What name is given to the hypothesized ancient ocean in Mars's northern hemisphere?

The hypothesized ancient ocean in Mars’s northern hemisphere is called Oceanus Borealis. The proposed ocean would have occupied much of Vastitas Borealis, the broad, low-lying basin covering the planet’s northern lowlands.

The idea developed from Mars’s striking hemispheric dichotomy: the northern plains are generally flatter and lower than the heavily cratered southern highlands. Researchers have also identified possible ancient shorelines, valley networks, deltas, sedimentary deposits and tsunami-like features that could be consistent with a former large body of water.

Estimates place the proposed ocean mainly in the Noachian or early Hesperian eras, roughly 4.1 to 3.8 billion years ago. Some studies have suggested it could have covered a substantial fraction of Mars’s surface, while atmospheric isotope evidence has been used to estimate a past water inventory comparable in scale to an ocean.

Oceanus Borealis remains a hypothesis rather than an uncontested historical fact. The proposed shorelines vary in elevation, and climate models must explain how early Mars could have remained warm enough for extensive surface liquid water. “Mare” names such as Mare Australe refer to regions or seas, not this proposed northern ocean.