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 ( ), would have filled the basin Vastitas Borealis 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 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.
History of observational evidence
Features shown by the Viking orbiters in 1976, revealed two possible ancient shorelines near the pole, Arabia and Deuteronilus, each thousands of kilometers long. Several physical features in the present geography of Mars suggest the past 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 lowlands. 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.
These observations led a number of researchers to look for remnants of more ancient coastlines and further raised the possibility that such an ocean once existed. In 1987, John E. Brandenburg published the hypothesis of a primordial Mars ocean he dubbed Paleo-Ocean. The ocean hypothesis is important because the existence of large bodies of liquid water in the past would have had a significant impact on ancient Martian climate, habitability potential and implications for the search for evidence of past life on Mars.
Beginning in 1998, scientists Michael Malin and Kenneth Edgett set out to investigate with higher-resolution cameras on board the Mars Global Surveyor with a resolution five to ten times better than those of the Viking spacecraft, in places that would test shorelines proposed by others in the scientific literature. Their analyses were inconclusive at best, and reported that the shoreline varies in elevation by several kilometers, rising and falling from one peak to the next for thousands of kilometers. These trends cast doubt on whether the features truly mark a long-gone sea coast and, have been taken as an argument against the Martian shoreline (and ocean) hypothesis.
The Mars Orbiter Laser Altimeter (MOLA), which accurately determined in 1999 the altitude of all parts of Mars, found that the watershed for an ocean on Mars would cover three-quarters of the planet. The unique distribution of crater types below 2400 m elevation in the Vastitas Borealis was studied in 2005. The researchers suggest that erosion involved significant amounts of sublimation, and an ancient ocean at that location would have encompassed a volume of 6 x 107 km3.
In 2007, Taylor Perron and Michael Manga proposed a geophysical model that, after adjustment for true polar wander caused by mass redistributions from volcanism, the Martian paleo-shorelines first proposed in 1987 by John E. Brandenburg, meet this criterion. The model indicates that these undulating Martian shorelines can be explained by the movement of Mars's rotation axis. Because centrifugal force causes spinning objects and large rotating objects to bulge at their equator (equatorial bulge), the polar wander could have caused the shoreline elevation to shift in a similar way as observed. Their model does not attempt to explain what caused Mars's rotation axis to move relative to the crust.
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.