New research suggests a vast ocean covered a third of the surface of Mars 3.5billion years ago.
It stretched around the planet’s northern hemisphere and held ten times more water than all of the Earth’s oceans combined, scientists believe.
It may also have provided a cradle for the emergence of extraterrestrial life.
Scientists have long debated whether oceans, seas, lakes or rivers ever existed on the Red Planet. Satellite images reveal features on the planet resembling river valleys and flood plains.
However, they may have been created by volcanic activity rather than flowing water.
The latest findings are based on a study of what appears to be ancient river delta deposits and valley networks. Researchers identified 52 delta regions fed by many river-like systems.
All lay at about the same height, suggesting they marked the shore of a huge river-fed ocean.
Dr Gaetano Di Achille and Prof Brian Hynek, from the University of Colorado, believe the ocean covered about 36 per cent of the planet and contained 125million cubic km of water.
‘Our findings lend credence to the hypothesis that an ocean formed on early Mars as part of a global and active hydrosphere,’ they wrote in the journal Nature Geoscience.
Martian delta systems are likely to be key targets for future exploration.
But a key unanswered question remains: Where did all the water on Mars go?
Some scientists think much of it may remain in frozen deposits underground.