The overcast had cleared completely the next day. The sky was preternaturally blue, and the wind had become a steady near-gale from the north. Leah and I worked inside. Tally still did her reconnaissance patrol outside, but I think that even she must have spent much of her time huddled in the wind screen of one or another of the boulders. Now we knew what had scattered the pieces of the habitat.

The missing iron, as it turned out, wasn't a mystery at all. Once Leah realized what to look for, she found it easily enough, in the form of grit scattered in with the rest of the habitat pieces.

"It's a sulfur rich planet," she said. "I should have thought of it. In the year and a half of exposure, everything iron or steel got converted to iron sulfide.

It looked just like part of the regolith, so I overlooked it the first time."

"In just a year?" I asked. "Isn't that kinda fast?"

Leah shrugged. "Seems fast to me, too, but don't forget the UV. The surface here is more reactive than we're used to."

I worked on deciphering their electronic records. They hadn't kept personal logs, or perhaps if they had, they were on some optical I hadn't found yet. The opticals I had were mostly data, with occasional notes about where or how the samples were collected. By afternoon I had enough to determine when the last data had been recorded, and could at least put a date to the disaster.

"Sometime on August tenth," I told Leah. "Two years ago."

"Really," Leah said. "That's interesting."

"Interesting?" I said. "Not really. But you asked me for a date."

"No, but it is interesting," Leah said. "Today is June 23rd."

"So?"

"That's Earth reckoning, of course. The Mars year is 687 Earth days long-- one year, ten months and a few weeks. So, in Mars reckoning, it's nearly the first anniversary of the disaster. Five days from now, in fact."

"Spooky," I said.

"No, I wouldn't call it spooky," she said. "But it is an odd coincidence."

I marked it on the calender.

I liked working alone with Leah, with Tally outside on patrol. I didn't exactly resent Tally, but I did sometimes envy her effortless camaraderie with Leah. I welcomed the chance to be alone with her, even though, for the most part, we worked in silence.

"Tinkerman," Leah said.

"Yes?"

"Once you start getting the data you've recovered indexed, do a search on weather for me."

I shrugged. "No problem." I looked at her. "You think it's relevant to the investigation?"

She shook her head. "Just curious."

They had, I discovered, not taken detailed observations of the Martian weather.

But occasionally there was a mention of conditions outside. Their own experience mirrored ours. About the same time in the Martian year, the overcast had cleared, and a steady wind had arisen out of the north. The day before the disaster, data had been marked with a note that samples from two sites had been missed; the wind had blown away the stakes marking the site locations.

On another optical I found satellite photos of Mars. I looked at these with interest. The weather clearing we'd seen wasn't local to the Syrtisian saddle; the photos showed the northern hemisphere completely obscured by cloud cover, and then a sudden clearing across the entire hemisphere. The view must have been an infrared falsecolor, since the ocean was white and the land areas, in contrast, looked nearly black. I checked the dates on the photos, and converted them in my head into Martian season. The clearing started at just about the end of northern hemisphere spring.

Leah nodded when I showed her what I'd recovered. She'd already radioed up to ask Langevin for orbital photographs, and he'd confirmed that the clearing of the clouds we'd seen was ubiquitous, starting with breaks in the cloud cover at northern mid-lat.i.tudes, then slowly spreading south. "Apparently it's a seasonal thing."

Langevin had also mentioned that the rover had arrived, after a long slow transit from the Moon. Did we still want it? Where should he set it down?

Oh, yes, we still wanted it.

"Time for a vacation!" Tally said, when the unpiloted utility lander had dropped the rover off and I had checked out the systems and declared it fully functional. The rover was the same awful shade of yellow-green as the lander had been, a color chosen for maximum contrast against the browns and purples of Mars. It had six webbed wheels mounted on a rocker-bogey suspension that would give it incredible hill-climbing ability; I had little doubt that it would have been able to crawl right over the hab-lab, if an incautious pilot had tested poorly on navigation. I said as much to the team after the brief test drive.

"Are you seriously suggesting that the habitat was crushed by a rover?" Leah said. "No tread marks were found on any of the pieces we found."

"A rover would have left tracks," Tally said. "Even after two years, we'd have see them."

I shook my head. "No," I said. "I was just giving an example of how robust the suspension is."

"I see."

"So," Tally said. "Time for a trip."

"A trip" Leah said. "Why not? Where did you want to go?"

"Why not go the beach?" Tally said. "Head north. See what a Mars ocean is like."

"Mmm," Leah said. "Not today. I'll still be busy tomorrow, too, I think. Maybe the next day."

"Copacetic," said Tally. "I wouldn't mind a day to do some long-range recon with the rover, anyway. That is, if Tink says it's checked out okay?"

"All systems in perfect shape," I said. "No reason for you not to drive around a bit."

A lot of the work Leah asked me to do seemed to have nothing to do with the investigation of the accident. She was conducting her own investigation, I decided, a scientific investigation of the progress of terraforming-- no, ecopoiesis-- on Mars. She had me decipher all the data I could out of the opticals; data on bacteria counts and atmosphere, and checked it against the measurements she could make herself. "Cripes, I wish I were a biologist," had become her favorite phrase, muttered as she stared into the screen of a microscope, counting bacteria, but she was clearly happy doing the work, and I was happy to a.s.sist, to do anything that made Leah happy.

More methane in the atmosphere, she said, at a break. Some ethane, ethylene, even acetylene. And quite a bit more oxygen than expected.

"Oxygen and methane? Isn't that explosive?"

"No, oxy is still way under one percent; all in all, it's still mostly a reducing atmosphere. The hydrocarbons are all greenhouse gases."

"Gaia," I said, suddenly realizing what she was getting at.

"Gaia," she agreed, a soft smile creeping slowly across her face. The bacteria were producing greenhouse gases, warming the planet up. Making it a better abode for life.

I was getting bored with the claustrophobic s.p.a.ces of the habitat, and the sameness of the landscape, and I was sure that Leah and Tally were as well. We were all looking forward to the jaunt north to the sh.o.r.es of the Boreal ocean.

So I was rather surprised when, at breakfast on the morning designated, Tally shook her head, and said, "It'll be just you two loverbirds. I'm not coming."

I pretended interest in my food. I never could guess how Leah would react. For me, the idea of a trip in the pressurized rover, a thousand-kilometers alone with Leah, was as close to heaven as I was likely to ever find.

"Why?" Leah said.

Tally smiled. "A trap."

Despite a.s.siduous searching, Tally had found no evidence whatsoever of sabotage.

Anybody else would have said, that means it was an accident. Tally said: that means that they were clever.

We made a great show of our departure, deliberately packing the rover slowly and openly with all the supplies for three people to take an extended trip. Then all three of us got in. From outside, through the bubble canopy, it would be clear that three people were in the piloting compartment, eagerly watching the terrain.

It would be impossible to tell that one of the three was no more than a dummy constructed of spare clothing.

Once aboard, I powered up the rover, and it rose up from its squatting position to its full height above the Martian terrain. I checked all the systems one more time, testing each wheel in turn for forward and reverse power, making skid-marks through the brown grit and tossing muck across the landscape. The bacteria would not care; they would thrive in one spot quite as well as another.

If somebody had bombed the first habitat, and was clever enough and subtle enough to betray no sign of themselves, they must be flushed out of hiding. They might be complacent enough to try the same trick again, if they were thoroughly convinced that n.o.body was watching. Tally wanted to give them that chance. Tally wanted to watch them set the bomb.

Systems all functional. I had a wild urge to wave goodbye to Tally, but that would never do. We set off with no ceremony.

For hundreds of kilometers we looked at brown rocks, covered with a thin veneer of slime.

The wind got stronger as we drove north toward the ocean. The landscape was monotonous; rocks and rilles and tiny rivers, broken by lakes, each lake in the form of a perfect circle, reflecting the too-blue sky. To our left, the ground sloped gently up toward the ancient volcano whose flanks we were skirting. The actual summit of the volcano was invisible over the horizon. When we crossed the peak of the Syrtis saddle the wind was coming straight at us at well over a hundred kilometers an hour. It was enough to slow the rover's progress considerably, and at places I almost worried that the wind would pick us up and blow us backwards, but the rover's six huge wheels held traction superbly, and kept us moving.

Once across the pa.s.s, the wind dropped a bit, but never let up entirely. It was constant, unwavering from the north.

The rover drove itself, if we let it, with infrared laser-stripers searching out obstacles in front of it and a mapping program in its computer brain that continually compared the view against the inertial navigation and the stored satellite maps, to compute an optimal traverse across the rippled terrain. For most of the first day, Leah and I took turns driving, following the computer's suggested path sometimes, diverting to a different route that looked smoother or more interesting when the whim struck. By the afternoon, the novelty of the drive had slackened, and we let the rover pick its own path.

Langevin had left Mars...o...b..t days earlier, but he had left behind him a little areosynchronous communications relay, so we could have stayed in touch with Tally at the habitat if we had desired to. We kept radio silence, though, by agreement: Tally had said that we should a.s.sume that any radio communications we made would be heard by the enemy. The relay had enough power to let us send reports directly to s.p.a.cewatch. We transmitted our daily report back, essentially just a "yes, we're alive" verification, and in the report we included a recorded snippet of Tally's voice, to maintain Tally's deception to her hypothesized snooping ears.

In the middle of the afternoon, the rover crested a rise and angled off to the west, finding a smoother traverse down the slope to avoid a field of boulders the size of skysc.r.a.pers. Leah was in the aft cabin, a.n.a.lyzing data she had brought with her, and I was alone in the c.o.c.kpit. At first I didn't know what I was seeing, looking north. The horizon was white.

This was the highest ridge between us and the ocean, so, looking north, I ought to be able to see the ocean. Was the ocean covered with ice? I overrode the autopilot and parked the rover for a moment, rummaging for binoculars to get a better view. Leah came up from the cabin.

"The ocean's white," I said.

"Odd." Leah looked at it, pondering. "Not ice; it's nearly northern summer, and the ice melted months ago. Whitecaps, from the wind, maybe. We'll see soon enough, if we keep driving."

I took that as advice, and brought the autopilot back on line. The rover started to roll. Leah reached out an arm to steady herself against a handbar, and kept on standing, looking out the bubble at the horizon.

We didn't reach the Boreal Ocean that evening. The autonavigation on the rover was perfectly capable of continuing its traverse after dark, but we were no more than thirty kilometers from the ocean, and we elected to shut down for the night, so that our arrival at the ocean would be in daylight.

After nine hours of motion, the cabin still seemed to rock with the motion of an imaginary traverse, although I had squatted the rover in the lee of a hundred-meter escarpment.

The workstations of the aft cabin folded away into panels on the walls, and two narrow cots folded out from the bulkhead, transforming the cabin into a small but cozy bedroom. I looked at the cots, and at Leah. The cots were narrow, but looked like they might be wide enough for two, if the two slept close. Leah gave me no hints. I folded the second cot back into its niche, and convinced myself that I saw just the faintest trace of a smile on Leah's face. In any case, she slid over silently, and I nestled myself in next to her.

We reached the ocean a bit before noon of the next day. The final few kilometers was a steep traverse down the bluffs, not quite steep enough to be called cliffs, but steep enough that the rover picked its way slowly, sidling nearly crabwise down the last few hundred meters. There wasn't much of a beach; just rocks. From above, the ocean was white. It moved with something more than just the rhythmic swell of waves. It writhed, and humped, looking almost alive. As we got closer, a fine spray peppered the bubble in erratic spurts. The spray dried to milky white flakes, smearing but not totally obscuring the view.

"Salt?" I said.

Leah shook her head. "Magnesium sulfate, mostly," she said. She spoke louder than normal to be heard over the whistling of the wind and a sudden patter of spray. "The ocean's got tons of it. It's another reason the ocean doesn't freeze solid in the winter; lowers the freezing point a few degrees."

I squatted the rover down behind a boulder, where it would be out of the worst of the spray, and we suited up with rebreathers and sunblock to go outside.

Outside, the constant wind was warm and damp. Between the wind and the spray, I think that it was the most miserable place on Mars. Leah, though, laughed and ran like a little girl, arching her back and spreading her arms, daring Mars to do its worst.

I took off one glove, raised my hand and caught a bit of spray on my fingers, then pulled up my rebreather mask slightly to put it to my tongue. It was slightly bitter. Leah looked back at me over her shoulder, and laughed. "Don't eat too much of it," she shouted.

"Why?" I shouted back. "It's not poisonous."

"You might regret it," she shouted back. "You know what they used magnesium sulfate for in the old days?"

"What?"

"Laxative for infants! You're standing right next to the universe's largest dose of baby laxative!"

With that she turned back, and started to pick her way past the rocks toward the ocean. I scrambled to catch up with her. I could hear the ocean now, but it wasn't the rolling of waves that I heard. It was a stranger sound, hissing and popping and splatting.

In a few moments we reached a final set of rocks, right at the edge of the ocean, and at last we could observe what we had been unable to see from further away.

The ocean was boiling.

From the pools at our feet to the farthest horizon, the entire ocean was aboil, bubbles rising up and breaking, spattering spray everywhere. Enormous bubbles rose burping out of the depths with a thunderous roar followed by a tremendous splatter; smaller bubbles rose with blurps and pops from everywhere; infinitesimally tiny bubbles fizzed and hissed in rocky pools.

An huge bubble burst in front of us, not five meters distant, and I instinctively flinched, antic.i.p.ating being hit with scalding spray. Leah laughed with delight. She pulled her glove off and, when the slosh came toward her, bent over and dipped her bare hand into the boiling water. Before I could scream at her, she cupped a handful of water and, with a grin so large I could see it even behind her rebreather, she dashed it in my face and giggled.

The water was lukewarm.

When we got back in the rover, our coveralls were so stiff with dried spray that it was difficult to peel them off. Our faces and hands were red from the wind, and itchy with dried ocean. Leah was still in her puckish good mood, and as we peeled down to undergarments, she was laughing.

"You know what?" she said, pulling off her rebreather, and she didn't bother to wait for an answer. "You know the great thing about it? Makes it worth the whole trip?"

"What's that?"

"You don't stink!"

I opened my mouth to say something, and suddenly realized she was right. The stench of Mars that we had gotten so used to every time we came in from the outside, was missing.

"What a great planet," she said.

We both stripped, and gave one another sponge baths. The water recycler would have the devil of a time pulling sulfate out of the water, but that was what machinery was for. I took a lot longer cleaning her off than I had any right to, and with one thing leading to another, it was nearly dark before either of us dressed.

I knew she was waiting for me to ask. At last I did. "Leah? The water was warm, but it wasn't hot. Why was it boiling?"