How did the Moon form?
October 17, 2022 Issue #474
Upcoming Appearances/Shameless Self-Promotion
Where I’ll be doing things you can watch and listen to or read about
ICYMI I have a new book coming out, called Under Alien Skies. I wrote about it in an issue of BAN last week, including a link to the publisher’s page which itself has lots of links on how to pre-order the book (it comes out in April 2023).
A lot of folks have asked if any of the links helps me more than others, like getting it through Amazon versus Barnes & Noble, etc. The answer is nope. Every seller counts as much as any other one, so if you want to get it find the bookseller you like best. It all works for me.
And I appreciate people trying to help out as much as possible! Honestly, any pre-order is good and they’re all the same, and they all help the publisher know the numbers better, and also could help the book get on lists of popular books (oh, a nerd can dream…).
So thanks! And I hope you like it. I’ll have a lot more to say about as the publishing date nears.
It’s a big Universe. Here’s a thing about it.
How did Earth’s Moon form?
Or a better question: How did it form in detail?
That second one is better because we have a pretty good idea of how it formed in general. Radioactive dating indicates it’s very old, forming within roughly 100 million years after the Earth itself. The exact date is still argued over; it used to be though it was less than 100 million years after the Earth, but some studies indicates the Moon is younger than that.
If it formed with the Earth then they’d be the same age, about 4.56 billion years. Since the Moon formed so much later there must have been a separate event that made it. There have been a lot of hypotheses over the years, but one that came about in the 1970s was the Giant Impact Hypothesis: A Mars-sized object, now called Theia, slammed into the Earth at high speed and at a grazing angle. This blasted a vast amount of material into orbit around the Earth, some of which formed the Moon.
A big question is: How long did this take? Once the material was ejected into space, what happened to it? The current idea is that it formed a disk of material around the Earth that slowly reaccumulated over years or decades.
But a new piece of research indicates it may have happened faster than that. Like, a lot faster: Much of the Moon’s bulk was in place within a day of the impact. A day! Needless to say, if true, this is a big deal, and a huge change in how we think the Moon got to be the way it is today [link to paper].
This idea was dreamed up in the 1970s to explain the Earth-Moon system’s high angular momentum. This is like normal linear momentum you’re familiar with, but instead is locked up in an object’s rotation, or one object’s revolution around another — for example, a steel ball rotating rapidly would be harder to slow down than one made of cardboard because the steel one has more angular momentum.
Compared to other planets, the Earth-Moon system has a lot of angular momentum, which is weird. One explanation is a collision with a decent-sized planet, which came in at high speed — probably 10 kilometers per second or maybe more. A grazing impact would have spun the Earth up, making it rotate faster, as well as flung off all that material into orbit which also contributes to the angular momentum.
Anyway, there are other observations supported by this giant impact idea. The Moon has largely the same elemental composition as Earth, though is less dense. In an impact you expect a lot of Earthly material flung away to form the Moon, which makes sense. Also, the Earth itself would have differentiated by the time of the impact, meaning heavy stuff like iron sank to the core while lighter stuff like silicates (rocks) floated to the surface. A grazing impact would’ve skimmed off the lighter stuff, which is why the Moon has less iron and is less dense than Earth.
Computer simulations of the physics of the impact do support the gross facts of the impact, You can get enough material to make the Moon, for example. But the details are maddeningly off. Most sims show the Moon should be made more from the material of Theia than Earth, but that’s not what we see.
[3D rendering of the new simulations showing the creation of the Moon. Theia (upper right) hits the Earth (lower left) creating an immense filament of material, some of which forms the Moon.]
The new work is similar to the old sims except in one crucial detail: It’s much higher resolution. Simulations of physics like this are extremely intensive in computer time, and would take months to run if you don’t make some compromises. For example, as things change with time, you can use big time steps in the sim, so you have fewer calculations to do. You also have to track a lot of material in space, which gets divided up into particles — think of them as clumps — and the fewer you track the faster the calculations.
But this can return misleading results, because a lot of physics happens on short timescales on over small volumes of space. The good news is computers get faster with time, and the coding used to run the sims gets better too, working faster and more accurately.
[The same impact as the video above but seen from a different angle. Note that the “small” blobs are still a hundred or so kilometer wide, and an impact by something that big today would be an extinction level event.]
In the new sims, they use more particles to get better resolution, and hoo boy it’s a lot more. Older ones used a hundred thousand to a million particles, but the new ones use one hundred million. It’s like going from a blurry pixelated image to one with super-high res.
The researchers varied the impact parameters to see what matched observations best. That mean, for example, using different velocities and grazing angles in each simulation run to see which ones work and which ones don’t. They ran 400 simulations in all.
Older sims had trouble getting the amount of iron right in the final Moon, as well as having trouble with the proto-Moon breaking up due to tides from the Earth, which stretch and rip apart anything big enough that gets too close.
[The actual particle simulator showing the results of the impact. The cores and mantles of the two impacting planets are shown to see where material ejected comes from. Note the timing shown at the top; this whole thing takes just hours.]
The new sims show that these issues were due to the low resolution of the older sims. They found that the impacts blew out a huge streamer of material from the proto-Earth, which forms a long finger-like trail that swings around the Earth. Most of it falls back to slam into the planet, but a blob at the end disengages from the tendril and forms the Moon.
The important thing here is that the new hi-res sims show the Moon forms directly from the material ejected, whereas old ones needed that material to first form a disk from which the Moon accumulates slowly over many years. Incredibly, the Moon appears to form in mere hours in the new simulations. Wow.
This makes a huge difference. In the older sims the Moon forms very hot as material falls onto it as it grows. In the new ones the material doesn’t get as hot, which means the Moon’s interior structure will form differently. That difference can be tested by probing the Moon’s interior using gravity maps by orbiting satellites as well as using seismometers, in much the same way we use them to probe the Earth’s interior. That’s a testable way to see if these sims work or not.
There are a lot of things the sims seem to get right, including the elemental composition of the Moon, which has bedeviled earlier work. There’s still a lot of work to be done here, including testing the results with observations, but the stuff it gets right is very encouraging.
But what gets me the most is just how fast the Moon forms! Planetary collisions are funny; even moving at 40 kilometers per second it would take a small planet five minutes to cross the Earth’s diameter, so collisions look slow when simulated. But then to have an object the size of our Moon — over 3,000 kilometers across — form in hours is just staggering.
I remember when we had no clue how the Moon formed, and when the Giant Impact Hypothesis was starting to become accepted in the 80s. Now we can map out the details of it using simulations and advances in physics.
Think on that the next time you see the Moon in the sky. For millennia it was beyond our reach, used as a metaphor for something unreachable. Now we can reach it, and we can understand it, including how it was born.
SCIENCE! I love this stuff.
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