I've often wondered how Mars lost it's atmosphere, and with it, most of it's moisture. Even though there may be significant water-ice frozen in permafrost near the poles, it's a puzzle that Mars started, like Earth, with significant water for lakes, sedimentary formations, river canyons, even glaciers--but now it's gone.
The best explanation--aha!--is that Mars, which is half the size of Earth--lost it's magnetic field when it's iron core cooled and hardened. It seems that the fluid dynamics of a molten core are what generates the Earth's magnetic field. This field shields us from cosmic rays and also the vast majority of the solar wind. It was the solar wind that stripped off the atmosphere and water vapor of Mars once it's smaller iron core cooled and hardened.
For a possible answer to your question of what became of the atmosphere and oceans of Mars, I recommend you read Graham Hancock's "Mars Mysteries". He puts forth a very intriguing theory. This is not Erich von Danikan stuff, but all too plausible.