Water vapour carrying capacity of the atmosphere is related mostly to pressure and temperature. (This is why Mars' atmosphere is so dry - because it has a miniscule pressure and a low average temperature). The dry valleys are low altitude precisely because there is no build up of ice there - in other parts of the ice cap, the elevation is 3-4 km and the air pressure substantially reduced.

At altitude, adiabatic cooling means that temperature is lower than on the ground. You can use this simple thermobarometer to connect temperature outside an airplane with altitude and ground temperature - I fly regularly between Australia and Malaysia where the ground temperature is between 10 and 30 C, but the air temperature at 10 km altitude is between -40 and -60 C i.e there is 6-7 C temperature drop per km.

The dry valleys are near sea level, compared to the 4 km altitude of the south pole, giving 26 C temperature difference - enough for a huge change in water bearing capacity.

This is what makes the dry valleys dry - cold dry air from the high plateau sweeps down into the valleys and WARMS UP through adiabatic compression, gaining 10 C or more. In doing so, even if it was saturated with water vapour before (and it probably was close, seeing as it snows up on the plateau), then it becomes highly undersaturated within the valleys and readily absorbs water vapour from ice, snow, and occasional trickles of meltwater.

IF YOU DON'T KNOW SOMETHING, DON'T RELY ON IT!

If you only lknow a little, don't keep talking about it, or you'll run out of what you know, before you even realise.

And don't ever try to lecture us about why you are right, when you are wrong and don't even know enough to know why you are wrong.

Earth is wetter than Mars. Period.

Equivalent parts of Earth are wetter than equivalent parts of Mars. Period.

Mars has more climatic extremes than Earth. Period.

It is true that the warmest bits of Mars are warmer than the coldest bits of Earth (and vice versa). It is also true that the driest bits of Earth are drier than the wettest parts of Mars.

But even the wettest parts of Mars are so G*dD@mn dry it would be a great place to make freeze-dried coffee in the open air.

Now instead of fantasising about what you LIKE Mars to be like, go learn about and THINK about what it is REALLY like.

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