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A few days ago, I was roaming around Wikipedia and came across something, this comet came extremely close to Mars for astronomical standards recently.

I'm also working on a science fiction universe and was wondering what would happen if this comet collided with Mars. From Wikipedia, it would have released 24 billion megatons of TNT.

Would this be enough to release the carbon dioxide on Mars and start a runaway greenhouse effect? Also, how would this comet be tweaked to collide with Mars, if even possible?

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  • $\begingroup$ I'm interested in this but don't feel like doing the math. So I'm hoping someone else will answer :) $\endgroup$
    – Jim2B
    Apr 15, 2016 at 19:13
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    $\begingroup$ I'm no scientist, but I understand that while it briefly will do things to Mars, because it has no magnetosphere, it will simply not be able to hold an atmosphere. I hope/wish someone will disprove me. $\endgroup$
    – Mikey
    Apr 15, 2016 at 20:08
  • $\begingroup$ Without a magnetosphere, it would disappear on timescales longer than human civilization has existed, and hopefully, an artificial magnetosphere can also be constructed. $\endgroup$
    – GaiusTheRoman
    Apr 15, 2016 at 20:16
  • $\begingroup$ That's what my calculations show. Although Mars won't retain an atmosphere over geologic timescales, over human timescales people won't notice the change. $\endgroup$
    – Jim2B
    Apr 15, 2016 at 21:09
  • $\begingroup$ Did you want a scientific answer to this question? The astronomy SE might be a better place to ask this question. $\endgroup$
    – Marion
    Apr 16, 2016 at 0:50

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A single massive impact will probably cause more problems than it solves. The huge release of energy is going to blow a lot of the water and gasses from the comet right back into space, and there is a good chance that the current atmosphere and whatever water is trapped under the surface will also be depleted rather than enhanced.

If you are going to use impacts to deliberately terraform Mars or any other small planet, it would be worthwhile to do some preliminary work on the comet before impact.

Rather than a single impact with a unitary body, the comet should be divided into many smaller bodies so individual impacts can be spread over a larger area, and the energy of each individual impact can be lowered. With good planning, the various parts of the comet should be separated and manoeuvred with solar sails. The other consideration should be to ensure the impact is slow enough that the atmosphere isn't blown off by energy being imparted by the comets. The calculations are a bit beyond me (and will depend on the size of the planet in question anyway).

By spreading the impacts over time and space, the terraforming process will be under the control of the terraformers and they can ensure the process happens where and when they want.

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  • $\begingroup$ Not to mention that you can "aim for effect" which might mean targeting the polar regions. This way the energy released from the impact could also be used to vaporize some of the greenhouse gases there. $\endgroup$
    – Jim2B
    Apr 16, 2016 at 5:05
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I believe that in "The Mars Trilogy" (Robinson), part of the terraforming process was to capture methane ice out of the Saturnian rings and slam them into the Martian atmosphere. In whichever of those novels that was described, it was many, many of them, and it happened over a period of years. Robinson, being a scientist by day and a sci-fi writer at night (at least then) probably would have worked out the real world details of such an effort.

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Greenhouse effect - not remotely.

The volume of the comet is about 1.4 x $10^9$ cubic meters. The area of Mars is about 1.45 x $10^6$ $km^2$, so each square kilometer will receive about 1000 cubic meters of comet.

Comets are primarily water and rock. Let's be optimistic and go for 1% CO2. Solid CO2 has a density of about 1600 kg/$m^3$, so the total CO2 load will be about 0.16 grams of CO2 per square meter of surface.

By comparison, a square meter on earth has an atmospheric column about 8.5 km high if converted to sea level density, with a density of 1.3 g/$m^3$ or about 11 kg equivalent. At 400 ppmv, that's about 5 grams/square meter.

In other words, whacking Mars with this comet might introduce a carbon dioxide layer totaling about 1/30th that of earth. Even if you make the unwarranted assumption that the comet is 10% CO2, the result is 1/3 that of earth.

So, no runaway greenhouse.

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  • $\begingroup$ Comets have more than CO2, a layer of organics, methane, ammonia, CO and of course H20. Which all would contribute to the greenhouse effect, not to mention the tremendous heat from the impact melting what's already there. (Unless as already described it's too much heat and you end up losing volatiles instead.) $\endgroup$
    – Brooks Nelson
    Aug 21, 2017 at 12:59

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