Mars ocean hypothesis
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 km (2.5–3 miles) 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 have required a denser atmosphere and warmer climate to allow liquid water to remain at the surface.
Several physical features in the present geography of Mars 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 suggestive 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 criterion. The Mars Orbiter Laser Altimeter (MOLA), which accurately determined the altitude of all parts of Mars, found that the watershed for an ocean on Mars covers three-quarters of the planet.
The unique distribution of crater types in the Vastitas 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.
Research published in 2009 shows a much higher density of stream channels than formerly believed. Regions on Mars with the most valleys are comparable to what is found on the Earth. In the research, the team developed a computer program to identify valleys by searching for U-shaped structures in topographical data. The large amount of valley networks strongly supports rain on the planet in the past. The global pattern of the Martian valleys could be explained with a big northern ocean. A large ocean in the northern hemisphere would explain why there is a southern limit to valley networks; the southernmost regions of Mars, farthest from the water reservoir, would get little rainfall and would develop no valleys. In a similar fashion the lack of rainfall would explain why Martian valleys become shallower from north to south. A study of deltas on Mars revealed that 17 are found at the altitude of a proposed shoreline for a Martian ocean. This is what would be expected if the deltas were all next to a large body of water. A large area of inverted stream channels have been interpreted to be a large delta that is along the proposed ocean shoreline.
Research with MARSIS, a radar instrument on board the Mars Express that is in orbit around Mars, supports the idea of a large, northern ocean. The instrument revealed a dielectric constant of the surface that is similar to with low-density sedimentary deposits, massive deposits of ground-ice, or a combination of the two. The measurements were not like those of a lava-rich surface.
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 elevations; at higher elevations water can exist only in solid or vapor form (assuming pure water). 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 retaining 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 have been sufficient to warm the mean surface temperature to a value above the freezing point of water. The atmosphere has since been reduced by sequestration in the ground in the form of 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).
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, the fate of the water requires explanation. 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 Vastitas 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.
The existence of a primordial Martian ocean remains controversial among scientists. The Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) has discovered large boulders on the site of the ancient seabed, which should contain only fine sediment. However, the boulders could have been dropped by icebergs, a process common on Earth. The interpretations of some features as ancient shorelines has been challenged.
Confirmation or refutation of the Mars ocean hypothesis awaits additional observational evidence from future Mars missions.
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