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Since I was in grad school there’s been a LOT of progress figuring out how the Moon formed. Some important objections had been raised against the giant impact hypothesis, but now they have been largely answered.
2/ The “Synestia” theory developed by @SarahTStewart & co-workers has successfully answered the main objection against the Giant Impact Hypothesis. ucdavis.edu/news/how-moon-…
3/ When collisions are inelastic (absorbing energy), the two colliders tend to stick together instead of bouncing apart. If they are giant planets, gravity then takes over so they merge into one planet. So how did we get a separate Moon??? (image credit Wikimedia) 
4/ Previously we knew of only one possible answer: the collision had to have a lot of angular momentum. It had to be a glancing collision. The debris would spin enough to avoid being absorbed into one planet. That would form a separate Moon. (Credit: Robin Canup/SWRI...?)
5/ This glancing impact hypothesis makes some testable predictions. The debris that gets flung out to form the Moon should have come mostly from Theia, so the Moon should be chemically different than the Earth. (A glancing collision won’t mix the two bodies together very much.)
6/ The problem is that the chemistry of the Earth & the Moon are too similar. (This new study shows that the chemistry of the water on the Moon and the Earth is also very similar.) This seemed to disprove the giant impact hypothesis. Therefore some scientists did not believe it!
7/ The genius of the research by @SarahTStewart & her co-workers was to understand the vital physical that had been omitted from simulations of colliding planets. She added the effects of high temperatures vaporizing material, making a giant cloud around Earth. The “synestia”.
8/ The synestia would create conditions around the Earth that allows material to not get sucked back into the Earth, so a moon could form even if Theia had collided with the Earth head-on. Glancing collisions are not necessary for formation of the Moon!
9/ The synestia hypothesis predicts produces different testable predictions than the glancing impact hypothesis. It predicts the chemistry of the Moon & the Earth will be very similar, because Theia and the early Earth mixed together thoroughly in the impact and in the synestia.
10/ This is in violent agreement with measurements of rocks brought back from the Moon compared to rocks on Earth (and comparison of the water they contain, according to the new study). This indicates it was indeed a head-on collision that formed the Earth-Moon system.
11/ So the major objections against the giant impact hypothesis have been removed. Four immediate thoughts come to mind...I think these are super cool take-aways...
12/ First, this illustrates pure science at its best. Science is all about creating models of nature to explain the governing dynamics, to explain how and why things are the way they are. Nature is too complex to grasp. The model is a *simplification* so we can grasp its essence.
13/ But if the model is too simple then it fails to explain nature. The earlier models of the giant impact were too simple. They failed to explain how an impact could form a moon that has the same chemistry as Earth. Scientists had not yet figured out the governing dynamics!...
14/ Sarah & co-workers developed insight about the missing physics. They changed the physics, & now the model successfully explains what we see in nature. It is still a simplified model, but it now contains the *essence* of the governing dynamics.
15/ Thus, the use of models in science allows us to identify the essential organizing principles of nature in a way that our minds can grasp. Science is all about creating models — models that explain.
16/ So bravo to the authors of the new study for showing the chemical similarity of water on the Earth & the Moon, and especially to @SarahTStewart and her co-workers for their amazing result in explaining the fundamental essence of the formation of the Moon.
17/ Another example of models in science (from my own work, which I know well enough for an example but isn’t nearly as cool as origins of the Moon 😅). We’ve been trying for 17 years to explain exactly how rockets blow soil during lunar & Martian landings. It’s not so simple!
18/ We did a lot of experiments with gas jets blowing down on regolith beds to measure the erosion processes. The pipe is at the top. The gas blows against a clear window so we can see inside the crater as it forms. The curve is the beveled edge of the window.
19/ This should be simple to model because it is just gas pushing sand grains. We already understand how to model the motion of blowing gas. Just add sand grains into the model, right? Not so simple! There are too many sand grains for a computer to be able to handle it.
20/ So we have to invent ways to simplify the model. In the same way that nature is far too complex for our minds to grasp all the details, it is also too complex for a computer to simulate all the details. Science is the process of creating a mental model of nature we can grasp,
21/...JUST LIKE computer simulation is all about simplifying the physics into something a computer can calculate. Well, in our experiments, we got the idea to study how the sand grains under the surface are moving when we hit them with a gas jet in a box like this:
22/ So we used layers of colored sand. After the experiment we filled the crater with black sand to stabilize it then filled it with clear epoxy then cut it open. We expected the sand to be pushed down under the crater. Instead, the sand came UP under and around the crater!
23/ The models were not predicting this! Clearly our models were too simplified. We thought we had simplified them in a way that encoded the organizing dynamics of nature, but obviously we had not. We had left off something important from the physics!
24/ We later realized that the missing physics was the diffusion of gas between sand grains. When gas seeps through the sand, it drags sand in a different direction than was predicted by the simpler models. So we had to add “gas diffusion” into the list of the governing physics.
25/ In my opinion we still don’t have all the key physics figured out for rocket exhaust cratering soil. We can predict where soil goes after it gets lifted off the lunar surface, but IMO we have not identified all the governing physics for erosion in these extreme conditions.
26/ BTW, another scientist working on planetary erosion phenomena, someone who tweets super cool science, is @LoriKFenton.
27/ So for the rocket exhaust problem we performing experiments, getting data from planetary landings, & trying to make the models predict what we see in nature. Lotsa work to do in this field! When the models are good we will believe we have identified the organizing dynamics.
28/ So that’s not nearly as cool as explaining how the Earth has a sibling planet, the Moon! But it is another case to show that science is really *all* about models.
29/ (By the way, I want to also mention @Doctor_Astro who has done a lot of seminal work on rocket exhaust blowing soil.)
30/ My second takeaway was that this shows the value of space exploration. This explanation for the Moon was possible because Apollo and Luna missions returned samples from the Moon. And there is so much more to come!
31/ For at least 10,000 years of human history & uncountable prehistory we looked up and wondered about our Moon. It wasn’t until we built rockets to fly up there and get a sample that we could unravel the governing dynamics to explain how the Moon formed. Amazing!
32/ And there’s so much more the Moon will tell us. I bet there are people who will deny that we can learn more from the Moon. They would’ve also denied that Moon rocks could enable us to figure out how the Moon formed. Science is so amazing it is sometimes hard for ppl to grasp.
33/ The Moon is an airless body without the tectonic cycle of Earth so it better maintains a long-term record of events in our solar system than the Earth does. It has data we need to figure out the larger history of our solar system. I’m excited about our plans on the Moon!
34/ 3rd takeaway. Sorry, I can’t help myself: I have to say this. 😅 Notice the article at the top of this thread calls Theia a “planet” even though by the IAU’s 2006 definition it would not be a planet since it didn’t clear Earth out of its orbit. Words are important in science,
35/ because (as argued above) science is *all* about creating simplified models, whether they are models in our minds or simulations on a computer. We create models in order to identify the organizing dynamics. Our minds are verbal. Mental models are *made* out of words.
36/ When I saw this article calling Theia a planet, I’m reminded that many planetary scientists are also ignoring the 2006 definition because it isn’t useful. It defines a hybrid geophysical/dynamical category that does not come into the governing dynamics of any models.
36/ The concept of “planet” since Galileo has usually been “geophysically complex world like Earth”. Kepler defined them by their gravitational rounding. Even moons like Europa & the Moon are called planets by many from Galileo until now (Kepler called them “secondary planets”).
37/ The image the public has about planets is similar to this, although they don’t usually realize many planetary scientists also consider large moons to be (secondary) planets. So it is narural to call Theia a planet. It is useful to convey the concept of a large, complex world.
38/ Fourth (last) takeaway. I regret making this last because this is more important than the above. Is it too much for me to suggest the following? I don’t think so, so here goes...
39/ ...I would love to see a Nobel prize go to @SarahTStewart & her co-workers for the work on Synestias as explanatory for the origin of the Moon. My sense is her work will stand up over time. Because it successfully resolved the problems with the Giant Impact Hypothesis, and...
40/...filled in a huge gap in the missing physics, fundamentally changed how the models behave, showed a revolutionary new view of the organizing dynamics behind lunar formation, and addressed one of the Big Questions that has faced us throughout all human history, I feel that...
41/...this has everything that is needed for an award of that magnitude. It shouldn’t be only the particle physicists and cosmologists who get Nobel prizes in physics. Planetary geophysicists are physicists, too, but are dealing with complex topics where elegance is rare.
42/ We have seen Nobel prizes in physics that is close enough to suggest the work on synestias is within-bounds. There were Nobel prizes for evolution of stars, for example, and discovery of pulsars. All these involve the explanation of the evolution of large bodies in space.
43/ In this case, humanity has stared at the Moon and wondered about it for as long as our collective memory can reach. It enters into our literature and our visual arts. It enters into theories about the types of planets that can support life and Are We Alone In The Universe?
44/ The work creating models of lunar formation has undergone an abrupt change in just the past few years, made possible because we now see how a head-on collision could form the Moon, explaining all the problems with prior theories. The result was elegant, fundamental, and deep.
45/ This was physics at its best. What else is needed beyond this? I think this is the moment for a geophysicist, at last, to be awarded a Nobel prize.
I hope people won’t think I’m wrong for suggesting it. This is how it seems to me.
I hope people won’t think I’m wrong for suggesting it. This is how it seems to me.
