THE CALLISTO INCIDENT
by Ken Wharton

Originally published in The Leading Edge, #39, Spring 2000.

"Nothing?"

John Stanford shook his head in response, still watching the cartoon molecules flash across the screen. 'Nothing' was an exaggeration; simple organics abounded as usual. But nothing he could call life, not even a broken piece of PNA.

Agnieszka cleared her throat. "So... I can use the rest of it?" John glanced up and saw her walking over to the cold box.

"Wait!" he yelled at her. "Just wait a second! I'll give it to you just as soon as--" John took a deep breath, realizing he had overreacted. She was only trying to do research, same as him. Unlike him, though, she was actually getting positive results. All the geologists were. It just seemed brutally unfair; he was the one who had gotten the primary funding, he was the one who had organized this whole project. The geologists were just parasites, spiriting away his core samples almost the instant they surfaced from the ice.

But at least Agnieszka had waited patiently through the whole procedure. He had to give her that much. His own research team had all left after the first ice sliver.

"No need to shout, John," said Agnieszka. "You did say this was the last one." She glanced at the cold box. "I don't mean to hurry you, it's just this is the deepest sample we've--"

"Why do you think I'm checking it so carefully?"

She sat down in a swivel chair and rolled over to him. "We're bound to hit water in a few more months, and then you'll know for sure. If anything's down there, you're going to find it. But you have to be--"

"Patient. I know, I know."

She shrugged sympathetically. "If it's not in the ice, it's not in the ice. You'll have to wait."

John frowned. She was probably right. They had tested ice from three different wells, seven small meteor impacts, and twenty different fault zones. But even samples from last month's ice volcano only yielded polypeptide chains up to 10 amino acids.

But how could it not be in the ice, he asked himself for the thousandth time. If there was life teeming below him in Europa's hidden ocean, why would there be no trace of it? Perhaps it was destroyed by the freezing process and high pressures? More likely it was confined to a few thermal vents and wasn't widespread. Whatever the explanation, John knew something was alive down there. And he was going to be the one to find it.

He was just about to tell Agnieszka to take the sample, to go off and do her bubble analysis and isotope measurements and god-knew-what-else, when Simon stepped into the lab. John blinked. He hadn't seen the sheriff on this level since before the orbital confrontation with the colonists.

"Callisto's off-line," Simon said calmly.

John blinked again. Callisto? he thought. Chandler just landed there a few weeks ago.

"The Kepler's not fueled," Simon continued, "and nothing else on Ganymede can beat us, so we're the rescue. I know you and Dr. Peterson are pretty close, so I thought you might like to join us. We leave for the ship in an hour."

"What do mean off-line?" asked Agnieszka, obviously concerned. "Their daily report...?"

"Ganymede didn't get one today," said Simon. "Nothing out of the ordinary yesterday, but they've only been there for a month and they do have two backup transmitters. It might be serious."

John glanced up at a map of the Jovian system. Callisto had always seemed just next door; suddenly he realized how far away the outer moon really was. "Nearest approach, what are we? Twenty R-jays?"

"Seventeen."

The math was simple enough. It was just over five R-jays to Ganymede, and those shuttles regularly took two days in transit. "We'll never get there in time. If it's something serious, I mean."

"It's an emergency, so we can max everything out. Two flybys, no extra fuel for a return. We'll land on Callisto in 58 hours."

John frowned. "That's still a long time. And then we'll be stuck there until someone else can come get us."

Agnieszka attempted a smile. "At the very least, John, it will keep you busy while they drill."

True enough, he thought, but what if they broke through the ice while he was gone? He instantly felt guilty about putting his research ahead of his old friend, Chandler Peterson. Chandler was one of only three people on that entire moon, and John knew something could have gone disastrously wrong.

"I'll go," John said at last. Simon nodded and spun out of the room.

Agnieszka exhaled sharply. "Wow. I guess..." She smiled, stepped toward the cold box. "I guess you're done with the ice, then."

"Yeah," said John. He massaged his forehead, not looking up. "For now, I suppose I am."

The truck rolled out of the freight elevator onto the gray ice of the Europan surface. John ignored the other three passengers, gazing out the side window at what he could see of the base. The only permanent settlement on Europa sat at the bottom of a small depression, one of the lowest points on the moon that wasn't seismically active. John himself had chosen the site, although he had been disappointed to find that almost none of Jupiter was visible from the bottom. For the midterm grant review he had been careful to include pictures taken from the western rim, giving the impression that half of the gas giant could seen hovering above the base.

As the truck drove out of the depression, John got a nice view of the structures below. The main feature was the superconducting magnet, a huge white ring that enclosed almost half of the depression floor. The three wells were positioned around the circle and looked like Arctic oil platforms mired in ice for the winter. A vague blue light emanated from the center of the ring, evidence that the magnetic field was indeed diverting the Jovian magnetosphere. The shielding kept the secondary radiation down to a somewhat tolerable level--still not quite low enough for children or colonists, but for a once-in-a-lifetime chance to answer the most important question in all of science... John didn't give the risks much thought.

At the top of the depression the truck immediately plunged into a tunnel and emerged in the center of a doublet ridge. Ridges ran everywhere on this moon, like a horribly complex freeway system. The older, safer ridges were all but useless--they were continually crossed over by more recent fractures that filled the passage with impenetrable mounds of ice. The only way to get anywhere was to live dangerously, driving along recently-active fault lines. Fortunately, the 'road' to the launching pads was well traveled; accidents usually happened far from base.

With nothing to see except tall hills of ice on either side of the truck, John leaned back into his seat.

"So, how far to the ship?" John looked sideways to see Samantha watching him, the one-woman emergency medical team coming to Callisto. John was hoping she'd be sufficient in event of a real crisis.

The sheriff responded from up front. "Another hour," Simon called over his shoulder.

Samantha nodded, then shrugged. "I still don't understand why you didn't build it any closer."

Simon didn't answer. The driver and pilot--a squat man named Tony--gave John a quick backward glance.

John knew what that meant. Tony made this drive all the time, often with tanks of biowaste in the back, and he had often complained about the excessive distance. John turned to Samantha to explain. "That was my policy. We can't have the ship's recoil shear off any of our drill pipe. Lost about ten kilometers to meteor hits already, so we have to launch from the next major plate."

"That's not the real reason," said Tony, "It's 'cause he didn't want us crazy pilots coming anywhere near his little base."

John smiled, shook his head. "Tony's actually one of our best pilots," he said to Samantha.

Tony snorted a laugh. "May be true, may be true..." Suddenly the truck swerved to the right and accelerated. Startled, John looked out the front window and saw they were heading straight for a two-meter drop in the 'road'. "But that doesn't mean I'm not crazy!" Tony finished, and then they were in free-fall. John braced himself, way too soon. His muscles had already started to relax when the impact came, with much less force than he had expected. The truck rolled onward, apparently undamaged.

John and Samantha exchanged wide-eyed looks, but didn't say a word. "Funny," commented Simon. "Don't do it again."

Three ships sat in a row on the blackened LT-Concrete, all freighters that had recently delivered supplies from Ganymede. Only the closest ship, the Falcon, was fully fueled. Tony backed the truck against the main hatch; moments later the sharp hiss announced they were linked.

John helped carry in a load of supplies, but it wasn't until he was in the ship that he noticed the labels on the boxes he was holding. Lead-lined wetsuits. Mental alarm bells sounded instantly.

"Simon? We're not going in the coffins, are we?"

"I told you. We're getting a gravity assist from Io. We'll submerge for the hour we're inside Io's orbit.

"Inside?" John gaped. As far as he knew, no one had ever flown into the Torus. Not on purpose anyway. The coffins were available for freak Jovian magnetic storms or emergency engine failure, but they were planning to use them? "Why can't we do the flyby on the outside?"

Simon just shook his head.

"Well... what sort of radiation?" John asked.

Simon looked upward, thinking. "No major eruptions on Io, last I heard, so the plasma density of the Torus won't be too high. Outside the ship... twenty kilorads, integrated. In the coffins it'll be roughly six months on Europa."

Six months? That was six months less research before he had to start spending half-time on radiation-sheltered Ganymede. The biological risks didn't bother him, but the safety officers wouldn't allow him to risk it. Did he really want to do this? After all, his training in the coffins had been his least favorite prep exercise.

Simon looked at him intently. "Do you not want to go? If you think you can drive the truck back..."

"I'm going," John responded with a confident voice. But what if this was just a communications failure, he wondered? He didn't want to sacrifice his rad-count for nothing.

They spent the next half hour exchanging gear with the truck. Every unnecessary piece of equipment had to go; they were going to be cutting the fuel requirements too close as it was. A few arguments ensued over what was truly 'unnecessary'; they didn't know what had happened on Callisto, John argued, so most of the equipment could conceivably be useful. But Tony stressed that the trajectory had been designed for a minimum mass ship, and the others seemed to agree. The only marginal equipment that John was able to salvage was a portable B-generator and an additional two weeks of supplies. Everything else went into the truck.

Tony remotely drove the truck to the parking lot when they finished, and before John knew it they were ready to launch. He strapped himself in with the others, and waited for the launch window. He didn't have to wait long; five minutes later the rockets lifted them off the surface. The acceleration felt even worse than leaving Mars, John thought, although it probably wasn't. Two years at an eighth of a gee had left him much weaker, despite his daily workouts.

The engines fired for perhaps a minute, cut out for a few seconds, and then came on again at half-thrust. With nothing to do but sit, John watched the orbital information change on the main screen. Earlier, he had taken a look at their complicated trajectory, and it somehow hadn't seemed right. Callisto was the outermost Galilean moon, but they were counter-intuitively accelerating almost directly towards Jupiter for the Io flyby. Orbital mechanics were complicated around here, though, and he could only assume that the computer had found the optimal route.

The burn seemed awfully long, longer than on any of his other intermoon trips. And Jupiter's gravity was helping the acceleration as well; he wondered how fast they would be going when they hit Io. Narrowly missed Io, John corrected himself. Narrowly missed Io while submerged in a dark, watery coffin.

It was as bad as he had remembered. Immersed in warm water and breathing into the mini-life-support system, the only thing that made the coffin different from a sensory-deprivation chamber was the small screen directly in front of his face. That was a small comfort, though, informing him exactly how much radiation both he and the outside of the ship were absorbing. More trajectory information also appeared, as well as a live picture of the approaching volcanic moon. But almost a third of the pixels in the image were saturated; the plasma torus was blasting the outer cameras with radiation.

Even so, Io was an impressive sight. The most interesting body in the solar system, the funding propaganda went. John didn't agree, of course, but maybe Io came in a close second to Europa. Sulfuric volcanoes, enormous tidal heating, and a continually evolving surface made for a lot of fascinating science.

He was mildly surprised that Chandler hadn't ended up on Io. There were four or five research teams on the moon at any given time, and they were still nowhere close to understanding the place. Plasma physicists, chemists, geologists: they all loved it. If Chandler had been so intent on changing fields, why couldn't he have picked Io? Why boring old Callisto?

Of course, John didn't even understand why Chandler had wanted to change fields in the first place. Their life-hunts on Mars together had been so stimulating, so encouraging. They had worked well together; John's methodical techniques nicely complimented Chandler's goal-oriented approach. Chandler intuitively seemed to know what direction to steer their research, but it was John who made sure they got there via careful scientific experimentation.

So John had been completely shocked when Chandler had turned down the chance to be on the Europa Life Project. John suspected it had had something to do with wanting to be a project leader himself. But what was more important: furthering his career or helping to answer the ultimate question? John shook his head, stirring up the water that surrounded him in the coffin. If it had gone the other way, if Chandler had been the project leader, John knew he would have happily jumped on board.

John's picture of Io was clearer now that the Falcon was partially shielded by the moon's own magnetic field. The radiation figures dropped; they had made it out of the Torus. But even though he knew the true danger had passed, the image of the fast-approaching moon felt like a more tangible threat. Already he could see the textured yellow-orange surface well enough to make out a tiny crater that hadn't yet been erased by volcanoes. He didn't want to know how close they were going to ultimately get.

John braced himself against the light/heavy water partition, ready for the moon to whip them away from Jupiter. But so far there had been no acceleration at all. He looked at the screen in surprise, reading the changing altitude numbers. When would he feel it? Soon Io actually began to recede, and John finally realized that he was waiting for something that wasn't going to happen. But that somehow didn't seem right to him. How could you accelerate without feeling it?

With nothing else to do, he puzzled it out for a minute, decided that it made perfect sense after all, and just then he did feel a force, pulling him right toward the outflow mesh. Confusion was followed by a relaxed smile. The water was being pumped out, and his time in the coffin was almost over.

Almost a full day passed before the gravity assist from the largest moon in the solar system. It seemed amazing to John that a rescue ship launched from Ganymede wouldn't have made it to Callisto faster, but Tony told him only the Kepler would have had a chance without the double flyby.

"You mind if I use the comm?" John asked the sheriff. "I know some people here from Mars, but I hate talking on Europa. The lightspeed delay, you know."

Simon nodded. "Just keep in mind we're not the two most popular people in the system right now."

It took a moment for John to realize he was referring to the incident with the unauthorized colonists. John looked puzzled. "Surely they don't hold that against us?"

"Come here," beckoned Simon, floating over to a window. "Look at it. What do you see?"

John peered down at the gray moon, still an hour from closest approach. Half of what he could see was dayside; ancient lava flows and impact scars decorated the complex landscape, remnants from a time when Ganymede had been much closer to Jupiter. On the dark half John could make out at least a dozen spots of light.

"Those are all crater communities," said Simon. "Big ones. We probably can't see most of them. The moon's getting crowded, John, and these colonists came to get away from the crowds. They want Europa."

John watched Ganymede grow larger. "Why not Callisto, then? Why does it have to be Europa?" Simon didn't answer, no doubt because John knew the usual tenuous arguments. Callisto had no magnetic field. The surface water-ice was dirty, poisonous, and difficult to filter. And it was too far away from Ganymede to keep a fledgling colony supplied without many more ships.

But those were just excuses. The real reason was that everyone was fascinated with Europa. Everyone wanted to live over an alien ocean, even if their presence would destroy the very research they found so intriguing.

"It was just one incident," said Simon. "But it's part of a larger problem. Don't make the mistake of thinking it won't happen again."

"Still?" asked John. "Even after what happened?"

Simon shrugged. "They know we can't shoot down every colony ship."

"We didn't shoot it down! That was a warning shot--"

"Which I fired closer than regs permitted. After you ordered me to."

"Well, if they hadn't jinked they wouldn't have been hit. And we rescued them afterwards."

Simon pointed a finger at John. "I was up in the patrol ship, I rescued them. But we killed that woman who didn't make it. Sylvia. It was our fault that she died. And their ship. They lost everything they owned."

John nodded. "It was a tragedy, I agree. But--"

"But it was worth it? Was it really?" Simon turned away, floated off to his bunk, leaving him by the window.

John looked down at all the crater communities again, somehow shining brighter than before. Was it worth it? Since the incident, no other groups had attempted landings. If the others had learned a lesson, John decided, then it had been worth it. Europa was a pristine research lab. And it had to stay that way, or they'd never learn anything.

Mars had been ruined from the start. The first colonists hadn't worried about proper biowaste disposal--none of them ever did--so it was impossible to be sure which of the Martian bacteria were truly Martian. Apart from the countless dead organisms, three live species of microbiota had been found that were probably native. John and Chandler had discovered one of the three during their work in the Parana Basin. But even if they weren't mutated colonist bacteria, they still weren't truly alien. The genes were so familiar that they must have had a common ancestor with life on Earth.

Now the top exobiologists on Mars were simply debating which planet the common ancestor had evolved on; had it gone from Mars to Earth or vice-versa? Ejecta meteors were much more likely to favor the former route, but life appeared on Earth so fast it was hard to imagine it hadn't evolved there first.

Regardless, that wasn't the type of research John found interesting. Answering that question wouldn't shed much light on whether life was 'hard' or 'easy', whether life was just an amazing local accident or if it was everywhere. As soon as it was feasible, John had planned, lobbied for, and launched the Europa Life Project; now on the verge of answering the question for once and for all. Assuming, of course, that he could keep the colonists from contaminating his moon.

John pushed away from the window, deciding not to use the comm after all. He'd be back here soon anyway. Four days after they arrived at Callisto a slower ship would arrive from Ganymede and bring them back. And ideally he'd be bringing Chandler back with him. He smiled, thinking about the two of them in the midst of a Martian reunion: Waldo, Christina, Raj and Sheila, Double-Back Dave, and a dozen others were down on Ganymede. Chandler probably knew many other colonists as well; he had stayed behind on Mars for a couple additional years, trying to get funding to go to Callisto.

Thinking about it now, John decided it was amazing that Chandler had ever succeeded. Limited money was available for the other large moons, but nothing for Callisto. The outer Galilean satellite was so boring that it was used as a benchmark for what the inner three would have looked like if it hadn't been for Jupiter's strong influence. Unlike Ganymede, Callisto had never been pulled into a close Jovian orbit. There was no internal differentiation of Callisto's core, there hadn't been surface activity for over three billion years, and if there were any scientific puzzles they were so mundane as to be irrelevant.

John recalled Chandler telling him about one Callistan mystery or another. Disappearing small craters, the strange chemical makeup of the black clay surface, the statistically unlikely crater-depleted regions. But it always sounded like he was reaching for something that simply wasn't there. And now he was in serious trouble, if not dead, all because he had abandoned exobiology.

Although... What had Chandler told him that one night, just before they split up? They had been up drinking, saying good-bye, arguing about some of Chandler's wild ideas. Chandler would often come up with crazy notions of what alien life might look like, and John would systematically shoot them down. Liquid lifeforms on Titan. Plasma-based lifewaves in Jupiter's ionosphere. Chandler could go on with his half-serious theories for hours. But this night he said he had a new idea. And for the first time, he wouldn't say what it was. All he said was something about alien life being everywhere except where they were looking. What did that mean? And why, for someone as obsessed with discovering life as himself, why did he give it up? John didn't know, and now it looked like he might never find out.

There was something missing from the Callisto daily reports. To fill the hours John had read all thirty-six of them, and although he could almost hear Chandler's voice speaking the written words, he could also read between them. Chandler was holding something back.

Chandler loved secrets, but he was notoriously bad at keeping them. You could always tell when he was hiding something; he simply chose another topic and talked about it incessantly.

In the Callisto reports, the other topic was the weather.

Of course, Callisto had no weather. No atmosphere to speak of; just the far reaches of Jupiter's magnetosphere, the solar wind, and the occasional molecule escaping from the surface. But Chandler had dutifully recorded ground temperatures, micro-torr scale pressures, plasma compositions. Just the sort of boring scientific data that Chandler detested. He was up to something else, but John had no idea what it might be.

The rest of the reports were filled up with the usual minutiae that inevitably accompanied a new settlement. They hadn't been able to fine-tune the watermaker for Callisto's heavy impurities, their primary generator was sporadically shutting down, the regolith-oxygen smelled so bad they were wearing makeshift filter masks, and so on.

The only interesting fact that John found in the reports was buried in the list of supplies they had asked for on the next drop. In the food section, between the sauerkraut and the Spam, Chandler had put in a request for five live hens. The request had later been denied.

John puzzled over that one for quite a while. Sure, they had farms on Ganymede, but live deliveries? In his two years on Europa he had never seen a live animal; they were always delivered frozen. Did he want a supply of fresh eggs? But Chandler never cared too much about food; he just ate to survive, sometimes even needing reminders for that. Maybe it was one of the other men, John speculated. Or maybe it was a clue to what he was really doing.

"John?"

He looked up from the reports; Samantha was just outside his cabin. She had mostly kept to herself for the trip, and as a relative newcomer to Europa he barely knew her at all. "Yes?"

She grabbed a handhold and swung through the door. "I was looking at the medical records for the Callisto group, but there's not much of a personal profile. I was hoping you could tell me a little bit about Dr. Peterson. What he was like to work with, any occasions he may have--"

"Personal?" John said with a smile. "What, are you trying to be a shrink all of a sudden?"

"Actually, I am," she said. "Sort of. Emergency Medicine came first, but colonist psychology was the main reason I came to Jupiter."

John nodded, then frowned as he realized what she was saying. "Look, whatever has gone wrong down there, it's a physical problem. I don't know what you're trying to accuse Chandler of--"

"I'm not accusing anyone of anything." She looked taken aback. "I'm just trying to get a feeling for the group dynamics. Think about it: these are the only three people on an entire moon, and their relationships surely have something to do with the situation."

John sniffed. "Chandler Peterson's a scientist. Working with him was, well..." John wondered what he should say. That Chandler tended to dump the hardest work on his subordinates? That he viewed human limitations as a nuisance to be overcome so the work could get done? None of that sounded too positive, but Chandler was a good person and John didn't want to give Samantha the wrong impression. True, Chandler's graduate students would often take ten years or more to graduate, but that was just his style, always using all of his resources to the utmost.

"He wasn't too hard to get along with," John continued. "As long as you were working towards the same goal. And if he's gotten himself into a scrape down there, he'll do what he needs to do to get out of it. How he gets along with others isn't what's going to matter in a crisis."

Samantha looked away and sighed. "Maybe. Maybe it's silly to even be speculating when we don't know any details." John didn't respond and soon she turned to leave. "I certainly hope they're okay," she said, then pushed off and soared out of the cabin.

"I do too, Samantha," he called after her. "I do too."

On the main screen, Callisto looked like a portal into another universe. The surface of the moon was primarily black, even where it was touched by the dim sunlight. Callisto wasn't as black as the surrounding space, of course, but somehow it didn't seem any lighter, either. Just a different sort of black; the blackness of a universe with slightly altered laws of physics.

Filling this dark circle were galaxies of craters, randomly distributed across the face of the moon. Meteors had blasted away the surface minerals and exposed the purer ice underneath, accumulating vast constellations of white speckles on the ancient crust.

"It's beautiful," remarked Samantha. She was in the seat next to him, but the voice came through his suit helmet.

John tried to shrug against his straps. They had been buckled in for five minutes now, awaiting the engines to nudge them into the correct landing trajectory. "There's not much down there," he said. "Old regolith doesn't make for a lot of interesting science."

"Simon said there were surface organics."

"Yeah. Simple tholins, just like on Ganymede," he said. "We've known that since one of the pre-Mil probes. Galileo, I think it was called."

"Well, that sounds interesting to me. How do you make organic molecules in a cold vacuum?"

The engines fired a brief pulse, then stopped. "UV," John said. "And it's not always cold. When meteors hit, things get really hot for short periods of time. Plus there are ion impacts that can do a bit of surface chemistry. Hydrogen and methane from Jupiter, sulfur and phosphorus from Io's volcanoes, carbon and oxygen from the solar wind, water-ice and ammoniated clays already on the surface. All the ingredients you need. And a lot of organics can be made in space. The smallest amino acid, glycine, has even been detected out in the middle of nowhere."

Samantha only nodded in response, so John focused his attention the main screen again. Over the next twenty minutes the moon grew quite a bit larger, and soon the image of the surface was sweeping along as they orbited towards their destination.

The engines fired up again, this time for a long burn, and John was pressed back against his seat, still watching the screen. The moon was much closer now, but the crater density hadn't gone up all that much. Had Chandler mentioned that? It certainly was curious; simple statistics required that for every large crater there should be many more small ones. Perhaps the ejecta from the smaller craters had devolatilized into space over time.

Eventually he got the feeling they were slowing down; the image stopped sweeping across the screen quite as fast, and soon it wasn't moving much at all. A medium-sized central-dome crater slowly filled the screen--Sarakka crater, if they had navigated correctly. Why Chandler had chosen this particular location on the outskirts of the Valhalla Impact Basin, John didn't know.

"You see it?" said Simon, his voice strained because of the acceleration. "Right on the bottom edge of the screen."

John saw. A small structure jutted out of the shadows: the Callisto Research Station. The white rectangle sat under the lip of the crater, and was growing fast. Soon he could make out the transmitter dishes on the roof. They looked to be intact.

"What's that to the right side?" asked Simon

John squinted at the screen. Some sort of light-colored debris lay outside the Station. In fact, one of the larger shapes looked like a human body, lying spread-eagle on the ground. He took a deep breath, hoping it wasn't Chandler.

"My god," muttered Samantha. As if on cue, the engines started hiccuping, cutting out for a half-second at a time and jerking John against his straps. The Station swung out of view on the screen, and John gripped the armrests tightly. Was this intentional? Were they losing control? Did they miscalculate the level in the fuel tanks and--

Thud!

John waited a cautious moment, then breathed a sigh of relief. They were down.

The four of them wasted no time; already suited up, they quickly unstrapped themselves and hurried to the airlock. John thought it was a bit silly, rushing around like every second counted when it had been almost three days since the original problem. But even as he rationally considered this, an undeniable sense of urgency kept him moving fast.

Simon was the first one on the surface, and John followed just after. They had landed in the same crater as the Station, somewhere between the edge and the central dome structure, the latter of which loomed behind the just-landed Falcon. John took an experimental jump, thinking that it was odd he didn't feel any heavier than normal. But he knew for a fact that Callisto was a much larger moon. John turned to ask Simon, but the sheriff was already running toward the station with long, practiced strides. Samantha was just behind him, and Tony was apparently waiting with the ship.

Okay Chandler, John said to himself. Here we come. He waved a quick good-bye to Tony and took off across the surface. The terrain was very different from Europa; dark, soil-like clumps instead of flat water ice. And it gave under his feet! As he jumped forward there was definitely a slight compression of the regolith. In confirmation, he noticed the sunken footprints of Simon and Samantha; apparently not all materials were rock-hard at 130 Kelvin.

He caught up to Samantha shortly before they reached the Station. Simon had stopped just outside, examining something on the ground. John slowed down, part of him not wanting to know what the object was. But inertia and curiosity drove him forward, and soon the three of them were standing over the frozen body of a dead man.

It was impossible to tell if it was Chandler. The decompression had exploded parts of the chest and face. Not-so-gray gray matter spilled out from the head, already dusted with the black surface material. Whoever he was, he hadn't been wearing a suit.

Remarkably, John found he wasn't nauseated at all. The body was frozen so solid that it was hard to imagine that this had once been a real person. John stared at the man for a few moments, then looked at the others. "Who is it?" he asked over the intercom. "Messner? Dupree?"

"Not sure," said Samantha. "Shouldn't be hard to figure out, though."

Simon nodded and turned toward the Station itself, only five meters away. John followed, noting that both airlock doors were wide open. Perhaps the dead man had been blown outside when a malfunction had opened them at the same time. He entered the Station; Samantha stayed with the body.

The airlock was mostly blocked with the remnants of a six-wheel rover. It had apparently been thrown against the outer wall; the plastic driver-bubble had crumpled with the impact. Stepping around it, John walked into the main room and instantly despaired of finding anyone alive.

The place was a disaster. It looked like a Callistan giant had picked up the station, shaken it thoroughly, and thrown it back down into the crater. Debris lay everywhere; broken glass and plastic littered the floor, two workbenches had been toppled, and several million dollars of brand-new scientific equipment lay in frozen pieces on the ground. A mattress lay against the far wall, and John saw that all three of the personal quarters were wide open. No survivors here, that was certain.

Simon completed his quick survey of the station, then returned to the entrance. "Tony?" John heard him say over the suit's intercom. "Can you hear me from there?"

"Copy that." came the response.

"Tell Ganymede to keep their eyes out for their ship. Don't know when they launched, but there should be one or two people on board."

Confused, John shook his head. "Simon? What are you talking about?"

Simon jerked his thumb toward the airlock. "Come on, John. Didn't you see their ship out there?"

"No. Where did you see--?" A burst of understanding allowed John to breathe a small sigh of relief. The ship that Chandler's group had originally arrived in was no longer here. Whatever had gone wrong, at least someone had escaped.

"It's Messner," announced Samantha's voice over the radio.

"In the ship?" asked Simon. "No, you mean the dead guy. How can you tell?"

"He's circumcised."

After an awkward pause, Simon responded. "Clever. But Samantha?"

"Yes?"

"Cover him back up. Please. Makes me cold just thinking about it."

The next hour led to a clearer picture of what had happened. It was not an electrical or mechanical failure, as John had assumed. Instead, someone had manually bypassed the interlocks, opened the airlock doors, and vented the station. Messner had blown right through.

The transmitters had all been destroyed, but not by the decompression. Apparently someone had savagely attacked them, ripping out circuitry and wiring well beyond the point of repairability. The only good news was that two spacesuits were missing, so perhaps Chandler and Dupree had survived and made it to the ship. But as John pointed out, if Dupree was responsible, he probably would have killed Chandler as well.

"Or the other way around," Samantha said. "But then there would be another body."

John nodded. "So maybe they're both still alive. Maybe Dupree left in the ship, Chandler might have been suited up, and he could still be around here somewhere! Couldn't he?"

Simon pointed to the oxygen racks. "I'm sorry John, but no one's been using the tanks. And the oxygen generators failed when the Station vented."

"But with the rebreathers, if he had a few tanks with him..."

Simon shook his head in the spacesuit. "It's been too long."

John bit his lip. If there was a way out of this, Chandler would have found it. Maybe he had been working out in the field. Maybe he had a portable oxygen generator out there somewhere. Or even just a stockpile of emergency tanks...

"Uh, guys?" It was Tony's voice, from back at the Falcon.

"Any word from Ganymede?" asked Simon.

"Yeah, but they don't see anything. They claim that if there's an extra ship in the system they'd see it. So I was thinking about what else might have happened, and I looked back over the vid of the landing."

"Yes?" prompted John.

Tony cleared his throat. "I think I found the missing ship."

The three of them made their way up the south wall of Sarakka crater. It was an old crater and some process had worn away the rim, so climbing out of it was simply a matter of walking up a steep slope. John noticed that they were following a worn path in the surface material, formed by countless footsteps and rover tracks. Apparently Chandler and the others had come this way many times.

The climb looked worse than it was; every time the slope got too dangerous they found a gentle switchback that had been trampled in the regolith. John wasn't even winded when they emerged between two ejecta mounds at the top.

John looked at the bleak landscape ahead of them. The tiny sun was up, but hardly any light reflected off the ground at all. The surface was different here than in the crater. Darker. He bent to the ground and decided it was almost like black frost. But John knew it wasn't dirty water ice; he had seen plenty of that on Europa. He touched it with his gloved fingers and flakes broke off; perhaps it was some sort of brittle clay network. He brought his hand up to his faceplate and studied the debris that stuck to it like iron filings. Was this the organic gunk that had been detected from orbit? If so, it wasn't at all like the tholins he had seen on Ganymede.

"Okay, Tony," said Simon. "We're at the top. We'll probably go out of range soon, so give us the rest of the directions now."

"Let's see," came the voice. "Looks like about five big ejecta mounds on your side of the crater. Number them clockwise, one to five. Then find number four and walk directly away from the crater. Shouldn't be more than a klick."

They quickly determined that they were standing between ejecta mounds number one and two, and started walking to their right. But even before they made it over to number four, they saw what Tony was talking about. Poking up from behind the horizon was a glint of metal that could only be the crashed spaceship.

Simon started running again, and John and Samantha followed close behind, their boots silently crunching through the strange regolith. Within five minutes they were standing before the wreck.

The ship was intact but horizontal, lying in a boulder field after apparently skipping across the surface a few times. A section of the main hull had been peeled back like a sardine lid, and one of the engines lay a good hundred meters away.

"Wow," said Simon. "This thing's in pretty good shape. Must not have gotten very high before it crashed."

John looked at him strangely; to him the wreck looked about as bad as he could imagine. He pointed to the gaping hole. "We going in?"

Simon shook his head. "Probably cut our suits. See if there's a bigger hole on the other side. Or maybe the main door still works."

As it turned out, the main door did work. They had to climb on top of the ship to get there, but the impact-plowed ice acted as a serviceable stairway. Simon was the first one inside.

"Damn." He waved his flashlight around. "These things are built pretty tough."

"What do you mean?" John lowered himself down behind him, and noticed the flashing red lights on the control panel. "Emergency power still works?"

Simon nodded. "Must have barely gotten the ship above the crater rim when something went wrong. Didn't have far to fall. If the hull hadn't ripped, the air would probably still be breathable."

"Which way to the cockpit?" asked Samantha, now inside the ship as well. "There?"

"That's it," said Simon. The three of them cautiously made their way forward, checking for sharp edges with their flashlights. Finally they emerged into the main cabin and found Dupree.

At least he had been wearing his spacesuit. It didn't seem to be punctured, but he was lying in an impossible position, upside down in the corner. And there was blood on the inside of his faceplate. Either the crash had killed him or he had run out of air afterwards.

"Shit," muttered Simon. "Why wasn't he strapped in?"

John nodded, looking around at the empty seats. "Good question. Probably would have saved his life."

"If we can get him back to the Falcon I can do an autopsy," said Samantha.

"Right," said Simon. "First I want to take a look around."

A quick search didn't turn up much. John was relieved that they couldn't find the last spacesuit; Chandler was most likely dead, but at least they couldn't write him off quite yet. Amazingly, the computer was still alive, and even though Simon couldn't interpret the autolog he downloaded a copy to show Tony.

"So what do you think?" asked John. "Dupree wrecks the station, kills Messner, then leaves in the ship but crashes? Maybe he didn't know how to fly the thing."

"John," said Simon, looking at him carefully. "Let's not go leaping to conclusions just yet. You're leaving Peterson out of the picture. However well you thought you knew him, I'm afraid he's currently suspect number one. And Dupree..." He gestured to the body behind him. "He was the best pilot that Dr. Peterson could find."

Back in the Falcon, Samantha discovered Dupree's personal e-log inside of the dead man's spacesuit. It still looked functional, so John took it to his bunk while Samantha started the autopsy.

"John?" It was Tony, peering his head into the small cabin. "If we're not back in three hours, come looking for us."

John looked up in confusion. "Where are you going?"

"To the crash site. Simon copied the wrong file, and I want to take a look at it anyway. Simon's coming with."

John nodded and returned to the log. The power came on right away and he started scanning through the entries. Unfortunately, Dupree didn't keep frequent notes or go into much detail.

00.14.5.10.23
Took Peterson up to the plain again, very excited about something. Messner still working on equipment, but should be set up before long. Air smells terrible. Messner says the biomechs can't handle the sulfur, but I hope he fixes it soon.

00.18.7.10.23
Equipment drop today. Damn beacon failed and it landed it two craters over, so I'm taking the rover. Probably need three trips to get everything back, but maybe I can do it in two if I leave some of the gear we don't really need. Peterson's barely slept since he looked at the first samples.

00.9.11.10.23
Peterson wants to survey the south plain for more samples. They're getting pretty excited about the results, but I guess it's all secret for now.

John scanned the text anxiously. What was the secret? He forwarded the pages, looking for an answer, and finally his eyes locked on a relatively long entry.

00.20.30.10.23
Peterson's insisting we always sterilize our boots in the airlock. I don't know why he's worried; just simple proteins they say. Messner said it's not alive or anything. I don't really know what the big secrecy thing is all about. I guess if there were some Callistan bugs that made proteins, yeah, that would be a big deal, but they haven't found any yet and Peterson doesn't think they will. They told me clay can make proteins by itself, so there's probably nothing alive. But if they already know clay can make proteins then what's the big secret? Even before we got here they told me that the surface is clay. They're both real excited, but I can't figure them out.

John read that passage over and over, his eyes widening. Proteins? On Callisto? They hadn't even found any true proteins on Europa yet! No, Dupree was just the pilot. He must have misunderstood what they were talking about.

Chandler had never liked the prebiotic soup concept, even though it was the standard theory for how life had developed on Earth. He always brought up the chain-length issue; with water present, hydrolysis would prevent long polypeptide chains from forming. And if they couldn't grow long enough, they couldn't make a viable genetic system.

John had gone over those arguments many times on Europa, as others invoked it to explain why they weren't finding any complex organics in the ice. One solution, he knew, was to throw clay into the equation. Clays could not only concentrate minerals to make amino acids and purines, but they had also been shown to form a template that could link amino acids together in long, entropy-defying chains. Some fringe scientists even speculated that clay could have been a primitive inorganic life; patterns of cation structures might have propagated as the clay grew, evolving "life" on a two-dimensional framework and eventually attracting organics in the same, complex patterns.

But it all required water. Water was the crucial ingredient to life, not clay. And there was no liquid water on Callisto, despite those early anomalous variations in the moon's magnetic field. True, some of the ice would have briefly melted during meteor impacts, but to wait around for enough impacts would take...

Forever?

But this was Callisto. The oldest landscape in the solar system. Was three and a half billion years of meteor impacts enough to do what happened on Earth in a few million?

No, John thought again, shaking his head. Chandler had probably just found a few short polypeptides, and then tried to explain it to Dupree in terms the pilot could understand. There wasn't a fine line between polypeptides and proteins anyway; once they got above a certain length the organic polymers were called proteins. But the shortest true protein, insulin, was a whopping 35 amino acids long. Most were much, much longer. There was absolutely no way Chandler had found anything close to that on this ancient, dry surface. Was there?

Shaking his head, he skipped forward to Dupree's last entry, recorded three and a half days ago, near the same time as the final daily report.

00.2.8.11.23
Something is happening to me. I'm sure of it now. Peterson won't listen. I think we should leave or at least call for help. But we've kept it a secret for so long, he won't do it. Every minute I feel myself getting worse. It's happening too fast. I'm not really in total control anymore. I just know it won't end well.

Just as John was thinking about going to look for Simon and Tony, their voices came over the radio. They were back in the crater.

"You guys okay?" John asked.

"Yeah," said Simon. "But strange news. Assuming the clocks were set right, it looks like the spaceship crashed before the Station was vented. Almost two hours before. Tony thinks the ship might have been sabotaged before it took off. Fuel cut out after a few seconds."

"But then..." John tried to process the new data. Even if Messner had sabotaged the ship, that meant that someone had killed Messner after Dupree was already dead. And that someone had to be... "Chandler?" he asked.

"Still no sign of him. But we have every reason to think he lived through this. Whether he's still alive or not, I don't know. If he turns up, be careful. He seems to be a very dangerous man."

John rubbed his forehead. He thought that it was clear from the log that Dupree had been the one who lost it, who killed everyone. In that last entry Dupree clearly admitted that he was losing control. Until now, John was thinking that maybe Dupree had snapped, killed Messner, and then crashed the ship. But if he had died first...

John had to admit all the evidence pointed to Chandler being responsible. But he didn't want to mentally convict his friend without thinking through all the possibilities. For example, what if...

"Simon?" John said into the radio. "We're assuming that Messner was killed by the station decompression. But maybe Messner was already dead, killed by Dupree before the crash. Then, after the crash, Chandler might have had some reason to open the airlock."

Tony's laugh came over the radio. "What possible reason could he have had for that?"

John glanced at Dupree's log. Maybe it had something to do with the secret research; destroying evidence or something.

"I like that idea." John looked around to see Samantha standing behind him. "I think Dupree was mostly to blame," she said.

"Why?" John asked.

She motioned back to where she had come from, where Dupree's body lay. "I want to show you something."

John held up a finger. "Simon? We'll talk about it when you return. I'm signing off." Flipping off the radio, he nodded to Samantha. "Let's go."

The autopsy room was just a cabin with a pair of unused bunks; Dupree's body had been laid on the bottom one. The spacesuit had been cut away from most of his upper torso, and various incisions had been made in his chest and head. Samantha had made no attempt to patch him up just yet.

"I think Dupree went crazy, killed the others and himself," she said. "He died instantly in the crash, but he wouldn't have made it much longer regardless. This is the most advanced case of CJD I've ever seen."

John shook his head. "CJD?"

"Creutzfeldt-Jakob Disease. His brain has wasted away. Comes with classic dementia and spongiform encephalopathy. Just look at it."

John swallowed and peered into the skull. The brain was there all right, but he hadn't seen enough brains in his lifetime to know what was the matter with this one.

Samantha handed him a hand-scope, the electronic lenses already charged up to 50x magnification. Then she picked up a scalpel and made an incision, peeling a layer back as John watched through the narrow scope. "See all those gaps? Like a sponge. He was a goner either way, probably went crazy as a result. Takes years to get this advanced, so it's pretty amazing he got through med screening on Mars. Still, it's possible they skipped the test, being so rare of a disease and all."

John looked up from the scope. Something didn't seem right here. "How rare?"

She shrugged. "A few cases a year? You can only get infected by eating human or animal brains. New Guinea cannibals used to get a version called kuru. You can also inherit it; Fatal Familial Insomnia, that's called. Pretty amazing, to find a case way out here."

"But... If it took years to get this far, why would he suddenly go mad in a day or two? His e-log was pretty coherent, right up to the end." He showed her the last few entries.

"Who knows? It's hard to say how much of the brain you have to destroy before you'll have any particular effect. He might have just been lucky for all this time."

"Well..." John nodded. "I guess that's it then. It would explain it if he killed Messner and crashed the ship. But where's Chandler, and why did he vent the Station?"

Samantha put a gloved hand on his shoulder. "I'm sorry John. But you have to know he probably didn't make it."

John turned off the scope, handed it to her, and left the cabin without a word.

John couldn't sleep. The others had passed out immediately--Tony was snoring in the bunk overhead--but John's mind was racing too fast to do the same.

Dupree, with an incredibly rare brain-wasting disease? It didn't make any sense. Sure, it was possible, but John never liked it when unlikely events happened. He wanted there to be a more plausible explanation.

It was sort of like his quest for life, he decided. Yes, it was possible that Earth had some incredibly rare set of circumstances that allowed the creation of life. But it would be a much better story if life was 'easy', if life could be anywhere. They were on the verge of discovering the answer on Europa. And if Chandler's research had actually found true proteins on Callisto, then perhaps the easiness of life had already been demonstrated.

Could there be a connection there, he wondered? What had Dupree written about sterilizing their boots? Maybe Chandler had been worried about some sort of alien infection. He almost laughed. Alien viruses would be out of the question. Sure, they might exist somewhere, but what were the odds that they would be tailored to churn out copies of themselves from human cells? Alien bacteria might be more of a problem, and they had talked about the problem on Europa, but that certainly wasn't an issue here on Callisto. If Callistan proteins were improbable, bacteria would be infinitely more so. Plus, Dupree had said they didn't find any life. No, whatever had gone wrong with Dupree, it wasn't related to the local organics.

But as he tried to sleep, he couldn't get that thought out of his head: Dupree had been infected by an alien disease that ate his brain. It sounded like a bad sci-fi movie, but despite the face-value ridiculousness, it was somehow a compelling explanation. Ignoring the sheer impossibility, John tried to reason through the consequences. What if all three of them had been infected simultaneously? Dupree might have gone crazy and killed himself in the ship, and then... And then Messner might have killed himself as well! What if they were both suicides? And Chandler might have killed himself, too, somewhere out in the middle of nowhere.

He bolted upright in bed. Chandler had been the one to insist on sterilization procedures. Maybe he had been more careful than the others, hadn't gotten infected at all. What would he have done? John tried to imagine himself, wandering on the surface of the moon in a spacesuit, both members of his research crew dead of suicides that destroyed the only two life support systems. All the transmitters were destroyed as well. It would be days until a rescue, maybe a full week if Chandler hadn't known about the flyby possibility from Europa. Where would he have gone? The small rover had been destroyed in the airlock. The suit couldn't last long without access to an oxygen generator. But there were the extra oxygen tanks in the Station; why didn't he stay there?

Power. Chandler would have needed power after a few days, and the Station was dead. The only working electrical system on the planet was in the crashed ship. But there was no life-support there...

Yes there was! Chandler jumped out of bed and all but ran to get in his suit. He wouldn't wake anyone else. It didn't matter whether this all had been caused by an alien disease or not. He knew exactly where Chandler would have gone. If he was still alive, John wanted to confront him alone.

John bounced past the Research Station, past Messner's body that still lay on the regolith. Tony had wanted to bury him, Simon had wanted to bring him back frozen, and Samantha had wanted to thaw him out and do another autopsy. John hadn't gotten involved in that particular argument, and the impasse had left the body untouched.

But walking past him now, John wondered if he could test his hypothesis. After all, Messner's brain was splattered all over the ground. He changed course and jogged back to the body.

The brain was impossible to cut. It was frozen so hard that his suit-knife didn't make a dent. He stabbed at it a few times, then brought down the heel of his boot on some of the gray matter. It shattered apart, and John picked through the fragments, realizing too late that he didn't have a scope with him. He couldn't tell if it was the same as Dupree's, but... He looked closer at the just-broken shard, catching his breath. There; bits of the surface fines had somehow gotten inside the brain!

John stepped back, looked at the whole body. Come to think of it, there were black grains all over the brain, but nothing else. He had originally assumed that the wind had blown the dust, but with no atmosphere that was impossible! Had the surface organics somehow gotten in his head and caused this? But even if the regolith did contain simple proteins, how could they turn a human brain into a sponge?

He considered it as he climbed out of the crater, again heading toward the crashed ship. What had Samantha called her diagnosis? CJD? And you could only get it by eating other brains. Why would that be? What sort of disease could you get by eating--

He came to a sudden stop as the answer hit him. Prions. CJD was a prion disease. Connections flooded through his mind. CJD was the human version of mad cow disease. They had had a scare on Mars, about ten years ago. Some stupid Martian farmers had been feeding ground-up pygmy cows to other animals, and the brain-prions had been passed on to a few people through the meat. He remembered it all now; a prion was simply a protein that was able to fold similar proteins into copies of itself. One particular class of them were found in the human brain and nervous system. Once you were infected, the prions would spread by folding those proteins into copies of themselves, with a resulting snowball-effect infection. It was almost impossible to treat, because it was an pathogen without genetic material; a disease agent that was not even truly alive.

John started running again. Chandler had been a genius if he had expected to find this on Callisto. He could only hope his friend hadn't been killed by his own discovery.

Lowering himself into the dark spaceship, John quickly made his way into the center. There, surrounded by walls of lead and concrete, lay the emergency radiation coffins. The coffins that came with their own, individual life-support systems. And, judging by the indicator lights, four of them were operational.

John tried to open one, but the lead door was too heavy, even in this low-g environment. They were designed to only be used in space, he knew, but if Chandler had gotten in here...

There. John saw an automatic opening mechanism. He hit the button, somehow knowing it would work, and sure enough one of the coffins opened up. It was empty.

The next one was also empty, but the third was not. A man in a spacesuit lay in the dry chamber, absolutely still.

"Chandler!" John called. The body didn't move. The oxygen hoses were connected to the suit, but there was no response. Looking more carefully, John saw that all of the suit's non-essential systems were powered down. He reached into the coffin and activated Chandler's radio. "Chandler?" he tried again. Still nothing.

John bent down, illuminated the suit with his flashlight. The readout on the wrist indicated that the oxygen percentage was dangerously low. But Chandler was still alive; he took in slow, deep breaths. Alive, but either asleep or in a coma.

Peering closer, he noticed that no oxygen was flowing into Chandler's suit. Frantically he reached down and connected their buddy-tubes together, giving Chandler access to his own tanks. Almost immediately the oxygen percentage started rising, and Chandler began to stir.

"Can you hear me?" John asked. "It's me, John Stanford. Stan. You okay?"

Chandler eyes opened and slowly focused on John. Then he looked at his own wrist monitor and seemed to jerk awake. He fumbled with his air-control system. "What are you trying to do," he muttered. "Kill me?"

"Huh?"

He gestured to the coffin door. "The air doesn't flow unless it's closed up."

John smiled. "Come on, Chandler. I save your life and this is the thanks I get? Your ox was already low when I opened the coffin."

Chandler focused on John again. "Stan. It's really you. I can't tell you how good it is to see you." He extended a hand and John helped pull him out. Chandler was smiling. "And don't go calling yourself a hero so fast. I dialed down the percentage for a timed amount. To put myself to sleep."

John seemed to remember learning something about that procedure in his early training, but he doubted if he would have remembered it in an emergency. "Well, we did come all the way to Callisto for you, ungrateful bastard." He laughed. "And you're actually alive. I don't believe it."

Chandler looked nervous. "Right. Messner, Dupree... I know how it must look, but--"

"Don't worry, Chandler. I figured it out."

"You did?" He looked amazed. "Great. I mean, that saves me a lot of--"

"Prions? You really found prion proteins on Callisto?"

This time Chandler actually gaped. "But how--?"

"We did an autopsy on Dupree."

"You did?! Where? It was positive? I have to see for myself."

John smiled. "Back at our ship. In the crater. Come on, let's get out of this wreck."

Chandler led the way, chuckling. "But you still don't understand. You didn't figure it out after all. Think about it. You haven't even asked how an alien protein could be compatible with our brain's proteins."

A frown formed on John's face. "You're right. I didn't think... But Dupree? How could--?"

"Let's go outside," Chandler said, disconnecting their buddy-tubes. "I'll show you."

Half a minute later they were standing on the toppled spacecraft, surveying the inter-crater plain. Jupiter hung in the sky to the east, incredibly small compared to what he was used to; only twice the size of his outstretched fist. The sun lit the scene from behind them, shading the dark plain with darker shadows.

"It's all clay," said Chandler at last, gesturing around them.

"I know it's all clay. But the proteins...?"

"Forget the proteins. The proteins are secondary. This is a clay ecosystem. And this particular version around us here, it's slowly taking over the entire moon."

"What?" Chandler made no sense him to him. "You mean it's altering the landscape?"

"Yes, yes." Chandler brushed off his suggestion as if it was irrelevant. "It modifies the regolith and degrades the smaller craters. Devolatilization was a poor explanation from the start. The clay does that, but that's secondary, too. The clay is alive."

"Come on Chandler. What are you really saying? Define life."

Chandler shook his head. "I'm not getting into one of those arguments with you again. In fact, those arguments are now obsolete. I've solved it. I've solved the old chicken-and-the-egg dilemma. Proteins came first. And this is how it happened."

John stared at his friend. The dilemma he was referring to was the protein-DNA question. How could one evolve without the other? Shifting the question to a RNA or even a PNA genetic medium didn't really solve the underlying issue. But if complex proteins could be created inorganically... "The clay makes proteins?"

"And the proteins build the clay. Callisto's a clay-world, Stan. It's barely evolving at all, but it's evolving. It's just taken a long time to get this far."

"But I still don't understand. What happened to Dupree? And what's so special about this particular area?"

Chandler smiled. "Oh, nothing except the crucial missing link between the organic and inorganic worlds. The particular cation pattern in this clay has made it a universal prion."

"A universal--?"

"Yes! It doesn't have to rely on its own proteins to grow. It can co-opt other proteins, fold them to suit its purposes! Usually folding alpha-helixes into beta-sheets, of course, because of the flat clay geometry. It's slowly spreading around the moon, taking over all the other clay-systems. The other proteins are being folded up and used by this universal prion clay."

"So Dupree and Messner, they weren't infected by the proteins. You're saying--"

"Exactly. The clay got in their systems, went to their brain. The clay itself acted as the prion, not the Callistan proteins. It started a chain reaction with the Earth proteins in their bodies. The clay increased the beta-sheet content, which unfortunately happens to be the way to make dangerous prions."

"Very unfortunately."

Chandler nodded sadly. "Still, I'm really interested to see Dupree's autopsy results." He began to clamber down from atop the ship. "I would have done it myself, but of course..."

"Chandler? Are you infected?"

He shrugged the question off. "No, no. I'm fine. Come on, my tanks are low."

John followed Chandler down and gingerly stepped onto the quasi-living clay. "Should we be walking through this stuff?"

"Don't have much of a choice, do we? We'll be fine, Stan. Just trust me on this one. Say, how did you get here so fast? And weren't you on Europa?"

"Yeah," responded John, but he knew something was wrong. Chandler had changed the subject. Was he really okay? If the others had breathed in bits of clay, how could he have avoided it? Still wondering if his friend was alright, John started relaying the events of the last few days as they walked back toward the crater.

The sheriff was waiting just inside the airlock, motioning for them to take off their helmets. John cracked his seal, and Chandler did the same.

"He's alive, Simon!" said John. "I found him in the--"

"Peterson?" interrupted Simon, ignoring John for the moment. A pair of handcuffs dangled from his hand. "I'm placing you under arrest. Get out of your suit. Now."

"Simon," started John. "You don't understand. It wasn't Chandler. It was the--"

"Listen, John. He'll get his trial. But for now, please just shut up."

"It's okay, Stan," said Chandler, pulling off his suit. "We'll get this all straightened out in a little bit." He held out his hands towards Simon, who quickly locked them together.

Tony and Samantha appeared in the passageway, both bleary-eyed. "What the--?" started Tony.

Chandler smiled at them kindly. "I'm Chandler Peterson," he said. "Thank you very much for coming all this way to rescue me."

"Well goddamn," said Samantha. "The man's still alive."

"The crash was my fault," Chandler deadpanned, then winced as Samantha put in an IV. The other three just watched the dirty, exhausted man tell his story. "I didn't realize how much fuel was left in the system when I locked off the main tank valve. I didn't think he'd even get off the ground."

"So you did sabotage the ship," said Tony. "I knew it. You killed Dupree."

"Dupree would have died soon anyway," Samantha reminded the pilot. "But why, Chandler? Why close the valve?"

Chandler shrugged. "Dupree was acting strangely, talking about leaving Callisto. I didn't know what was wrong with him, but I thought I'd play it safe and lock off the fuel so we wouldn't get stranded. I figured if he wanted to leave he'd try to get the keys from me..."

"And Messner?" asked Simon suspiciously.

"He wasn't behaving normally, either. I left him in the Station while I ran out to the crash site. To see if Dupree had survived, of course. When I got back... Well, it was pretty much as you found it. Airlock was wide open. If you don't believe me, I bet you could find his fingerprints on the manual interlocks."

"It's the clay, you see?" added John. "I've been trying to tell you. This is the scientific discovery of all time. The surface is practically alive. It's the real killer, here."

"How dangerous is it?" Simon directed the question at Chandler.

"It's nothing we should worry about. I've been working with the clay for weeks, and I'm okay."

Simon shook his head. "I don't buy that. Then how did the others get infected?"

Chandler opened his mouth to speak, then shut it again. He waved his hands in the air. "I guess I don't know. They brought in the first samples when we arrived. Maybe they got infected right away, and it still hasn't hit me yet. I guess I might be in trouble after all."

"Shit," muttered Samantha. "You can breathe it in. I've been working on Dupree, and I might have gotten a dose."

"My suit!" John exclaimed. "I passed by Messner's body and stomped on his brain to see if it was the same as Dupree's. I never cleaned off the boot; it's still in the airlock."

"And we've all been walking through the clay," said Simon. "It's probably all over the ship by now."

Chandler looked nervous. "Maybe. But I don't think you can breathe it in. I don't know why, but it's just a feeling I have."

A long silence passed between the five of them. Finally Simon spoke. "Okay. It's almost four days until the ship gets here from Ganymede. I don't want anyone leaving the Falcon for anything. I want life-support scrubbed down, I want the filters changed, and I want all the used suits sealed in the airlock."

"But my work, my notes, the samples," said Chandler. He patted his belt-pack. "I put a few personal things here, but most of it's still in the Station, and--"

"And that's where they'll stay until the ship gets here. We'll decide what to bring later. Samantha? Is there any way to diagnose this disease?"

She shook her head. "There is a test for the prion protein, but I won't be able to do it until we get to Ganymede. We should all keep tabs on each other, see if anyone acts strangely. Especially Dr. Peterson. But with this equipment here, positive identification would require a brain biopsy."

Tony grimaced. "I hope it doesn't come to that."

"It won't," said Chandler, shaking his head. "I'm sure it won't."

An hour later, John stepped into Chandler's private cabin. His old friend had just finished a shower and was wearing a fresh set of clothes. He still looked pretty haggard, but at least he was clean. John closed the door behind him.

"What's going on, Chandler? Talk to me."

Chandler looked blank. "You said it, back there. The most important scientific discovery of all time. The missing link in the creation of life."

"But you said... You said you didn't know what was wrong with Messner and Dupree. Why would you lie about that?"

Chandler smirked. "I wasn't lying; I didn't know what was going on. I mean, I had a hunch, but I figured most of it out afterwards. In the coffin."

"The hens," John said. "I know about the hens."

"The...?"

"You knew perfectly well that a universal prion might be dangerous. I bet that when we find your notes there will be something in there about it. You knew the danger, and you wanted to test your theory on some live animals."

Chandler sat on his bunk, nodding. "Yeah. But--"

"So when Messner and Dupree started losing it, that would have clinched it for you. I know how you think. You would have known instantly that it was the clay."

"Okay, so I knew. I jumped to a conclusion before a good scientist might have, but I knew. So what's your point?"

John backed toward the door. "You pretended like you didn't know. You're not usually one to hide your bursts of insight, Chandler. Not unless you have a reason."

He laughed nervously. "And what sort of reason do you think I might have?"

The door opened and Simon stepped into the cabin, his eyes fixed on Chandler. "John thought you might have injected your co-workers with a deadly alien substance to further your own research." He turned to John. "You were right. Samantha just found the microneedle puncture on Dupree. If he was asleep he wouldn't have felt a thing."

Chandler sat down on the bunk, eyes on the ground. He took a deep breath, but didn't speak.

"It's true, isn't it?" asked John. "But why? Why would you--"

"I didn't think it would kill them," he interrupted. "I thought it wouldn't do anything at all. But there was that slight chance... I had to know, and that was the only way."

"The only way?!" John exclaimed. "What about telling me? I could have diverted all sorts of resources--"

"And taken all sorts of credit," shot back Chandler.

John shook his head emphatically, walking forward. "You know I wouldn't have, Chandler. But what you did, putting human lives at risk for your research--"

"No worse than what you did on Europa."

"That's completely different," snapped John. "You... you had no reason to act. You had no legal standing. You just killed them to test a theory. Two people!"

Chandler smiled. "I see. If I had just killed one of them, like you did, then you might approve?" He shook his head. "Dupree's small dose wasn't doing anything, so I gave a larger one to Messner. Both kicked in at the same time."

John took a deep breath. "Look, Chandler. I'm sorry you don't see the distinction, but I'm sure the courts will spell it out for you when we get back."

Chandler stared at him for a moment, then reached into his belt-pack and pulled out a small black case.

"What's that?" asked Simon nervously.

"Just something I kept with me, for emergencies." Chandler opened it to reveal a small syringe, filled with a black liquid. John took a sharp breath.

"You can't have it both ways, Stan," said Chandler. "Your research was worth more than a life, and so is mine." He picked the syringe out of the case and held it above his own arm. "Isn't it?"

"What are you doing?!" yelled John.

Simon was much calmer. "Put it down, Chandler," he said, slowly walking toward the bunk.

Chandler held up the syringe and waved it threateningly. Simon froze, then backed away.

"Just think," continued Chandler. "Just think of all the knowledge we could gain if I did this. You could keep me under observation, see how it progressed--"

"Don't pretend like you'd be making some great sacrifice for science," John said. "You know how it works. You've killed for that knowledge already. If you're stupid enough to do it again--"

"I could drink it, then. That would be something different." He held the syringe next to his face.

"Don't do it, Chandler," said John. "You know it'll kill you. That's how you get CJD. By eating--"

"It looks like you're bent on ending my career anyway," said Chandler. "Might as well do research while I still can. After all, our research outweighs everything else, right?"

John clenched his fists. "No, dammit. You're worth more than the damn clay. So don't--"

With a disappointed scowl, Chandler shook his head. "From you, Stan, I was hoping for the truth." He raised the syringe and squirted a jet of liquid into his mouth.

"Don't--!" yelled John, but it was too late. Chandler coughed briefly, then smiled. It was done.

Simon exhaled sharply. "If you start to go mad," he said, "we'll have to restrain you."

Chandler nodded. "Fair enough." He looked deep into John's eyes. "But we both know you're jealous." He raised the half-filled syringe. "This isn't just one life. This is all life."

John bit his lip, turned, and stalked out of the cabin. After a moment, Chandler's laughter followed.