Okay, not with the usual smaller 10 km size comets. If you could deflect giant 200 km comets, say from the Kuiper belt, or some of the smaller icy moons from Jupiter or Saturn, you could do it.
The thing is, that Mars is very dry and probably has a depleted hydrosphere also, so any water you add will accumulate at the poles as ice.
If it gets to the equatorial regions and is still liquid it will sink kilometers into the desert sand. Like pouring water into the Sahara desert, but drier to much more depth than the Sahara.
There is enough water theoretically at the poles and in reserves already to cover it to some depth in water, but that's not taking account of the dry sands in the equator which would take up all that water.
Anyway no matter how much water you add, Mars is so cold, because it is further from the sun, that it would turn into ice, whatever was left on the surface.
When it had liquid oceans long ago, first it had a lot more water, not just a few cubic kilometers. It had twenty million cubic kilometers of water in its ocean. Nasa finds evidence of a vast ancient ocean on Mars
To recreate that, you'd need around 400,000 comets of diameter 10 km. Or you could hit it with 170 km diameter comet you could do it in one go.
You would need more water than that though, because Mars in the equatorial region is though to be now dry to a depth of 100 meters or more, (see also Page on si.edu) though with some relic areas of ice sheets somehow preserved perhaps because covered with a less porous layer.
So you'd have to fill that dry layer with water too. I'm not sure how to estimate how much water you'd need for that.
Mars currently has a global equivalent layer of ice of around 30 meters. Originally had several hundred to a thousand. See Page on utinam.cnrs.fr
So to recreate ancient Mars you'd need at least 100 meters of water depth over the entire surface of Mars.
With a surface area of 144.8 million square kilometers, (Mars) you would need 14.5 million cubic kilometers for each 100 meters of of the original global equivalent layer of water or ice. (If it was originally the upper estimate of 1 km, then you'd need 145 million cubic kilometers of water to recreate it)
To recreate the ancient Mars though, you'd also need a much thicker atmosphere - CO2 and nitrogen, lots of it. Which is now gone, possibly it's locked up in nitrates, if so maybe it can be liberated somehow.
At any rate, though it's fun to think about this theoretically - perhaps not quite ready to try something like this yet :).
Lots to go wrong, see my
Trouble With Terraforming Mars
It's probably far easier to start building large space habitats, and if anything goes wrong, you can scrub the atmosphere, replace the soil, import water, or even start again from scratch. With a kilometer scale habitat we can do things like that in hours (for scrubbing atmosphere) to years, which would take centuries with a planet. And can get to the final results, a working fully functional biosphere with breathable atmosphere and generating all the food needed by humans, on the timescale of decades, while with a planet, it would take thousands of years to get that far, if it works at all.
And if you can deflect a 200 km scale comet from the outer solar system to hit Mars, why not instead bring it into the inner solar system, and put it into orbit around the Earth as a vast store of water for propulsion and creating habitats closer to Earth?
It doesn't happen naturally as there are no craters this big anywhere on Mars. The last time something this big hit Mars was over 3 billion years ago.
See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths
The thing is, that Mars is very dry and probably has a depleted hydrosphere also, so any water you add will accumulate at the poles as ice.
If it gets to the equatorial regions and is still liquid it will sink kilometers into the desert sand. Like pouring water into the Sahara desert, but drier to much more depth than the Sahara.
There is enough water theoretically at the poles and in reserves already to cover it to some depth in water, but that's not taking account of the dry sands in the equator which would take up all that water.
Anyway no matter how much water you add, Mars is so cold, because it is further from the sun, that it would turn into ice, whatever was left on the surface.
When it had liquid oceans long ago, first it had a lot more water, not just a few cubic kilometers. It had twenty million cubic kilometers of water in its ocean. Nasa finds evidence of a vast ancient ocean on Mars
To recreate that, you'd need around 400,000 comets of diameter 10 km. Or you could hit it with 170 km diameter comet you could do it in one go.
You would need more water than that though, because Mars in the equatorial region is though to be now dry to a depth of 100 meters or more, (see also Page on si.edu) though with some relic areas of ice sheets somehow preserved perhaps because covered with a less porous layer.
So you'd have to fill that dry layer with water too. I'm not sure how to estimate how much water you'd need for that.
Mars currently has a global equivalent layer of ice of around 30 meters. Originally had several hundred to a thousand. See Page on utinam.cnrs.fr
So to recreate ancient Mars you'd need at least 100 meters of water depth over the entire surface of Mars.
With a surface area of 144.8 million square kilometers, (Mars) you would need 14.5 million cubic kilometers for each 100 meters of of the original global equivalent layer of water or ice. (If it was originally the upper estimate of 1 km, then you'd need 145 million cubic kilometers of water to recreate it)
To recreate the ancient Mars though, you'd also need a much thicker atmosphere - CO2 and nitrogen, lots of it. Which is now gone, possibly it's locked up in nitrates, if so maybe it can be liberated somehow.
At any rate, though it's fun to think about this theoretically - perhaps not quite ready to try something like this yet :).
Lots to go wrong, see my
Trouble With Terraforming Mars
It's probably far easier to start building large space habitats, and if anything goes wrong, you can scrub the atmosphere, replace the soil, import water, or even start again from scratch. With a kilometer scale habitat we can do things like that in hours (for scrubbing atmosphere) to years, which would take centuries with a planet. And can get to the final results, a working fully functional biosphere with breathable atmosphere and generating all the food needed by humans, on the timescale of decades, while with a planet, it would take thousands of years to get that far, if it works at all.
And if you can deflect a 200 km scale comet from the outer solar system to hit Mars, why not instead bring it into the inner solar system, and put it into orbit around the Earth as a vast store of water for propulsion and creating habitats closer to Earth?
It doesn't happen naturally as there are no craters this big anywhere on Mars. The last time something this big hit Mars was over 3 billion years ago.
See Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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