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If there are cities on Mars in the second half of this century with say a million people, they will be living in engineered structures and sustained by agriculture in protected, controlled environments.

If there was an asteroid impact on Mars with the mass and velocity that would classify it as an Extinction Level Event if it had hit Earth, would they also become extinct?

While Mars is slightly smaller than Earth, it's a lot closer to the asteroid belt, so these kinds of events are just as relevant to "them" as they are to "us". Even more so perhaps.

Assume the impact is not necessarily a direct hit on the city itself, or that they are widely spaced enough that at least some are far from the impact.

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    $\begingroup$ That's actually a pretty interesting question. After all, a large part of an extinction of that level is atmospheric and volcanic effects. And since Mars doesn't have an appreciable atmosphere nor any volcanism, getting hit by an asteroid might not hurt as much. $\endgroup$ – Phiteros Sep 28 '16 at 2:48
  • $\begingroup$ Wouldn't they be using molten salt thorium reactors for energy by then so even more contained and less affected by atmospheric events . Part of colonizing space would be to have greatly advanced abilities to stop such events happening .Asteroid harvesting equipment at space hubs which may be utilized to alter paths of threatening asteroids.. Chris $\endgroup$ – user17187 Oct 7 '16 at 11:33
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I would guess no. What makes the extinction level events so dangerous for us isn't the impact or the shockwave. A shockwave on Mars won't do much damage since the atmosphere is very thin anyway, and it's likely that colonists are mostly living underground (using the ground as radiation shielding).

What makes the big impact strikes so dangerous on Earth is the dust thrown into the atmosphere which drops the global temperatures long afterwards. On Mars, later this century, open-air agriculture is probably only in the experimental phases, with most of the population still living on food produced in protected, indoor farms. The colonists will likely still live in heated, pressurized habitats rather than on a terraformed surface.

A catastrophic environmental collapse is unlikely just due to dust, since there's not going to be much environment outdoors. Since Mars already experiences periodic dust storms which would interfere with solar power generation, the colonists will have backup power sources. I suspect more dust in the atmosphere would be an inconvenience for the colonists, rather than life threatening

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  • $\begingroup$ It sounds like maintaining structural and environmental integrity, along with either sunlight or electricity is important. Would it still be too dark to grow plants, thereby requiring a few years stored energy for refrigeration and/or grow lights? Would the seismic shock carried by the crust rather than the atmosphere also be a concern? $\endgroup$ – uhoh Sep 28 '16 at 13:54
  • $\begingroup$ Seismic shock might be a real concern if the habitats are located underground. I guess it depends on where the impact occurs. I guess plants would be grown under electric lights as sunlight can be obscured for a long time. A dust storm in 2007 lasted a few weeks and forced the Mars rovers to go into a low power survival mode since the sunlight was reduced. This might kill off your colony plants if they were totally dependent on sunlight. But since the radiation there will likely damage plants, i think farms could be located underground too, and run on electric lights and hydroponics. $\endgroup$ – Innovine Sep 28 '16 at 14:02
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    $\begingroup$ Much will come down to whether your colonists energy comes from solar power, or if they have some nuclear reactor or other power sources they can rely on for a while (years?). Given that they will be prepared for natural dust storms which last weeks or months, alternatives to solar power would be very nice. $\endgroup$ – Innovine Sep 28 '16 at 14:07
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    $\begingroup$ I'm thinking that it simply won't be dark long either - with 1% of the atmospheric pressure (and similar scale height) won't the dust settle out a lot quicker? Won't it be a "nuclear fortnight" instead of a few years of "nuclear winter"? Pardon the term-mangling. $\endgroup$ – uhoh Sep 28 '16 at 14:59
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    $\begingroup$ You might get an estimate for that by looking at how quickly the martain dust storms settle down again. I don't really know. The darkness leads to a big drop in temperature, which might encourage more storms, so it could sustain itself. Just guessing now.. :) $\endgroup$ – Innovine Sep 28 '16 at 15:08

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