Scientists have long debated whether or not Mars may have once had oceans. In particular, one of the most obvious surface features on Mars is a large plain surrounding the north pole that resembles a sediment-filled ocean basin with shoreline-like features.
However, the observation that the purported shoreline isn't level has been used as an argument against the presence of an ocean. A recent study by NASA scientists shows that the undulations can be explained by movement of Mars' spin axis, and thus its poles,
and that a liquid water ocean could indeed have existed there. As for the atmosphere, there is a discussion of the amount of carbon dioxide in the Mars atmosphere at http://www.nasa.gov/mission_pages/MRO/news/mro20110421.html