So they set off back along the flagged trail, moving from one emerald dot to the next. But visibility continued to decrease, until they couldn't see from one marker to the next. Phyllis said, "Here, let's get into the shelter of those icebergs."

She struck off toward the dim shape of an ice prominence, and Sax hurried after her, saying, "Be careful, a lot of seracs have creva.s.ses at their base," and reaching forward to take her hand, when she dropped as if falling through a trap door. He caught an upflung wrist and was jerked down hard, hitting his knees painfully on the ice. Phyllis was still falling, sliding down a chute at the end of a shallow creva.s.se; he should have let go of her but instinctively held on, and was dragged over the edge head first. Both of them slid down into the packed snow at the bottom of the creva.s.se, and the snow gave under them so that they dropped again; crashing onto frosty sand after a brief but terrifying free-fall.

Sax, having landed mostly on Phyllis, sat up unhurt. Alarming sucking sounds came over the intercom from Phyllis, but it soon became clear that she had only had the wind knocked out of her. When she controlled her gasping she tested her limbs gingerly, and declared she was okay. Sax admired her toughness.

There was a rip in the fabric over his right knee; but otherwise he was fine. He took some suit tape from his thigh pocket and taped the rip; the knee still bent without pain, so he forgot about it and stood.

The hole that they had punched through the snow above them was about two meters over his outstretched hand. They were in an elongated bubble, the lower half of a creva.s.se that had a kind of hourgla.s.s shape. The downstream wall of their little bubble was ice, the upstream wall ice-coated rock. The rough circle of visible sky overhead was an opaque peach color, and the bluish ice wall of the creva.s.se gleamed with reflections of the dusty sunlight, so that the net effect was somewhat opalescent, and quite picturesque. But they were stuck.

"Our beeper signal will be cut off, and then they'll come looking," Sax said to Phyllis as she stood up beside him.

"Yes," Phyllis said. "But will they find us?"

Sax shrugged. "The beeper leaves a directional record."

"But the wind! Visibility may go right down to nothing!"

"We'll have to hope they can deal with it."

The creva.s.se extended to the east like a narrow low hallway. Sax ducked under a low point, and shone his headlamp down the s.p.a.ce between ice and rock; it extended for as far as he could see, in the direction of the east side of the glacier. It seemed possible that it might reach all the way to one of the many small caves on the glacier's lateral edge, so after sharing the thought with Phyllis he set off to explore the creva.s.se, leaving her in position to be sure that any searchers who found the hole would also find someone at the bottom of it.

Outside the glary cone of his headlamp's beam, the ice was an intense cobalt blue, an effect caused by the same Rayleigh scattering that blued the color of the sky. There was a fair amount of light even with his headlamp off, which suggested that the ice overhead was not very thick. Probably the same approximate thickness as the height of their fall, now that he thought of it.

Phyllis's voice in his ear asked if he was all right.

"I'm fine," he said. "I think this s.p.a.ce might have been caused by the glacier running over a transverse escarpment. So it very well might run all the way out."

But it didn't. A hundred meters farther on, the ice on the left closed in and met the ice over the rockface to the right, and that was it: dead end.

On the way back he walked more slowly, stopping to inspect cracks in the ice, and bits of rock underfoot that had perhaps been plucked from the escarpment. In one fissure the cobalt of the ice turned blue-green, and reaching into it with a gloved finger, he pulled out a long dark green ma.s.s, frozen on the surface but soft underneath. It was a long dentritic ma.s.s of blue-green algae.

"Wow," he said, and plucked a few frozen strands away, then shoved the rest back into their home crack. He had read that algae were burrowing down into the rock and ice of the planet, and bacteria were going even deeper; but actually to find some buried down here, so far from the sun, was enough to make one marvel. He turned off his headlamp again, and the luminous cobalt blue of the glacial light glowed around him, dim and rich. So dark, so cold, how did any living thing do it?

"Stephen?"

"I'm coming. Look," he said to Phyllis when he returned to her side, "it's blue-green algae, all the way down here."

He held it out for her to look at, but she only gave it the briefest glance. He sat down and got out a sample bag from his thigh pocket, and put a small strand of algae inside, then stared at it through his 20ayen; magnifying lenses. The lenses were not powerful enough to show him all he wanted to see, but they did reveal the long strands of dentritic green, looking slimy as they thawed out. His lectern had catalogs with photos at similar magnifications, but he couldn't find the species that resembled this one in every detail. "It could be nondescript," he said. "Wouldn't that be something. It really makes you wonder if the mutation rate out here is higher than the standard rates. We should work up experiments to determine that."

Phyllis did not reply.

Sax kept his thoughts to himself as he continued to search through the catalogs. He was still at it when they heard scratchy squeals and hisses over their radio, and Phyllis began calling out over the common band. Soon they could hear voices on the intercom, and not long after that, a round helmet filled the hole overhead. "We're here!" Phyllis cried.

"Wait a second," Berkina said, "we've got a rope ladder for you."

And after an awkward swinging climb they were back on the surface of the glacier, blinking in the dusty fluctuating daylight, and crouching over to meet the gusts of wind, which were still powerful. Phyllis was laughing, explaining what had happened in her usual manner-"We were holding hands so we didn't lose each other, and boom, down we went!"- and their rescuers were describing the brute force of the strongest gusts. All seemed back to normal; but when they got inside the station, and took off their helmets, Phyllis gave him a brief searching glance, a very curious look indeed, as if he had revealed something to her out there which had made her wary- as if he had somehow reminded her of something, down in that creva.s.se. As if he had behaved down there in a manner which gave him away, without hope of contradiction, as her old comrade Saxifrage Russell.

Through the northern fall they worked around the glacier, and saw the days grow shorter, and the winds colder. Big intricate ice flowers grew on the glacier every night, and only melted at the edges briefly in the midafternoons, after which they hardened and served as the base for even more complex petals that appeared the next morning, the small sharp crystalline flakes bursting away in every direction from the larger fins and tines beneath. They could not help crushing entire fractal worlds with every step as they crunch-crunched over the ice, looking for the plants now covered in frost, to see how they were coping with the coming cold. Looking across the b.u.mpy white waste, feeling the wind cut through one of the thicker insulated walkers, it seemed to Sax that a very severe winterkill was inevitable.

But looks were deceptive. Oh there would be winterkill, of course; but the plants were hardening, as the overwintering gardeners called it, acclimatizing to the onset of winter. It was a three-stage process, Sax learned, digging in the thin hard-packed snow to find the signs. First, phytochrome clocks in the leaves sensed the shorter days- and now they were getting shorter fast, with dark fronts coming through every week or so, dumping dirty white snow out of black low-bellied c.u.mulonimbus clouds. In the second stage, growth ceased, carbohydrates translocated to the roots, and amounts of abscisic acid grew in some leaves until they fell off. Sax found lots of these leaves, yellowed or brown and still hanging from their stems, hugging the ground and providing the yet living plant with some more insulation. During this stage water was moving out of cells into intercellular ice crystals, and the cell membranes were toughening, while sugar molecules replaced water molecules in some proteins. Then in the third and coldest stage, a smooth ice formed around the cells without rupturing them, in a process called vitrification.

At this point the plants could tolerate temperatures down to 220K, which had been approximately the average temperature of Mars before their arrival, but was now about as cold as it got. And the snow which fell in the ever more frequent storms actually served as insulation for the plants, keeping the ground that it covered warmer than the windy surface. As he dug around in the snow with numbed fingers, the subnivean environment looked to Sax to be a fascinating place, especially the adaptations to the spectrally selected blue light that was transmitted through as much as three meters of snow- another example of Rayleigh scattering. He would have liked to study this winter world in person for the entire six months of the season; he found he liked it out under the low dark waves of cloud, on the white surface of the snowy glacier, leaning into the wind and stomping through drifts. But Claire wanted him to return to Burroughs, to work with the labs there on a tundra tamarisk they were close to succeeding with in the Mars jars. And Phyllis and the rest of the crew from Armscor and the Transitional Authority were going back as well. So one day they left the station to a little crew of researcher-gardeners, and got in a caravan of cars, and drove back south together.

Sax had groaned when he heard that Phyllis and her group would be going back with them. He had hoped that mere physical separation would end the relationship with Phyllis, and get him away from that probing eye. But as they were going back together, it looked like some sort of action would have to be taken. He would have to break it off if he wanted it to end, which he did. The whole idea of getting involved with her had been a bad one to begin with; talk about the surge of the unexplainable! But the surge was over, and he was left in the company of a person who was at best irritating, and at worst dangerous. And of course it was no comfort to think that he had been acting in bad faith the entire time. No step along the way had seemed more than a little thing; but altogether it came to something rather monstrous.

So their first night back in Burroughs, when his wrist beeped and Phyllis appeared to ask him out to dinner, he agreed and ended the call, and muttered to himself uneasily. It was going to be awkward.

They went out to a patio restaurant that Phyllis knew of on Ellis b.u.t.te, west of Hunt Mesa. Because of Phyllis they were seated at a corner table, with a view over the high district between Ellis and Table Mountain, where the woods of Princess Park were ringed by new mansions. Across the park Table Mountain was so gla.s.s-walled that it looked like a giant hotel, and the more distant mesas were not much less gaudy.

Waiters and waitresses brought by a carafe of wine, and then dinner, interrupting Phyllis's chatter, which was mostly about the new construction on Tharsis. But she seemed very willing to talk with the waiters and waitresses, signing napkins for them, and asking where they were from, how long they had been on Mars, and so forth. Sax ate steadily and watched Phyllis, and Burroughs, waiting for the meal to come to an end. It seemed to go on for hours.

But finally they were done, and taking the elevator ride to the valley floor. The elevator brought back memories of their first night together, which made Sax acutely uncomfortable. Perhaps Phyllis felt the same way, for she moved to the other side of the car, and the long descent pa.s.sed in silence.

And then on the streetgra.s.s of the boulevard she pecked him on the cheek with a swift hard hug, and said, "It's been a lovely evening, Stephen, and a lovely time out at Arena as well, I'll never forget our little adventure under the glacier. But now I have to get back up to Sheffield and deal with everything that's been piling up, you know. I hope you'll come visit me if you're ever up there."

Sax struggled to control his face, trying to figure out how Stephen would feel and what he would say. Phyllis was a vain woman, and it was possible she would forget the entire affair faster if she was avoiding thought about the hurt she had caused someone by dropping him, rather than brooding over why he had seemed so relieved. So he tried to locate the minority voice inside him that was offended to be treated in such a manner. He tightened the corners of his mouth, and looked down to the side. "Ah," he said.

Phyllis laughed like a girl, and caught him up in an affectionate hug. "Come on," she admonished him. "It's been fun, hasn't it? And we'll see each other again when I visit Burroughs, or if you ever come up to Sheffield. Meanwhile, what else can we do? Don't be sad."

Sax shrugged. This made such sense that it was hard to imagine any but the most lovelorn suitor objecting, and he had never pretended to be that. They were both over a hundred, after all. "I know," he said, and gave her a nervous, rueful smile. "I'm just sorry the time has come."

"I know." She kissed him again. "Me too. But we'll meet again, and then we'll see."

He nodded, looking down again, feeling a new appreciation for the difficulties actors faced. What to do?

But with a brisk good-bye she was off. Sax said his own goodbye to a look over the shoulder, a quick wave.

He walked across Great Escarpment Boulevard, toward Hunt Mesa. So that was that. Easier than he had thought it would be, certainly. In fact, extremely convenient. But a part of him was still irritated. He looked at his reflection in the shop windows he pa.s.sed on the lower floors of Hunt. A raffish old geezer; handsome? Well, whatever that meant. Handsome for some women, sometimes. Picked up by one and used as a bed partner for a few weeks, then tossed aside when it was time to move on. Presumably it had happened to many another through the years, more often to women than to men, no doubt, given the inequalities of culture and reproduction. But now, with reproduction out of the picture, and the culture in pieces.... She really was rather awful. But then again he had no right to complain; he had agreed to it without conditions, and had lied to her from the very start, not only about who he was, but about how he felt toward her. And now he was free of it, and all that it implied. And all that it threatened.

Feeling a kind of nitrous oxide lift, he walked up Hunt's huge atrium staircase to his floor, and down the hall to his little apartment.

Late that winter, for a couple of weeks in 2 February, the annual conference on the terraforming project took place in Burroughs. It was the tenth such conference, t.i.tled by the organizers "M-38: New Results and New Directions," and it would be attended by scientists from all over Mars, nearly three thousand of them all told. The meetings were held in the big conference center in Table Mountain, while the visiting scientists stayed in hotels all over the city.

Everyone at Biotique Burroughs went over to attend the meetings, hurrying back to Hunt Mesa if they had experiments running that they wanted to check in on. Sax was intensely interested in every aspect of the conference, naturally enough, and on its first morning he went down early to Ca.n.a.l Park and grabbed a coffee and pastry, and walked up to the conference center and was nearly the first in line at the check-in table. He took his packet of program information, pinned his name tag to his coat, and wandered through the halls outside the meeting rooms, sipping his coffee, reading the program for the morning, and glancing at the poster displays set in designated parts of the halls.

Here, and for the first time in more years than he could remember, Sax felt supremely in his element. Scientific conferences were all the same, at all times and in all places, even down to the way people dressed: the men in conservative, slightly shabby professorial jackets, all tans and browns and dark rust colors; the women, perhaps thirty percent of the total population, in unusually drab and severe business dress; many people still wearing spectacles, even though it was a rare vision problem that was not correctable by surgery; most of them carrying around their program packets; everyone with their name tag on their left lapel. Inside the darkened meeting rooms Sax pa.s.sed talks that were beginning, and there too all was the same as ever: speakers standing before video screens that displayed their graphs and tables and molecular structures and so on, talking in stilted candences timed to the rhythm of their images, using a pointer to indicate the parts of overcrowded diagrams that were relevant.... The audiences, composed of the thirty or forty colleagues most interested in the work being described, sat in rows of chairs next to their friends, listening closely and readying questions that they would ask at the end of the presentation.

For those fond of this world, it was a very pleasant sight. Sax poked his head into several of the rooms, but none of the talks intrigued him enough to draw him in, and soon he found himself in a hall full of poster displays, so he kept on browsing.

"Solubilization of Polycyclic Aromatic Hydrocarbons in Monomeric and Micellar Surfactant Solutions." "Post-Pumping Subsidence in Southern Vast.i.tas Borealis." "Epithelial Resistance to Third-Stage Gerontological Treatment." "Incidence of Radial Fracture Aquifers in Impact Basin Rims." "Low-voltage Electroporation of Long Vector Plasmids." "Katabatic Winds in Echus Chasma." "Base Genome for a New Cactus Genera." "Resurfacing of the Martian Highlands in the Amenthes and Tyrrhena Region." "Deposition of the Nilosyrtis Sodium Nitrate Strata." "A Method for a.s.sessing Occupational Exposure to Chlorophenates Through a.n.a.lysis of Contaminated Work Clothing."

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 geographic.u.m Rhizocarpon geographic.u.m in the East Charitum Montes," detailing the high-alt.i.tude 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 Altoc.u.mulus Clouds in Cyclonic Vortexes in North Tharsis," a meteorological study of some importance. in the East Charitum Montes," detailing the high-alt.i.tude 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 Altoc.u.mulus 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 c.u.mulative 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 220K, 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 273K. Raising the average surface temperature of an entire planet by more than 53K was a very intimidating challenge, requiring, Sax had figured, the application over time of no less than 3.5 X 106 joules to every square centimeter of the Martian surface. Sax in his own modeling had always aimed to reach a mean of about 274K, 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 0.05K. And he could find nothing wrong with the various a.s.sumptions 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, h.e.l.las." "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 c.o.c.ktail," which was what his Echus Overlook team had called it in the old days. The halocarbons in the c.o.c.ktail 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 s.p.a.ce, and Sax had decided early on to attempt to close it, by releasing enough of the c.o.c.ktail so that it would form ten or twenty parts per million of the atmosphere, following the cla.s.sic early modeling on the subject by McKay et al 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-alt.i.tude 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 12K.

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 c.o.c.ktail. 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 warming 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 ma.s.sive 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 5K 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 5K to the mean temperature.

The reduced albedo, an effort which had never been very vigorously pursued, had added some 2 degrees. The two hundred or so nuclear reactors scattered around the planet had added another 1.5 degrees.

Then Borazjani came to the c.o.c.ktail of greenhouse gases; but instead of using the 12K figure from Simmon's poster, he estimated it was 14K, and cited a twenty-year-old paper by J. Watkins to support his a.s.sertion. Sax had spotted Berkina sitting in the back row near him, and now he sidled over and leaned down until his mouth was by Berkina's ear, and whispered, "Why isn't he using Simmon's work?"

Berkina grinned and whispered back, "A few years ago Simmon published a paper in which he had taken a very complex figure of the UV-halocarbon interaction from Borazjani. He modified it slightly, and that first time he attributed it to Borazjani, but after that when he used it he only cited his own earlier paper. It's made Borazjani furious, and he thinks Simmon's papers on this subject are derivative of Watkins anyway, so whenever he talks about warming he goes back to the Watkins work, and pretends Simmon's stuff doesn't exist."

"Ah," Sax said. He straightened up, smiling despite himself at Borazjani's subtle but telling little payback. And in fact Simmon was there across the room, frowning heavily.From BorazjaniBorazjani had not even included the windmill heaters, so on his lectern Sax did. Altogether it came to 37.55K, a very respectable step, Sax thought, toward their goal of 53+. They had only been going at it for sixty years, and already most summer days were reaching temperatures above freezing, allowing arctic and alpine plant life to flourish, as he had seen in the Arena Glacier area. And all this before the introduction of the soletta, which was raising insolation by twenty percent.

By now Borazjani had moved on to the warming effects of the water vapor and CO2 that had been released into the atmosphere, which he estimated together as adding another 10K. "Some of this might be called a synergistic effect," he said, "as the desorption of CO2 is mainly a result of other warming. But other than that I don't think we can say that synergy has been much of a factor. The sum of the warming created by all the individual methods matches pretty closely the temperatures reported by weather reports from around the planet."

The video screen displayed his final table, and Sax made a simplified copy of it into his lectern: From Borazjani 2 February 14, 2102: 2 February 14, 2102: Halocarbons: 14 H2O and CO2: 10 Moholes: 5 Pre-Soletta Mirrors: 5 Reduced Albedo: 2 Nuclear Reactors: 1.5 Borazjani had not even included the windmill heaters, so on his lectern Sax did. Altogether it came to 37.55K, a very respectable step, Sax thought, toward their goal of 53+. They had only been going at it for sixty years, and already most summer days were reaching temperatures above freezing, allowing arctic and alpine plant life to flourish, as he had seen in the Arena Glacier area. And all this before the introduction of the soletta, which was raising insolation by twenty percent.

The question period had begun, and someone brought up the soletta, asking Borazjani if he thought it was necessary, given the progress being made with the other methods.

Borazjani shrugged in just the way Sax would have. "What does necessary necessary mean?" he replied. "It depends how warm you want it. According to the standard model as initiated by Russell at Echus Overlook, it is important to keep CO2 levels as low as possible. If we do this, then other warming methods are going to have to be applied to compensate for the loss of the heat that CO2 would have contributed. The soletta might be thought of as compensating for the eventual reduction of CO2 to breathable levels." mean?" he replied. "It depends how warm you want it. According to the standard model as initiated by Russell at Echus Overlook, it is important to keep CO2 levels as low as possible. If we do this, then other warming methods are going to have to be applied to compensate for the loss of the heat that CO2 would have contributed. The soletta might be thought of as compensating for the eventual reduction of CO2 to breathable levels."

Sax was nodding despite himself.

Someone else rose and said, "Don't you think the standard model is inadequate, given the amount of nitrogen we now know we have?"

"Not if all the nitrogen is put into the atmosphere."

But this was an unlikely achievement, as the questioner was quick to point out. A fair percentage of the total would remain in the ground, and in fact was needed there for plants. So they were short on nitrogen, as Sax had always known. And if they kept the amount of CO2 in the air to the lowest levels possible, that left the percentage of oxygen in the air at a dangerously high level, because of its flammability. Another person rose to state that it was possible that the lack of nitrogen could be compensated for by the release of other inert gases, chiefly argon. Sax pursed his lips; he had been introducing argon into the atmosphere since 2042, as he had seen this problem coming, and there were significant amounts of argon in the regolith. But they were not easy to free, as his engineers had found, and as other people were now pointing out. No, the balance of gases in the atmosphere was turning out to be a real problem.

A woman rose to note that a consortium of transnats coordinated by Armscor was building a continuous shuttle system to harvest nitrogen from the almost pure nitrogen atmosphere of t.i.tan, liquefying it and flying it back to Mars and dumping it in the upper atmosphere. Sax squinted at this, and did some quick calculations on his lectern. His eyebrows shot up when he saw the result. It would take a very great number of shuttle trips to accomplish anything that way, that or else extremely large shuttles. It was remarkable that anyone had thought it worth the investment.

Now they were discussing the soletta again. It certainly had the capability of compensating for the 5 or 8K that would be lost if they scrubbed the current amount of CO2 from the air, and probably it would add even more heat than that; theoretically, Sax calculated on his lectern, it could add as much as 22K. The scrubbing itself would not be easy, someone pointed out. A man standing near Sax, from a Subarashii lab, rose to announce that a demonstration talk on the soletta and the aerial lens would occur later in the conference, when some of these issues would be greatly clarified. He added before sitting down that serious flaws in the singlephase model made the creation of a two-phase model nearly mandatory.

People rolled their eyes at this, and Borazjani declared that the next meeting in the room needed to begin. No one had commented on his skillful modeling, which had sorted out so plausibly all the contributions of the various warming methods. But in a way this was a sign of respect- no one had challenged the model either, Borazjani's preeminence in this area being taken for granted. Now people stood, and some went up to talk with him; a thousand conversations broke out as the rest filed out of the room and into the halls.

Sax went to lunch with Berkina, in a cafe just outside the foot of Branch Mesa. Around them scientists from all over Mars ate and talked about the events of the morning. "We think it's parts per billion." "No, sulfates behave conservatively." It sounded like the people at the table next to theirs were a.s.suming there was going to be a shift to a two-phase model. One woman said something about raising the mean temperature to 295K, seven degrees higher than Terra's average.

Sax squinted at all these expressions of haste, of greed for heat. He saw no need to be dissatisfied with the progress that had been made so far. The ultimate goal of the project was not purely heat, after all, but a viable surface. The results so far certainly seemed to give no reason for complaint. The present atmosphere was averaging 160 millibars at the datum, and it was composed about equally of CO2, oxygen, and nitrogen, with trace amounts of argon and other gases. This was not the mixture Sax wanted to see in the end, but it was the best they had been able to do given the inventory of volatiles they had to begin with. It represented a substantial step on the way to the final mix Sax had in mind. His recipe for this mix, following the early Fogg formulation, was as follows: 300 millibars nitrogen 160 millibars oxygen 30 millibars argon, helium, etc.

10 millibars CO2 = Total pressure at datum, 500 millibars All these amounts had been fixed by physical requirements and limits of various kinds. The total pressure had to be high enough to drive oxygen into the blood, and 500 millibars was what was obtained on Earth at about the 4,000-meter elevation, near the upper limit of what people could live at permanently. Given that it was near the upper limit, it would be best if such a thin atmosphere had more than the Terran percentage of oxygen in it, but it could not be too much more or else fires might be hard to extinguish. Meanwhile CO2 had to be kept below 10 millibars, or else it would be poisonous. As for nitrogen, the more the better, in fact 780 millibars would be ideal, but the total nitrogen inventory on Mars was now estimated at less than 400 millibars, so 300 millibars was as much as one could reasonably ask to put into the air, and perhaps more. Lack of nitrogen was in fact one of the biggest problems the terraforming effort faced; they needed more than they had, both in the air and in their soil.

Sax stared down at his plate and ate in silence, thinking hard about all these factors. The morning's discussions had given him cause to wonder whether he had made the right decisions back in 2042- whether the volatile inventory could justify his attempt to go straight for a human-viable surface in a single stage. Not that there was much that could be done about it now. And all things considered, he still thought they were the right decisions; shikata ga nai shikata ga nai, really, if they wanted to walk freely on the surface of Mars in their own lifetimes. Even if their lifetimes were going to be considerably extended.

But there were people who seemed more concerned with high temperatures than breathability. Apparently they were confident that they could balloon the CO2 level, heat things tremendously, and then reduce the CO2 without problems. Sax was dubious about that; any two-phase operation was going to be messy, so messy that Sax couldn't help wondering if they would get stuck with the 20,000-year time scales predicted in the earliest two-phase models.

It made him blink to think of it. He couldn't see the need. Were people really willing to risk such a long-term problem? Could they be so impressed by the new gigantic technologies that were becoming available that they believed anything was possible?

"How was the pastrami?" Berkina asked.

"The what?"

"The pastrami. That's the kind of sandwich you just ate, Stephen."

"Oh! Fine, fine. It must have been fine."

The afternoon's sessions were mostly devoted to problems caused by the successes of the global warming campaign. As surface temperatures rose, and the underground biota began to penetrate deeper into the regolith, the permafrost down there was melting, just as hoped. But this was proving disastrous in certain permafrost-rich regions. One of these, unfortunately, was Isidis Planitia itself. A well-attended talk by an areologist from a Praxis lab in Burroughs described the situation; Isidis was one of the big old impact basins, about the size of Argyre, with its northern side completely erased, and its southern rim now part of the Great Escarpment. Underground ice had been creeping off the Escarpment and pooling in the basin for billions of years. Now the ice near the surface was melting, and in the winters freezing again. This thaw-freeze cycle was causing frost heaving on an unprecedented scale; it was pretty near the usual two-magnitude enlargement compared to similar phenomena on Earth, and karsts and pingos a hundred times the size of their Terran a.n.a.logues were big holes, and big mounds. All over Isidis these giant new holes and hummocks were blistering the landscape, and after her talk and a sequence of mind-boggling slides, the areologist led a large group of interested scientists to the south end of Burroughs, past Moeris Lacus Mesa to the tent wall, where the neighborhood looked like it had been devastated by earthquake, the ground having heaved up to reveal a rising ma.s.s of ice like a bald round hill.

"This is a fine specimen of a pingo," the areologist said with a proprietary air. "The ice ma.s.ses are relatively pure compared to the permafrost matrix, and they act in the matrix the same way rocks do- when the permafrost refreezes at night or in winter, it expands, and anything hard stuck in this expansion gets pushed upward toward the surface. There's a lot of pingos in Terran tundra, but none as big as this one." She led the group up the shattered concrete of what had been a flat street, and they stared out from an earthen crater rim, onto a mound of dirty white ice. "We've lanced it like a boil, and are melting it and piping it into the ca.n.a.ls."

"Out in the country one of these coming up would be like an oasis," Sax remarked to Jessica. "It would melt in the summer, and hydrate the ground around it. We ought to develop a community of seeds and spores and rhizomes that we could scatter on any sites like this out in the country."

"True," Jessica said. "Although, to be realistic, the permafrost country is mostly going to end up under the Vast.i.tas sea anyway."

"Hmm."

The truth was Sax had temporarily forgotten the drilling and mining in Vast.i.tas. When they had returned to the conference center, he deliberately looked for a talk describing an aspect of that work. There was one at four: "Recent Advances in North Polar Lens Permafrost Pumping Procedures."

He watched the speaker's video show impa.s.sively. The lens of ice that extended underground from the northern polar cap was like the submerged part of an iceberg, containing some ten times as much water as the visible cap. The Vast.i.tas permafrost contained even more. But getting that water to the surface... like the retrieval of nitrogen from t.i.tan's atmosphere, it was a project so ma.s.sive that Sax had never even considered it in the early years; it simply hadn't been possible then. All these big projects- the soletta, the nitrogen from t.i.tan, the northern ocean drilling, the frequent arrival of ice asteroids- were on a scale that Sax found he was having trouble adjusting to. They were thinking big these days, the transnationals. Certainly the new abilities in design and in materials science, and the emergence of fully self-replicating factories, were what made the projects technically feasible; but the initial financial investments were still huge.

As for the technical capabilities involved, he found himself adjusting to the idea of them fairly rapidly. It was an extension of what they had done in the old days: solve some initial problems in materials, design, and homeostatic control, and one's powers grew very considerable indeed. One might say that their reach no longer exceeded their grasp. Which, given the directions their reach sometimes took, was a frightening thought.