Geoscience Reference
In-Depth Information
8.3.4
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 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.
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-year-old (2 Gyrs) 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 has been put forward as the most
likely cause.
8.3.5
Present Water Ice
A significant amount of surface hydrogen has been observed globally by the Mars
Odyssey's neutron spectrometer and gamma ray spectrometer. This hydrogen is
thought to be incorporated into the molecular structure of ice, and through stoichio-
metric calculations, the observed fluxes have been converted into concentrations
of water ice in the upper meter of the Martian surface. This process has revealed
that ice is both widespread and abundant on the modern surface. Below 60 degrees
of latitude, ice is concentrated in several regional patches, particularly around the
Elysium volcanoes, Terra Sabaea, and northwest of Terra Sirenum, and exists in
concentrations up to 18 % ice in the subsurface.
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