Who helped this deep pyroxene escape, and why is it only on one side of the Moon?
Part 3: A KREEPy Sandwich
Let’s look at crystals in water for a second. When water freezes, ice forms at the surface and gets thicker over time, reaching deeper down. In contrast, sugar crystals sink to the bottom, and build upward over time, like rock candy. Inside magma, both actions can happen at once. That was the whole point of our game: we learned that some crystals like pyroxene sink, while others like plag float.
What does that mean for the ancient Moon? Let’s shrink the magma ocean into a single cup. As the magma cools, green olivine and dark pyroxene settle on the deep bottom, building upward like sugar. At the same time, pale plag on the surface grows thicker, reaching farther down into the sea like icebergs.
Evolution of the Moon’s magma ocean. Dark pyroxene sinks, plag floats.
We’ve basically formed a hot, gooey sandwich: the top slice is pale plag, the bottom slice is dark pyroxene, and the remaining magma ocean is squeezed in between. Our final section will focus on this middle magma layer because it’s extremely weird. It’s not plag, it’s not pyroxene, it’s something else entirely.
This magma filling contains the “rejects” of its’ neighbors. There is a long list of elements in this leftover magma soup, so geologists use a helpful acronym: K-REE-P, or KREEP with a K. Easy to remember.
The K stands for potassium, the P stands for phosphorus, and the REE in the middle stands for Rare Earth Elements, the 17 sisters we met in Episode 27, in Earth’s oldest rocks.
We don’t have time to describe every element in KREEP, but all together, this material has two very important properties, two facts that will make the Man in the Moon.
1: KREEP is very hot. You might say, “No, duh- KREEP is magma,” but it’s not just molten heat, it’s radioactive heat, coming from uranium, thorium, and even potassium. For a brief side-tangent, when most folks think potassium, they think bananas. So, are bananas radioactive? Very, very, very slightly: a lethal dose would require 35,000,000 bananas, which would crush you first. Also, your body removes potassium, so it doesn’t build up. Also also, bananas are not actually potassium-rich, as I learned writing this episode: a baked potato or a cup of lima beans has twice as much. Some food for thought.
Back on the Moon, the KREEP layer has the same radioactive potassium, but a lot more than your bananas, pumping out a lot more heat. We can measure this heat from the Earth- the Man on the Moon has a lot of KREEP underneath, pumping out 10x more heat than the far side. As you can imagine, this heat helps make volcanos. But volcanos are not enough to make the Man in the Moon. We need to pull up the dark pyroxene from deep below, which brings us to Point 2.
2: KREEP is very dense. Over time, it grows denser than the olivine and below it. This difference leads to instability. For context, imagine you have a very broad, very thin sheet of metal. This metal is definitely denser than water, but maybe if you’re careful you can get that sheet to float for a few seconds. But any major wave will unsettle the system and make the metal sink like it wants to.
The same thing happened in the ancient moon. The KREEPy magma was denser than the dark pyroxene layer below. Eventually, the two layers began to mix like fluids in a lava lamp, with dark pyroxenes rising much closer to the surface than before.
Thorium distributions on the Moon, one of the elements in KREEP.
In short, KREEP helped heat and stir the Moon’s interior, bringing up deep material into hot surface volcanos. These alien lavas slowly filled in the deepest craters, cooling into black, pyroxene-rich basalts, forming the Man in the Moon.
You might think all those meteors from last episode helped stir the pot, blowing holes in the Moon’s surface to make new volcanos. but that doesn’t seem to be the case. Remember, the largest craters on the moon are from January and February on the Earth Calendar. In contrast, the vast plains of basalt, the maria, ramp up in March, starting around 3.8 billion years ago. So while meteor impacts didn’t instantly make volcanos, they thinned and weakened the crust for later eruptions.
Most KREEP got recycled a long time ago, but there’s still a pocket sitting beneath today’s Man in the Moon. That hot KREEPy pocket fueled volcanic activity for the next two billion years, until September on the Earth Calendar, Season 6 on this program. We’ll probably check in with the Moon before then, but honestly, things will be stable for a long time to come. From here on, the Moon is essentially the neighbor we know and love. Earth still has many changes to make, but the Moon will always be looking on, a memory of a distant, violent time long, long ago.
Summary:
How did the Man in the Moon form?
First, asteroid impacts carved titanic craters on the Moon’s surface, the size of nations and even oceans, slowing down around 3.8 billion years ago. At the same time, the Moon’s interior was roiling and changing. Different layers of magma and crystal rearranged themselves, vomiting deep material onto the Moon’s surface in volcanic eruptions. Hot lava cooled into dark basalt, filling in the giant craters and making the Man in the Moon. Eruptions will continue for the next two billion years, but by the end of this season, the Moon would be recognizable to any human today. As alien as our world will become, you can always look up and remember this is our home.
Next episode, we’ll return to the Earth for the final arc of Season 2. We’ve seen Earth’s oldest rocks, and the oldest rocks that formed on Earth’s surface. Our last location will be the widest window onto the Eoarchean, the most rocks we’ve seen to date. Where and when are these rocks? Stay tuned to find out!
Riccioli labeled more than 200 features on the moon, most of which we still use. We now know there’s no liquid water on the moon, but his names have stuck. To this day, we call the dark, flat plains “maria”, the plural form of “mare”. Again, that’s MAria, not maREEa like a person’s name.
The other major feature of the moon are the pale highlands, the areas between the low, dark maria. The highlands are the moon’s original face of the moon, the first surface that cooled down way back in Season 1. I’ve described this surface like a golf ball- white and full of craters. These craters range in scale from nearly invisible to the size of the Arctic Ocean, some of the largest in our Solar System.
Many of the Moon’s largest craters are filled with dark maria, which brings us full circle. What is the “Man in the Moon”? A series of titanic craters filled in with dark rocks we call maria. Today, we’ll learn about the ancient craters themselves, before they got filled in. Next episode, we’ll focus on those newer dark seas of stone.