Could a ~150 mile (240 kilometers) crater form from a small object impact?

So the object impacting the site of the crater is a human shape and not able to be destroyed. The location of the impact is in an ocean which is on average 1000 feet (304 meters) deep with the ocean floor made of mostly granite. The atmosphere is also similar to earths. The gravity is half that of earth.

New info

The impactor can be as dense as you like and go from a human foot sized object to a refrigerator if that helps.

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    $\begingroup$ Wait... The impact location is the ocean, but you want a 150 mile impact basin? At the bottom of the ocean? The impact velocity would probably vaporize most of the ocean (after igniting the atmosphere), killing everything on the planet. Did I understand that correctly? $\endgroup$ – JBH Jan 7 '19 at 4:31
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    $\begingroup$ I scratch my head for a science based question involving an indestructible bullet... $\endgroup$ – L.Dutch - Reinstate Monica Jan 7 '19 at 4:40
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    $\begingroup$ Yes you understand correctly. But at this point there wouldn't be life at all. The story is a deity of sorts falls into the planet at an extremely high speed and the impact creates the only land on the planet. $\endgroup$ – SlothsAndMe Jan 7 '19 at 4:43
  • $\begingroup$ "The impactor can be as dense as you like and go from a human foot sized object to a refrigerator if that helps." It does not help. With the conditions you provided there is very little difference whether it is human-sized or 18-wheeler sized, and whether it is made of uranium or unobtanium. $\endgroup$ – The Square-Cube Law Jan 7 '19 at 10:44
  • $\begingroup$ Not possible - comparable craters are usually caused by objects the diameter of a fairly large city. Such a tiny object must be either laughably dense or traveling laughably fast. Example: A super-dense deity will punch entirely through the granite, drift down to the center of the Earth, and live out eternity there (bit dull) . $\endgroup$ – user535733 Jan 7 '19 at 13:23

I used Down2Earth's impact calculator to try and find out what it would take to get what you want.

The only parameter I could get even close to what you want was the target site: the bed of a 300m deep water body, on an Earth-like planet. I used the densest projectile and the smallest available size: a 100m wide piece of iron. The angle with the widest crater is that of a head-on crash.

This is not going to end well

This is what we get:

Told ya

If instead of Iron we used uranium, or even osmium (the densest element), we would get approximately thrice the energy on the impact. The crater would still be one or two orders of magnitude smaller than what you want. But that's with the 100m wide impactor.

As you can see, for even an osmium man-sized projectile to give you the juice you need, it will need to impact at a considerable fraction of the speed of light. It will open a hole in whatever tectonic plate it hits, besides melting most of the crust around the impact, triggering a new Hadean era. If the planet has an atmosphere, it will evaporate into space. Gasified rocks will form a new, venusian atmosphere that may last a billion years. The debris from the impact will form a new Moon; the planet will have its mass reduced due to losing that debris to form the Moon, so the planet will have a smaller gravity after the impact. Also, its orbit will probably change excentricity around the sun.

If the solar system is already depleted of icy bodies (i.e.: comets) going around the orbit of that planet, it will never get enough water to develop life again. Otherwise give it a billion years for the crust to cool off and the atmosphere to renew itself, and maybe some microbial life will appear. Billions of years later, any intelligent life will never be able to tell that the impact happened.

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    $\begingroup$ maybe try with something denser like neutronium. $\endgroup$ – Jasen Jan 7 '19 at 7:05
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    $\begingroup$ (+1) Any kind of impactor goes down before exploding sideways. An object so small to make a crater so large is going to make it very deep also. So deep, in fact, that's going to nearly destroy the planet or, at the very least, uncover the upper mantle, from where magma would flood to fill the crater and thus, no crater would remain. $\endgroup$ – Rekesoft Jan 7 '19 at 8:47
  • $\begingroup$ @Rekesoft The mantle is not liquid. Magma only occurs in a very small part of the volume. If you removed a few km of the rock above and added water from the ocean, some of the material will probably melt - but I'm pretty sure it wouldn't be anywhere near the amount of rock that was expelled and vaporized by the impact. If anything, it would help make the crater bed very smooth. $\endgroup$ – Luaan Jan 7 '19 at 9:07
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    $\begingroup$ @Luuan I'm no geologist but we are talking about highly speculative geology here. The mantle behaves as a viscous fluid due the extreme pressures of both the inner mantle and the crust. If we remove the crust above some point of the mantle I would expect some new tectonic rift to appear in there, quicky becoming a crust-creating point. The inner sections of the mantle would literally push up there, being the path of less resistance. $\endgroup$ – Rekesoft Jan 7 '19 at 9:12

The Chicxulub impactor is roughly 6.8-50.3 miles in diameter and in this event it released $5.8 \cdot 10^{25} J$ of energy.

To figure out how fast a body would have to travel to impart that energy the equation is $E=(1/2)mv^2$.

Assuming average mans body ~80kg (ASSUMING) we get a velocity of $~1.2 \cdot 10^{12} m/s$ which is about 4000 times the speed of light.

To scale that out to 150 mile wide crater would be far beyond anything even worth mentioning. Now if this god being were super massively dense, maybe the math would work out.

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    $\begingroup$ It would be better if you used the relativistic formula for kinetic energy $\endgroup$ – L.Dutch - Reinstate Monica Jan 7 '19 at 6:48
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    $\begingroup$ Gotta agree with @L.Dutch. If using the classical formula gives an answer like that then it’s time to move up a gear. $\endgroup$ – Joe Bloggs Jan 7 '19 at 6:53
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    $\begingroup$ For example, a quick estimate using this calculator pegs the required speed at 0.9999999999999 times the speed of light. Don’t get me wrong, that’s still obscene, but at least it’s physically possible! $\endgroup$ – Joe Bloggs Jan 7 '19 at 7:01
  • $\begingroup$ So what you guys are saying is that it wouldn't be possible? $\endgroup$ – SlothsAndMe Jan 7 '19 at 8:16
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    $\begingroup$ @SlothsAndMe Not with kinetic energy alone. You can always add some protective shell around the god that explodes on impact if you want to. $\endgroup$ – Luaan Jan 7 '19 at 9:08

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