Images obtained 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. In 1987, John E. Brandenburg published the hypothesis of a primordial Mars ocean he dubbed Paleo-Ocean. 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 × 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, 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. A 2010 study of
deltas on Mars revealed that seventeen of them 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. Research presented at a Planetary Conference in Texas suggested that the
Hypanis Valles fan complex is a delta with multiple channels and lobes, which formed at the margin of a large, standing body of water. That body of water was a northern ocean. This delta is at the dichotomy boundary between the northern lowlands and southern highlands near
Chryse Planitia. Research published in 2012 using data from
MARSIS, a radar on board the
Mars Express orbiter, supports the hypothesis of an extinct large, northern ocean. The instrument revealed a dielectric constant of the surface that is similar to those of 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. In 2026. researchers found that topographic shelves rather than shorelines may be better indicators of long-lived oceans on Mars. On Earth, the most prominent topographic sign of a global ocean is not a shoreline, but a band of low slope and curvature values that comprises coastal plains and the continental shelf, with an elevation range of −410 m to −15 m. Mars also a comparably flat zone between approximately –1,800 m and –3,800 m elevation, potentially marking a partially preserved Martian coastal shelf. In March 2015, scientists stated that evidence exists for an ancient volume of water that could comprise an ocean, likely in the planet's northern hemisphere and about the size of Earth's
Arctic Ocean. This finding was derived from the ratio of water and
deuterium in the modern
Martian atmosphere compared to the ratio found on Earth and derived from telescopic observations. Eight times as much
deuterium was inferred at the polar deposits of Mars than exists on Earth (VSMOW), suggesting that ancient Mars had significantly higher levels of water. The representative atmospheric value obtained from the maps (7 VSMOW) is not affected by climatological effects as those measured by localized rovers, although the telescopic measurements are within range to the enrichment measured by the
Curiosity rover in
Gale Crater of 5–7 VSMOW. Even back in 2001, a study of the ratio of molecular hydrogen to
deuterium in the upper atmosphere of Mars by the NASA
Far Ultraviolet Spectroscopic Explorer spacecraft suggested an abundant water supply on primordial Mars. Further evidence that Mars once had a thicker atmosphere which would make an ocean more probable came from the MAVEN spacecraft that has been making measurements from Mars orbit. Bruce Jakosky, lead author of a paper published in
Science, stated that "We've determined that most of the gas ever present in the Mars atmosphere has been lost to space." This research was based upon two different isotopes of argon gas. For how long this body of water was in the liquid form is still unknown, considering the high greenhouse efficiency required to bring water to the liquid phase in Mars at a heliocentric distance of 1.4–1.7 AU. It is now thought that the canyons filled with water, and at the end of the
Noachian Period the Martian ocean disappeared, and the surface froze for approximately 450 million years. Then, about 3.2 billion years ago, lava beneath the canyons heated the soil, melted the icy materials, and produced vast systems of subterranean rivers extending hundreds of kilometers. This water erupted onto the now-dry surface in giant floods. The impact that created the crater
Lomonosov has been identified as a likely source of tsunami waves. ESP 028537 2270tsunamischannels.jpg|Channels made by the backwash from tsunamis, as seen by
HiRISE. The tsunamis were probably caused by asteroids striking the ocean. 28537 2270tsunamisboulders.jpg|Boulders that were picked up, carried, and dropped by tsunamis, as seen by HiRISE. The boulders are between the size of cars and houses. Tsunamisstreamlinedp20008931.jpg|Streamlined promontory eroded by tsunami, as seen by HiRISE. Research reported in 2017 found that the
amount of water needed to develop valley networks, outflow channels, and delta deposits of Mars was larger than the volume of a Martian ocean. The estimated volume of an ocean on Mars ranges from 3 meters to about 2 kilometers GEL (
Global equivalent layer). This implies that a large amount of water was available on Mars. (ESO) in 2015 shows how Mars may have looked like about four billion years ago. In 2018, a team of scientists proposed that Martian oceans appeared very early, before or along with the growth of
Tharsis. Because of this the depth of the oceans would be only half as deep as had been thought. The full weight of Tharsis would have created deep basins, but if the ocean occurred before the mass of Tharsis had formed deep basins, much less water would be needed. Also, the shorelines would not be regular since Tharsis would still be growing and consequently changing the depth of the ocean's basin. As Tharsis volcanoes erupted they added huge amounts of gases into the atmosphere that created a global warming, thereby allowing liquid water to exist. In July 2019, support was reported for an ancient ocean on Mars that may have been formed by a possible
mega-tsunami source resulting from a
meteorite impact creating
Lomonosov crater. In January 2022, a study about the climate 3 billion years ago on Mars shows that an ocean is stable with a water cycle that is closed. They estimate a return water flow, in form of ice in glacier, from the icy highlands to the ocean is in magnitude less than the Earth at the last glacial maximum. This simulation includes for the first time a circulation of the ocean. They demonstrate that the ocean's circulation prevent the ocean to freeze. These also shows that simulations are in agreement with observed geomorphological features identified as ancient glacial valleys. In a paper published by the
Journal of Geophysical Research: Planets in 2022, Benjamin T. Cardenas and Michael P. Lamb asserted that evidence of accumulated sediment suggests Mars had a large, northern ocean in the distant past. In a study, published in January 2026, a group of researches found some boundaries of a northern ocean in Valles Marineris. They determined where the sea level was by studying fans. These fans look like river deltas on Earth. They represent the mouth of a river into an ocean. So, the ancient ocean that was located in the North, and according to this study, extended all the way to the Valles Marineris. ==Issues==