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09/26/2004 Archived Entry: "Gordon on Mars"

Gordon P., our resident Mr. Science, provides some fact-based whimsy that springboards from an article entitled "Mars, Once Warm and Wet, Left Some Clues."

Gordon writes, "Since the early Mariner missions, Mars has presented contradictory evidence regarding the past existence of large amounts of water. Despite the existence of the "permanent" water-ice icecaps, features that look like ancient river- valleys and flood-washes, the neutron spectrometer evidence suggesting vast amounts of hydrogen-containing compounds near the surface over most of the planet, and the recent martian rover observation of "sedimentary" features and hematite "blueberries" that probably needed water to form, it has been recently argued that Mars could not have had large amounts of _liquid_ water, because despite an atmosphere composed mostly of carbon dioxide, we see no evidence of the large amounts of carbonate rocks that _should_ have formed, and significant amounts of rocks such as olivine that should not have been able to survive in a "wet" environment.

A group of spanish scientist may have solved at least the first of the above two problems: The lack of carbonate rocks. The Earth's oceans form carbonate rocks becase they are slightly alkaline (pH of about 6), and therefore react to neutralize the carbonic acid formed when CO2 dissolves in water to form insoluable carbonates. However, Alberto Fairen et al of the Universidad Autonoma de Madrid have found a spanish river,
the Tinto, in which the water is naturally acidic (pH of about 8) --- and under such conditions, carbonate rocks do not form. (Indeed, the acid will react with carbonates to _release_ CO2!). The key to preventing carbonates from forming on both planets, according to Fairen et al's analysis, are sufficient concentrations of iron and sulfates, or salt. We know that the Martian surface-environment has lots of iron --- the whole planet is tinged red with iron oxides! --- and Mars' huge volcanoes suggest large amounts of sulfur compounds were probably also present. There are also evidences of large amounts of salts present in the Martian surface. Hence, it seems likely that all the ingredients required by Fairen et al's hypothesis were probably present in the early martian enviroment, neatly reconciling the simultaneous presence of large amounts of CO2 and liquid water in the past with an absence of any signficant amounts of carbonate rocks in the present.

Fairen notes that despite having a pH arounf 8, the Tinto river has a thriving ecosystem, so an acidic martian ocean is not incompatible with possible life. Fairen et al's paper appeared in the Sept. 23 issue of the journal _Nature_.

Now --- There is the remaining problem of the large amounts of olivine and other water-sensitive rocks in the martian highland regions, which are not compatible with liquid water. My own observation: The Martian highlands are so far above the martian "sea level" suggested by other studies that they are above most of the martian atmosphere --- and would _still_ have been above most of the martian atmosphere, even back when it might have been warm, wet, and thick near "sea level." (By comparison, the tops of the Himalayas are mere foothills!) Hence, the liquid phase of water could not have existed in the martian highlands --- only the ice and vapor phases. Also, even under a "warm, wet" martian climate, the martian highlands would have still been so bitterly cold that the atmospheric humidity would have been negligible. Hence, I do not consider the presence of olivine and other water-sensitive rocks in the martian highlands to necessarily be incompatible with liquid water in the martian lowlands.