Would we all perish due to excessive heat? Or would that be limited to the area near the impact while the people on the rest of the earth would die from other phenomena such as mega earthquakes, volcanic activities, tsunamis etc.? Does it matter where the impact is - if it landed in Antarctica, would we have massive floods, but if it landed in the middle of the Eurasian continent, would the effect be different?
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$\begingroup$ By an impact of such magnitude earth's tilt as well as orbit would be displaced causing dramatic changes in temperature, guaranteed to make earth unhabitable for life as we know it. This however would not be the culprit that would annihilate you, it would be cause by the scattering of the planetesimal's rock. $\endgroup$– AlanZ2223Commented Aug 3, 2014 at 4:03
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$\begingroup$ Just some minor details. Something the size of the Moon would be called a dwarf planet, not an astroid. Also, we'd be lucky if there was any flooding at all. More likely, the oceans would all boil away and Earth's orbit would change, etc. A regular large asteroid will have different effects depending what part of Earth's crust is hit. Something that large will have no difference because there won't be a crust when it hits. $\endgroup$– JimCommented Aug 4, 2014 at 17:30
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3 Answers
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First of all, there are no Moon-sized asteroids in the (sufficiently inner) Solar System. The largest asteroid has radius 450 km which is about 4 times smaller (64 times smaller volume) than the Moon. Moons of planets are not counted as asteroids.
A collision with a Moon-sized object would of course be a terminating catastrophe for the Earth. If you look at the explanation of the Torino scale of such impacts
you will see that the largest diameter even mentioned in the graph is smaller than 10 km, and those already exceed the highest Torino 10 scale which corresponds to the global destruction of the civilization.
You are talking about a collision with an object that is 400 times larger in linear dimensions which means 64 million times larger in volume and mass.
If such an object had the usual velocity, and it would have to have because no similar large objects are "synchronized" with the Earth near its orbit (which is needed for the relative speed to be much lower), the impact energy would be so high that the whole Earth would melt.
This is not something unprecedented. After all, the Moon was (probably) created after a similar collision. The Earth used to be smaller at that time. A large celestial body collided with the Earth. The whole Earth melted and a sufficient piece of the Earth – and perhaps with pieces of the other body – went into space and became the seed of the Moon. The Earth itself ended up larger than before the collision.
Another collision like that would probably create another Moon from the escaping material (most likely, a smaller one than the Moon we already have because the attraction of the now-larger Earth is stronger and keeps most of the material here) while the rest of the Earth would grow even bigger than it is now.
The geological processes would start from scratch. I don't know where and how life could survive. Perhaps some bacteria in our satellites etc. would survive and could return, to speed up the evolution. Just to be sure, no one would survive by hiding in a basement, not even a heavily fortified one.
answered Aug 3, 2014 at 5:40
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$\begingroup$ "Another collision like that would probably create another Moon from the escaping material" - wouldn't the escaping material merge with the existing moon instead? $\endgroup$ Commented Aug 5, 2014 at 12:37
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$\begingroup$ Unlikely, the escaping material would have to shoot in a very precise direction to achieve your goal. $\endgroup$ Commented Aug 6, 2014 at 18:08
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The University of Arizona had a neat website that addressed precisely such questions. It's moved here now.
You can choose all kinds of parameters - kind of projectile (ice / rock), impact angle, velocity, landing site and get a prognostication, expected damage etc. Really cool stuff.
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According to this newly published paper such an impact would create deathly effects on earth. In the paper the authors calculate/simulate effect of impact of an object with 1000 km radius (radius of moon is 1700 km).
In the abstract of the paper they say
After the Moon’s formation, Earth experienced a protracted bombardment by leftover planetesimals. The mass delivered during this stage of late accretion has been estimated to be approximately 0.5% of Earth’s present mass...
That is, such an impact would even change the chemical composition of earth. Somewhere in the paper they make a calculation and claim that a minimum of 400 K increase in global temperature would be expected.
Look at this nice animation* they have created. You could easily tract the waves created on the earths surface and the change in temperature. What is even more cool is that the core of the planetesimal is "injected" in to the core of earth, just like a bacteriophage.
So answering your questions:
Would we all perish due to excessive heat?
Yes, we all would turn to grilled chicken. If not fully evaporate.
Or would that be limited to the area near the impact while the people on the rest of the earth would die from other phenomena such as mega earthquakes, volcanic activities, tsunamis etc.?
As the animation shows the waves from the impact go all over the world. Therefore for sure there would be horrible volcanic activities and earthquakes. But, I doubt about tsunamis because all water would evaporate.
Does it matter where the impact is - if it landed in Antarctica, would we have massive floods, but if it landed in the middle of the Eurasian continent, would the effect be different?
Again, based on the animation for sure the effect would be worst at the area of impact (temperature reaches to 10000 K) but everywhere else life as we know would disappear.
*https://phys.org/news/2017-12-collisions-moon-formation-remodeled-early.html
