[Fozzie (OP)]

There is a famous picture of a lake of ice on Mars which can be seen at http://weirdsciences.net/2010/08/07/analysis-of-evidence-of-life-on-mars/mars-ice

Rather than simply sublimating, it looks like a large body of water has flowed out from somewhere, formed a lake and then frozen, however, water in it's liquid state cannot exist on mars due to the low air pressure. So my qustion is - how has this formed?

[Soul Surfer]

It is considered that these events happened a very long time ago in a period  when the atmosphere was thicker than it is now allowing liquid water to be stable  this could have been associated with the early life of the planet or briefly after a major asteroid impact or volcanic eruption.   Because Mars has no magnetosphere to protect its atmosphere from the erosive effects of the solar wind.  the atmosphere on mars requires continual replenishment to avoid being reduced to something like the level it is today.