Scientists believe the moon was formed by a collision into planet earth by another celestial body. Why didn't:
1. The other celestial body leave a very obvious elemental makeup in the collision point on earth? We can detect other asteroids, but this one was 100% identical to Earth's crust?
2. Why didn't the debris left by the collision form a ring around earth like Saturn? Instead it all coalesced into one compact body and left zero debris in a ring around earth.
I am not an astrophysicist but these seem like obvious questions with no clear answers online after my quick Google search.
1. The other celestial body leave a very obvious elemental makeup in the collision point on earth? We can detect other asteroids, but this one was 100% identical to Earth's crust?
Earth's crust was rubble and magma. Sort of like how you couldn't find an impact point if you threw a water balloon into a can of paint.
Why didn't the debris left by the collision form a ring around earth like Saturn?
It did. Earth had very impressive rings for thousands of years after the impact.
The short answer is "math". The material either coalesced into the moon or eventually fell back to earth. It's actually a pretty short lifetime for the rings, compared to ring systems like Saturn. Earth has several disadvantages when it comes to maintaining a set of rings. It doesn't have gravity strong enough for tidal forces to tear orbiting bodies apart at far remove from the planet and it doesn't have "shepherd moons" to keep ring material in a stable orbit.
AFAIK, all this discussion is in the realms of hypothesis.
It is. It's just currently the best hypothesis, and it's refined to better match observation over time. For example, the impact with Theia was originally thought to be a glancing blow by a planet spinning similarly to earth, but more recent simulations indicate it was a head-on collision and that Theia had relatively little angular momentum. This hypothesis does make predictions about the moon's core that are falsifiable but it may be some time before we figure out a way to make the necessary measurements.
Saying it' maths is simplifying it a bit. They use simulations. They can simulate how an object orbits and eventually fall down to earth. They can test those simulations everyday by predicting where a know object in orbit will be in the future. Multiply that by a billion and you can predict that earth's rings would have fall down eventually.
>I say you can use maths to prove whatever you like.
You're technically right, but only when compared against the exact way the comment you're replying to was phrased. Scientists don't just write down whatever equations give them the answers they feel like getting that day, they first figure out which equations give correct answers, and then use only those.
The stability of ring systems can be worked out using the same old laws that Newton figured out, which describe inertia and gravity. They can then be tested in particular by observing rings around other planets, and in general by every observation that confirms Newton's laws.
> but I say you can use maths to prove whatever you like.
> AFAIK, all this discussion is in the realms of hypothesis.
it is unfortunate that you are being downvoted. I have been increasingly taking note of things that "science" says that the general public then takes for granted as fact, when they're entirely unfalsifiable, unprovable, and just conjecture. of course there is nothing wrong with conjecture, and especially nothing wrong with mathematically-backed conjecture. but there is a huge gulf between making conjectures about Earth possibly having had rings in the past and accepting as fact that Earth most certainly had rings in the past.
it seems to me that a big part of this is the (at least contemporary) human "need" to be absolutely sure about everything. most people come away from reading e.g. this comments thread either "knowing" that Earth definitely had rings in the past, or rejecting the rhetoric here and believing Earth never had rings. few people allow themselves the "mental gray area" of not really believing or disbelieving something, instead leaving that "did the Earth ever have rings or not" factoid field in their mental database with the value of "idk, maybe?"
once you start training yourself to embrace this "mental gray area" for unprovable (or merely unproven) facts, you start to notice a lot of things that many people take for granted as being absolutely True, when in fact there is ample possibility for them to be False. another example is the composition of the Earth's core—there's all sorts of ways we can inductively determine this, but ultimately we don't definitively know what the Truth of the matter is.
this "lack of 'mental gray area'" phenomenon is what leads to many people today taking not just proven scientific research as gospel, but also all sorts of theories and conjectures and unproven hypotheses. how many ardently atheist "science-believers" literally believe in the existence of dark matter, because "the science says so" (even if that's not really the case)? awhile ago I posted a comment on this website about how an atheist friend of mine believed that a government policy definitely had no downsides or issues because a social "science" Study had been done saying no such issues existed, and he took this just as equally As Gospel as he would F=ma—it's Science, Scientists say so, so therefore it must be True—as though the world were a computer simulation and Scientists are reading the assembly language and/or database entries that comprise the totality of the Universe verbatim. intuitively realizing that this explicitly is not and cannot be True has really opened my eyes to the nature of how many (most?) people take lots of scientific conjecture and theory and social "science" as Accepted Fact, and how that's Probably Not The Best Way To Go About Things.
The way I put it sometimes, is that people don't know what 'knowing' is. It is an epistemological question. Individuals seem to think 'we' can know something - or that knowing is a group activity! That you can accept authoritative sources without personally verifying it AND call it knowledge.
Really, they are just hypotheses, or beliefs. There is actually no difference between these beliefs and religion that I can see... but people are so adamant that science is not religion!
making conjectures about Earth possibly having had rings in the past and accepting as fact that Earth most certainly had rings in the past.
Earth having rings is not really special. In fact right now we have "dust moons" orbiting us. Any large-scale impactor is going to create rings of orbiting debris. The question is how much mass was in orbit and how long it took for the amount of mass in orbit to return to an average amount. Rings are quite ephemeral things despite their striking visuals.
Those rocks identifiable as moon rocks on the surface of the earth are from ejected debris from the later meteor collisions that formed the craters that we can see on the moon. The early collision debris would have fallen on the molten surface of the earth and been absorbed.
> but this one was 100% identical to Earth's crust?
Given the size of this impact, the crust[1] didn't really survive. Most of the mantle material ended up vaporized and either falling back down or coalescing onto a single fragment, becoming the moon.
It was a collision "of" the planet, not "on" the planet.
[1] Which in any case wouldn't have survived this long anyway. The differentiation process that produces the crust we're standing on takes billions of years. There aren't any surface rocks that old, and wouldn't be even if the collision hadn't happened.
This isn't 'the place the moon hit', it's material from the colliding object (Theia) that has been added to the Earth's mantle. The mantle convects, and this material is a little more dense, and has pooled up at the bottom of the mantle, but because the mantle convects (the material circulates) there is no real reason to believe that these bodies are at the location of collision, especially considering that much of the mantle (and the parts of Theia that stayed behind) would have been reduced to flying bits of lava in orbit around the core, before coalescing.
My personal opinion, but informed by arguments from a number of "real" scientists: because that explanation is bogus. It never made much sense. A collision of a magnitude to break off a moon-sized chunk of mass would be enormous. And why wouldn't the chunk either escape or fall back down, rejoining the main mass? Do we have any evidence that such collisions ever result in significantly-sized orbiting bodies?
Or maybe I'm just not understanding that "theory" well enough, because my current preferred hypothesis could be described in much the same way. Let's say there was a large collision, but it was when the Earth was hotter and more molten, and it added enough heat and instability (and perhaps angular momentum) that large globs were thrown off. Some escaped, some fell back down, and some were swept up by the largest glob, now the Moon.
But either way, I think the answers to your objections are: (1) it was molten enough to meld together, OR it wasn't, in which case plate tectonics regularly replace the entirety of the crust (and underneath the magma layer is molten); and (2) the Moon cleared its orbit and the Earth is small enough to not support a large area to hold crud that makes up a ring? (All of this is guesswork.)
At these scales, there are no rigid bodies, and so no single "chunk". If the giant-impact account is correct, the moon coalesced out of ejecta in orbit.
> And why wouldn't the chunk either escape or fall back down, rejoining the main mass?
Some material likely did escape, but that takes much more energy than getting into orbit. You need to leave the surface of the Earth at about 11km/s to escape, but only 6km/s to reach lunar orbit.
It wouldn't necessarily "break off" anything. If the angular momentum of the post-impact planetary mass reaches or exceeds orbital velocity, and its temperature is high enough, you get a novel type of celestial body - a synestia [1], a planet-sized hurricane of boiling rock. As the rock cools, magma rains inward re-fornming the Earth (Earth V2?) over decades, while everything far enough out (beyond the Roche limit [2]) would coalesce to form the Moon.
>Or maybe I'm just not understanding that "theory" well enough
From what you've written, I think you are misinterpreting the standard theory, which is much more like 'your idea' (the second paragraph) than your conceptualization of it in the first paragraph, i.e. there was no moon-sized solid chunk removed. I don't think any scientists working on this think that.
Although there is some argument on the size of the impacting body (Theia), most/all the impact-origin theories hold that much of the planet was reduced to tiny liquid fragments and/or vaporized, including the material that became the Moon.
Something that is important to keep in mind, but is non-intuitive, is that all of the Earth with the exception of the upper crust (say, the upper 10-30 km of the Earth, radius 6378 km) is hot enough to melt if it was not under the confining pressure of the weight above it. Remove material from above the lower crust or mantle, and it undergoes decompression melting. So even if the material that coalesced to become the moon was removed through gentle caresses, without producing heat due to friction, that material would melt, and so would the rock surrounding the area that it was removed from. So you don't need to place the collision happening in a context where the Earth was much hotter (which it probably was to some degree regardless). Add in the enormous friction caused by collision, and you are in a situation where much of the Earth and possibly Theia are reduced to tiny particles of liquid (lava) or gas (vaporized basically), though these would cool to basically volcanic glass very rapidly in space. Then the Moon and the Earth's mantle were reassembled from these tiny particles.
Caveat: I have a PhD in geology but my research is not in this area and I don't follow it closely.
the diameter of the moon as 2,400 miles (actual ~2,160)[6] and the distance between the moon and the earth to be 258,000 miles[6] (now known to vary: 221,500–252,700 miles (356,500–406,700 kilometres).[11]
Agreed. If someone argued that geology began in the 1800s and neglected to mention Eratosthenes’ calculation of Earth’s size 2000 years earlier, I’d hardly say they’re showing an anti-Greek bias.
In this case it’s more likely either ignorance or a different definition of “study”.
The article doesn't state that "astronomy" as a whole was invented by Galileo, but rather that the moon itself was never studied scientifically until his time. That's a patently false statement that requires dismissing the millennia of Indian observations and research on the moon. I'm pretty sure we could make similar arguments for the Middle East or Asia.
It's like when they teach that everyone thought the world was flat back then. They didn't. That's a fairy tale. Some people thought that, but the idea that the Earth was in fact round was not a shocking finding to contemporary people.
Likewise, geology as a field may have begun in the 1800s, but you wouldn't claim that no one tried to study any aspect of the earth before then. And if you did, I would describe that as being at least ignorant of Eratosthenes. (Actually, no I wouldn't, because I almost certainly wouldn't have thought of that precise example. But I digress.)
Is the issue that "racist" just seems too harsh of a label for what may be a simple error? One can definitely argue there is a western bias in claiming Galileo as the starting point for studying the moon. Maybe conflating "western bias" with "racism" is a step too far.
> a patently false statement that requires dismissing the millennia of Indian observations and research on the moon
Every culture observed and most rendered calculations on the moon's motion. Forecasting eclipses is a motif of early mathematics.
> geology as a field may have begun in the 1800s, but you wouldn't claim that no one tried to study any aspect of the earth before then
"Studying any aspect" of something isn't scientifically analyzing it. To my knowledge, Siddhanta and Aryabhata calculated the motion of celestial bodies remarkably early. But many others independently repeated those calculations, albeit with varying accuracy, across the world and millennia. None of those methodically delved into lunar makeup and origins. The tools didn't exist. The scientific method didn't exist.
Aryabhata did comment on the moon's luminosity. But that's a far cry from attempting to prove it. Which is fine. It doesn't diminish him or his work. But it fails as a counterpoint to the rigor and volume of post-Galilean research.
> Every culture observed and most rendered calculations on the moon. Calculating eclipses was similarly attempted and solved by practically every major civilization.
Just to be clear, I'm not suggesting it was only India. I was saying there's something western-centric about dating things to Galileo. However...
> Nobody, however, methodically delved into lunar makeup and origins in a way remotely scientific, in part because the scientific method is a relatively recent invention.
Fair point. If you're saying that anything prior to the scientific method (or that doesn't utilize the scientific method) doesn't count as science, I can see this argument and am happy to concede the point.
> anything prior to the scientific method (or that doesn't utilize the scientific method) doesn't count as science, I can see this argument
Not quite arguing this. The article says it wasn't until Galileo that "scientists really [began] to study" the moon. Within the context of the moon's origin, it's a legitimate statement.
Ancient astronomers weren't following the scientific method, but I wouldn't dismiss them as not being scientists. If someone said scientists didn't really start studying subatomic particles until the 19th century, it would be asinine to claim offense on the basis of Lucretius having mentioned indivisible building blocks of reality.
Put another way, the same comment would have been stronger without the accusation of racism. (Qualifying with "mild" does nothing. Calling someone mildly idiotic isn't somehow less provocative than saying they're an idiot.)
Because OP used a trigger word and HN instinctually downvotes that word instead of taking the more charitable action of benevolently and accurately interpreting OP as having meant “cultural bias.”
I didn't downvote you, but I think "implicit or unconscious ethno-centrism" might be a more suitable term than "mildly racist". At least it could have been more palatable to the HN hive mind.
From the article:
> Humankind has maintained an enduring fascination with the Moon. It was not until Galileo’s time, however, that scientists really began study it.
I'd agree with you that this statement ignores how scientists around the world, including European and "non-Western" cultures, have studied the moon - more or less scientifically and mathematically - for centuries before Galileo. (In the case of Aryabhata, a millenium earlier.)
54 comments
[ 3.2 ms ] story [ 112 ms ] thread1. The other celestial body leave a very obvious elemental makeup in the collision point on earth? We can detect other asteroids, but this one was 100% identical to Earth's crust?
2. Why didn't the debris left by the collision form a ring around earth like Saturn? Instead it all coalesced into one compact body and left zero debris in a ring around earth.
I am not an astrophysicist but these seem like obvious questions with no clear answers online after my quick Google search.
Having an outsized moon might also degrade the orbit of any material that would have been left over after coalescing into the earth and moon.
Earth's crust was rubble and magma. Sort of like how you couldn't find an impact point if you threw a water balloon into a can of paint.
Why didn't the debris left by the collision form a ring around earth like Saturn?
It did. Earth had very impressive rings for thousands of years after the impact.
How do we know we had rings around earth for so long? And where did they go?
For comparison, the dinosaurs existed before than the rings did.
I know it doesn't, but I say you can use maths to prove whatever you like.
So, 'maths' as an answer, is not satisfactory to me.
AFAIK, all this discussion is in the realms of hypothesis.
It is. It's just currently the best hypothesis, and it's refined to better match observation over time. For example, the impact with Theia was originally thought to be a glancing blow by a planet spinning similarly to earth, but more recent simulations indicate it was a head-on collision and that Theia had relatively little angular momentum. This hypothesis does make predictions about the moon's core that are falsifiable but it may be some time before we figure out a way to make the necessary measurements.
You're technically right, but only when compared against the exact way the comment you're replying to was phrased. Scientists don't just write down whatever equations give them the answers they feel like getting that day, they first figure out which equations give correct answers, and then use only those.
The stability of ring systems can be worked out using the same old laws that Newton figured out, which describe inertia and gravity. They can then be tested in particular by observing rings around other planets, and in general by every observation that confirms Newton's laws.
> AFAIK, all this discussion is in the realms of hypothesis.
it is unfortunate that you are being downvoted. I have been increasingly taking note of things that "science" says that the general public then takes for granted as fact, when they're entirely unfalsifiable, unprovable, and just conjecture. of course there is nothing wrong with conjecture, and especially nothing wrong with mathematically-backed conjecture. but there is a huge gulf between making conjectures about Earth possibly having had rings in the past and accepting as fact that Earth most certainly had rings in the past.
it seems to me that a big part of this is the (at least contemporary) human "need" to be absolutely sure about everything. most people come away from reading e.g. this comments thread either "knowing" that Earth definitely had rings in the past, or rejecting the rhetoric here and believing Earth never had rings. few people allow themselves the "mental gray area" of not really believing or disbelieving something, instead leaving that "did the Earth ever have rings or not" factoid field in their mental database with the value of "idk, maybe?"
once you start training yourself to embrace this "mental gray area" for unprovable (or merely unproven) facts, you start to notice a lot of things that many people take for granted as being absolutely True, when in fact there is ample possibility for them to be False. another example is the composition of the Earth's core—there's all sorts of ways we can inductively determine this, but ultimately we don't definitively know what the Truth of the matter is.
this "lack of 'mental gray area'" phenomenon is what leads to many people today taking not just proven scientific research as gospel, but also all sorts of theories and conjectures and unproven hypotheses. how many ardently atheist "science-believers" literally believe in the existence of dark matter, because "the science says so" (even if that's not really the case)? awhile ago I posted a comment on this website about how an atheist friend of mine believed that a government policy definitely had no downsides or issues because a social "science" Study had been done saying no such issues existed, and he took this just as equally As Gospel as he would F=ma—it's Science, Scientists say so, so therefore it must be True—as though the world were a computer simulation and Scientists are reading the assembly language and/or database entries that comprise the totality of the Universe verbatim. intuitively realizing that this explicitly is not and cannot be True has really opened my eyes to the nature of how many (most?) people take lots of scientific conjecture and theory and social "science" as Accepted Fact, and how that's Probably Not The Best Way To Go About Things.
The way I put it sometimes, is that people don't know what 'knowing' is. It is an epistemological question. Individuals seem to think 'we' can know something - or that knowing is a group activity! That you can accept authoritative sources without personally verifying it AND call it knowledge.
Really, they are just hypotheses, or beliefs. There is actually no difference between these beliefs and religion that I can see... but people are so adamant that science is not religion!
It's funny to watch.
Earth having rings is not really special. In fact right now we have "dust moons" orbiting us. Any large-scale impactor is going to create rings of orbiting debris. The question is how much mass was in orbit and how long it took for the amount of mass in orbit to return to an average amount. Rings are quite ephemeral things despite their striking visuals.
Given the size of this impact, the crust[1] didn't really survive. Most of the mantle material ended up vaporized and either falling back down or coalescing onto a single fragment, becoming the moon.
It was a collision "of" the planet, not "on" the planet.
[1] Which in any case wouldn't have survived this long anyway. The differentiation process that produces the crust we're standing on takes billions of years. There aren't any surface rocks that old, and wouldn't be even if the collision hadn't happened.
Or maybe I'm just not understanding that "theory" well enough, because my current preferred hypothesis could be described in much the same way. Let's say there was a large collision, but it was when the Earth was hotter and more molten, and it added enough heat and instability (and perhaps angular momentum) that large globs were thrown off. Some escaped, some fell back down, and some were swept up by the largest glob, now the Moon.
But either way, I think the answers to your objections are: (1) it was molten enough to meld together, OR it wasn't, in which case plate tectonics regularly replace the entirety of the crust (and underneath the magma layer is molten); and (2) the Moon cleared its orbit and the Earth is small enough to not support a large area to hold crud that makes up a ring? (All of this is guesswork.)
> And why wouldn't the chunk either escape or fall back down, rejoining the main mass?
Some material likely did escape, but that takes much more energy than getting into orbit. You need to leave the surface of the Earth at about 11km/s to escape, but only 6km/s to reach lunar orbit.
1. https://en.wikipedia.org/wiki/Synestia 2. https://en.wikipedia.org/wiki/Roche_limit
From what you've written, I think you are misinterpreting the standard theory, which is much more like 'your idea' (the second paragraph) than your conceptualization of it in the first paragraph, i.e. there was no moon-sized solid chunk removed. I don't think any scientists working on this think that.
Although there is some argument on the size of the impacting body (Theia), most/all the impact-origin theories hold that much of the planet was reduced to tiny liquid fragments and/or vaporized, including the material that became the Moon.
Here is a video of a simulation of the the 'standard' theory: https://www.youtube.com/watch?v=wfImQOZp3hE
Something that is important to keep in mind, but is non-intuitive, is that all of the Earth with the exception of the upper crust (say, the upper 10-30 km of the Earth, radius 6378 km) is hot enough to melt if it was not under the confining pressure of the weight above it. Remove material from above the lower crust or mantle, and it undergoes decompression melting. So even if the material that coalesced to become the moon was removed through gentle caresses, without producing heat due to friction, that material would melt, and so would the rock surrounding the area that it was removed from. So you don't need to place the collision happening in a context where the Earth was much hotter (which it probably was to some degree regardless). Add in the enormous friction caused by collision, and you are in a situation where much of the Earth and possibly Theia are reduced to tiny particles of liquid (lava) or gas (vaporized basically), though these would cool to basically volcanic glass very rapidly in space. Then the Moon and the Earth's mantle were reassembled from these tiny particles.
Caveat: I have a PhD in geology but my research is not in this area and I don't follow it closely.
It is 2022 and yet we see mildly racist falsehoods like this.
https://en.m.wikipedia.org/wiki/Surya_Siddhanta
https://en.m.wikipedia.org/wiki/AryabhataThe earlier learnings are interesting; the accusation that racism has something to do with the omission is not.
In this case it’s more likely either ignorance or a different definition of “study”.
It's like when they teach that everyone thought the world was flat back then. They didn't. That's a fairy tale. Some people thought that, but the idea that the Earth was in fact round was not a shocking finding to contemporary people.
Likewise, geology as a field may have begun in the 1800s, but you wouldn't claim that no one tried to study any aspect of the earth before then. And if you did, I would describe that as being at least ignorant of Eratosthenes. (Actually, no I wouldn't, because I almost certainly wouldn't have thought of that precise example. But I digress.)
Is the issue that "racist" just seems too harsh of a label for what may be a simple error? One can definitely argue there is a western bias in claiming Galileo as the starting point for studying the moon. Maybe conflating "western bias" with "racism" is a step too far.
Every culture observed and most rendered calculations on the moon's motion. Forecasting eclipses is a motif of early mathematics.
> geology as a field may have begun in the 1800s, but you wouldn't claim that no one tried to study any aspect of the earth before then
"Studying any aspect" of something isn't scientifically analyzing it. To my knowledge, Siddhanta and Aryabhata calculated the motion of celestial bodies remarkably early. But many others independently repeated those calculations, albeit with varying accuracy, across the world and millennia. None of those methodically delved into lunar makeup and origins. The tools didn't exist. The scientific method didn't exist.
Aryabhata did comment on the moon's luminosity. But that's a far cry from attempting to prove it. Which is fine. It doesn't diminish him or his work. But it fails as a counterpoint to the rigor and volume of post-Galilean research.
Just to be clear, I'm not suggesting it was only India. I was saying there's something western-centric about dating things to Galileo. However...
> Nobody, however, methodically delved into lunar makeup and origins in a way remotely scientific, in part because the scientific method is a relatively recent invention.
Fair point. If you're saying that anything prior to the scientific method (or that doesn't utilize the scientific method) doesn't count as science, I can see this argument and am happy to concede the point.
Not quite arguing this. The article says it wasn't until Galileo that "scientists really [began] to study" the moon. Within the context of the moon's origin, it's a legitimate statement.
Ancient astronomers weren't following the scientific method, but I wouldn't dismiss them as not being scientists. If someone said scientists didn't really start studying subatomic particles until the 19th century, it would be asinine to claim offense on the basis of Lucretius having mentioned indivisible building blocks of reality.
Put another way, the same comment would have been stronger without the accusation of racism. (Qualifying with "mild" does nothing. Calling someone mildly idiotic isn't somehow less provocative than saying they're an idiot.)
Studying orbits is not just math.
From the article:
> Humankind has maintained an enduring fascination with the Moon. It was not until Galileo’s time, however, that scientists really began study it.
I'd agree with you that this statement ignores how scientists around the world, including European and "non-Western" cultures, have studied the moon - more or less scientifically and mathematically - for centuries before Galileo. (In the case of Aryabhata, a millenium earlier.)
https://www.smithsonianmag.com/science-nature/ancient-greek-...
https://en.wikipedia.org/wiki/Tom_Dooley_(song)
https://en.wikipedia.org/wiki/Theia_(planet)