... MarLy 1-metre telescope . ... About four billion years ago, the young planet would have had enough water to cover its entire surface in a liquid layer about 140 metres deep, but it is more likely that the liquid would have pooled to ... hemisphere, and in some regions reaching depths greater than 1.6 kilometres. тАЬ Our study provides a solid estimate of how much water Mars once had, by determining how much water was lost to ... Mars: the planet that lost an ocean's worth of water . ...