Quarterdeck Magazine Online -- Texas A&M Oceanography

Texas A&M University
Department of Oceanography

Summer 1999 - Vol. 7, No. 1

Carbonates' role in the chemical evolution of oceans on Earth & Mars

by John Morse

Certainly one of the greatest scientific, philosophical, and probably even religious questions facing humankind is whether or not life exists elsewhere in the universe.
Although we are not close to answering this question for planets beyond our own solar system, in the next few years we may see the question answered for Mars and the moons of Jupiter, Europa and Titan. These three bodies are the primary targets for investigation because they possibly contain or did contain liquid water. The presence of liquid water is critical to the formation and persistence of life as we know it.

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Mars facts

Mars is currently being investigated by the Mars Global Surveyor satellite and Mars Pathfinder, a scientific lander. Data from these instruments indicate growing evidence of the existence of ancient martian oceans.
Also, the potential (and highly controversial) existence of fossil evidence of life in meteorites that are believed to have come from Mars has heightened scientific and public interest in the question of life and water on Mars. Biogeochemical oceanographers at Texas A&M are interested in answering the question: If there were oceans on Mars, what might their chemistry have been like, and how might it be related to their disappearance?

Above: The origin of canyon Nanedi Vallis, about 2.5 kilometers wide, is enigmatic. Features such as canyon terraces and the small, 200 meter-wide channel near the top of the image suggest continual fluid flow and downcutting. Other features suggest formation by collapse.

Goldilocks and the three planets

If we look at Earth's two nearest neighbor planets, Venus and Mars, as they currently exist, a situation is observed reminiscent of Goldilocks and the three bears. Venus is too hot for liquid water, Mars is too cold, and Earth is just right for liquid water -- and 70 percent covered with it.
But was this situation always the case? For Venus, the answer is almost certainly yes, it has always been too hot for liquid water. However, for Mars, there is growing consensus in the scientific community that the answer is no. Mars Global Surveyor's observations of steep-walled valleys and wide channels suggest the existence of major flows of water, and Mars Rover's observations of rocks and terrain indicate that floods occurred.
The primary evidence for liquid water on ancient Mars is the presence of these valleys and channels, whose formation by running water seems the most reasonable explanation. The valleys generally appear to be older than the channels, and their form closely resembles the erosional features of Earth's rivers. Martian valleys are as long as 1,000 kilometers--about the distance from El Paso to Texarkana.
The mechanism by which the large outflow channels were formed is not well understood. Possibly, they may have formed when Mars' surface conditions were similar to today's. Discharge rates were as high as one billion cubic meters of water per second-more than 10,000 times the discharge rates of the Earth's largest rivers. One hundred thousand cubic kilometers of water flowing at speeds approaching 150 meters per second could form such channels on Mars.

Take a closer look at Mars' valleys and channels.

Evidence for lakes and oceans

In addition to the evidence for flowing water, considerable evidence supports the existence of large bodies of water in Mars' northern hemisphere. Although the best evidence is associated with modestly sized lakes that fill gigantic impact craters (Hellas and Argyre), some scientists estimate that much of the northern lowlands were covered with water at some point in time. These lowlands cover roughly 30 percent of Mars' surface, and if surface water were concentrated in them it could have reached a depth of 100 to 2,800 meters. A liquid ocean could most easily have created large-scale erosional escarpments (steep slopes or cliffs), and the widespread presence of weathered surface rocks and soil, plus salts, supports the presence of extensive surface waters over a fairly long time. Landforms resembling Earth's glacial features suggests that glaciers existed on Mars, requiring a hydrologic cycle on early Mars involving liquid water that evaporated, condensed, and precipitated.

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What is carbonate?

The impact of atmospheric carbon dioxide on ocean pH

On Earth, no rocks are known to have survived prior to the Archean Eon (starting 3.8 billion years ago), and only very limited information from this period is currently available from Mars. However, it is still possible to calculate approximate values for important aspects of seawater chemistry during this time period, based on other sources of information, experiments, and reasonable assumptions about processes such as weathering reactions.
The approach used is largely based on application of the Pitzer equation, as developed by Texas A&M scientists Shiliang He and John Morse for the carbonic acid system in concentrated electrolyte solutions, and experimental studies of carbonate mineral precipitation from seawater-like solutions and brines in Dr. Morse's laboratory.
For Earth, seawater composition was calculated for water at various temperatures-from nearly boiling to 70 degrees Celsius-and various levels of atmospheric carbon dioxide, from 10 to 0.03 atmospheres pressure. (The current pressure of carbon dioxide is about 0.00036.) Over these ranges, the influence of temperature on seawater composition is relatively small. Changes in atmospheric carbon dioxide, however, result in large variations in the chemistry of Hadean seawater. In the early Hadean Eon, seawater was probably moderately acid, about pH 5.8. Dissolved inorganic carbon may have been nearly 50 times the current value, and alkalinity was perhaps close to 12 times the current value. By the late Hadean, seawater pH probably had changed close to neutral (about 6.8), and dissolved inorganic carbon and alkalinity were much closer to present-day values.
These calculations support the hypothesis that a carbonate chemistry of seawater roughly similar to that of modern oceans could have been acquired very early in Earth history, and the composition of late Hadean to early Archean seawater on Earth was not vastly different from that of today. (Diagram) Thus, at least by the end of the Hadean Eon, environmental conditions at the Earth's surface, including temperature and seawater composition, were sufficiently stable for the evolution of life. *

* Possibly, the concentration of calcium in seawater did not reach levels like that of modern seawater until the late Precambrian (about 600 million years ago) and thus may have constrained the timing of the "Big Bang" of organic evolution, the emergence of the shelled invertebrates at the beginning of the Phanerozoic, 550 million years ago.

The change in atmospheric CO₂ influenced the ocean chemistry
on Earth
and Mars.

View a diagram of the change in Earth's atmospheric carbon dioxide.

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Mars on the web

From liquid water to ice

A similar evolution of the early martian atmosphere would result in freezing conditions, and the hydrologic cycle would largely cease. During the period of freezing, the oceans would act as a source of carbon dioxide, rather than a reservoir for its removal. This would further slow the rate of climate change on Mars, extending the persistence of liquid water on the surface and giving life a greater time period to evolve. (Diagram)
The alkalinity of the freezing seawater would probably be sufficient to result in the removal of almost all calcium as precipitated calcium carbonate minerals, followed by the removal of magnesium and some sodium, also as carbonate minerals. The removal of these metals as carbonate minerals has a major influence on the final temperature at which liquid brines would be able to persist on the surface of Mars. An example of a late martian-like ocean on Earth may be the calcium chloride-rich Don Juan Pond in Antarctica, which does not even freeze during the Antarctic winters.

Future investigations

In summary, the chemical environments in the oceans and atmospheres of early Earth and Mars were similar, but Mars was probably considerably cooler-which raises the possibility that conditions for the formation of primitive life were more favorable on Mars than on Earth. Chemical weathering and removal of atmospheric carbon dioxide resulted in the oceans on both planets becoming less acid, and the major cooling during the planets' first 0.5 to 1 billion years, resulted in climate conditions that may have been similar to conditions found on these planets today. On Earth, life has flourished in the past four billion years, but too much cooling has turned Mars into a "frozen" planet.
The next major scientific questions about Mars concern the existence and persistence of life. Did life form in the early martian oceans? Does life persist in subsurface regions on Mars? Hopefully, these questions will be answered in the not-too-distant future.

Dr. John W. Morse is a biogeochemical oceanographer at Texas A&M University. His e-mail address is morse@ocean.tamu.edu.

View a diagram of the geochemical cycle on early Mars for carbon.