Water on Mars?

We've seen a hint that Mars may have water:

Polar ice caps. (A mixture of CO₂ ice and H₂O ice.)
Spacecraft imaging: Flow channels indicative of ancient water flows.
Recent Mars Orbiter Camera images: possible recent flow channels.
We also know the Earth has water. Why wouldn't Mars?

Questions:

Where is it now?
How could it ever have existed?
Was it in long-lived bodies of water, or transient flows? How could you tell?

1. Where is it now?

Outflow channels suggest the presence of large amounts of water
Accretion theory for planets also would predict lots of water.
Polar caps hold a very small fraction of this inferred total of water (<1%).

Atmospheric losses: D/H ratios
Think about deuterium (D): ²H (one proton + one neutron [+ one electron]). So water molecules can come in two types:

regular (H₂O), and
heavy (HDO).

In the upper atmosphere, water vapor can be dissociated and the hydrogen lost to space. Which type is most easily lost? So what happens to D/H over time?

On Mars, D/H ~ 8x (D/H)_Earth -- Mars lost water through atmosphere!

How much? D/H is a measure of fractional loss. We also have a measure of how much water vapor is currently in Mars' atmosphere (it's small). These two combined can tell us the total amount of water lost through atmospheric processes, and it's much less than the total amount needed to explain the channel flows.

Where's the rest?

2. How could it ever have existed?

Mars has a temperature of 200-250 K.
Mars has an atmospheric pressure of ~ 5 millibars.

Look at an atmospheric phase diagram for H₂O and CO₂:

Conditions aren't right for liquid water!
What does this mean?

3. Long-lived bodies of water or transient flows? How could you tell?

2004/5: Mars rovers find nodules of jarosite and hematite in Mars rocks -- minerals that form over time in the presence of water.

Jarosite nodules (greenish-blue)

Hematite nodules