From Newsgroup: alt.fan.heinlein

C Stuart Hardwick
-+
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Scifi author and science nerd.Updated 7y
If the Moon hit Earth over four billion years ago, where is the crater
here on Earth?
The Moon did not hit the Earth. A Mars-sized proto-planet (which some
people call Theia) hit the proto-Earth. The impact practically turned
the two bodies inside out and partially re-melted them both. Their
nickel-iron cores merged, as did most of their mantles. A good chunk of
the crust (very likely of both of them) was ejected into space, where it eventually coalesced into a moon, or more likely, two moons which later
merged at relatively low speed.


Such an impact does not leave a crater. It leaves a new, bigger planet,
with a brand new surface. It also altered our axis of rotation, setting
it 23 degrees from the plane of the solar system, and giving us far
stronger seasons than we would otherwise have.

And the Moon? When it formed, it was as close as it could be and remain
a moon. Until tidal forces pushed it away (as they still do today at a
greatly reduced rate) it would have created tides of lava across the
molten Earth, then daily, cyclical quakes as it dominated the sky.

If you like space science, you might enjoy my free award-winning scifi sampler.

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Mark McDermott
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Mark McDermott
-+ 7y
Having read about the proto-planet collision theory, I realized it gave
the resulting Earth some unique characteristics, mainly a satellite
almost a quarter of the planetrCOs size, instead of the tiny asteroidal objects around Mars. Not only do we have more variable seasons, but we
also have tides, which, some argue, created tidepools where early sea
life could be safe from predators and do some procreating and evolving.
It seems very unlikely other planets with a big buddy moon like ours
would be hospitable to life for over a billion uninterrupted years,
suggesting there may be less alien life out there than werCOve calculated before.

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Arni Highfield
-+ 7y
It also made our metallic core much larger than would be expected for a
planet our size, since we appear to have absorbed that of Theia too.
This has implications for the magnetic field, vulcanism and mass. The
first of these is almost certainly important for life on Earth, helping
to protect our atmosphere from stripping, and life from solar radiation.

I am one of those that believes ET life is much less likely and probably
very rare simply because of the over 100 special features planetary
scientists have identified for Earth. If Abiogenesis required a
particular combination of these at just the right time, which I suspect
it did, life starting elsewhere would be incredibly rare,
notwithstanding the number of exo-planets.


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Doug Callahan
-+ Jan 17
This subject is a hobby of mine. Do you have a link or reference for
these 100 circumstances?

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Arni Highfield
-+ Feb 4
I canrCOt find the specific link, itrCOs something I read. However, I know
of many features of this planet that are certainly rare, and in
combination, perhaps unique.

This Quora topic is about the early collision with a proto-planet we
call Theia. It had to be just the right size, strike us at just the the
right angle at just the right point in our history. Had it been any
different we would not have the resulting features; a large moon, a
tilt, seasons, a very large core, be the densest planet in the Solar
System, and probably near the very dense end of the spectrum in the galaxy.

Then had a wandering Jupiter not cleared this area of the system, had we
not been amongst the inner planets with a protective screen of gas
giants outside us, (which is VERY rare, we havnt discovered other
planetary systems like ours.) Had our sun been different etc etc.

Look up

papers by planetary scientists and you will find many more
peculiarities. Each individual feature might, perhaps, be found
elsewhere, albeit we already know them to be exceedingly rare as we
havnt found any like that, but in combination, at the right time? The
odd against it are staggering, which undermines the argument that rCythere
are so many exo planets, there must be life elsewhererCO

And though there may be other planets suitable for life, were they ever suitable for abiogenesis?


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Rick D
-+ Apr 3
The odds against are staggering, but there are probably trillions of
planets just in this galaxy, and probably trillions of galaxies in the universe. Not to mention that there could be moons that life could also
start on. So the odds against might be staggering, but the number of opportunities are staggering too.

Arni Highfield
Statistics, odds, are very strange things. Combinations of probabilities
can quickly turn into really large numbers. If planetary scientists are correct, and there are over a hundred unique or very rare features to
Earth, and they had to occur in a limited number of combinations, then
the numbers would get so large as to make the trillions of planets trivial. Profile photo for Doug Callahan
Doug Callahan
-+ Feb 4
Thank you!

I read the book Alone in the Universe by John Gribbin, and he discusses
many of these criteria.

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Powl Smith
Excellent book
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Kurt Klingbeil
-+ Jul 28 https://www.perplexity.ai/search/100-special-features-planetary-eZxIObzvRfSyYrtK.Dd.fA


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Farallon
-+ Aug 28
I agree that life is probably much less common than we previously
thought. As you said, Earth has many very unique features. Another
factor supporting the rare life hypothesis is that prior to a few
billion years ago, the Universe may not have been able support life at
all; it was too hot with too much radiation and the stars and gas clouds
that would form them were too poor in heavy elements to produce rocky
planets. We may be the first, or one of the first planets to develop intelligent life.

Terry Denton
Or we may be someone else's ant farm and just don't know it.
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C Stuart Hardwick
-+ 7y
Could be, but I strongly suspect (in my inexpert opinion) that the moon
is not nearly as critical to life as some have suggested.

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Gabriel Salmon
-+ 7y
It does stabilize our rotation, which is certainly useful to how our particular types of life survived. Maybe life would be able to find a
way around that, but that is a commonly cited reason for how the moon
was essential to life on Earth.

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Adam Strover
-+ 7y
Man, donrCOt you just wish it was possible to witness something like that first hand? There would be an upside to being a god.

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Terry Denton
-+ Fri
I think if soneone was around to see it, I think you would be in
trouble, because there is nowhere to watch it from.

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Jamie
-+ 7y
I didn't know that's what causes the tilt in the Earth. That's fascinating.

So had we an axis more akin to the Solar plane, what would the seasons
be like?

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James H. Kelly
-+ 7y
There would be no seasons at all. It would just be warmer at the equator
and cooler at the poles, but the days would be the same length all year
long.








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