What if the Moon-forming impact also determined which…
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What if the Moon-forming impact also determined which planet became Earth—and where it ended up?
***One clarification:*** *I’m not necessarily proposing that the body we call “Theia” became the present Earth, or that proto-Earth was completely destroyed/ejected. I’m suggesting that we don’t necessarily know how the identities of the two bodies map onto the objects we have today. The present Earth could be predominantly proto-Earth, predominantly Theia, or a combination of both, while some material from either or both bodies could have ended up in the Moon or elsewhere. My point is that we tend to label the pre-impact bodies as “proto-Earth” and “Theia” and then implicitly assume which one became the Earth we have today, when the actual collision could have been much more complicated.*
I’ve been reading about the giant-impact hypothesis and one thing that caught my attention is the remarkable isotopic similarity between Earth and the Moon. One of the difficulties this creates for some versions of the hypothesis is explaining why material from Theia could be so similar to Earth.
It got me wondering whether we’re making an assumption about the event that doesn’t necessarily have to be true: what if proto-Earth wasn’t originally at \~1 AU?
What if proto-Earth and Theia were originally on different solar orbits, and the collision altered the trajectory of the resulting planet, ultimately leaving the Earth–Moon system near Earth’s present orbit?
In that scenario, the impact wouldn’t simply be *Theia hitting Earth and producing the Moon*. The collision could have fundamentally rearranged the system’s composition and its heliocentric orbit. The Moon could have formed from the resulting ejecta, while the present Earth and Moon would share material from the same highly energetic, mixing event.
I’m genuinely curious whether this is physically possible, and especially whether there are problems with the orbital mechanics or isotope evidence that I’m overlooking.
#space
source
***One clarification:*** *I’m not necessarily proposing that the body we call “Theia” became the present Earth, or that proto-Earth was completely destroyed/ejected. I’m suggesting that we don’t necessarily know how the identities of the two bodies map onto the objects we have today. The present Earth could be predominantly proto-Earth, predominantly Theia, or a combination of both, while some material from either or both bodies could have ended up in the Moon or elsewhere. My point is that we tend to label the pre-impact bodies as “proto-Earth” and “Theia” and then implicitly assume which one became the Earth we have today, when the actual collision could have been much more complicated.*
I’ve been reading about the giant-impact hypothesis and one thing that caught my attention is the remarkable isotopic similarity between Earth and the Moon. One of the difficulties this creates for some versions of the hypothesis is explaining why material from Theia could be so similar to Earth.
It got me wondering whether we’re making an assumption about the event that doesn’t necessarily have to be true: what if proto-Earth wasn’t originally at \~1 AU?
What if proto-Earth and Theia were originally on different solar orbits, and the collision altered the trajectory of the resulting planet, ultimately leaving the Earth–Moon system near Earth’s present orbit?
In that scenario, the impact wouldn’t simply be *Theia hitting Earth and producing the Moon*. The collision could have fundamentally rearranged the system’s composition and its heliocentric orbit. The Moon could have formed from the resulting ejecta, while the present Earth and Moon would share material from the same highly energetic, mixing event.
I’m genuinely curious whether this is physically possible, and especially whether there are problems with the orbital mechanics or isotope evidence that I’m overlooking.
#space
source
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