As always, the posters were a deliciously mixed bag. They were posters rather than talks for a variety of reasons-often the work of graduate students at the university in Sabishii, or concerned with topics peripheral to the conference-but anything might be there, and it was always very interesting to browse. And at this conference there had been no strong attempt to organize the posters into hallways by subject matter, so that “Distribution of Rhizocarpon geo-graphicum in the East Charitum Monies,” detailing the high-altitude fortunes of a crustose lichen that could live up to four thousand years, was facing “Origins of Graupel Snow in Saline Particulates Found in Cirrus, Altostratus and Altocumulus Clouds in Cyclonic Vortexes in North Tharsis,” a meteorological study of some importance.

Sax was interested in everything, but the posters that held him the longest were those that described aspects of the terraforming that he had initiated, or once had a hand in. One of these, “Estimate of the Cumulative Heat Released by the Underhill Windmills,” stopped him in his tracks. He read it through twice, feeling a slight dampening of spirits as he did.

The mean temperature of the Martian surface before their arrival had been around 220°K, and one of the universally agreed-upon goals of terraforming was to raise that mean temperature to something above the freezing point of water, which was 273°K. Raising the average surface temperature of an entire planet by more than 53°K was a very intimidating challenge, requiring, Sax had figured, the application over time of no less than 3.5 X 10” joules to every square centimeter of the Martian surface. Sax in his own modeling had always aimed to reach a mean of about 274°K, figuring that with this as the average, the planet would be warm enough for much of the year to create an active hydrosphere, and thus a biosphere. Many people advocated even more warming than that, but Sax did not see the need.

In any case, all methods for adding heat to the system were judged by how much they had raised the global mean temperature; and this poster examining the effect of Sax’s little windmill heaters estimated that over seven decades they had added no more than 0.05°K. And he could find nothing wrong with the various assumptions and calculations in the model outlined in the poster. Of course heating was not the only reason he had distributed the windmills; he had also wanted to provide warmth and shelter for an early engineered cryptoendolith he had wanted to test on the surface. But all those organisms had in fact died immediately upon exposure, or shortly thereafter. So on the whole the project could not be said to be one of his better efforts.

He moved on. “Application of Process-Level Chemical Data in Hydrochemical Modeling: Dao Vallis Watershed, Hellas.” “Increasing CO₂ Tolerance in Bees.” “Epilimnetic Scavenging of Compton Fallout Radionuclides in the Marineris Glacial Lakes.” “Clearing Fines from Piste Reaction Rails.” “Global Warming As a Result of Released Halocarbons.”

This last one stopped him again. The poster was the work of the atmospheric chemist S. Simmon and some of his students, and reading it made Sax feel considerably better. When Sax had been made head of the terraforming project in 2042, he had immediately initiated the construction of factories to produce and release into the atmosphere a special greenhouse gas mix, composed mostly of carbon tetrafluoride, hexafluoroethane, and sulphur hexafluoride, along with some methane and nitrous oxide. The poster referred to this mix as the “Russell Cocktail,” which was what his Echus Overlook team had called it in the old days. The halocarbons in the cocktail were powerful greenhouse gases, and the best thing about them was that they absorbed outgoing planetary radiation at the 8-to 12-micron wavelength, the so-called “window” where neither water vapor nor CO₂ had much absorptive ability. This window, when open, had allowed fantastic amounts of heat to escape back into space, and Sax had decided early on to attempt to close it, by releasing enough of the cocktail so that it would form ten or twenty parts per million of the atmosphere, following the classic early modeling on the subject by McKay et al. So from 2042 on, a major effort had been put into building automated factories, scattered all over the planet, to process the gases from local sources of carbon and sulphur and fluorite, and then release them into the atmosphere. Every year the amounts pumped out had increased, even after the twenty parts per million level had been reached, because they wanted to retain that proportion in an ever-thickening atmosphere, and also because they had to compensate for the continual high-altitude destruction of the halocarbons by UV radiation.

And as the tables in the Simmon poster made clear, the factories had continued to operate through 2061 and the decades since, keeping the levels at about twenty-six parts per million; and the poster’s conclusion was that these gases had warmed the surface by around 12°K.

Sax moved on, a little smile fixed on his face. Twelve degrees! Now that was something!-over twenty percent of all the warming they needed, and all by the early and continuous deployment of a nicely designed gas cocktail. It was elegant, it truly was. There was something so comforting about simple physics…

By now it was ten A.M., and a keynote talk was beginning by H. X. Borazjani, one of the best atmospheric chemists on Mars, concerning just this matter of global warming. Borazjani was apparently going to give his calculations of the contributions of all the attempts at wanning that had been made up until 2100, the year before the soletta had come into operation. After estimating individual contributions, he was going to try to judge whether there were any synergistic effects taking place. This talk was therefore one of the crucial talks of the conference, as so many other people’s work was going to be mentioned and evaluated in it.

It took place in one of the biggest meeting rooms, and the chamber was packed for the occasion, a couple of thousand people in there at least. Sax slipped in right at starting time, and stood at the back behind the last row of chairs.

Borazjani was a small dark-skinned white-haired man, speaking with a pointer before a large screen, which was now showing video images of the various heating methods that had been tried: black dust and lichen on the poles, the orbiting mirrors that had sailed out from Luna, the moholes, the greenhouse gas factories, the ice asteroids burning up in the atmosphere, the denitrifying bacteria, and then all the rest of the biota.

Sax had initiated every single one of these processes in the 2040s and ‘50s, and he watched the video even more intently than the rest of the audience. The only obvious warming strategy that he had avoided in the early years was the massive release of CO₂ into the atmosphere. Those supporting this strategy had wanted to start a runaway greenhouse effect and create a CO₂ atmosphere of up to 2 bar, arguing that this would warm the planet tremendously, and stop UV radiation, and encourage rampant plant growth. All true, no doubt; but for humans and other animals it would be poisonous, and though advocates of the plan spoke of a second phase that would scrub the CO₂ from the atmosphere and replace it with a breathable one, their methods were vague, as were their time scales, which varied from 100 to 20,000 years. And the sky milk white however long it lasted.

Sax didn’t find this an elegant solution to the problem. He much preferred his single-phase model, striking directly toward the eventual goal. It meant they had always been a bit short on heat, but Sax judged that disadvantage worth it. And he had done his best to find replacements for the heat that CO₂ would have added, as for instance the moholes. Unfortunately Borazjani’s estimate of the heat released by the moholes was fairly low; altogether they had added perhaps 5°K to the mean temperature. Well, there was no getting around it, Sax thought as he tapped notes into his lectern- the only good source of heat was the sun. Thus his aggressive introduction of the orbiting mirrors, which had been growing yearly as sunsailers came out from Luna, where a very efficient production process made them from lunar aluminum. These fleets, Borazjani said, had grown large enough to have added some 5°K to the mean temperature.