Mars ocean theory

The Mars ocean theory states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet's geologic history. This primordial ocean, dubbed Paleo-Ocean or Oceanus Borealis (/ˈsənəs ˌbɒriˈælɪs/ oh-SEE-ə-nəs BORR-ee-AL-iss), would have filled the basin Vastitas Borealis in the northern hemisphere, a region that lies 4–5 km (2.5–3.1 mi) below the mean planetary elevation, at a time period of approximately 4.1–3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere. Early Mars would have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.