![]() ![]() Eco-engineers had invaded the Wall’s liveable area with terran genestocks deftly altered in orbital labs. ![]() Yet two hundred kilometres higher the wall was a diaphanously thin membrane only microns wide completely invisible except when rare optical effects made it hang like a frozen aurora against the stars. The ramparts were hundreds of meters thick, dark as glacial ice, sinking great taproots deep into the lithosphere to harvest the ores needed for the Wall’s continual growth. The Wall needed to be very tall because the low Martian gravity meant that the column of atmosphere was higher for a fixed surface pressure than on Earth. There were no craters deep enough, so the Wall had been completely artificial: a vast ring-shaped atmospheric dam designed to move slowly outward, encompassing ever more surface area at a rate of a twenty kilometres per year. ![]() Instead of modifying the whole atmosphere at once, the Wall allowed the initial effort to be concentrated in a relatively small region, at first only a thousand kilometres across. The logic had been inescapable: a means to avoid the millennia-long timescales needed to terraform Mars via such conventional schemes as cometary bombardment or ice-cap thawing. The Wall was the most audacious and visible of Voi’s projects. Once, hundreds of communities had sheltered inside that cell of warm, thick, oxygen-rich atmosphere. ![]()
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