Like Greg - I'll say up front, I hope I've not screwed up any decimal places here.....
1kg of water per m^3 of soil.
They said figures like 1mm, 2mm, a few mm. I'll go with 2mm.
Thus - 1000 x 1000 x .002 m (i.e. farming 1 sq km) - is 2,000 kg of water.
Looking at something like the Mars Direct ISRO numbers - taking 8T of H2 and working with in-situ CO2 to make methane and O2. - 8T of H2 as water would be a further 64T of O2 - for a total of 72T of equivalent water, as it were.
That's 36 sqkm of farming, or, from a landing site - every scrap of surface regolith to a radius of 3.4km
Alternatively - taken an MSL sized rover - with some sort of soil harvesting combine harvester style rig on front - shall we say 3 x 1m wide grabbers ( like a big gang-lawn mower).
It would have to travel a total of 12,000km of 3m wide stripes to cover 36 sqkm. Quite by chance - that's about 1000km further than a circumnavigation of the whole moon.
At a brisk rover of, say 2.5m/sec (just over 5mph) - operating a 50% duty cycle for the day/night cycle - 110 days. But of course, you can't have a rover that just end up dragging a 70 ton sack - it'll have to get it bit, return it to be stored, go get some more, return it. Say you farm a 6km square, and can get one 3m x 6,000m stripe in one 'store'. It would be an average drive out of 3km, an average drive back from the end of 6.7km. Plus farming of 6km. 2000 times - 31,400km. Basically - a year of driving.
Harvesting at that higher figure, though, of 2.5m/sec - perhaps taking the top cm of soil (can't imagine how you'd take the top 2mm) - and a regolith density of 2.9g/cm^3 - that, amazingly, is 4.5 CuM or 13 tons of regolith per minute, producing (as only 1/5th of our 1cm harvest is 1% water) basically, 1 litre per minute.
I have no idea what sort of energy will be involved in getting that water out. Latent heat of vaporization is 2257 kJ/kg - 37 kWatts of energy required. (330x the RTG power of MSL, or a solar array about 9x9m at 30% effic)
As a comparison - to get 72,000kg of water up at Phoenix's landing site - taking, say, 30cm trough, at 500kg / m^3, at 3m wide, is only 160km of trough - 75x less than the lunar combine harvester.
QUOTE (Phil Stooke @ Sep 24 2009, 08:43 PM)
I'm not convinced we could use the water that is being described at low latitudes.
Nor me. Interesting - but not a resource.
I want my solar powered MSL sized water farming soil munching robot