We now believe that there was liquid water on the surface of Mars for long periods of its history. It is assumed that there would have to be some 'greenhouse gas' in the atmosphere, to help retail heat, as Mars would other wise have been too cold for liquid water.

It was widely supposed that this gas would be carbon dioxide. Evidence of sunsantial amounts of such gas in Mars' past was looked for, in the form of carbonate mineral deposits. These were not found.

The question now is, what made Mars warm enough for liquid water at its surface? The other candidate greenhouse gasses, such as methane and sulfur dioxide, have a problem. They are unstable over a relatively short period of time. To warm Mars, they would have to be replenished frequently.

This might have been accomplished by volcanic activity , but it's an open question if there was enough of this in Mars' history, on a regular basis, to do the job.

Another possibility is that Mars' environment was modified by the presence of certain methane generating bacteria. This could have provided a steady, self-renewing supply of methane. To this day, we see small bursts of methane gas released on Mars, which could be evidence of remnant microbial life.

Link to article on this topic, below:

http://www.space.com/35595-ancient-mars-atmosphere-liquid-water-mystery.html