The Past: Mars has water, and it used to have a lot more.  If modern Mars had the ocean it once had, it would evaporate off into space quickly because there is no heavy atmosphere to help keep it pressurized and in liquid form.  Mars would have had a thicker atmosphere in addition to it’s magnetic field in order to keep all that water in one place.  So where did the atmosphere go? And if there was such a thick atmosphere, how does it account for the fingerprint of excess Carbon-13 and a lack of Carbon-12 found on the red planet...