Mars: the planet that lost an ocean's worth of water

This artist's impression shows how the planet Mars lost most of its surface water over the past four billion years. 

The young planet Mars 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 form an ocean occupying almost half of Mars’s northern hemisphere, and in some regions reaching depths greater than 1.6 kilometres.

The water remaining on the surface of Mars is now frozen in the polar caps.