Many sources indicate that there is ice water on Mars today, but it is mainly traces, here and there, but not that much.
However, solid hypothesis state that there were oceans on Mars, see Wikipedia's Mars ocean hypothesis or at least large aquifers on early Mars. They all concern early Mars so around 3.5 to 4 billions years ago. These theories suggest that Mars oceans could have formed Mesa landscapes (just likeGrand Canyon or Monument Valley).
So my question is simple, if all these hypotheses are true, where did that water go?
Is it possible that so much water could have been evacuated from the planet? I guess it would have needed A LOT of energy.
So it may have been transformed on the planet.
Is it possible that the water has oxidized iron, creating Iron oxide (ie rust) and that it is the main reason for Mars red color? The Mars ocean hypothesis page suggests some leads about the fate of that ocean, but none suggests the oxidation of iron consuming the entire oceans. Moreover, oxidation would imply the presence of water and dioxygen...