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Assuming that Starship needs to fully refuel on the Martian surface to return to Earth, that will require about 260 tons of methane->66 tons of hydrogen->600 tons of water, which you would need to gather in 800 days. This raises the question of how much equipment (mass, power, cost, volume) will be required, because it seems like a substantial task to gather and store that much water.

Relevant: Landing sites being considered by SpaceX. Masten Aerospace has a design for a moon rover that can theoretically mine 420 tons of ice, but that's Moon not Mars. Some notes on Mars water mining in here. Previous question on mining water on Mars.

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  • $\begingroup$ My math could be wrong, but 600 tons of water would require a container something like 19,000 cubic feet to store. That's a cube 26-27 ft to a side, or many, many smaller containers. $\endgroup$
    – BMF
    Dec 16, 2021 at 15:24
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    $\begingroup$ Having watched the Masten Aerospace video showing their proposed ice/water mining method for the Moon, that method could be used at some locations on Mars. Obtaining the water from each each "flamed" pit might be efficiently done but I don't like the technique because it does not efficiently mine the entire resource. By proposing a "flame pit" method, the parts of the resource between each pit are not being extracted by the technique. The technique would produce a honey combed surface. How does one mine the remnant material between pits? That material still contains valuable water. ... $\endgroup$
    – Fred
    Dec 16, 2021 at 16:49
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    $\begingroup$ ... If the remnant material were to be mined using some form of mechanical digging, having to deal with the "flamed pit" voids could be a problem. I'd like to see a more efficient way of extracting as much of the resource as possible. $\endgroup$
    – Fred
    Dec 16, 2021 at 16:51
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    $\begingroup$ As to how much equipment would be needed to mine 600 t of water ice, that would depend on mining method & the equipment that would be used for that method. It would also depend on the time frame required to mine the ice. Longer time frames would required less equipment than short time frames. It's a question of throughput, how may tonnes per hour needs to be mined will dictate how much of particular mining & processing technology would be required. $\endgroup$
    – Fred
    Dec 16, 2021 at 17:07
  • $\begingroup$ Too many variables. How much mass is being returned to Earth? Are they going to use a Martian orbital tanker? What is the nature of the matrix in which the water is is embedded in / combined with at the landing site? How deeply buried is the water ice layer? What is the concentration of water ice in the matrix of other materials? How is the water to be extracted? How much power is available? Will any attempt be made to gather water robotically ahead of schedule? But whatever the answer to these questions it is unlikely that they would mine all the water first. It will be processed as they go. $\endgroup$
    – Slarty
    Dec 19, 2021 at 14:03

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Unknown

This is very, very mission dependent. Where does it land? How long do they stay? What is the mission hoping to accomplish, beyond producing enough fuel to return?

Add in all the other unknowns (How much water is locked in the Martian soil? How deep? Does that change over time? How?) and this question is very hard to answer.

But, Perhaps Less Equipment Than You'd Think

Hohmann transfer windows suggest a Mars mission will be ~18 months on the ground. Let's call that 500 Martian days. If they can mine 1.5 tons of water a day, they'll easily have >600 tons at the end of the mission.

The Korolev Crater has 530 cubic miles of surface ice - if they landed there, a team of 10 or so could probably mine 1.5+ tons of ice a day with shovels.

I... don't know that I'd sign up for that mission.

Pre-Mining Might be Better

If instead you landed one or more rovers in advance, you could feasibly have collected all or most of the water prior to the main mission touching down. This seems a much more reasonable answer to me, but implies we won't be seeing a manned Martian mission any time soon, since they'd have to launch the rover mission at least one Hohmann transfer window earlier than the manned flight, and Hohmann windows only come every two years.

EDIT: Let's Add Some Numbers

If our rovers arrive one Hohmann window ahead of the the manned mission, they'd have ~1,200 days to harvest material - they'd need to produce 0.5 tons water a day on average.

From there, we have to make many assumptions. Let's assert that Martian soil near the surface is 1% water by mass. Our system needs to extract the soil, separate out the water, and dispose of the slag. For 0.5 tons a day of water, we need to process 50 tons of soil.

If a rover can dig up and carry a "load" of 200 pounds of soil, and each rover is capable of 10 loads per day, one rover collects one ton per day. We need 50 rovers to process 50 tons of soil per day.

Recent Martian rovers have weighed around 1 ton. Since these new rovers will be performing more "industrialized" work, let's double the weight. This implies 100 tons of rover, plus whatever we need to separate the water from the soil, and whatever we use to store the water.

Obviously, these end numbers are very sensitive - if we assume that the soil is 10% water by mass, that changes the outcome by an order of magnitude - only 5 rovers are needed for a total of 10 tons. Similarly, if the rovers are more mobile, or carry more weight per trip, then the total weight scales linearly with those changes.

So the answer depends heavily on information we just don't have.

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