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Set in the 23rd Century, space tourism has matured. The scientists gathered at a summit to discuss plans for terraforming Mars, therefore the Mars colonists living inside a temporary biosphere will have to be evacuated. The plan is called "Project Noah" using unmanned robots to capture comets and hurl/divert them towards Mars, the idea is since comets contain lots of frozen water and other gases we can burn/melt them during entry and extra atmosphere to improve greenhouse effect for precipitation. My question is, assuming by then we have the technology, can this plan work to bring rivers and seas to Mars?

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instant habitable planet. Just add water.

Well, that would be nice, wouldn't it?
One of the bigger problems with terraforming Mars is it's lack of magnetic field. Our magnetic field helps greatly to prevent solar winds from stripping away the atmosphere.

As o.m. already pointed out, you need an atmosphere to get rain, and you need rain to keep rivers flowing and lakes from drying out. Of course, having an atmosphere, especially one that is breathable for humans, also helps with colonizing and with your tourism project.

So, in addition to adding water you would need to provide a magnetic field, too. (And beaches, and cocktails.)

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  • $\begingroup$ A previous similar question on SE:Worldbuilding, mentioned that it was possible to restart the magnetic field by hurling comets at the planet restarting the core, how is your thoughts on that ? $\endgroup$ – Magic-Mouse Jul 1 '15 at 8:01
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    $\begingroup$ @Magic-Mouse To generate a magnetic field the planet requires a liquid iron core because of the moving charges due to convection current, unfortunately Mars already cooled. I think if you bombard the planet hard and often and uniform enough the resulting quakes will scatter throughout the interior of the core as friction and friction is heat... and it lasts so long you don't stop hurling comet punches! $\endgroup$ – user6760 Jul 1 '15 at 8:08
  • $\begingroup$ @Magic-Mouse i think i remember the question. From what (very little) i know, it could be possible to install a moon, to help generating an em-field, which in turn might help liquefying parts of the core, and subsequently generate an earth-like dynamo. But that is no more than an educated guess. $\endgroup$ – Burki Jul 1 '15 at 8:18
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    $\begingroup$ The effects of magnetic fields on atmosphere retention is coming under some critism: space.com/11187-earth-magnetic-field-solar-wind.html it may not be as dramatic as once thought. $\endgroup$ – Innovine Oct 9 '16 at 7:25
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    $\begingroup$ Even if you do not have magnetic field, atmospheric loss is negligible in human terms. An asteroid every now and then will be able to keep up with the loss. $\endgroup$ – Cem Kalyoncu Oct 9 '16 at 10:26
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I think the answer is yes, but ...

  • You stipulated that we have the technology. But what scale would be required to affect a planet? How many thousand robotic comet-catchers?
  • Open water (or for that matter rain) requires a reasonably dense atmosphere. Will the atmosphere remain considering the low gravity and the lack of biological processes? Over a geological timeframe, the atmosphere would either escape to space or go into the ground.
  • When you crash enough large comets into Mars, will there be a stable surface where rivers can flow? How much energy are you adding to the planet?
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    $\begingroup$ This has more questions than answer! $\endgroup$ – Antony D'Andrea Jul 1 '15 at 7:08
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    $\begingroup$ I wrote "yes, but" in the first line. Generally speaking, yes, but the devil is in the details. $\endgroup$ – o.m. Jul 1 '15 at 15:18
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The biggest issue with simply diverting comets to Mars is the impact energy is as likely to blow away as much water and atmosphere as is delivered. People on Mars are not going to appreciate being told to hunker down in underground earthquake proof shelters for decades or centuries only to discover they are back to square 1.

For this to work properly, the comet engineers will have to do a lot more work; calculating the delta-V needed to get the comet to Mars, and then doing some elaborate orbital manoeuvres to bleed off all the energy and do a relatively "soft" landing. Coming in from Martian orbital velocity will still be fairly energetic, but far less than the potentially 72Km/s possible for a body in Solar orbit. (any faster and it will fly off into interstellar space).

To save on mass and energy, the best way to steer comets in on a controlled but not too energetic trajectory would be to attach them to huge solar sails, and spiral inwards from deep space, using the sails to shed velocity once you are close to Mars.

Once landed on Mars, the extra water vapour in the atmosphere will help both raise the atmospheric pressure and act as (the) most powerful greenhouse gas, raising the temperature to something closer to what you would need for the rest of the terraforming effort. As noted, the lack of a magnetic field will cause lots of long-term difficulties, and some almost Clarketech engineering would be needed to melt the core and restart the magnetic field. As well, an atmosphere of essentially steam is not going to be very useful, so waves of algae or other engineered organisms will be needed to start creating a breathable atmosphere right away. Do this right and real estate speculators will have a field day early in the 33rd century!

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    $\begingroup$ Don't crash large comets into the planet. Instead throw snowballs which aerobrake, and either vaporize in the atmosphere, or hit the ground quite gently. $\endgroup$ – jamesqf Jul 1 '15 at 21:34
  • $\begingroup$ The issue is the kinetic energy. Even a very small comet moving at interplanetary velocity will simply punch through the atmosphere before melting or vaporizing. You have to physically slow the thing down to Martian orbital velocity (or less, if possible) to minimize the effect of stripping off the atmosphere and getting the effect you want. $\endgroup$ – Thucydides Jul 1 '15 at 23:10
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    $\begingroup$ Yes, that's why I said to throw snowballs. Something that size/density will aerobrake in the Martian atmosphere, either vaporizing or hitting the ground at low speeds. Heat from kinetic energy would warm the atmosphere some, but that has to be done anyway (through orbital mirrors and greenhouse effect) to make it habitable. $\endgroup$ – jamesqf Jul 2 '15 at 21:13
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Mars is not very dense. While it's half the diameter of the earth, it's only 1/3rd of the gravity. and so you'd need three times as much gas in the atmosphere to have the same pressure (umm, my figures are probably off, this is just speculation) which is impossible given the lack of magnetic field.

In order to convert mars into prime real-estate, you'll have to do more than hurl a few comets. You're going to have to INJECT the core with massive amounts of material. Lots of Iron. You're essentially increasing the gravitational strain on the core and enduing radio-active decay to liquefy the core and then you'd be able to build the atmosphere you want.

Luckily there just so happens to be a large mass of astroids nearby . . ..

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The Rosetta mission struggled to show a trace of watery ice on the comet, and the composition is still under a lot of debate. Suffice to say that the 'dirty iceball' theory is under a lot of attack. It's entirely possible comets don't have water in sizable amounts.

Kim Stanley Robinson, in his trilogy on Mars, the final book of which was called BLue Mars, explored several different techniques for terraforming, among them aerobraking comets into the atmosphere. However, the primary water source came from detonating nuclear devices deep within the crust to melt the frozen aquafirs and polar ice caps. Moholes were drilled (essentially just giant deep holes) to release internal warmth into the atmosphere. Greenhouse gasses and engineered bacteria and lichens were also produced and released

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  • $\begingroup$ I've heard icey dirtball is now considered more accurate a description for comets. $\endgroup$ – Brooks Nelson Sep 23 '17 at 14:06

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