5 ms·

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

by sfink 4y ago

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.)

hither_shores 4y ago
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.
GolfPopper 4y ago
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. 1. https://en.wikipedia.org/wiki/Synestia https://en.wikipedia.org/wiki/Synestia 2. https://en.wikipedia.org/wiki/Roche_limit https://en.wikipedia.org/wiki/Roche_limit
deleted 4y ago
[deleted]
cossatot 4y ago
>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. Here is a video of a simulation of the the 'standard' theory: https://www.youtube.com/watch?v=wfImQOZp3hE 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.

by sfink 4y ago

hither_shores 4y ago

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.

GolfPopper 4y ago

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. 1. https://en.wikipedia.org/wiki/Synestia https://en.wikipedia.org/wiki/Synestia 2. https://en.wikipedia.org/wiki/Roche_limit https://en.wikipedia.org/wiki/Roche_limit

deleted 4y ago

[deleted]

cossatot 4y ago

>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. Here is a video of a simulation of the the 'standard' theory: https://www.youtube.com/watch?v=wfImQOZp3hE 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.