Previous spacecraft investigations have pointed to the possible presence of an ancient ocean, with supporters for the idea that there is still a substantial amount of water under the surface as liquid or ice. Climate change over millions of years might have led to the disappearance of the atmosphere, which would mean that any water on the surface would boil away.
Volcanic activity is among other explanations for apparent gullies, river valleys, flood plains, lakes, seas and other signs of water that have vanished.
Gaetano Di Achille and Bryan Hynek, of the University of Colorado, led a team that analysed the distribution of supposed ancient delta deposits and river-valley networks on Mars. They found many of the deltas were at a similar elevation and suggested these might ring an ancient shoreline, providing strong support for a vast ocean once covering the northern plains of the planet.
If there was that much liquid water, there had to be life. When will we start finding real Martians?