It has been brought up here a few timeshttps://forum.nasaspaceflight.com/index.php?topic=33716.msg1145185#msg1145185https://en.wikipedia.org/wiki/WAVAR(google also returned this link, not sure what message it was attached to)http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=33716.0;attach=567424I think the conclusion here was that it was doable. The energy requirement is high but only a fraction of the energy to turn the water and CO2 into propellant.. I think the reason discussion moved away from it is that extraction from the ground is looking very promising. (on that topic, I noticed the Insight mission plans to dig 5 meters into the ground with what looks to me like a pretty small device. This makes something like a Rodriguez well sound pretty straight forward to me.. in the right location http://www.gdargaud.net/Antarctica/Water.html#Well )