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 nobody 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 pass, 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 orbit 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 we should assume that any radio communications we made would be heard by the enemy. The relay had enough power to let us send reports directly to Spacewatch. 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 skyscrapers. Leah was in the aft cabin, analyzing data she had brought with her, and I was alone in the cockpit. 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 online. 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, anticipating 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 r
ebreather, 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?”
“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 hell of a lot.
Over the long Martian winter, the temperature of the northern ocean dropped to near freezing, and the ocean 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 god, 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. R.I.P.
But Tally didn’t answer the radio, not the regular channels, not the emergency channel.
“Damn,” 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 space, 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 online. 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, 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 online and punched for speed.
There was nothing more we could do. There was no way that I could outpilot 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 bumpy, 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. Nobody 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 r
ises. 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. Nobody 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 scrape 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 Hellas 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.
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