Mars current atmosphere is 0.006 bar. It is theorized that the atmosphere was thicker in the past possibly thicker than Earths 1 bar atmosphere. There are many processes that could have done this such as having no magnetic field, lack of geological activity, a planetoid crashing into it and escape velocity of lighter molecules. According to the link below Mars can hold CO2, oxygen and nitrogen now but water vapor will escape and this is at 210 K. http://astro.unl.edu/naap/atmosphere/animations/gasRetentionPlot.html Mars early in its life was likely much warmer which would have caused gases to escape much easier, this coupled with the lack of geological activity would have made things even worse for Mars. There is also the heat from planetoid and asteroid impacts that would cause the temperatures to soar hundred of degrees not just from the initial impact but from the reentry of materials. As you see on the link if you increase the temperature the gases will escape including CO2. Therefore its my believe that the triple effect from lack of a magnetic field, lack of geological activity and the escape velocity from both its warmer period and the heat from its many asteroid collisions caused the gases to escape. Would this make any sense?