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"To have been awash with water, Mars had to have two things, the needed temperature range and sufficient atmosphere."

And the problem with sufficient atmosphere is that Mars is not large enough to have enough gravity to hold a respectable atmosphere in place. Which, to me, points to the idea that Mars was once larger than it is now. That would seem rather hard to explain, but here's my science-fiction crack at it.

There was an intelligent civilization on Mars long ago. Like Earth, it had a Nitrogen-Oxygen atmosphere, but in a different ratio and with other elements as well. They reached a level of science approaching ours, including the technology to initiate a nuclear fission reaction.

Those familiar with the development of nuclear technology may be aware that the guys who initially began a reaction at Trinity had some major cojones. See, there was a sizable number of scientists at the time who believed that a nuclear reaction in our atmosphere would be self-sustaining, and that it would use our atmosphere as fuel. Thank God it didn't happen... but what if that was only due to the particular makeup and percentages of the elements of our atmosphere? What if a different ratio of nitrogen to oxygen -would- trigger such a chain reaction, or a third element could provide a catalyst to keep the reaction going?

So. They set off a nuke. It began a chain reaction in their atmosphere. The primary element, nitrogen, fused. A whole -hell- of a lot of the mass of the planet was blown off in the reaction, accounting for it's smaller size and inability to hold an atmosphere now.

Additional: do you know what you get when you fuse two nitrogen atoms together? You get silicon. That would be... sand. Mars has a whole lot of sand.

Basically, what I'm getting at is that if the scientists pre-Trinity had been right about a nuclear reaction in the atmosphere setting off a chain reaction that would fuse the nitrogen in our atmosphere, Earth would look very very similar to Mars a million years from now.

It's a theory I play with when I think about writing my Great American Science Fiction Novel :)

Qwinn

The ancient predecessors of John Carter, Warlord of Mars, teleported back here with all the water.

...

Actually, the Oort cloud contains massive amounts of cometary material, and I'd suspect the odds are not great that we actually have a reliable notion of what ratios of heavy water they contain, since we haven't sampled much, and their sources seem likely to have been diverse. If there's someplace in the solar system where water is in abundance, then odds are, that's where our water came from. I'm betting on the steady accrection theory--the water's here because it is steadily (or unsteadily) renewed from cometary material.

"And the problem with sufficient atmosphere is that Mars is not large enough to have enough gravity to hold a respectable atmosphere in place. Which, to me, points to the idea that Mars was once larger than it is now."

I submit another possibility. Mars' core was once liquid, which would mean that Mars at once had a magnetic field. Such a magnetic field would keep the solar wind from abrading away the atmosphere. Look at sketches of Earth's magnetic field in relation to the solar wind, and such sketches show a bow shock wave. The charged particles of the solar wind are deflected by the magnetic field. Now granted the solar wind is not very dense, but going on the order of a million miles per hour it does things like throw space probe trajectories off a bit (measureable) and over time (geologic scales mind you) can have some significant impact.
When the iron core of Mars solidified, the field was lost, no bow schock to shield the atmosphere.