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For every asteroid or meteorite that hits dry land, two hit the sea. If a Manicouagan-sized asteroid struck the deep ocean (but not above a trench),how would the H2O vapour, chlorine, iodine bromine and other elements affect the climate, and for how long? Also, if it had thousands of feet of water to penetrate, would it leave a large crater on the sea bed? For sure it would create massive tsunamis, so is there any evidence for these in the geological record? The Manicouagan crater is far from being the largest on Earth, but is only slightly smaller than the Chicxulub example. It was caused by an asteroid 5km in diameter traveling at 25km per second, though friction with the atmosphere probably reduced this slightly before it struck the ground.

Manicouagan, however, is just an example, we are talking here about a hypothetical asteroid of this size striking the deep ocean. Don't be misled by the word hypothetical; given the number of large impacts which have happened on land there must have been some which landed in deep ocean. How deep? Let's call it 6,000 feet, which is nowhere near a record depth.

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  • $\begingroup$ And define 'deep ocean'. If you are excluding trenches and ask about the sea bed, you should at least specify its depth (why else exclude trenches). A specification of Manicouagan-sized would also help so that we don't have to go look it up. $\endgroup$ – Jan Doggen Jun 17 at 12:46
  • $\begingroup$ How about 1,500 fathoms? Users are free to choose their own depth if they want to to,but it has to be very deep. Manicouagan is a very large astrobleme in Quebec,but is reputed to have been caused by a slightly smaller asteroid than the Chicxulub example. $\endgroup$ – Michael Walsby Jun 17 at 12:57
  • $\begingroup$ note Chicxulub was a shallow ocean impact. The Eltanin impact might be what you are looking for, it is deep ocean even if it is much smaller. en.wikipedia.org/wiki/Eltanin_impact your problem is techtonics wipe out any impact events older than about 150mya. $\endgroup$ – John Jun 21 at 1:28
  • $\begingroup$ I'm not actually looking for any particular impact;I mentioned Manicouagan because it was a really big one but by no means a record. All I really want to know is what the consequences would be of a major impact in mid ocean. Choose a different depth if you like, 6000 feet is not particularly deep as deep ocean goes,but it's a likely sort of depth to be struck at random. I exclude trenches because the chances of a major impact directly above a trench are remote,whereas impacts in deep ocean must have happened. Choose a slightly smaller asteroid if you want to,but not a lot smaller. $\endgroup$ – Michael Walsby Jun 21 at 7:59
  • $\begingroup$ here is one that made quite a splash when it hit en.wikipedia.org/wiki/Mj%C3%B8lnir_crater and there is a list of others on this page. $\endgroup$ – trond hansen Jun 25 at 4:29
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A 5km impactor would make a very, very global mess.

Here's a discussion of tsunami formation from asteroid impacts. It presents numbers for impactor size up to 2km, and posits a rule-of-thumb that indicates a 5km impactor would generate a coherent wave (one that doesn't dissipate much over great distances) as long as it impacts in water less than 20-30km deep -- in other words, in any ocean on Earth. The tsunamis would propagate right around the globe.

Here's a discussion of more general effects from larger impactors. Fireball radiation from a 5km impactor would ignite flammables within a 600km radius, and the secondary radiation from returning ejecta would cause a firestorm across a 5000km radius. Earthquakes and hurricanes would take care of whatever the tsunamis couldn't reach. The skies would be completely dark for months (once the glowing stuff finished landing).

About our best hope would be for a polar impact, where global atmospheric circulation patterns might spare the opposite hemisphere the worst effects.

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I submit the argument that it wouldn't have much an effect on the scale you're inquiring. A 5km wide impactor (like the Manicouagan was calculated to be) in the massive ocean, wouldn't do much damage. I do think you're right in suggesting there would be some effect, but I don't think it would be on a scale to change the climate. I also think that such an impact would completely obliterate the impactor. The velocities at which impact occurs would have similar consequences on land vs water, in that it would be akin to the impacting body slamming into land. You know how water can feel hard when you slap it with just your hand? Imagine it being an asteroid moving at several km/s. It would be like slamming into a solid surface, and thus I submit the asteroid would vaporize upon impact.

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  • $\begingroup$ I think the result would be similar to a land impact in some ways, different in others. enormous quantities of water vapour would be released,& great quantities of halogens: chlorine, bromine & iodine. $\endgroup$ – Michael Walsby Jun 25 at 8:12

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