"That's an easy one. It wasn't."

"But--"

"Carbon dioxide," she said. "I should have known, but it wasn't obvious until I saw it. Mars has mostly carbon dioxide in the atmosphere, so it should have been obvious that the oceans would be saturated with dissolved CO-two. It wasn't boiling-- it was fizzing."

That made sense, all but one thing. "But, wouldn't it be in equilibrium? Why should it be fizzing?"

"Summer. The ocean is warming up in the summer sun. Carbon dioxide has a solubility in water that strongly decreases when it gets warmer. So, as summer comes to the northern hemisphere, the Boreal ocean releases carbon dioxide."

"Oh."

And it wasn't until the middle of the night that she suddenly stiffened and sat bolt upright. "Oh," she said, in a tiny voice. I opened my eyes and watched her sleepily. "The wind," she said. "The wind."

She got up, and in a moment there was a glow as her computer came alight. She was beautiful, limned in pale fire by the glow cast by the screen backlighting.

"What is it?" I said.

"Nothing. Go back to sleep."

"It must be something."

"Just-- I had a thought, that's all."

"What?"

"I wonder." She bit her lip. "Just how much carbon dioxide, exactly, do you think is dissolved in the Boreal Ocean?"

By the time the sky started to brighten with dawn, Leah was distinctly bedraggled, but she had it mostly worked out. The answer was, a lot. A h.e.l.l of a lot.

Over the long Martian winter, the temperature of the northern ocean dropped to near freezing, and the ocean had served as a sponge for carbon dioxide. A peculiar convection served to stir the ocean as it cooled: as the surface layers cooled and became saturated with carbon dioxide, they got denser, and sank, turning over the ocean until the entire ocean was uniformly cold and saturated with carbon dioxide.

When the spring began, the surface layers of the ocean warmed up, and the dissolved carbon dioxide began to come out of solution. But the warmer water, free of its heavy carbon dioxide, stayed on the surface; the cold, saturated water stayed below. With only two tiny moons, there was little in the way of tides to stir the deeps. The water got warmer, but in the deep water, the dissolved carbon dioxide was under pressure. The water warmed a little, but the supersaturated carbon dioxide stayed in solution.

But it was an unstable situation, and ever more precarious as the season moved toward summer. Eventually, something must trigger the inevitable. Somewhere, a little of the carbon dioxide came out of solution, at pressure, and formed bubbles. The bubbles stirred the water, expanding as they rose, and the stirring let more carbon dioxide out of solution. The warm surface waters turned over, and supersaturated cold waters from the depths warmed up. Like a chain reaction, the release of supersaturated carbon dioxide was almost explosive, and it took only days for the reaction to spread across the entire width of the Boreal ocean. A whole winter's worth of atmosphere was coming out of the ocean, and coming out with vigor.

The wind. We had felt the wind from the ocean, a clue blowing right in our faces, and we'd ignored it.

"They weren't murdered, Tinkerman," Leah said. "They were-- my G.o.d, Tally's still back there, in the habitat. She doesn't know-- The radio. We can get her on the radio, warn her."

"Doesn't know what?"

"I'll explain everything when I talk to her. Quick, what day is it?" She grabbed my calender and looked at it. In neat letters, on the bottom corner of the square marked June 28, I had completely forgotten that I'd written a note: One Martian year. RIP.

But Tally didn't answer the radio, not the regular channels, not the emergency channel.

"d.a.m.n," I said. "It's Tally and her blasted radio silence. She won't answer."

Leah shook her head violently. "I know Tally better than that. She would listen to the emergency channel no matter what, and she'd answer when she heard us break silence. Tinkerman, I think the wind must have torn away the radio aerial.

The hab was designed for s.p.a.ce, not for Mars, and the antenna wasn't that strongly mounted. Probably blew over the high-gain antenna as well. "

"So?"

"So how fast do you think this thing can go?"

It took longer to get moving than I had expected. The autonavigator wouldn't come on line. Over the night the spray had fogged over the lenses of the laser stripers, and the autopilot wouldn't budge without its obstacle-recognition system working. As I took the rover up the bluff on manual control, climbing only centimeters at a time over the rough spots, Leah fidgeted with clear agitation, but she stayed silent, knowing that distracting me from piloting would only slow us down. As soon as we had climbed a few hundred meters above the ocean, I put on a rebreather and, using half our supply of clean water, I carefully washed the laser striper and the bubble.

The steel parts of the rover looked matte, almost corroded. When we got back, I would have to take the rover down for inspection and overhaul. In fact, I would have preferred to do a thorough inspection right then, but I knew Leah wouldn't let me stop for that. The rover's autodiagnostic checked out green, so I put the autopilot back on line and punched for speed.

There was nothing more we could do. There was no way that I could out-pilot the autonavigation system over a course it had run before; it had all the bad terrain memorized in detail and had learned exactly which parts to detour around and which were smooth running. The ride was b.u.mpy, but that was only to be expected. I turned to Leah, and waved a hand.

"I'm ready to listen," I said.

"It was all there in front of us," Leah said. "All the clues, if only we'd really seen them. The pieces of the habitat, that should have tipped us off right there. The habitat modules, they weren't originally designed for Mars. We knew that. n.o.body ever goes to Mars, so how could there be hab modules designed for it? It's a lunar habitat design.

"The air pressure on Mars is five hundred millibars, just about half that of Earth. So we set the pressure in the hab to five hundred millibars, and forgot about it. With a nearly fifty-fifty mixture of oxygen and nitrogen in the air mixture, the oxygen in the habitat was just what it is at standard conditions, and after a week I bet you didn't even remember that it wasn't Earth standard.

"But there's one critical diffence. Lunar habitat modules are designed to withstand pressure from the inside. They're plenty strong, against internal pressure. But what about external pressure?"

"It imploded."

"Right. The air pressure on Mars is not a constant! All that gas dissolved in the northern sea-- when it comes out of solution, the air pressure rises. It rises a lot. The wind, that constant wind from the north-- that was our second clue. The habitat was set to maintain a constant pressure of five hundred millibars inside. n.o.body ever designed it with the idea that the outside pressure might increase. Somewhere there was a weak joint, maybe a seam that wasn't reinforced against an unexpected pressure from outside. It blew."

"But there was an explosion. We saw the marks."

Leah shook her head. "You saw the piece, the one with the tiny sc.r.a.pe of blue paint on it. What does blue paint mean to you?"

I only had to think for an instant. "Blue. Oxygen."

"Right. The implosion must have punctured an oxygen tank in the habitat. Pure oxygen, under pressure, spurting out into the Mars atmosphere.... the Martian atmosphere is mostly carbon dioxide, but a good component is methane, and it's got noticeable amounts of other hydrocarbons as well. In a pure oxygen leak, of course it will burn."

"It must have happened at night," I said. "They never knew what hit them. The one man was killed instantly. The other was tossed out of the hole in the side of the habitat, without a rebreather, to die of suffocation."

Leah nodded. "And now the same thing is happening. The atmospheric pressure is rising. Tally's there in the habitat, alone... and she's waiting for the wrong enemy."

We were over the peak of the Syrtis saddle and a good way into the long, slow downhill toward the h.e.l.las basin, only a hundred kilometers from the hab, when the wheel fell off. Leah was on the radio, in the unlikely hope that perhaps the synchronous relay was the problem, and now that we were approaching line of sight conditions, direct communication might raise Tally. The wheel came off with a resounding snap, and the rover lurched.

The autopilot diagnosed the problem, instantly rebalanced the suspension to keep the weight away from of the missing wheel, and smoothly braked us to a stop, blaring alarms.

The alarms were a little late.

We both went outside to look. It was the right rear wheel that had failed; we found it a few dozen meters further on, where it had rolled up against a rock.

The wheel itself was a t.i.tanium-alloy mesh, light enough to carry in one hand, for all that it was nearly two meters in diameter. The wheel bearing was steel.

Or, it had originally been steel, when it had been there at all. There was little left of it.

"Well," Leah said.

"Well," I said. There was no way to replace a wheel; they weren't supposed to come off. "I think maybe we can rebalance the rover. Shift the loading to the front left side. Five wheels ought to be enough. We might have to go a bit slower."

Leah nodded. "It's a plan."

We piled rocks onto the rover, and strapped them down with bungees, to move the center of gravity forward off of the missing wheel. Then we piled more rocks inside the rover, in the front left pilot's seat. I didn't mention that we would never get the Mars stink out of the rover; it was too late to worry about that, and we barely noticed it by then anyway. The autopilot refused to budge so much as a meter without an overhaul, so I piloted it on manual. This was good for less than a third the speed of the autopilot, but still, even that pace covered ground. Leah went back into the aft cabin to examine the samples she had sc.r.a.ped off of the wheel.

It was only a hundred kilometers. We finished more than fifty of them before the second wheel fell off.

We were going more slowly this time. There was no lurch, and no noise. The rover just slowly careened to the right, and kept on rolling until it slid to a stop on its side.

Leah came out of the hatch after I did. She didn't bother looking at the axle, or at the rover. No need; it was obviously not going anywhere, even if we had a crane to put it back right-side up. The rocks we had piled onto the rover had cracked the bubble when it rolled. "Sulfur-reducing bacteria," she said.

"Say again?"

"Sulfur-reducing bacteria," she said. "Convert iron to iron sulfide. There's energy in free iron; in the presence of free sulfur, enough energy for a bacterium to exploit. The lack of iron at the site; I should have figured that ordinary weathering wasn't enough to account for it."

"Oh," I said.

"Not that it matters now," Leah said. "We don't have time to waste. We've got to get to Tally and warn her." With a matter of fact att.i.tude, she hopped up onto a rock and stared across the horizon. "So how far do we have to walk?"

I tried the radio one more time. Come on, Tally. What was she doing, I wondered.

Did she even know that the antenna was down, or did she just think we were scrupulous in keeping radio silence? Was she standing at the door of the habitat with a gun? Hiding behind the rocks, waiting for enemies that would never come?

If only she would answer, it would only take an instant to tell her about the dangerously low habitat pressure.

Fix the antenna, Tally, I thought, just fix it, and listen to the radio. But she wouldn't. Fixing the antenna would be too obviously a sign that the habitat was still occupied. I threw down the radio.

The inside of the rover was a mess, but we managed to scrounge two spare sets of replacement packs for the rebreathers. I downloaded the bearing to the hab out of the rover's computer, and set the inertial compa.s.s. Once we got close, we would be able to use the habitat's come-hither beacon to home in. I grabbed a set of portable radio transceivers and checked that they were working. I couldn't think of anything more to carry. Before we left, Leah snipped two pieces of t.i.tanium sheeting away from internal part.i.tions of the rover, and snapped them free.

"Ready," she said.

We ran.

The Mars gravity makes it easy to run, and the unwavering wind was, for a change, on our side. Still, after an hour of running I was winded, and the second hour was more trudging than running. Our cold-suits trapped sweat all too well, and it ran down my back and down my legs, like ants with clammy feet.

Mars narrowed in on us. Ridges, followed by valleys; valleys followed by ridges.

Another hour.

"Bear further to the right here," Leah said.

"That's not the most direct route."

"I know."

We were walking pretty slowly by now. Her route followed the contour, instead of cutting downhill, and was a bit easier, even if it was less direct. I was beginning to worry that we wouldn't make it to the habitat by nightfall. It would be impossible to continue after darkness fell--Mars's moons shed almost no useful light--and by the morning, we couldn't even be certain that the habitat would still be there.

In another hour we had reached the edge of a long downhill. There, tiny in the distance was a glint of metal: our goal, the habitat.

It was impossible to tell from the gleam whether it was still in one piece.

Without a word, Leah handed me one of the two sheets of t.i.tanium. I looked at the downhill. It was a long, smooth grade, with the usual cover of Martian slime. I grinned, and Leah grinned back at me, her face in the rebreather mask like some painted mechanical demon, and then we both stood on our sleds, grasped the lanyards, and, at the same moment, pushed off.

We would arrive in style.

My sled skidded to a stop in a spray of slime a hundred meters or so from the habitat, and Leah stopped close behind me.

The habitat was apparently empty. But at least it was still apparently in a single piece. I ran toward it, shouting for Tally. I reached the airlock, and was just reaching out for the handle when I felt the gun pushed gently between my shoulder blades.

"Moving real slowly, friend, keep your hands in sight, and turn around. Slowly."

Tally was painted the same color as the Martian slime, bits of sand and rock sticking to her randomly. The projectile rifle was in her left hand, aimed steadily at my middle. I could see the crinkling at the edges of her eyes as she smiled behind the rebreather. "Tinkerman. Welcome home."

She lowered the gun, and turned to greet Leah. "Didn't expect you to come back on foot. What brings y'all back so sudden?"

"The air pressure," Leah said. "It's going to--"

"Yeah," Tally said. "I noticed something going on with the air. Could feel it in my bones, like a thunderstorm. Fact, I had to dial up the pressure in the hab three times in four days."

Leah stopped, thunderstruck. "You increased the hab pressure?"

"Why, sure," Tally said.

We just looked at each other.

"What?" Tally asked. "Something wrong with that? I figured that if the hab pressure wasn't increased, there could be trouble."

Leah shook her head. "No, nothing wrong. Nothing at all."

It was our last night on Mars. We had filed a preliminary report with s.p.a.cewatch, and in the morning, Langevin would bring the lander down to take us home.

I was looking out the tiny window of the hab at the Martian landscape. In the evening twilight the browns had turned to purple. Tiny puddles of water caught the skylight and reflected it back at us. Even the slime looked fragile and ethereal "It is beautiful," I said "in its way."

"Ask me, it still stinks," Tally said.

"It's dying," Leah said.

"Dying?" I turned away from the window.