The Moon has had a huge impact on Earth and its cultures. That’s pretty much inarguable. Pretty much every society to have ever form has attributed some significance to the moon or the lunar cycle. Even in the physical sense, it controls our tides, shields us from asteroids, and provides the dominant source of light at night among other things. Arguably one of the greatest accomplishments of humanity in the twentieth century was actually managing to put people up there. But what if it was even more important?
What if the moon can, and did, support life?
This is probably going to be a three part series. We’re going to look at what would need to happen for the moon to be habitable and what it would be like, then what life on the moon would might look like, then finally what that would mean for human civilization. I may at some point write a novel set in a world where this is true, or it may just be a fun idea I play with here.
Oh, and quick aside: If you believe we never set foot on the moon, or that going to the moon is impossible, or anything that disagrees with normal scientific consensus: I don’t care. Okay? Okay.
So let’s get into it. What would a habitable moon need?
First and foremost, it would need an atmosphere. Well, technically it has one but its so thin that it doesn’t matter. It needs a thick, dense atmosphere. That’s pretty obvious. While there’s a huge amount of debate about what is needed for life to form and survive on other worlds, from heat to radiation levels to even alternative biochemistries, no one really argues that life could survive on a world without air. So, let’s give it one.
Aaaand in a few million years, it’s gone.
See, there are a couple barriers to the moon having an atmosphere. The first is mass. The moon is much less massive than Earth. Everyday, Earth loses some of its atmosphere to space as molecules get bounced out of Earth’s gravity and float away. On the moon, that would happen even quicker, because its smaller. But we can make it just a bit bigger and get away with it.
There is a moon in our solar system that has an atmosphere, one that’s actually ninety-one percent as thick as Earth’s, so it’s really close to being exactly what we know. That moon currently belongs to Saturn, and its called Titan. It can hold an atmosphere, in part, due to the fact that it is eighty percent as massive as our moon. (Pay attention to that ‘in part”, it becomes important later). So lets beef up our moon, give it eighty percent the massive of Titan.
If this magically happened today, we’d definitely notice it, but it wouldn’t be a world breaking disaster (although human civilization would probably get pretty wrecked in the initial massive earthquake would happen). The sudden addition of almost a second moon’s mass would shift things noticeably. First of all, the baycenter (that is, the center of gravity) of the Earth-Moon system would now be shifted outside the Earth, which means that the Earth and Moon would be rotating around a point slightly above the Earth’s surface.
Technically the Earth-Moon system already rotates around such a point, but because the baycenter is within the Earth, we don’t really notice it. If it was outside the Earth, we probably wouldn’t notice it all that much, although it would impact the weather in unpredictable ways. The addition of a Titan-mass moon would also increase how much tides raise and lower by about 180%. That sounds like a huge amount, but in reality it probably wouldn’t have too much of an impact except for some coastal cities that have a very low coastline.
Also, it would be significantly brighter than the moon. Depending how how reflective the atmosphere was, it could be two or three times brighter. So night would suddenly become much, much less frightening.
Now, here’s the thing – I’m mostly interested in this scenario for how a Habitable Moon would impact life on Earth. Specifically, human life in a modern era similar to our own. So while it’s interesting to note those changes, I don’t want to spend too much time on the exact math or figuring out how the mass would impact Earth. We’ve established the important detail – it would destroy the Earth, so we can plop it up there safely, atmosphere and all.
And the atmosphere is gone. Again. Crap.
Remember when I said that “in part” was going to be important? That’s where this comes in. See, the other reason Titan has a nice thick atmosphere is because it’s all the way out by Saturn, making it really, really cold. We’re talking “go to antarctica to warm up” cold. It has an average temperature of -179 celsius, or -290 Fahrenheit. That means that the atmosphere doesn’t have much energy, making it harder for gas to escape. But if out here, orbiting the Earth? It’ll be about the same temperature as Earth, since it’ll be getting the same amount of sunlight. On top of that, it’s getting 99% the solar radiation it was before. Those two factors will strip away its atmosphere long before life could develop.
While this won’t fix the heating problem, there’s one thing we can do: we can give it a molten core, and thus give it a magnetosphere.
I can’t find any good data for how two magnetospheres being as close as the Earth and the moon would interact, but that’s okay, because we’re pretending Earth isn’t going to be impacted too much physically. What matters is how the New Moon would even have a molten core.
So first of all, how does Earth?
Earth’s core is molten because of three factors. First of all is because, billions of years ago when the solar system was first forming, everything was hot. Super absurdly hot. At one point it was so hot, the Earth was molten. It eventually cooled down enough to have a crust, and that’s part of why the Earth’s core is molten – it was so absurdly hot billions of years ago, the interior of the Earth is still hot from back then. Which is crazy when you really think about it. Imagine your Hot Pocket, after coming out of the microwave, needed billions of years to cool down. I don’t know why you’re imagining that, it’s absurd, but that’s how absurdly hot the solar system used to be. Incidentally, it would take the Earth about 91 billion years for it to cool down.
The second reason the core is still molten is that radioactive elements within the core are, well, being radioactive and therefore releasing energy. Energy hits rest of core, energy converts to heat, rocks start melting and stay melting. Fairly simple. The third factor is friction, as heavier elements keep sinking deeper and deeper into the Earth. Eventually they’ll sink all the way down, removing this source of heat.
Our New Moon, while much bigger than the OG Moon, is still fairly small. Much like what happened with Mars and Mercury and many other bodies in the solar system, it would have cooled to the point of being solid all the way through by now. On top of that, because it’s smaller, frictional heating lasts for a shorter amount of time, since the heavier elements have a shorter distance to go. So to fix that, lets just say our New Moon has a much more radioactive core than Earth. It got a bunch more uranium and thorium compared to its size early in the solar system’s formation, and has been riding that wave since it formed. It will cool down before Earth does, but it hasn’t yet.
That solves the problem of solar radiation stripping away the atmosphere. It also gives our New Moon plate tectonics, earthquakes (moonquakes?), and volcanoes, all of which are great for life and biodiversity. Incidentally, this is why it’s worth putting the effort in to figure these things out, because you come up with things that might not have occured to you otherwise.
The final problem, the fact that a hotter atmosphere escapes more quickly…I’ve got nothing for. Without adding even more mass, so much so we’re not talking about a moon but a second planet, I have not been able to figure out a way to solve this problem. And this is where narrative trumps logic. For the idea to work, we need the moon to have an atmosphere. We’ve solved two out of three problems to get there, but the third isn’t something we can solve. So, we handwave it by saying that solving the first two meant the third won’t be a problem for long enough for the moon to still be habitable in the modern era.
(If you happen to know of a way to solve that problem, I’d love to hear it)
So, we have our new moon, which deserves a more regal name. Let’s call it Luna from here on out, and whenever we say “the moon” we’re referring to our real, less interesting moon. Would Luna be as big as Titan?
Remember that we are keeping Luna as massive as Titan because it’s a good point of comparison, but Titan isn’t very dense by the standards of rocky bodies. For Luna to have a molten core, however, we had to add a lot of heavy, and therefore dense radioactive elements to keep things toasty in there. Titan is about 50% larger than the moon. Luna is going to be larger, but not as large as Titan. I’m doing very rough math here, but it’ll probably be about 25% larger than the moon. Now, we can have life there.
Wait, crap, we forgot water. Let’s do what happened to Earth and slam some comets into Luna early in its life.
There we go. Now, we can have life there. But would we?
We could argue back and forth about whether or not life will inevitably arise from the conditions needed to create life, but the good news is, for Luna, we don’t need to. Luna would most likely have life, because it’s so close to Earth. The concept is called panspermia. Essentially, at some point Earth (or another body with life on it,) in the process of getting struck by asteroids and whatnot, could shoot out debris that lands on other worlds and that debris could still have microbial life on it that could colonize new worlds.
With Luna being so close to Earth, it’s pretty much inevitable that this would happen. In fact, we can even pick a particular event: evidence has been found of a massive asteroid impact in South America that’s 3.2 billion years old. There absolutely one unicellular life 3.2 billion years ago. The date is very important, because that’s close to the same time that the first photosynthetic life evolved, meaning Luna could get seeded with early photosynthetic life as well as nonphotosynthetic.
This means that a lot of sci-fi tropes (aliens that eat people, people eating alien plants, alien diseases) become much more reasonable since life on Earth and Luna share identical base biochemistry. They came from the same source, they have the same origin. They’ll go down vastly different paths, as we’ll see next time, but for right now, we can at least know they started from the same place.
So that’s part one of this, and it was longer than I planned. Tune in later for part two, where we look at what life on Luna could look like, and how all the math and science we did in this earlier part will play a role in shaping that. Subscribe to email updates, twitter, reddit, or facebook for the next part, and leave me a comment on any of those platforms to discuss.
And as always, enjoy.