Greetings, my fellow travelers on this tiny little planet. We received an interesting question here on Skepchick about the formation of the Moon. I got a little busy with the whole dissertation-writing thing, but I’m back to tell you about the somewhat violent formation of the Earth’s famous natural satellite.
Here is the question from Barry:
So I was watching a rerun of the Universe on TV about the
Moon and its origins. One of the theories of the origins of our moon
is that a Mars sized impactor hit Earth in its early days and the
debris that acretted around the bulk of the Earth that was left formed
the Moon. I was on board with this idea until they said that the
model simulations they do puts the formation of the moon taking about
a standard year, maybe 2. This feels so ludicrous to me because
everything that happens in the universe (creation-wise) takes million
and billions of years. Can you look into this particular area and see
what these models are based on?
The currently favored hypothesis of the Moon’s formation is also known as they “Giant Impactor Hypothesis” or the “The Big Whack” hypothesis. Yes, really. Astronomers: not so good at naming stuff.
According to this scenario, as the question says, something about half the size of the Earth whacked into the proto-Earth early on in the formation of the solar system. This is not so much of a stretch to believe as the early solar system was a veritable shooting gallery full of debris that was coalescing into what we now know as the planets. This glancing blow would have scattered debris everywhere, but some would have stayed in orbit to coalesce to form the Moon. This would explain why the Moon is slightly different in composition from the Earth, particularly that it has a tiny iron core and is overall less dense than planet Earth. This makes sense if it was primarily formed from this impactor and the upper layers of the Earth’s crust and mantle.
It is true that this whole event happens over a “short time scale” as compared to the age of the solar system (about 4.5 billion years). The actual time depends on the exact circumstances of the cosmic smash, and computer models don’t always agree. The disk of debris that would become the Moon probably took about a hundred years to come together, which is still an incredibly short period of time on cosmic scales.
But why so short cosmologically? Stars live out their lives over billions of years, and planetary systems are formed over millions of years. However, some dynamical processes can still occur on shorter timescales. Impacts, in particular, take just minutes, hours, or days for the dust to settle. A great example is the Shoemaker-Levy 9 impact of a comet onto Jupiter in 1994. It only took a few months for the damage to disappear, since Jupiter is so much larger than the comet, but it shows that short events can happen.
Of course, the currently accepted hypothesis is not perfect, and some recent lines of evidence have challenged its dominance. The Skeptics Guide to the Universe reported on one such story recently about titanium isotopes in Earth and Moon rocks, and how they seem too similar to have come from the collision of two bodies. However, these minor details probably have more to do with the initial conditions of the impact, which are not well known, and aren’t enough to throw out this hypothesis completely. The “Big Whack” is still the most viable hypothesis and the one that makes the most physical sense. One early hypothesis had the Earth spinning so fast that part of it was flung off to form the Moon, but without an implausible Superman movie plot, there is no way the planet could have spun up that quickly.
One thing that we do know is that the Moon continues to surprise us. Not just a dead hunk of rock, the Moon has been shown to have water in its shadowy craters, and spacecraft such as the Lunar Reconnaissance Orbiter (LRO) are measuring the physical properties of our nearest planetary neighbor with unprecedented accuracy. The best part of this is that you can participate.
LRO is sending back tons and tons of high resolution images of the Moon, and planetary scientists can’t possibly comb through all that data. Enter Moon Mappers, where you can identify craters and even pit yourself against computer algorithms to pick out faint structures on the Moon. A part of the growing CosmoQuest empire, of which I am proudly a part, this project is invaluable to making modern Moon research happen. This is just one of several projects that we’ll be rolling out, allowing anyone with a bit of time and interest to do cutting edge science. And that is going to help us get better and better at explaining how the Moon, Earth, and rest of our little Solar System came to be, and came to be our home.
Thanks, Barry, for the question!