In a series of three papers, each produced at intervals of about 10 years apart starting in 1981, Stephen Gillet has suggested that the Oberg-Fogg scenario might be modified with respect to its hydrogen-importation requirements. Indeed, he argues that perhaps only 1019 kg of H2 might need to be imported (a quarter of the amount prescribed in the original scenario) to terraform Venus. The atmospheric carbon dioxide would still need to be reduced, however, and this Gillet suggests might be achieved by importing calcium (Ca) and magnesium (Mg) especially mined from the surface of Mercury (where a rich and relatively nearby supply of such materials can be found). Once the calcium and magnesium had been deposited within the Venusian atmosphere, Gillet envisions a two-step process that will take place, whereby, for example, two calcium atoms will interact with an oxygen molecule to produce a calcium oxide (schematically: 2Ca + O2 ) 2 CaO). The calcium oxide will then react with a carbon dioxide molecule to produce calcium carbonate (schematically: CaO + CO2 ) CaCO3). A similar chain of reactions with magnesium will produce magnesium carbonate MgCO3. Through this chain of reactions, an inert carbonate dust will fall to the surface of Venus. Once the atmospheric CO2 has been predominantly locked away in the carbonate layer, the final terraform-ing and seeding for new life phase can proceed in a similar manner to that described by Oberg and Fogg.
In many ways, the scenario outlined by Gillett reduces one importation problem (that of the H2) only to replace it with another, potentially more severe, one (the importation Ca and Mg from Mercury). Certainly one might imagine that most of the work will be done by autonomous robot systems, but the process is perhaps a good deal less than ideal. Indeed, Gillett was well aware of this problem and consequently suggested in a paper published in 1999 that there is perhaps one way in which the entire Venusian atmosphere can be purged of its CO2 without importing any additional elements. This rather miraculous possibility comes about through the production of a crystalline structure (yet to be synthesized in the laboratory) called carba.
The atomic structure of carba resembles that of the tetrahedral arrangement of carbon atoms in diamond, but in the carba situation each of the carbon-carbon bonds (C-C) are replaced by a C-O-C chain of bonds (the O being an oxygen atom). Gillett envisions that the carba might be grown, just as a mollusk grows in a calcium carbonate shell, by genetically modified microorganisms. These microorganism workers would be introduced into the Venusian atmosphere, grow their carba shells, and then upon completing their lifecycle fall to the surface of Venus, where they could be left to accumulate or mined for export as a raw material.
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