The far side of the moon as pictured with Earth in the background; taken from the Deep Space Climate Observatory spacecraft. (NASA/NOAA)

Do we owe our existence to the Moon?

Planetary geologist Erik Asphaug on where the moon came from — and why it made life (as we know it) possible


Keith A. Spencer
October 13, 2019 11:30PM (UTC)

Science fiction can be oddly prescient. Though "Star Trek," "Star Wars," and "Lost In Space" all depicted life on other planets in distant solar systems, scientists had no evidence that there were any other planets outside of our solar system until 1992, when the first exoplanet was discovered. That means that those films and TV shows, which debuted long before 1992, foretold the discovery of the diversity of planets and moons that exist in our vast universe.

Now that scientists have documented thousands of exoplanets, we are getting closer to answering the most profound questions about our existence, namely: are we unique in the universe? Does life exist on Earth because our solar system and planet are special, and an unlikely confluence of events led to our existence? Or rather, are solar systems with Earth-like bodies common, meaning we are merely an unexceptional instance of a habitable planet?

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Professor Erik Asphaug, a planetary geologist at the University of Arizona, has penned a new book about precisely this. Titled "When the Earth Had Two Moons," Asphaug's book is a history tale, essentially, of the origin of our solar system. Though a story that is constantly in flux as the science changes, Asphaug notes how observation and simulation have coalesced around certain theories that have approached the realm of certitude — namely, the notion that Earth's moon was created when a Mars-sized planet named Theia struck a glancing blow on proto-Earth, ejecting massive amounts of rock that later coalesced into the thing that we call "The Moon."

I spoke to Dr. Asphaug by phone about the origins of the Moon, and why life as we know it on Earth seems to be dependent on this humble tidally-locked rock.

Keith A. Spencer:  I remember as a kid growing up in the 1990s, we didn't actually know if there were planets in other solar systems. Now that we've detected so many, my question for you is: how unique is our solar system? How unique is Earth? 

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Dr. Erik Asphaug: That's kind of the key question right now in astronomy. We have all this raw data from this Kepler Telescope — it has this one hundred and fifteen megapixel camera staring at a bunch of stars, and then whenever one of those pixels gets dim, that's a sign that a planet is passing in front of a star.

It's precise data, but it's also very biased when you think about it — it's only stars that happen to have a planet winking in front of it during the timeframe of the mission which is like three years.  So it's kind of biased to find things that are, like, big Jupiter sized planets that are pretty close in [to their parent stars]. 

So if you just look at the raw data — you plot all the planetary systems we know about — it's just mind blowing. We didn't know any of this in the 1990s or earlier. For example, as late as the 1960s and 1970s, the reigning hypothesis for our own solar system forming was that another star had come close to colliding with our star —and it was like a freak accident, and [this] is why we have planets [and] nobody else has planets.

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It's true that if you if you plot the planets that we know of against ours, we do look pretty freaky. Most of the [exo]planets [we've observed] orbit [their stars closer than] Mercury does to our star. Which is kind of bizarre to think about — you know, planets two or three times the mass of Jupiter orbiting their star every five days. That's a pretty typical planet, apparently. And that's nothing like we observe in our solar system, where we see this little dribble of planets fairly far from our star — fortunately that's where liquid water is found — and then Jupiter out there at five times the distance of the earth, and then Saturn is ten times the distance of the earth, and these ice giants farther out still — and that's still anomalous. But then, you have to remember, we probably wouldn't have discovered those planets yet around around those stars [with our telescopes like Kepler]. Jupiter orbits every 10 years Saturn or every 30 years. Neptune, Uranus you know a hundred years or so  — so that's kind of how long it's going to take to have the luck to happen to see one of those kinds of planets passing in front of their stars. 

So you're saying that there's this there's a selection bias in our list of the exoplanets observed? Like, the ones we can detect easily orbit really close to their stars? 

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Absolutely. And the astronomers — the ones who work with data, the observationists — are getting a handle on this, but it still looks like we're kind of anomalous.

What is the reigning scientific theory today as to what was here before our solar system, and how do we know that? 

You know if you look around in the nearby universe, in our little sector of the galaxy where we can see pretty well with telescopes, you see these dark clouds And you won't really notice them if you're a backyard astronomer —  you'll think it's just an area where there aren't many stars, but there are actually plenty of stars in [these areas]. In the infrared you can see that these clouds are actually kind of warm. And it doesn't take a whole lot of math to say, "hey that's a cloud of dust that's massive enough that it's in the process of condensing."

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So it would be like looking at a storm cloud and saying, "hey that's that's going to be a rain cloud." That's kind of the analysis. So we see what star formation looks like at the beginning and then we see what it looks like in the very early stages, when the star has formed, because these are easy to see using infrared telescopes.

And the point is, you can see this warm gas in or in orbit around these stars, and now you see the gas blob that wants to collapse to become a star — you know that's the next step. We also see we see little glowing pods of gas where a star has just lit up and just starting to pump out radiation,  it's like a neon tube it's pumping out ultraviolet radiation and it's irradiating the gas around it, and it glows.

And then the next step is you're actually seeing discs, and you see a star starting to burn in the middle of it and it might be a sun-like star. And then you see globs in the disc and you see gaps in the disc and you're like, "those must be planets." So we we see the process happening, [but] where we're missing the dots is with what occurs next. We can't really pinpoint what we see in a time sequence very well to say, "this is the next stage," and you can't see much later than that because the next stage the dust clears away.

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At that point, the planets are too small, too cold. Nothing to see.

So now it's left for the theorists to kind of connect the dots between those observations and what we see here in our solar system.

I want to get into the title of the book. Nowadays it is widely believed that the Moon formed when a Mars-sized body, Theia, smashed into the Earth, causing a bunch of stuff to eject and form a disc around the Earth that later coalesced into the Moon. Why has science coalesced around this theory, and what is the evidence? 

If you go back to the Apollo Era, when we were about to head to the Moon, there were two camps on this intellectually speaking. One camp was of the opinion that the moon was a primitive object left over from solar system formation, that had not been created by the Earth. Remember, there wasn't a lot of computer-modeling back then — there was none. So people just sort of had to use pen and paper to see if stuff made sense. 

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We've known for a couple of hundred years that the moon is a lot less dense than the Earth. It's got a density of about three-fifths that of the Earth. That's about the density of meteorites.

The other camp was a bit more physics-based and geophysics-based. They looked at detailed images of the craters and they saw what they suspected was widespread volcanism on the near side of the moon. Mathematicians asked, "OK let's say you want to have the moon captured, what does that mean?" And this is kind of where the Giant Impact Theory comes along, supported, as you say, entirely on the back of theory. You know you're trying to make sense of of little scraps of data and you know the modelers saying, "hey I think capturing the moon is a great idea let's do it." And they try to make it sort of come in for a close encounter with the earth and in order to do that it's like if the Earth had a mass of atmosphere maybe you could slow it down just right... But when they tried to model this, the Moon would just get destroyed.

So you're saying with the Capture Hypothesis, there wasn't any way to get the model to work — which means scientists start to think, "maybe something else happened," right?

Exactly. I think there was a bias against admitting the Giant Impact Hypothesis — no one wants to imagine something hitting Earth and possibly destroying it. You'll break it!

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It's a very calamitous event that reshapes the whole planet but it doesn't destroy [the Moon or Earth], it builds them. And the idea of the Moon being formed in this manner was not really on the forefront of anybody's minds heading into the Apollo missions — the Apollo missions just proved that the moon is volcanic. But what that gave us was permission to think about the moon as being formed in a very violent way. And that led to people paying more attention to this theory, and then computational power came of age and that was really a story of science in the 80s. Giant Impact Theory is really you know a story of science following what the computers tell us.

It is interesting when you think about it that we are the only terrestrial planet with a big moon. I mean, Mars has those glorified captured asteroids [Phobos and Deimos], but no other terrestrial planet has a moon like ours. I think back to that analogy of selection bias you mentioned earlier — we are on this planet, so it's hard it's hard for us to get a sense of how unusual or normal we are, right? I know the moon has some protective and stabilizing effects for us on Earth, and that may make the planet more habitable. Do we owe our existence to the Moon? 

That's a primary question, and the more we look at it in detail, the more it persists. When you think about Moon and theories for the origin of life — well, like you said, the moon stabilizes the Earth['s rotation]. So we don't get seasons that are drastic, [the axial tilt] is never more or less than 23.5 degrees. If you didn't have the moon there, the Earth could very well end up doing what Mars does every five million years or so — Mars ends up pointed towards the sun on its side, and goes through crazy seasons where the poles are pointed at the Sun, and then  it tilts back again.

So Earth had this moon that stabilizes its spin axis, it raises tides in the oceans, that is, you know, if not an incubator of the origin of life, it's certainly a resource for spawning life.

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Then there are these sort of cosmic things  — like the fact that the moon covers up the sun precisely [during an eclipse]. I make something about that in the book, because [a total solar eclipse] is the most remarkable thing any human being is going to see. Why is it that way? I don't know if that has anything to do with that with life, but it might have something to do with consciousness. I wonder — it feels like a sign, a symbol of something that might trigger some kind of a self-awareness that occurred in the Stone Age.


Keith A. Spencer

Keith A. Spencer is a senior editor for Salon. He manages Salon's science, tech, economy and health coverage. His book, "A People's History of Silicon Valley: How the Tech Industry Exploits Workers, Erodes Privacy and Undermines Democracy," was released in 2018. Follow him on Twitter at @keithspencer, or on Facebook here.

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