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The Mars Ocean Hypothesis 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 Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4-5 kilometers below the mean planetary elevation, at a time period of approximately 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 require a warmer climate and thicker atmosphere to allow liquid water to remain at the surface.

Observational Evidence
There are several physical features in the present geography of Mars that suggest the existence of a primordial ocean. Networks of gullies that merge into larger channels imply erosion by a liquid agent, and resemble ancient riverbeds on earth. Enormous channels, 25 km wide and several hundred meters deep, appear to direct flow from underground aquifers in the Southern uplands into the Northern plains. Much of the northern hemisphere of Mars is located at a significantly lower elevation than the rest of the planet (the Martian dichotomy), and is unusually flat. Along the margins of this region are physical features indicative of ancient shorelines. Sea level must follow a line of constant gravitational potential. After adjustment for polar wander caused by mass redistributions from volcanism, the Martian paleo-shorelines meet this criteria.

The unique distribution of crater types in the Vastitus Borealis below 2400 m elevation suggest erosion that involved sublimation, and an ancient ocean that would have encompassed a volume of 6 x 107 cubic kilometers.

Primordial Martian Climate
The existence of liquid water on the surface of Mars requires both a warmer and thicker atmosphere. Atmospheric pressure on the present day Martian surface only exceeds that of the triple point of water (6.11 hPa) in the lowest regions of the planet; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently less than 210 K, significantly less than what is needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to harboring liquid water at the surface.

Early Mars had a carbon dioxide atmosphere similar in thickness to present day earth (1000 hPa). Despite a weak early sun, the greenhouse effect from a thick carbon dioxide atmosphere, if bolstered with small amounts of methane or insulating effects of carbon dioxide ice clouds, would be sufficient to warm the mean surface temperature to above the freezing point of water. The atmosphere has since been reduced by sequestration in the ground as carbonates through weathering, as well as loss to space through sputtering (an interaction with the solar wind due to the lack of a strong Martian magnetosphere).

The obliquity (axial tilt) of Mars varies considerably on geologic timescales, and has a strong impact on planetary climate conditions.

Chemistry
Consideration of chemistry can yield additional insight into the properties of Oceanus Borealis. With a Martian atmosphere of predominantly carbon dioxide, one might expect to find extensive evidence of carbonate minerals on the surface as remnants from oceanic sedimentation. An abundance of carbonates has yet to be detected by the Mars space missions. However, if the early oceans were acidic, carbonates would not be able to form. The positive correlation of phosphorus, sulfur, and chlorine in the soil at two landing sites suggest mixing in a large acidic reservoir. Hematite deposits detected by TES have also been argued as evidence of past liquid water.

Analysis of molecular hydrogen to deuterium ratios in the upper Mars atmosphere from the NASA Far Ultraviolet Spectroscopic Explorer spacecraft suggests an abundant water supply on primordial Mars.

Fate of the Ocean
Given the proposal of a vast primordial ocean on Mars, one may inquire as to the fate of the water. As the Martian climate cooled, the surface of the ocean would have frozen. One hypothesis states that part of the ocean remains in a frozen state buried beneath a thin layer of rock, debris, and dust on the flat northern plain Vastitus Borealis. The water could have also been absorbed into the subsurface cryosphere or been lost to the atmosphere (by sublimation) and eventually to space through atmospheric sputtering.

Alternate Explanations
The existence of a primordial Martian ocean remains controversial among scientists. Large boulders have been discovered on the site of the ancient seabed by the Mars Reconisance Orbiter's High Resolution Imaging Science Experiment, which should contain only fine sediment. 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.