PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
The Moon-Forming Impact - Q&A with Dr. Rachel Phillips aka GeoGirl
Use Left/Right to seek, Home/End to jump to start or end. Hold shift to jump forward or backward.
In this episode, we're back with the fabulous Dr. Rachel Phillips, also known as Geo Girl on YouTube, diving into part two of our intriguing Q&A session. This time, we cover listener questions about early Earth geology, the moon-forming impact, geochemistry, and much more. Spoiler alert: we journey into the cosmic and tectonic weeds, so grab your scientific hats as we ponder if the moon-forming whammy kickstarted plate tectonics and its broader implications on our planet! The center of our discussion revolves around a burning question: Did the moon-forming impact trigger plate tectonics? From vigorous debates on early Earth’s behavior to the staggering differences between Earth and its 'toxic twin' Venus, we leave no stone unturned—or unmelted. Dr. Phillips and Jesse examine the evidence, debunk some age-old myths (bye-bye, late heavy bombardment), and think about how a cataclysmic impact billions of years ago might have reset our planetary clock. Oh, and don't get us started on whether Earth or moon rocks are older—trust us, it's complicated. As we approach the end of our cosmic conversation, we speculate about how such colossal impacts might have played a role in developing early life on Earth. Did these monumental collisions cook up the perfect conditions for life's early proliferation, or did they serve as the ultimate reset button? Join us on this epic journey through time, magma oceans, and cratered landscapes. And hey, if you have more burning questions or just want to say hello, hit us up on all the social medias at Planet Geo Cast or visit our website planetgeocast.com. Cheers to more geological deep dives in the future!
Download the CampGeo app now at this link.
On the app you can get tons of free content, exclusive images, and access to our Geology of National Parks series.
You can also learn the basics of geology at the college level in our FREE CampGeo content series - get learning now!
Like, Subscribe, and leave us a Rating!
——————————————————
Instagram: @planetgeocast
Twitter: @planetgeocast
Facebook: @planetgeocast
Support us: https://planetgeocast.com/support-us
Email: planetgeocast@gmail.com
Website: https://planetgeocast.com/
Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. Hey folks, today we are happy to bring you part two of our question and answer period with Dr. Rachel Phillips, aka GeoGirl on YouTube. In this episode, we are again answering questions that came from Rachel and Chris and I's discussion of early Earth geology, geochemistry, what we know about it, what we don't know about it, and listener questions that came to her from her YouTube channel. A lot of these ones had to do with the moon forming impact, and so that's kind of the center of our conversation. Again, there was lots of really great questions in this, and we're happy to answer them. Please reach out if you have questions for us. You can hit us up on all the social medias at Planet GeoCast. Shoot us an email. Planetgeocast.com is our website. There's a contact us link there. Go to our Camp Geo Mobile app, first link in your show notes, and check out all of the content we have there. And please shoot us an email. We're happy to do more of these. And uh Rachel and I will be taking more of your questions. So please reach out. Enjoy.
SPEAKER_03We are going to jump right into the weeds of it with this first question. Then this was this was a very, very commonly asked question. Did the moon forming impact cause plate tectonics?
SPEAKER_00Okay. Can I ask a quick question about the question? How common? I mean, you said you had like over 800 comments or questions about on this video. How many times, I mean, how many times did this come up broadly question like this?
SPEAKER_03Well, especially, so we had the two-parter, the first video, not as much, but the second video we talked about the moon forming impact with Chris. And so then it came up a lot more because we talked about the moon forming impact. We talked about early plate tectonics and how on early Earth plate tectonics may not have been the tectonic regime. And so then that brought up, I think, a lot of questions of well, what did cause the beginning of plate tectonics? Could it have been this large impact?
SPEAKER_00So Right, right. Okay, gotcha. Well, all right. I'm curious your thoughts on this. But my short answer is no. No, in the sense that the impact did not kickstart plate tectonics in that way. But it is a really interesting question. Why does Earth have plate tectonics and something like Venus does not? They're very similar kind of planets. Venus is Earth's toxic twin. And does it have something to do with having a moon uh in tidal forces or the planetary resetting event that the moon forming impact caused? Maybe that reset the planet uh at a time when it was cooler. And so may so maybe there's some triple down effects that led to plate tectonics, but the event itself did not kick start it. I think is probably what the question is kind of implicitly uh assuming.
SPEAKER_03Which is yes, and that's my understanding as well, because the the moon forming impact, I mean, it was very large, and to my understanding, it melted everything pretty much. Like it was like a global magma ocean. Okay.
SPEAKER_00This is a a global planetary resetting event, even if it's like uh an indirect hit, you know, if say uh the the uh the the impactor hit the proto-Earth, even if it hit a glancing blow or something, it's really a planetary resetting event. This is melting the entire mantle if it wasn't melted already. It's remelting it, it's resetting. Cores are merging, perhaps. So the impactor's core maybe merges with Earth's core, and some of the core material gets spit out. And so there's all these things, but it's a massively energetic event that is a planetary resetting event. Okay. I kind of think of like, you know, Star Wars Death Star kind of stuff. Like, you know, this is this is a serious event for a planetary.
SPEAKER_03Yes. And just to put some sort of timescale on it, it's also my understanding that plate tectonics, whether you go with the really, really old potential start date of like four billion years ago or almost four billion years ago, or even the newer, you know, 2.5 billion year ago start date of plate tectonics, both of those are millions, hundreds of millions of years after the moon forming impact. So plate tectonics started much later.
SPEAKER_00Yeah, it depends. Yes, I I think as with all these things, depends who you're talking to. But yeah, we we have, you know, we have a couple of fragments of mineral grains preserved from a period 100 million years after the moon forming impact. Like that's it. That's all of our record on Earth. So we really don't know. My opinion or my take on it, my interpretation of the current data, let's say, is that you know, 3.8 billion, which is 300 or 500 million years after the moon forming impact. You know, so so there's a significant amount of time here. But yes, you're right that there's a there's a bit of a time lag.
SPEAKER_03Yeah, a a very long time after, yeah, at least on on human type scales. Even on early Earth, that was relatively fast. But um, but yeah, and so the other kind of follow-up onto if the moon forming impact didn't directly cause plate tectonics to start on Earth, could it have maybe triggered mantle convection? And then, you know, plate tectonics was a result of that. Could it have started like the chain of events that led to plate tectonics?
SPEAKER_00Mm-hmm. Again, no. And I think we have to sort of think about what mantle convection is, and that'll explain the answer. No, is mantle convection is just the the earth losing heat. It's a lava lamp. You know, it's the lava lamp getting heated up from below, and then it's circulating. Hot stuff rises, cold stuff sinks, broadly. I mean, that's an oversimplification, but it broadly works here. So, so back in time, we talked about last time. I think that the mantle is probably a lot hotter. So convection was probably happening maybe even faster or more, more convection is happening. So the mantle's convecting. Any planet that is a quote unquote tectonically live planet has some kind of convection probably happening on it, or some transport of hot stuff from down below up to up to high.
SPEAKER_03Yeah. And I actually, after having looked through our notes on this and and what you had to add in, I had came up with another question that I'm sure would have asked if I just missed it. Because everything that I read says the same thing. Essentially, any body with heat should have some sort of you know, convection. And so I'm curious, was there a time when like the moon had convection? Because when you look at the simulations of the moon forming impact, they were both like pretty molten for a while. Yeah, yeah, yeah. And and so why didn't if our kind of if our Earth, if the progression of events was kind of the convection of the mantle, eventually plate tectonics, why didn't the moon ever evolve plate tectonics?
SPEAKER_00It's a great question. There are models, I think, for this, and it and it has to do with how quickly the planet cools effectively, and whether there's water or not water. And the classic thing, the classic petrologic thing is that the moon has this northocyte crust, which is plagiarically salt spark crust. And the idea there is that when the moon accreted after the impact, you know, it's fully molten, magma ocean planet, planetary body out there, and it cooled down, surface cools really quickly because it's exposed to space with little atmosphere or did not have too much of an atmosphere. So that cools pretty quickly, but basically the magma ocean starts to crystallize. There's debate about whether it starts top down or bottom up or middle out crystallization. But basically, the plagioclase in a magma like the moon, a basalt or an ultramafic composition that has very little water on it, plagiarist will float. And so that stuff floats to the top and becomes kind of a flotation cumulate. That's one model for the plagueplace float. So the reason we have more rocks from the moon that are older than almost all the rocks on Earth is that it didn't have plate tectonics and it kind of maybe fully crystallized and was not big enough to maintain this thermal engine that might have kept an atmosphere around. You probably have to have an atmosphere to have water to have plate tectonics. Like there's all these feedback loops, you know. There's not one defining feature that says this is a this makes a planet that has plate tectonics, but the moon probably didn't. And it took 10 to 20 million years, roughly, to crystallize the magma ocean on the moon, likely is kind of what some just like heat loss mechanisms.
SPEAKER_02So pretty quick compared to Earth, which is still cool.
SPEAKER_00Compared to Earth, and it depends a lot on what you think about the atmosphere. If there's a an atmosphere, if there's like a silicon vapor, if you have an atmosphere, it's an insulating blanket. So if there's any little atmosphere around there, it helps insulate and keep the magma ocean um melted for longer. Because it's basically like a heat, another heat exchanging layer on top of it, a little insulating blanket. If it's exposed just to space, it cools uh much quicker.
SPEAKER_03So for that time when it was cooling, it had some sort of maybe mantle convection, some sort of tectonic regime where it was losing heat, but it wasn't plate tectonics, it was just loss of heat mechanism.
SPEAKER_00That's right. That's right. Melt moving up and volcanism, and and we there is younger volcanism on the moon. So we know that there's there's heat getting out because we know that there's volcanism there um happening. So that's one indication that there's still melting going on down at depth in some shape or form, and heat is being lost. So there, yeah, there are other ways to do that. But back to the moon forming impact and Earth, what that did do is dump a bunch of heat into the mantle. So the mantle was probably convecting, it was already hot, but it re-melted the mantle, which is adding heat, and it probably fully melted the earth, the at least the mantle part of the earth.
SPEAKER_03Actually, that brings up one point. I feel like the mantle is often depicted as this red magma.
SPEAKER_00I hate the color schemes. The the color scheme we use in the textbooks as putting the mantle as red or orange drives me nuts because it gives you this idea that it's magma, right? Yes. And it's it's not today. Okay. I mean, there is a little hints of it around, but it's mostly solid. Solid. It's it's you know, hot plastic is kind of the analogy for what it what it behaves like. Yeah.
SPEAKER_03That's a good, yeah, that's a good analogy. So it was solid or or becoming mostly solid before the impact, and then the impact hit and it re-melt. Is that what you think?
SPEAKER_00Yeah, that was that would be one idea. Yeah, exactly. Dumps a bunch of heat back into it, and that does remelting. So this impact is so violent, so much heat is added that it does planetary remelting, planetary resetting.
SPEAKER_03Okay, so it did reset things, but to our understanding, Earth would have probably had it would have absolutely had some sort of mantle confection, no matter what, and then it would have also probably had plate tectonics without the moon forming impact. Everything probably would have happened the same, even if the moon forming impact didn't happen in terms of tectonics on Earth.
SPEAKER_00Maybe it's a it's an open question, right? I mean, what yeah, like how much volatiles and atmosphere, that's an important aspect of this. If you have one magma ocean event, yeah, that's one uh one time for the mantle to lose all of its gases. If you have two magma ocean events, that's a very different like mantle gas concentration world you live in. So a lot of this has to do with where did our volatiles come from? Where did our atmosphere come from? Yeah. Because that to maintain a habitable planet for a long time, you probably require something like that. So but it's really, to my knowledge, and and again, I haven't been kept up on the habitability literature for the last five years or so, but to my knowledge, that was very much a big, big question in the Earth sciences is how important was the moon's forming impact or even just the presence of a relatively large moon, how important is that for habitability of the planet? And plate tectonics is kind of tied into that habitability question a bit. So, you know, it wasn't the the starting gun for plate tectonics, right? But it could have done something, maybe forming a moon, maybe bringing a bunch of volatiles, maybe outgas in the mantle, but done something that helped eventually maintain the planet for longer in the way that Venus did not. Venus could not have maintained. So the difference between Earth and Venus, one big difference is hey, we got a moon and we had a moon-forming impact. Like, so how important is that? I think it's a big open question for sure. But it's uh one of those, one of those things that is a major difference between Earth and Venus at the moment.
SPEAKER_03Oh, okay. So maybe it had changed something that set us on this track. We just don't really know exactly what.
SPEAKER_00We don't understand the dynamic, yeah, the feedback. How important are really, yeah, exactly. Exactly.
SPEAKER_03Very interesting. Okay, so you know, aren't there students out there that's interested in this? Oh very open research.
SPEAKER_00There's a lot of uh it's a lot of job security because we don't understand a lot, right?
SPEAKER_03So we need more research.
SPEAKER_00Yeah, amazing.
SPEAKER_03Okay, so and we kind of hit on a little bit of this, but another viewer question that I only saw one time, but was very intriguing to me because I think it brings up an interesting distinction was which is older, the moon or Earth? And other viewers also asked a different but related question. How quickly did Earth start solidifying after the moon forming impact? And you talked a little bit about how quickly the moon started solidifying after this impact. But I think what's really interesting here, I think the answer to what is older, the moon or earth, is different than what's older Earth rocks or moon rocks. Uh yes.
SPEAKER_01Okay, that's a great point.
SPEAKER_03Because, like again, the earth was there before the moon hit. So technically the earth is older, but then when it hit, it reset the rocks, and so the rocks took longer to cool, and then earth rocks are technically younger than moon rocks because of the cooling after the event. But then in our notes, you put that Earth there before moon is actually called Proto-Earth. Yes. So is proto-Earth different than Earth Earth?
SPEAKER_00Well, okay, so proto-Earth, I'm gonna forget, I'm gonna get the numbers a little bit wrong here because I forget exactly what they are. But proto-Earth is Earth before the moon forming impact, okay? And that's smaller. We think that's substantially smaller than Earth as it is today. I don't remember the numbers, 70%, 80% smaller, right? And then mass was added, then this impactor, which we call Thea or Thaya, came in and hit. That's the body, the modest size body that hit Earth, and then a whole bunch of materials kind of blasted off during that impact way back up. The moon today has a smaller core as a proportion of its size than Earth does. And that's always kind of been hard to explain. One way that the giant impact model explains this very nicely is that the impact or Thaya's core would have, because it's so big and dense, it's kind of like a bullet, it would have just penetrated the mantle and merged with Earth's core mostly. So there's models that kind of say the core is trying to merge together. There's a whole bunch of stuff that's thrown off during this impact, and you know, Earth keeps spinning. Now we have Earth, so the combination of the two is Earth, but there's a whole bunch of debris around Earth, and that stuff aggregates to form the moon, and there's less iron in that stuff than there is in Earth because a lot of the Thaya's core went into Earth's core, and some of it got spun out. So this is like a homogenization mixing and then separating the two bodies kind of an idea. And the reason that the impact model is a really, really compelling one is that we look at the chemistry of Earth and Moon, and they're chemically nearly identical. There are small differences, small but important differences in the chemistry of Earth. It's different than Mars, it's different from Venus, it's different from Mercury, it's different from a bunch of meteorites. The isotopic composition in a bunch of different isotope systems, there's very small but very significant differences between the planetary bodies that have to do with where the planet formed in the radius of the sun. And so the Earth and Moon being so similar probably means that we had to have we mixed them together at some point. That the moon came from some mixture of Earth or you know, some combination of the two. So we call Earth before the impact proto-Earth, the impact or the thing that hit is Thea. Then we have after the impact, we have Earth and the Moon. That's kind of the and so Proto-Earth, we have no information. We have nothing left from Proto-Earth. We have nothing left from Thaya because they got mixed together, melted, totally destroyed. The moon then cooled quickly, had this magma ocean body, cooled relatively quickly, 10 to 20 million years. We have a bunch of rocks from there because not much has happened to the moon since that time. Since it cooled, impacts have hit it, there's been some volcanism, but there's no plate tectonics, it hasn't been resetting itself for four billion years. Earth, on the other hand, probably crystallized some some rocks on the surface very quickly after the moon swarming impact, had some crust around, but that stuff has been totally reset by plate tectonics over the last three, three point eight billion years of planetary resetting that plate tectonics. So the reason we don't have older rocks from Earth is almost certainly due to plate tectonics since that time, rather than like anything to do with the magma ocean phases there. Sorry, okay, that was a long diatribe. But I just want to kind of set the stage in the terminology that's used here, uh, because it can be massively confusing.
SPEAKER_03Okay, okay, okay. Hold on, back up a second. You said that Earth, Proto-Earth, was 70% smaller.
SPEAKER_00I might not get these sizes right. Um smaller, because there's like a Mars-sized body that hits, and the moon right now is smaller than that. So the Earth was not its full complement. You know, the impact or added material to Earth and what was spun out, some of that started to orbit around and then kind of aggregate into the moon. Relatively quickly. Yeah.
SPEAKER_03Yeah. So I'm curious. Now I'm trying to remember how much smaller Mars is compared to Earth.
SPEAKER_01Oh, that's a good question. Yeah.
SPEAKER_03I've always pictured this as being Earth was the larger body, and then the Mars size impact was the Mars-sized object larger or smaller.
SPEAKER_00No, Proto-Earth was still larger. Oh, okay, okay. But you know, more of the ended up in the Earth than in the moon.
SPEAKER_03Oh, that makes sense. And we know they're very chemically similar, as you mentioned, and this is one of the big proponents of the impact hypothesis.
SPEAKER_01Yeah.
SPEAKER_03Are there any slight or strong chemical differences? And if so, what do we think they mean?
SPEAKER_00There are tiny, tiny ones. I think it's debated what they actually mean. I think it's in the triple oxygen isotope system. There's some chemical differences. But the Earth and the Moon are very, very similar in a bunch of different ways, apart from maybe one or two chemical signatures. And I believe one of them might be triple oxygen isotope composition. So the relationship between the 18, 17, and 16 isotopes of oxygen. Again, this is going back a couple years, so I'm not entirely sure about that. But I actually did see a recent paper this fall, I think came out. They looked at, I think it was iron isotopes from Earth, moon rocks, and meteorites. And their model was that Thaya, this this Mars-sized thing, actually came from the inner solar system. So it was kind of an inner solar system body that came in and hit, which kind of makes some sense. It's a rocky body with a with a core, probably.
SPEAKER_03So we know we think that Thaya had a core before hitting.
SPEAKER_00Probably just because most meteorites have most differentiated meteorites of some body size end up having to be a little bit more. A certain size.
SPEAKER_03Right. Okay, okay.
SPEAKER_00Well they get to basically what happens is they get to a certain size early on and they can melt then. There's enough heat production in the rocks, there's enough radioactive decay that they can melt. And the melting removes the iron from the chondritic meteorites and puts it in the core and leaves behind ultramasic rock or silicate.
SPEAKER_03Right, right. Okay, interesting. So the short answer to is is the moon or earth older. Proto-Earth is older and Earth is technically younger?
SPEAKER_00Then the moon, Earth and the Moon, let's think of them as the same.
SPEAKER_03About the same size. Okay, okay, okay.
SPEAKER_00The the same, I mean the rocks themselves are not, but the bodies themselves are the same age. If this, you know, if we're with our working model of the moon forming by an impact, if that's right, then the earth and the moon are the same size. They've just had different resetting histories. The moon has had very little resetting. The earth has been plate tic tacks for a long time, so therefore lots of things.
SPEAKER_03Okay, that makes sense. And we talk about more in the first part of this follow-up. We talk about what defines the age of earth, the birth of Earth. So go watch that for more on our philosophical take on this. But we also mentioned a little bit ago in this conversation, the young moon versus old moon, old moon. So when was this moon forming impact?
SPEAKER_00Okay, so you have to think about. I mean, this is a good thought exercise for people to think about. Like, how would you date this process? How would you attempt to date it in any way, shape, or form? And given what we've just described, this moon forming impact, the oldest rock on either the earth or the moon has to be younger than the moon forming event, right? Than the impact, because this was a planetary rescinding event. So any rocks we find have to be older. The ages of the rocks that we find or minerals on Earth, it's like 4.35 or 4.38 billion, is the oldest fragment of a little zirpound grain that we find on Earth. The oldest rocks on the moon are around that same kind of time frame. Like they're 4.35, 4.3 something on the moon are the oldest rocks that we have. So that if you just take that and you say that's the minimum age that the moon forming impact could be, that 4.35 number, that's what I would put into the young moon camp. People would say, okay, that's the age of the moon, because that's the age of the rocks that we see, is 4.35, 4.38, something like that. That's the age of the moon forming impact. The other older ages, which are like 4.45 or 4.5, even, these rely on mostly on isotopic model ages, which are saying I have this rock in my hand, that's 4.35. What came before it? And was that a first stage rock or a second stage rock? And if it's a second stage rock, then that means there's a second stage there, which is time. And you can kind of model time, but it's all model-based. Uh isotopic model-based. I'm not saying it's model-based and then it's wrong. It's just a different way to kind of approach the question.
SPEAKER_03But could it be possible that the younger, like actual rock geochemical ages are when that body cooled versus the model ages are when the impact was?
SPEAKER_00Is that that that's true? Yeah, yeah, that's exactly right. Is is did the impact produce some chemical reservoirs that sat there and evolved asotopically, which then later on, 100 or 150 million years later, formed rocks?
SPEAKER_01Right.
SPEAKER_00Or do the rocks represent the first stages of magmosia crystal. And that's the kind of a question that people go to. So there's these these rocks called, I love this this acronym creep, creep basalt or creep rocks, which is potassium, rarosoma, phosphorus rich rocks. Those rocks are what people think are lunar magma ocean crystallization products. So therefore, if that's forming from the crystallization of magma ocean, then when you date that rock, you're dating the magma ocean phase. And and there are a group of people who think that that is like 4.35-ish, somewhere around about there, not 4.5 or 4.45. So there's like a hundred to 150 million year debate here about young moons 4.35, old moons 4.5 or 4.45. Wow. Billion years ago. Yeah.
SPEAKER_03Yeah. When you go back that many billions of years ago, you kind of think, eh, 4.3, 4.4. But then when you really think about how many years those point, you know, 0.1 is 100 million years, that's a massive amount of time.
SPEAKER_00Massive amount of time. Yeah. Yeah, absolutely.
SPEAKER_03Okay. So the next question you kind of hit on a little bit, but it was how did the moon forming impact change Earth? And how much do we know about what proto-Earth was like compositionally? You know, had it started to cool and solidify. Was there any crust before this impact? How could we tell? I mean, from what you've said so far, it sounds like we don't know because everything was reset. And is that the case for the moon too?
SPEAKER_00I think that's right. The everything's reset. It's kind of a short answer. It's a massive planetary resetting event. And so before that, we don't really know what it looked like. I mean, did it have some crust? Probably. You can't have like a magma ocean planet. And if you think of a magma ocean planet, we often like picture magma oceans, like actually oceans. But that's not gonna exist for very long. You have to have a super thick atmosphere with like basically a silicate vapor atmosphere to preserve liquid rock at the surface of the earth. That's like what that gotta be, like 1200 degrees Fahrenheit or something. You have to have an incredible atmosphere above that to preserve that temperature on the surface. So most likely there was like a basaltic crust on whatever planets were there. Just because other it's really hard to maintain for any length of time actual magma oceans, like what we think. Yeah. So when we say magma ocean planet, we mean like a thin crust of of rock. Okay. With we mean a melted mantle, is really what we mean by a magma ocean planet, is a melted mantle. So it's a bit of a confusing terminology. We should have covered that up front, but yeah.
SPEAKER_03So maybe kind of like, well, depending on different hypotheses, maybe like Io, if it did have the melted magma ocean. Okay.
SPEAKER_01Yep, yep. Okay, interesting. Exactly.
SPEAKER_03And I'm also curious, given the chemical similarities between the Earth and the Moon, do we know why the moon doesn't have an atmosphere or didn't evolve an atmosphere? Is it just the size? It just couldn't be gravitational.
SPEAKER_00You know, that's a great question. And I don't I would assume it's the size, and this is probably a textbook kind of level thing in planetary science that that we know, and I just don't know the answer to. I guess it was had a lot to do with the size. That's my assumption. And maybe Earth, Earth's gravity, you know, like attracted all the atmospheric stuff away from the moon or something like that, too. The solar wind, you know, gravity has a lot, you need to have a body of a certain size to keep it atmosphere because the solar wind kind of wants to blow it away all the time. So you gotta have some gravity to keep the atmosphere around it. Yeah.
SPEAKER_03Oh, and actually that brings up a question. You know, Earth developed its magnetosphere very early, pretty much immediately, because it would have had the liquid iron.
SPEAKER_00Yeah, I think it again, I don't know what the modeling says for the formation of the magnetosphere. Probably it was pretty early. I know that there's a significant debate in the paleomagnetic community about when the strength of the magnetic field through time, like how early did kid do what's the earliest? It's kind of like the early life debate. What's the earliest data point we have that detects a magnetic field and what is the strength of that magnetic field? So there's a big debate there about whether there's a magnetic field recorded in the Jack Hill Zirkon grains, for instance, or in early rocks. Early rocks have been metamorphosed so much it's hard to say, but I don't know the details of that. That magnetic evolution.
SPEAKER_03I would have assumed that it would have been immediate because everything was melted, but I guess the iron has to coalesce and start.
SPEAKER_00Yeah, and there's something to do with yeah, there's something to do with crystallizing the inner core. Like you gotta have an inner core so you can generate the outer core. I don't know. I'm I'm speaking of turn. We should talk to somebody else. Yeah.
SPEAKER_03My question was gonna be Earth developed this magnetic field. Does this mean that could the moon have had one if it had liquid iron initially and then lost it?
SPEAKER_00Another great question. Um, there is, oh gosh, it's a couple years ago I saw a talk on the lunar magnetic field. And again, it's kind of a thing where people debate what the strength of it was. It was probably there. Oh, okay if I remember correctly, it was probably there a billion years ago. Going back further, it was hard to tell. So debated literature, there's like very few data points from but there's some. All right.
SPEAKER_03Well, anyway, that was just a side question. We can have somebody else on to describe it.
SPEAKER_00Yeah, that's exactly I think that's right. I think that's right. Figure out talk to somebody who studies this stuff, I think would probably be best to get them on.
SPEAKER_03Well, other than the Proto-Earth, you know, what what it might have looked like in terms of crust or magma ocean, do we know if oceans might have existed before the moon forming impact? And do we know how long after impact they started to form or reform?
SPEAKER_00Great question. Okay, the first one, no idea. I would say that's what I figured. You know, before the moon forming impact, we just have no data from which to kind of address this question. Um, the other question depends on when you think the moon formed. So basically, we know that we have pretty good evidence from the zircon record, these ancient detrital zircons, they have uh an elevated oxygen isotope composition. So the oxygen isotope composition is high. The way you get high oxygen isotopes is by rocks interacting with low temperature water. So that isotopic signature has been used to argue that there was oceans or water, liquid water, let's say, on the surface of the earth 4.2, 4.3 billion years ago. To me, that's very compelling. That's kind of like textbook level stuff now. We we believe that to be true. So the surface was at a low enough temperature so that you could have liquid water, and you can have liquid water anything 100 degrees C, right? Well, it's not boiling water, it could be hot water, but it's not boiling, it's not atmosphere, it's not steam atmosphere. So that is 4.2, 4.3, something like that. So it depends on when you believe if you think the moon swarming impact happened 4.35 or 4.45 of you know, it changes the time between impact and liquid water on the surface. But I think you know the current paradigm has shifted a lot. 20 years ago, probably maybe 30 years ago, people thought of the early Earth as this really inhospitable hot world. And now the evidence is pointing towards liquid water on the surface. And there might have been a it might have been really hot on the inside of the planet, but the surface was relatively hospitable.
SPEAKER_03That's what I've noticed as well.
SPEAKER_00Liquid water, yeah, sediments, some thin sediments being deposited. So yeah.
SPEAKER_03Okay. And another few questions also were wondering or had seen a hypothesis of a potential global ocean. Can we know whether this liquid water was globally widespread?
SPEAKER_00No. Well, do we know if it's globally widespread? I mean, if one part yeah, no, but if one part of the earth is cool enough to have liquid water, then probably the most to the surface is at that temperature, apart from big impacts that hit. So big impacts could hit, and we're talking not moon-forming impact, not planetary resetting impacts, but really big. Like, you know, hurt the planet, but didn't reset it kind of thing. These would create probably transient or temporary temperature pulses on the surface of the Earth. So it would kind of reset that area, it would make it not habitable in that one area. But if one area on the surface is cool enough, the temperature gradients are probably not huge. So I don't think you're gonna have one side that's cool enough to have liquid water and one side that has you know molten rock at the surface necessarily for any long period of time.
SPEAKER_03Theoretically, if those isotope you know compositions do indicate that there could have been liquid water at the Earth's surface at that point, then it could have been everywhere. Like it could have existed in terms of conditions. Whether it did is another question.
SPEAKER_00That's right. So clearly. The evidence here is like 4.2 billion years. We have zircons from one part of the earth that have uh elevated oxygenized field compositions that indicate low temperature water interactions. So that's one thing. The next data point is 4.0 billion years old in the Acostan ice complex. Another, a different location, maybe a different location, has a similar isotopic signature for you know water interactions on Earth. So there's two data points. Then you go to 3.8 billion years, there's another rock suite, two rock suites, probably at that time interval at least, that have evidence for sediments, band and iron formations and and other types of sediments that are water-laying sediments. So that's again good evidence for a large body of water sitting there somewhere depositing sediments. So basically the the idea is we have little hints of a record, and what record we do have indicates low temperature, relatively low temperature surface environments. Liquid.
SPEAKER_03Yeah, and I think the it's an important distinction to make that the reason that we have such little amounts of, you know, oh, a grain here, a grain there isn't because there was little water, it's because all of those rocks have gone away or been altered or recycled, or you know, we just don't have access to rocks four plus billion years old. If we had access to all of them, we'd probably see globally, you know, concordant data. So that I think is an important distinction is we just have one grain here, one grain there, that's all we can look at. But those grains do indicate these super old oceans. So pretty, pretty cool.
SPEAKER_01That's right. Yes, exactly.
SPEAKER_03Okay, so we talked about moon forming impact, could have impacted the way tectonics evolved on Earth, could have impacted the way oceans evolved on Earth. So the follow-up question to that, people were wondering is could the moon forming impact have sparked life on Earth?
SPEAKER_00Um unlikely to have sparked life, I think. It's probably something that is more of a planetary resetting event, is probably damaging to any potential life. If it has anything to do with life, it probably reset it at in the moment, but then maybe it set the stage again for Earth to maintain this habitable. It created a moon. So it might have had something to do with maintaining habitability for a long period of time, um, but is unlikely to have sparked life, I think was the word. So yeah, I would say not.
SPEAKER_03That's my understanding as well, is it most likely sterilized the planet rather than sparked life. But the, you know, the heat that came from that and the plate tectonics which came later, which aren't directly, you know, correlated with the impact, but that may have played a role in helping life along. And that brings us to our next question, which was you know, speaking of early life on Earth, a lot of you guys were wondering the timing of the great oxidation event and the start of plate tectonics, well, one hypothesized start of plate tectonics are both around 2.5 billion years ago. And so a lot of you guys were wondering, is this a coincidence? And I don't know, I don't think we know fully the answer to that question directly, but I do think it's important to note, and I think you mentioned this too, Jesse, in the last conversation we had in the previous follow-up video, is that plate tectonics and life have been very linked throughout time. And it's not just that plate tectonics helps drive these nutrient cycles that drive life, but also that life and the rocks that it produces may in a way, indirectly maybe, help drive some mechanisms of plate tectonics. So it's my understanding that this may not be a coincidence, at least the timing of life blooming and plate tectonics starting to operate a little bit more globally. What do you think?
SPEAKER_00Yeah, I think um I think the the life in plate tectonics questions, gosh, you know, the there's so many different ways to create like temperature gradients or chemical gradients that could potentially initiate life, but it's more about like creating environments that are good for life to kind of propagate, I think. The link at the great oxidation event, like I kind of said before, I think plate tectonics was operating way before 2.5 billion years old. So I don't think that the GOE had anything to do with plate tectonics per se. It could have something to do with continental emergence and continents kind of emerging above sea level and becoming real stable blocks of of land, stable land above sea level. That could be something that I I could I could see happening more. But uh yeah, so it's a tough, it's a tough thing to take to make the link directly. But it's like the earth has so many of these feedback loops in it the carbon cycle, the life cycle. It's rare where one thing directly relates to another, but it's very common where one thing through a series of processes could impact another process.
SPEAKER_03So I I did see one hypothesis when I was trying to research for this specific question that suggests this kind of positive feedback loop within specifically how plate tectonics might have helped drive the GOE, or at least this positive feedback loop of increasing the rate of oxidation. It essentially suggests that the increase in atmospheric oxygen initiated by the photosynthesizers might have started to oxidize Earth's mantle because at early Earth was not oxidized, it was reduced. And then as oxygen formed, it started to oxidize all these chemicals, which is why we call it the great oxidation event. And then as plates were recycled through subduction and through plate tectonic mechanisms, that would have changed the volcanic gases release to oxidized gases instead of reduced gases. And that would have caused less oxygen consumption and then finally allowed oxygen to build up kind of on its own, molecular oxygen in the great oxidation event. Okay. So I'm I'm curious, and that might have been, you know, happening long before the actual oxidation event when we see the big spike in the atmosphere, you know, to finally allow it to build up on its own. But I I don't know. I hadn't seen that before, but it kind of makes sense if plate tectonics, if subduction zones began getting more more laterally continuous, more larger at the time and more able to bring down that material. I don't know. I don't it was interesting.
SPEAKER_00I think that's right. Whenever we're dealing with that's a good, that's a good point. Whenever you're dealing with the climate feedbacks or atmospheric feedbacks like you're talking about, the length of a subduction zone matters a lot and how consistent it's going down, like how much carbon is going down in a trench. Like that, that that's a mass thing. What we're dealing with in our signals when we're looking for plate tectonics or mobile lid tectonics going back in time is like, is there chemical evidence from for surface stuff getting down at depth at any point in time? Right. So in that, as we talked about before, it could be like a punctuated subduction environment, which has a very different probably impact on the environment and the sort of global oxygen carbon cycles than a steady state slab, like the entire cordiering spist fill with one slab going down or something like that. The more we talk.
SPEAKER_03Yeah, the more we talk about the beginning of plate tectonics on earth, the more that I think it was less the beginning of plate tectonics on earth, and more so an early type of plate tectonics, a middle type of plate tectonics, a newer type of plate tectonics, and finally our type of plate tectonics. And I feel like it was, you know, again, more protracted on a smaller scale and then became larger, more laterally continuous. And I think this was a very gradual thing. And it's hard to put 2.5 billion years or 4 billion. It's hard to put a specific time on it because it's so gradual. I mean, it's a planet, it's not one subduction zone equals plate tectonics, you know. So I think that is a hard thing to talk about. And we can't necessarily put one singular date on it. But it's yeah, yeah.
SPEAKER_00I agree completely with that. I agree completely. It's a great question and a tough one to answer for it. But I think thinking about it, I agree with you. Thinking about it as a spectrum is definitely the the right one.
SPEAKER_03But I do have, yeah, I do have a video on my channel that I made about can life induce mountain building? Because I saw this paper about how organic carbon in the rock record can act as kind of like a lubricant to uh oh yeah, okay. Is that have you seen this? Is this is this?
SPEAKER_00I I haven't seen the one about organic carbon. I've seen it with regard to to plate tectonic cycles that you know seduction zones are kind of lubricated by the hydrated upper plate. But uh no, I haven't seen that. That'd be interesting. You should send it to me. I'd be interested reading it.
SPEAKER_03I uh again, I make these videos and then I forget what I said, so I don't really remember much.
SPEAKER_00Yeah, I know the feeling.
SPEAKER_03But I'll link it down below if you want to check it out. There you go. More knowledgeable of Rachel. Okay.
SPEAKER_00Umledgeable, Rachel. That's great.
SPEAKER_03So finally, getting to last couple questions, kind of back to impact-related things. Is there evidence for on early Earth? Is there evidence for other collisions, other big impacts, other than the moon forming impact? And if so, how do we do we know how this affected old rocks, first continents, things like that?
SPEAKER_00So, is there evidence for big impacts on Earth? Yes, there is, not going back to the early Earth, though, because again of this planetary resetting process of plate tectonics. So the pro the proton Earth, we have no information about that. The Earth after the moon forming impact, we have very little information for a very long time. We have big impacts on Earth. We know that the rate of fort in South Africa impact structure, Sudbury up in Canada and Ontario, is a big impact structure. We know that these things happened. Our best record for the meteorite bombardment history of a planet, though, comes from the moon. And the moon is the calibration point for the entire inner solar system because we have rocks from the moon, we can look with our telescopes, we can map, and our rovers and stuff, we can map the craters, we can work out the the relative chronology of the craters, and then we can potentially date them. So the moon is what we kind of fall back on for our impact record of the inner solar system to calibrate that. And there used to be this idea of a late heavy bombardment, people call LHB, some people call it a terminal cataclysm, that suggested that there's a big spike in meteorites at 3.9 billion years ago. That idea has over the past decade or 15 years, has been completely squashed, I think. It's safe to say by that. Yeah. It's one of these weird things that it's still in, you know, it's in textbooks, it's in your like intro to level textbook. They see some stuff about this. But but the current planetary science video does not think that this happened at 3.9. There might have been a little bit of a spike at 4.2 billion years old, but I like there's this diagram or there's this phrase of a sawtooth-like bombardment history, which is basically like you had a lot more meteorite impacts very early on, and it decreased through time, and you might have had little spikes on it that look like a sawtooth, but but basically there's no big late heavy bombardment that's not really regarded as a thing. There's a whole literature on this, and people have written review papers that are kind of talking about how did the scientific community go off in this direction that we don't think there's much evidence for anymore. Like, how does science evolve? It's very interesting for uh to to look at, but the late heavy bombardment, probably not a thing at 3.9 billion years ago.
SPEAKER_03Okay, interesting. So there was probably a more consistent, higher rate of impacts. Almost certainly.
SPEAKER_00And it just depends on what that what that curve, what that curve looks, looks like. And so for like we published a paper a few years ago where we looked at the zircons, and other people have done similar things for different locations, but zircons can, as we talked about before, can record impact events in their structures. We we don't see any impact structures in the four billion-year-old zircons from the Adostanites complex. So, you know, what does that mean? You could try and do some statistical calculations, but basically it kind of fits with the the no late heavy bombardment model. We don't see it. Doesn't mean it didn't happen, not necessarily, but it's again more evidence that suggests it probably isn't likely. So there's things like that we can do. But yes, there was almost certainly more impacts further back in time.
SPEAKER_03Right, right. But we have, again, limited amounts of rocks and data from that. So it's hard to tell.
SPEAKER_00The answer would be go to the moon, sample the heck out of the moon, and then we know because the moon has a really nice crater record.
SPEAKER_01Yeah.
SPEAKER_00Like we we can map the craters really well. We can see here's the big ones, here's the little ones, here's the young one, here's the old ones. And we could go if we could, if we could like do field trips to the moon and collect a whole bunch of samples and date them, we'd be able to figure this out.
SPEAKER_03That's a great point. I feel like the moon is this, like on Earth, we're like, gosh darn it, we don't have all these old rocks because everything is tectonics keeps resetting everything and recycling everything, and it's so annoying. And then the moon is just sitting there, like preserving this really old history, and we're like, Hey, we have it. It's right there.
SPEAKER_00And it has a record of what a magma ocean actually looks like. You know, we drill down on the moon, we get a magma ocean record, uh, right? We so it has the records for the early planetary history sitting there uh out in our night sky.
SPEAKER_03Oh, yeah. Well, we better thank our lucky stars that it didn't involve plate tectonics in an atmosphere. That's great. Awesome. That's exactly that's right. I did see a really interesting presentation at GSA, or part part of one. I probably would be able to explain this better if I saw the whole thing. But um, I caught part of this presentation where this uh this woman was explaining how they're doing research on you know early Earth after moon forming impact, but early Earth, you know, Hadian to Archean transition time range of research on the impacts that were happening then, on specific impacts that they have records of, which is very few. But essentially how they affected or may have affected the evolution or the origin of life on Earth, or the at least the conditions that would have potentially supported an origin of life on Earth, which is super interesting. And a lot of research that they Doing was pointing to instead of this idea in people's heads that impacts would harm early life, it may have actually helped provide these, you know, heated hydrothermal areas where life could have started, which was really, really interesting. And I, you know, just went on this field trip, GSA field trip, about the KPG impact and how in the crater life came back soonest because of these hydrothermal ecosystems. And it was really, really interesting. I would have thought that that would have been the latest because that was molten for a million years after the impact.
SPEAKER_00Yeah, totally.
SPEAKER_03Yeah. So it's really interesting that somewhere where the surface was still molten, you could have ecosystem microbial, but ecosystems start to replenish so soon. And that was, of course, after life had evolved. But I don't know. I it was an interesting, uh, interesting research on early Earth impacts and their link to the origin of life and kind of early life on Earth. So I I'll link more below about it. Um, but yeah, it was really cool. So last question is, and you mentioned this earlier, how the moon forming impact hypothesis is strong because of the chemical similarities between Earth and the moon. But some people were curious are there alternative hypotheses and what is like the evidence for and against these?
SPEAKER_00So the best supported or the working model for sure is the moon forming impact hypothesis. So the the the sort of thing that I outlined before. There are other models sort of that they accreted together, like the Earth and the Moon are the same kind of age, and they accreted together, and and therefore they would both be like 4.5 something billion years old. Capture that the moon was kind of passing by and got captured in Earth's gravitational well, that the moon was kind of spun off of Earth at some level. You know, those have a lot more flaws, major flaws, critical flaws in them than the giant impact models. But the giant impact model matches, you know, most features of the planetary system. And the isotopic similarities are really, you know, and the fact that it has a smaller core is a big one. So the the moon's forming impact explains the isotopic similarities, the small size, relatively small size of the moon's core. Uh it explains kind of the orbital system, the dynamics of the orbital system really well, and and uh the isotopic similarities are kind of one of the biggest ones, I think, that it's really hard to argue for capture or co-accretion given the isotopic similarities here. So uh that's that's my opinion. The isotopic similarities are really compelling for the fact that these two planetary bodies were mixed together at one point in time.
SPEAKER_03Right, right. Okay, so it's still the leading, the leading one.
SPEAKER_00Yes, it's certainly the leading one. Absolutely. The leading, if not the, I mean, I would say dominant.
SPEAKER_03Very cool.
SPEAKER_00So it's cool. It's it's an arm wavy field. Again, we just talk about arm wavy stuff when we get together here, don't we, Rachel? I know.
SPEAKER_03I this this conversation was so interesting. So many things I didn't know. And yeah, sorry to talk a lot about non non-Earth things, but there were so many non-Earth questions.
SPEAKER_00Well, I I you know, I'm happy to the way I like I said I haven't paid attention to this literature for the last couple of years. It was fun to fun to kind of rethink through it a little bit um and and uh go back a couple years to when we was I was paying attention to this a lot.
SPEAKER_03So yeah, for sure. Well, one last suite of questions that we got on our previous sets of videos was specifically the second one was about the great oxidation event, these banded iron formations that formed, these biffs. And we talked a little bit about kind of the very simple, simplified explanation of why they formed, but we didn't talk about, and a lot of you guys asked, why are they banded? And there's a lot that goes into that. And you asked a lot of other questions related to the BIFs and and why they formed and the timeline of that. And so if you want us to make, I was gonna kind of smush that into this, but it didn't seem relevant to the impact. So if you want us to make another follow-up with potentially another expert to talk about biffs and how they formed and why they're banded, let us know in the comments below. And also let us know any other questions you have. And if you'd like to see us do what more, we're thinking of doing more, you know, answering your question type episodes or you know, long rants.
SPEAKER_00Absolutely.
SPEAKER_03So if you like these deep dives, long rants, exactly.
SPEAKER_00If you like long rants with uh Rachel and Jeffy, leave a comment.
SPEAKER_03We have that. We can do that. Well, a lot of my uh a lot of my videos I try to say relatively surface level, make it broadly applicable and understandable to everybody. And I think those are great, but I also love doing these really, really deep dives into you know newer research and isotopes and whatever, you know, these very specific details that we as geologists talk a lot about, but the public doesn't get to hear a lot about. So I love to do these. And if you guys like them or want, you know, have any questions you'd like us to do follow-ups on, let us know below.
SPEAKER_00Absolutely. Thanks, Rachel. Okay, that's a wrap. Please reach out if you have other questions you want Rachel and I to tackle coming in the new year at Planet Geocast and all the social medias. You can send us an email. There's a contact us link on our website, planetgeocast.com. Follow us on all the social medias and download our mobile app, Camp Geo mobile app. First link in your show notes. Cheers.