Many of the extensively surveyed geological features of Mars suggest that the planet once had an extensive liquid water ocean.
The ocean has been named as Paleo-Ocean and Oceanus Boreali , and some researchers suggest that it could have covered up to 30% of the planet's surface. The concept is not as yet universally accepted.
The existence of a primordial Martian ocean remains controversial among scientists. The Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) has discovered large boulders on the site of the ancient seabed, which should contain only fine sediment. However, the boulders could have been dropped by icebergs, a process common on Earth. The interpretations of some features as ancient shorelines has been challenged.
Alternate theories for the creation of surface gullies and channels include wind erosion, liquid carbon dioxide, and liquid methane.
Confirmation or refutation of the Mars ocean hypothesis awaits additional observational evidence from future Mars missions.
Source : Wikipedia
If there was an ocean, there is an additional problem regarding the fate of the 6 x 107 km3 of water which it is estimated to have contained. Some research teams suggest that most of it now remains underground, or that it evaporated into the atmosphere and was subsequently lost into space.
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