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 Paleo-Ocean and 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.
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 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.
These observations led a number of researchers to look for remnants of more ancient coastlines and further raised the possibility that such a 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 onboard the Mars Global Surveyor with a resolution five to ten times better than Viking, in places that would test shorelines proposed by others in the scientific literature. Their analysis 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 miles. These trends casted doubt on whether the features truly marked 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.
Scientists Taylor Perron and Michael Manga, proposed in 2007 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. Brandenburgin, meet this criterion. The model indicates that these undulating Martian shorelines "can be explained by the movement of Mars' spin axis". Because spinning objects bulge at their equator, the polar wander could have caused shoreline elevation shifts similar to those observed on Mars. Their model does not attempt to explain what caused Mars' 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 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 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.
- Brandenburg, John E. (1987), "The Paleo-Ocean of Mars", MECA Symposium on Mars: Evolution of its Climate and Atmosphere, Lunar and Planetary Institute, pp. 20–22, retrieved 19 February 2014
- Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier. NY
- Clifford, S. M. and T. J. Parker, 2001: The Evolution of the Martian Hydrosphere: Implications for the Fate of a Primordial Ocean and the Current State of the Northern Plains, Icarus 154, 40-79.
- Baker, V. R., R. G. Strom, V. C. Gulick, J. S. Kargel, G. Komatsu and V. S. Kale, 1991: Ancient oceans, ice sheets and the hydrological cycle on Mars, Nature, 352, 589-594.
- Read, Peter L. and S. R. Lewis, “The Martian Climate Revisited: Atmosphere and Environment of a Desert Planet”, Praxis, Chichester, UK, 2004.
- "Mars Probably Once Had A Huge Ocean". University of California - Berkeley (Science Daily). 13 June 2007. Retrieved 2014-02-19.
- "Mars Ocean Hypothesis Hits the Shore". Astrobiology Magazine. 26 January 2001. Retrieved 19 February 2004.
- Malin, M. C., and Edgett, K. S., 1999. Oceans or Seas in the Martian Northern Lowlands: High Resolution Imaging Tests of Proposed Coastlines, Geophys. Res. Letters, V. 26, No. 19, p. 3049—3052
- Smith, D. et al. 1999. Science: 284.1495
- Boyce, J. M.; Mouginis, P.; Garbeil, H. (2005). "Ancient oceans in the northern lowlands of Mars: Evidence from impact crater depth/diameter relationships". Journal of Geophysical Research (American Geophysical Union) 110 (E03008): (15 pp.). Bibcode:2005JGRE..11003008B. doi:10.1029/2004JE002328. Retrieved 2 October 2010.
- Zuber, Maria T., 2007: Planetary Science: Mars at the tipping point, Nature, 447, 785-786.
- Perron, J. Taylor; Jerry X. Mitrovica, Michael Manga, Isamu Matsuyama and Mark A. Richards (14 June 2007). "Evidence for an ancient martian ocean in the topography of deformed shorelines". Nature 447: 840–843. doi: 10.1038/nature05873. Retrieved 2014-02-19.
- Dunham, Will (13 June 2007). "Evidence seen backing ancient Mars ocean shoreline". Reuters. Retrieved 2014-02-19.
- "Martian North Once Covered by Ocean". Astrobiology Magazine. 26 November 2009. Retrieved 19 February 2014.
- "New Map Bolsters Case for Ancient Ocean on Mars". Space.com. 23 November 2009. Retrieved 2014-02-19.
- DiAchille, G and B. Hynek. 2010. Ancient ocean on Mars supported by global distribution of deltas and valleys. nat. Geosci. 3, P. 459-463, doi:10.1038/ngeo891
- DiBiasse, A. Limaye, J. Scheingross, W. Fischer, and M. Lamb. 2013. Deltic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars. Journal of Geophysical Research Planets 118, 1285-1302
- Mouginot, J., A. Pommerol, P. Beck, W. Kofman, S. Clifford. 2012. Dielectric map of the Martian northern hemisphere and the nature of plain filling materials. Geophysical Research Letters, Vol. 39, L02202, doi:10.1029/2011GL050286
- Carr, Michael H., 1999: Retention of an atmosphere on early Mars, Journal of Geophysical Research, 104, 21897-21909.
- Squyres, Steven W. and James F. Kasting, 1994: Early Mars: How warm and how wet?, Science, 265, 744-749.
- Forget, F. and R. T. Pierrehumbert, 1997: Warming Early Mars with Carbon Dioxide Clouds That Scatter Infrared Radiation, Science, 278, 1273-1276.
- Kass, D. M. and Y. L. Yung, 1995: Loss of atmosphere from Mars due to solar wind-induced sputtering, Science, 268, 697-699.
- Carr, M and J. Head III. 2003. Oceans on Mars: An assessment of the observational evidence and possible fate. Journal of Geophysical Research: 108. 5042.
- Abe, Yutaka, Atsushi Numaguti, Goro Komatsu, and Yoshihide Kobayashi, 2005: Four climate regimes on a land planet with wet surface: Effects of obliquity change and implications for ancient Mars, Icarus, Volume 178, Pages 27-39.
- Fairen, A.G., D. Fernadez-Remolar, J. M. Dohm, V.R. Baker, and R. Amils, 2004: Inhibition of carbonate synthesis in acidic oceans on early Mars, Nature, 431, 423-426.
- Greenwood, James P. and Ruth E. Blake, 2006: Evidence for an acidic ocean on Mars from phosphorus geochemistry of Martian soils and rocks, Geology, 34, 953-956.
- Tang, Y., Q. Chen and Y. Huang, 2006: Early Mars may have had a methanol ocean, Icarus, 181, 88-92.
- Krasnopolsky, Vladimir A., and Paul D. Feldman, 2001: Detection of Molecular Hydrogen in the Atmosphere of Mars, Science, 294, 1914-1917.
- Janhunen, P., 2002: Are the northern plains of Mars a frozen ocean?, Journal of Geophysical Research, 107, 5103.
- Kerr, Richard A., 2007: Is Mars Looking Drier and Drier for Longer and Longer?, Science, 317, 1673.
- Fairén, A. G.; Davila, A. F.; Lim, D.; McKay, C. (2010). "Icebergs on Early Mars". Astrobiology Science Conference. Retrieved 2010-10-02.
- Chol, Charles Q. (2010-10-01). "New Evidence Suggests Icebergs in Frigid Oceans on Ancient Mars". Space.Com web site. Retrieved 2010-10-02.
- Carr, M. H. and J.W. Head, 2002: Oceans on Mars: An assessment of the observational evidence and possible fate, Journal of Geophysical Research, 108.
- Leovy, C.B., 1999: Wind and climate on Mars, Science, 284, 1891a.