From the mapped contact levels, the ocean-filling factor can be determined, and the Arabia shoreline would constrain an ocean volume of some 108 km3. The smaller Deuteronilius shoreline would constrain an ocean averaging some 560 m deep and a volume of 107 km3. The ocean bounded by the Deuteronilius shoreline dates from the mid-Noachian to mid-Hesperian, a timespan covering perhaps 500 million years. The longer Arabia shoreline is believed to be younger, dating from the late Hesperian to the early Amazonian, representing a time span of about 1.5 billion years.
A long-running problem associated with the identification of the paelaeoshorelines on Mars has only recently been resolved in light of a detailed mathematical study by Taylor Perron (University of California) and co-workers.2 It was noticed soon after the two shorelines were first mapped out using visual analysis that they did not actually follow a contour of constant gravitational potential. In other words, the seas would not have been level, which, of course, is not physically possible.
What Perron and co-workers have shown, however, is that the shorelines are consistent with a constant gravitational potential if the spin axis of Mars has shifted from 30° to 60° over the past 2 billion years. That is, the current Martian north pole location was not the same as the north pole position when the oceans existed. The mechanism invoked by Perron et al. to account for this change is known as true polar wander (TPW), which is a spin-axis reorientation effect that comes into play whenever a significant mass redistribution takes place on and in a planet. It is not exactly clear what might have produced the TPW on Mars, however, but Perron and colleagues suggest it might have been the formation of the Elysium volcanic region (see Figure 6.8, at the two o'clock position in the upper diagram).
Another issue that has puzzled researchers, and may have recently been solved, relates to the predominance of sulfur-rich minerals on the Martian surface. Both orbital spacecraft and the Martian rovers Spirit and Opportunity have found that there are almost no calcium carbonate (limestone) deposits on Mars but that there are plenty of sulfur-rich ones. On the Earth, silicate rocks remove carbon dioxide from the atmosphere and in the presence of water produce limestone. On Mars, however, although there is abundant evidence for past surface water, there is very little surface limestone. The solution to this puzzle may reside in the Red Planet's past volcanic activity. Writing in the journal Science, for 21 December 2007, Itay Halevy (Harvard University) and co-workers suggested that on Mars volcanically outgassed sulfur dioxide substituted for carbon dioxide in the weathering process to produce sulfates rather than limestone. Not only this, while on the Earth, sulfur dioxide is quickly destroyed by oxidization, on Mars it would have served as a long-lived, atmosphere-warming greenhouse gas.
Figure 6.9 indicates that since about 2.5 billion years ago Mars has been an essentially dry and frozen world; small, high-salinity, and isolated lake systems may have existed near the pole in the northern lowland region for an additional billion years, but even these would have frozen over by the mid-Amazonian epoch. The questions that we need to ask now are, ''How did this change of state come about, and what happened to all the water?'' The point of these questions, of course, from the terraforming perspective, is to ask if large water-ice reserves exist on Mars, and might the heating of its atmosphere result in the reappearance of its long-lost oceans?
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