More than three billion years ago, the northern lowlands of Mars were covered by a vast ocean, suggests a study online this week in Nature Geoscience. Although previous spacecraft investigations have pointed to the possible presence of an ancient ocean on Mars, this past evidence has been uncertain.Gaetano Di Achille and Bryan Hynek analysed the distribution of ancient delta deposits and river-valley networks on Mars. Their investigations indicate that many of the deltas sit at a similar elevation. The authors suggest that these deltas ring an ancient shoreline, providing strong support for a vast and ancient ocean in the northern plains of Mars.

The elevation of the newly proposed shoreline is also consistent with the outlets of ancient river-valley networks, indicative of a globally uniform water level on ancient Mars.