Mars Ocean Hypothesis
The Mars Ocean Hypothesis proposes that the Vastitas Borealis basin was the site of an ocean of liquid water at least once, and presents evidence that nearly a third of the surface of Mars was covered by a liquid ocean early in the planet's geologic history. This 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. Two major putative shorelines have been suggested: a higher one, dating to a time period of approximately 3.8 billion years ago and concurrent with the formation of the valley networks in the Highlands, and a lower one, perhaps correlated with the younger outflow channels. The higher one, the 'Arabia shoreline', can be traced all around Mars except through the Tharsis volcanic region. The lower, the 'Deuteronilus', follows the Vastitas Borealis formation.
A study in June 2010 concluded that the more ancient ocean would have covered 36% of Mars. Data from the Mars Orbiter Laser Altimeter (MOLA), which measures the altitude of all terrain on Mars, was used in 1999 to determine that the watershed for such an ocean would have covered about 75% of the planet. Early Mars would have required a warmer climate and denser atmosphere to allow liquid water to exist at the surface. In addition, the large number of valley networks strongly supports the possibility of a hydrological cycle on the planet in the past.
However, the existence of a primordial Martian ocean remains controversial among scientists, and the interpretations of some features as 'ancient shorelines' has been challenged. One problem with the conjectured 2 billion years old (2 Ga) shoreline is that it is not flat — i.e. does not follow a line of constant gravitational potential. This could be due to a change in distribution in Mars' mass, perhaps due to volcanic eruption or meteor impact; the Elysium volcanic province or the massive Utopia basin that is buried beneath the northern plains have been put forward as the most likely causes.
Read more about this topic: Water On Mars, Extinct Water Bodies
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