SyFy explores where the water and and possible Martians have gone.

We know the current D/H ratio on Mars by looking at water in its atmosphere, and we can also see what it was like billions of years ago by looking at Martian meteorites, chucks of rock from Mars that fell to Earth after a large asteroid impact sent them into space. Sure enough, the meteorites show a lower ratio, meaning Mars used to have more water.

However, combining that information with how water escapes from Mars to space, that implies the amount of water on Mars long ago was around 50 – 240 meters depth if spread out over the whole planet, which is at the very lowest part of the range of 100–1,500 meters depth the geological evidence indicates. This in turn implies a lot of that ancient water is missing.

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