I have to wonder if the recently discovered ice deposits on mars would do the job

https://www.nasa.gov/feature/jpl/mars-ice-deposit-holds-as-m...

If this really is just a metre down and 80% water it's got to be an attractive target - relatively easy to mine (once surface is cleared, just use heat) and a hollowed out underground structure with 1m of ice above would be an effective radiation shield:

http://space.stackexchange.com/a/1826

... and once you're there, water is water, plus water can be used to make fuel.