A vast ocean covered the northern lowlands of Mars some 3.5 billion years ago, suggest planetary scientists.

In the current Nature Geoscience journal, Gaetano Di Achille and Brian Hynek of the University of Colorado, Boulder, looked at Mars Orbiter Laser Altimeter (MOLA) measurements made from NASA's 1996 Mars Global Surveyor. The pair find 17 of 52 delta features appearing in the data correspond to a common sea level, suggesting an ocean once covered about 1/3 of Mars.

"Collectively, these delta front elevations approximate an equipotential surface at the mean elevation of -2,540 m (-8,333 feet) with a standard deviation of 177 m (580 feet). Indeed, a contour traced at this elevation effectively outlines acomplete closure within and along the margins of the northern lowlands, delineating the boundary of the basin within which the deposits formed," says the study.

The contour corresponds to an "Arabia shoreline" feature noted by earlier studies. Valleys feeding the ocean all seem to occur at about the same heights with distance from the vanished sea:

"The latter observation probably suggests a progressive retreat of the ocean after its maximum extension during the Late Noachian. Furthermore, the distribution and outlet elevation of the 210 known martian palaeolakes within open basins is also consistent with the elevation of the enclosure: a negligible number of lake outlets show terminations below this level," says the study.

All told, the ocean would have covered 36% of Mars to an average depth of 1,805 feet. "Collectively, these results support the existing theories regarding extent and formation time of an ancient ocean on Mars and imply that surface conditions during that time probably allowed the occurrence of a global and active hydrosphere integrating valley networks, deltas and a vast ocean as main components of an Earth-like hydrological cycle," conclude the authors, at about the time that life was getting started in oceans on Earth.

By Dan Vergano