The Mars Ocean Hypothesis states that nearly a third of the surface of Mars

Mars

Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

 was covered by an ocean of liquid water early in the planet’s geologic history

Geology of Mars

The geology of Mars is the scientific study of the surface, crust, and interior of the planet Mars. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is fully analogous to the field of terrestrial geology. In planetary science, the term geology is...

.
This primordial ocean, dubbed Oceanus Borealis, would have filled the Vastitas Borealis

Vastitas Borealis

Vastitas Borealis is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the Northern plains or Northern lowlands of Mars. The plains lie 4–5 km below the mean radius of...

 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.

Observational evidence

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

Martian dichotomy

The most conspicuous feature of Martian surface geology is a sharp contrast, known as the Martian dichotomy, between the rugged southern highlands and the relatively smooth northern basins. The two hemispheres differ in elevation by 1 to 3 km...

), 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

Polar wander

Polar wander is the movement of the North and South Poles of the Earth with respect to the continents. This motion can be divided into two components: that due to continental drift , and true polar wander, in which the mantle and the crust rotate together into new orientations....

 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.

Recent research published in the Journal of Geophysical Research — Planets, 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 as you go from north to south.

Primordial Martian climate

The existence of liquid water on the surface of Mars requires both a warmer and thicker atmosphere

Atmosphere of Mars

The atmosphere of Mars is relatively thin and is composed mostly of carbon dioxide . There has been interest in studying its composition since the detection of trace amounts of methane, which may indicate the presence of life on Mars, but may also be produced by a geochemical process, volcanic or...

. Atmospheric pressure on the present day Martian surface only exceeds that of the triple point of water

Triple point

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...

 (6.11 hPa) in the lowest elevations; 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 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

Faint young Sun paradox

The faint young Sun paradox or problem describes the apparent contradiction between observations of liquid water early in the Earth's history and the astrophysical expectation that the Sun's output would be only 70% as intense during that epoch as it is during the modern epoch. The issue was raised...

, the greenhouse effect

Greenhouse effect

The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 from a thick carbon dioxide atmosphere, if bolstered with small amounts of methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 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

Sputtering

Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...

 (an interaction with the solar wind due to the lack of a strong Martian magnetosphere).

The obliquity (axial tilt

Axial tilt

In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

) 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

Deuterium

Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 ratios in the upper Mars atmosphere from the NASA Far Ultraviolet Spectroscopic Explorer

Far Ultraviolet Spectroscopic Explorer

The Far Ultraviolet Spectroscopic Explorer is a space-based telescope operated by the Johns Hopkins University Applied Physics Laboratory. FUSE was launched on a Delta II rocket on June 24, 1999, as a part of NASA's Origins program...

 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.

Alternate explanations

The existence of a primordial Martian ocean remains controversial among scientists. The Mars Reconnaissance Orbiter

Mars Reconnaissance Orbiter

Mars Reconnaissance Orbiter is a NASA multipurpose spacecraft designed to conduct reconnaissance and Exploration of Mars from orbit...

'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 iceberg

Iceberg

An iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice...

s, a process common on Earth. 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.