Hello everyone, my name is Danielle Piskorz, and I'm one of the graduate TA's for the course. Today we're going to take a break from Mike Brown's lectures, and I'm going to tell you guys about the formation of the moon. Some of my research is actually on the formation of the moon. I find it particularly interesting, because learning about the formation of the moon gives us important clues as to the environment of the early solar system, as well as some hints for how planet formation, in general, works. The moon is also interesting, because as Mike mentioned earlier in the lectures, the presence of the moon orbiting the Earth helps stabilizes the Earth's obliquity. Some scientists believe that if it weren't for the moon, the Earth's obliquity would wobble a lot more than it currently does. And this might mean that life would never have enough time to evolve in stable environment, and so if they weren't a moon there might not even be a us. And so, I am going to begin this lecture by telling you guys about the observables that we use to constrain our theories of moon formation, and then how we believe the moon formed, and what happened after the formation of the moon. So the observables we have to start with are, first, any theory for moon formation has to abide by the law of conservation of angular momentum. This means that the angular momentum of the Earth moon system, before there was a moon, over 4 billion years ago, must equal the angular momentum of the Earth moon system today. Another constraint is the interior structure of the moon. It turns out, that the core of the moon is very small. The radius of the moon's core is about a quarter of the total radius of the moon. For comparison, the radius of the earth's core is about half of the total radius of the earth. And so any theory for moon formation, would have to account for why the moon has such a small core, or even why so little iron was present at the time of moon formation. Another constraint we have for the formation of the moon, is the isotopic ratios we observe. You can see here, we have plotted on the x-axis delta O18 parts per thousand relative to SMOW, where SMOW is standard mean ocean water. This delta O18 value represents the ratio of the isotope Oxygen 18 to the isotope Oxygen 16. On the y axis, we have delta O 17, part per thousand relative to SMOW, standard mean ocean water. So this is the ratio of the Oxygen 17 isotope to the Oxygen 16 isotope. And you can see, that if you plot these 2 values, delta O 18 versus delta O 17 that each set of samples falls onto a line. So the Mars meteorites falls on it own line, the Vesta parent meteorites fall on a line, and the Earth and the Moon fall on a line as well. And so this suggests, that the material that made up the Earth and the Moon must have came from the same reservoir or experienced similar fractionation processes as they formed, and so this is an important clue as well. One of our last clues rather, is the surface composition of the moon. When you look at the moon you see these dark areas, which are Basaltic maria, and some light areas, which are the anorthositic highlands. And the important part here for deducing the formation of the moon, is the anorthosite. Now what is an anorthosite? Well, it is a rock, made of the mineral Plagioclase feldspar, and a smattering of mafic minerals. Now, Plagioclase feldspar is a feldspar made of Aluminum, Silicon, Oxygen, and Calcium. Your mafic minerals are very magnesium iron rich minerals, which you can kind of get from the name, mafic, the ma from Magnesium, and fic from ferron Iron. So, you get your Plagioclase feldspar, you get your mafic minerals, you put it together, and you get you an Anorthosite rock. And many of the rocks brought back by the Apollo astronauts where these Anorthosites. And so, this photo here, was a rock brought back by Apollo 15 and it's now called the Genesis Rock. When scientists dated this rock using samarium-neodymium dating, they found that the rock was 4.46 billion years old. At the time, that was the oldest rock that we had in our moon rock collection. And so everyone is really excited that we found this very, very old rock that could provide a very important clue to the formation of the moon. And even further, scientists realized that this rock is 98% plagioclase which is a crazy amount of plagioclase. On Earth we rarely find rocks this pure in plagioclase, so whatever process formed this rock, should be an important indicator for what happened to the moon as a whole. Now an important clue about plagioclase, is that it's very light, and so it have the tendency to float in say, a liquid magma. And so, how would you get this global layer of anorthosite covering the whole moon, if we know it's also very light? Well, maybe we had a magma ocean. And how would you cause a magma ocean? You could have a giant impact. And so today, the leading theory for moon formation is the giant impact hypothesis. According to this hypothesis, you have a Mars sized impactor hitting a proto-Earth, spewing off pieces of the impactor, and parts of the Earth's mantle into what's called a proto-lunar disk. Out of this proto-lunar disk, moonlets formed and those moonlets coalesced into our moon. Studies suggest that this process would have taken as little as a month. Now we should go back and double check that this theory for moon formation, does in fact, adhere to all the observables. Well first, we had the conservation of angular momentum. This observable can be satisfied by, allowing for the correct math and velocity of the impacting body. Second, we have the small core of the Moon. This makes sense, because in the giant impact formulation, you have your impactor and bits of the Earth's mantle making up the moon, and you could imagine that there would be little Iron available in that set of starting materials to make a very large core. Our third observable was the Oxygen isotope ratio. This observable can be adhered to, by allowing for the proper ratio of impactor material and earth mantle material and the proto-lunar disk. It's a bit more complicated than that, and I'll come back to that later. And finally, our last observable was the anorthosite crust on the moon. And so, how does a giant impact lead to an anorthosite crust? The answer is a magma ocean. And so here is a photo of what maybe a magma ocean could've looked like. The idea of a magma ocean, is you have an entire liquid surface of the moon of magma about 100 kilometers deep. At first, this magma ocean would have been exposed directly to the cold of space. So the first ,about 80% or so, of a magma ocean would have solidified in only 1,000 years. But once the magma ocean hit about 80% solidification, our favorite mineral, plagioclase, hit its liquidus. This means that our plagioclase mineral would start to crystallize out of the magma, and those crystals being lighter than the rest of the magma, would float to the surface, creating a flotation crust. This flotation crust would act as an insulating lid, and slow the cooling of the moon, so that the rest of the magma ocean, that last 20%, would take tens of millions of years to solidify. The moon in particular is unique, that it allows for the stability and flotation of plagioclase, because it's such a small body, and has such little gravity. And so this is how anorthosite crust formed on the moon. We look at the moon today, and we see the light regions, which are the lunar highlands, made of anorthosite. Be aware that this is a an oversimplification saying that the moon is only anorthosite, and only basaltic maria. There are of course many other rocks on the moon, but this is how we're explaining the overall surface of the moon. Like I said, these lunar highlands make up about 75% of the surface, and their ages cluster around 4.4 billion years old. Which gives us a good data point for constraining the actual timeline of moon formation. About 18% of the moon surface is covered by Lunar Maria. These regions formed when impactors hit the surface of the moon, causing the basaltic magma to erupt from the interior. Of course, everything I said is constantly in flux. We're constantly getting new information about the moon, that any theory for moon formation would have to explain. For example, the moon seems to have more volatiles, than the giant impact model would allow for. The moon doesn't have many volatiles, but it has enough, that they should have been lost in such a high energy event like a giant impact. In addition I explained earlier about the Oxygen isotope ratios, but it isn't really that simple. Just in late 2012, 2 new variations on the theory for moon formation were published, and those 2 variations were trying to explain how the Oxygen isotope ratios of the Earth and the moon could be similar. And so like I said, these entire fields is currently in flux, so stay tuned to lunar science.
