The Lunar Lichen
(1960)*
by Hal Clement

	If Ingersoll were telling the truth, he had indeed made a radical find, here on the moon. But Dr. Imbriano had doubts, and the destruction of the samples made him wonder even more if the geologist were trying to perpetrate a hoax. But ... if so, what was Ingersoll's motive? And what would his next move be? 

Chapter I

	Kinchen looked out and down from the observation port, watching the suited figure absorbed in its task about the trailer. He watched until the big number stencilled on the suit became visible, and he could be sure of the worker's identity; then he turned abruptly to the men seated behind him. His eyes sought out one of these. 

	"You admit they were—and are—alive." It was more a statement than a question. Imbriano took it so. 

	"They are." 

	"And you don't recognize the species." 

	"I don't—but that's ..." Kinchen raised a hand impatiently. 

	"I understand that you don't know by sight every fungus, lichen, or what have you that's ever been described. You can, though, recognize classes. And you think you recognize this one as belonging to whateveryou-call-it "

	"Hysteriales. And that's not ..." 

	"Never mind. I didn't mean to get technical about orders and phyla and whatever you call them. I'm no biologist. The point is or I think it is—that you used fairly gross characteristics for identification, and such characteristics might very well be duplicated by parallel evolution. Right?" 

	"That's true." 

	"Very well, then. Will you tell me why, except for a natural reluctance to believe there's any life at all on the moon, you feel so strongly that Ingersoll is pulling a Piltdown on us? Don't you like the fellow or what?" 

-

	Jack Imbriano hesitated, and frowned. 

	"It's true that I don't like him very much," he admitted finally, "but I don't think that's what had given me the idea. It's the whole set-up. He came back from a trip, which he'd made alone, well past our normal exploring range, with these specimens of lichen—or pseudo-lichen if you prefer. He had taken pictures of the site, but he says he took them after collecting the specimens, and the pictures certainly don't show any of the plants. They hardly could, of course, since the plants themselves are so small. He objects to going back to the site to find more ..." 

	"He didn't object. I did," Kinchen pointed out. "We have just so much working juice for ground travel, and Ingersoll used too much of it as it is. We could draw a little from the main tanks, but I don't want to cut our return allowance too fine." 

	"All right, you objected. But he also said there was no use going back, because he'd collected all he could find in the vicinity. That's ridiculous, on several counts. First of all, they're so small he couldn't be sure he'd found all that were there, any more than you pick all the raspberries from a patch the first time through. Secondly, he shouldn't have done it. Even a geologist leaves some of his material in site so that his work can be checked, as a standard working procedure. Under the circumstances, I want to go back to that region and hunt for more of what he found—if he found it." 

-

	The Director pondered for a minute or so. 

	"Your point is well taken, but the fuel question remains," he said at last. "We can do it, of course, though it means cancelling some other part of the program. Aren't there any more checks you could make right here, first? How about the rock the stuff is attached to? Don't lichens have some effect on the stuff they grow on —stick roots into it, and so on? How about checking that with the microscope." 

	"Lichens don't have true roots ..." 

	"Stop quibbling. They keep from being blown and shaken off rocks and trees somehow." 

	"You're right—but these were growing on the dust layer, according to Ingersoll. He brought some of the dust with him, but it's not possible to say whether or not it's the original substrate of the plants." 

	"Well, if, as you imply, he brought them from Earth with him, there should be traces of Terrestrial soil mixed in with the things. Can't you identify that?" 

	"I can't. We have geologists here, but who thought we'd need a soil specialist?" 

	"True enough. All right—how about this? Put some of the plants outside, and see whether they live, and grow. You say they're alive now." 

	"They seem to be—as nearly as one can tell with a lichen. There is protoplasm, or something like it, in their cells. And it shows streaming at times." 

	"Then do what I suggest. Ask Ingersoll whether he found them in full sunlight or in shadow—so he can't say you didn't reproduce conditions properly—put them out for a few hours, and see what happens." 

	"A few hours wouldn't produce detectable change in one of our lichens. Most of them take years to do much growing, as I remember." 

-

	Kinchen chuckled. "I'm just an astronomer and ballistics engineer," he said, "but I'll bet that a few hours of this environment will do something detectable to any Terrestrial life form. If that thing is still alive, after a few hours outside, then it's genuine —whether it shows any growth or not. I know people have talked for years about lichen-like growths being possible here, but I never heard a competent man say that actual Terrestrial lichens themselves could stand it. They'd be cooked, irradiated to death, and desiccated in a matter of minutes, and you'll have a hard time convincing me otherwise. That's why I doubt that Milt could possibly be trying a fake. He'd know there are too many easy ways to check on him." 

	"Why would he know it? He's just a geologist." 

	"Why would I know it? I'm just an astronomer. I don't see how anyone sharp enough to make a name for himself any one science can be completely ignorant of the rest." 

	"But Ingersoll hasn't made much of a name, even in his own profession." 

	"Then how come he's with us here?" 

	"How come I'm here? I passed a Civil Service exam." 

	"Hmph." Kinchen might have been impressed; it was hard to tell. "Get on with your check, anyway. If those things stay alive outside, I'll authorize another trip to the place he found 'em—where was it? Other side of Short, somewhere, didn't he say?" 

	"Right." Imbriano was already on his way down the hatch from the "main" deck. 

	At an observation port beside the main airlock there was a microphone, which was tied to the suit-frequency transmitter. The doctor snapped it on. "Milt? You read me?" 

	"Clear enough. What is it?" Ingersoll's voice came back instantly. 

	"I was wondering whether you'd found these plants in sunlight or shadow. It's a rather small sample, and it occurred to us that if we put some of them back outside—planted 'em, you might say—we could grow more before we have to leave, and learn more about them at the same time:" 

-

	"I see." There was a pause, and Imbriano wondered whether the other was pursing his lips in his usual pontifical manner when asked a question, or trying to decide what answer would suit the situation best. "They were in sunlight when I found them," he said after a moment, "but I can't remember whether they were in spots which had been out of shadow for long, or not. None of them was very far from some sort of shadow—but of course nothing is, in this part of the moon. It's as rough on a small scale as it is on the large one of astronomical photographs." 

	"That's true." The doctor was suspicious of the answer—it sounded like hedging to him. Of course, almost any other answer would have been equally suspicious, and Imbriano might have been broad-minded enough to admit this if someone had taxed him with the idea. 

	"Certainly they'd been in the sun for hours, anyway, and maybe days," the voice from the radio resumed. "I guess your stunt is worth trying. From what little I know of lichens, though, they won't do much in the few hours the ship will be in the sun. Remember, we came down just about south of the central peak of this crater, and we'll be in its shadow before long." 

	"That's true. Well, the few hours will do for an initial test—maybe I'll be able to find out how the plants keep from drying out in this pressure and temperature, anyway. I'll be out shortly." 

	Imbriano broke the connection without waiting for an answer, and went back to the main deck. The specimens were on the small table which served him for a laboratory. He had distributed them, together with the lunar dust which had been brought in with them, over several plastic Petri dishes. He glanced over these, picked up two which seemed to have healthy cultures in them, and carried them back down to the air-lock deck. There he suited up, tested his gear, picked up the dishes again, and went through the air-lock. 

	Getting down the ladder with his burden took some skill, the gripping attachments of the suits being what they were, but he managed it at last. Ingersoll's suited form was fifty yards away, still working over one of the tractor-trailer combinations; he did not seem too interested in the doctor's work. They exchanged a brief word over the suit radios, but the geologist did not leave his job. 

-

	Imbriano looked around for a suitable place to expose the specimens. The neighborhood of the ship was littered with gear which had accumulated during the five days of their stay so far. Some of it was apparatus which would have to be returned to Earth; some, like auxiliary fuel tanks, was doomed to stay on the moon. He thought of setting the dishes in sunlight on top of one of the tanks, where it could easily be found again; then he remembered that the radiation equilibrium temperature of the polished metal was a good deal higher than that of the lunar rock, and he would hardly be duplicating natural conditions. 

	He finally selected a spot about thirty yards north of the ship, a small open area floored with the omnipresent lunar dust, set the dishes down, and removed their covers. He watched them for a minute or two; they showed no visible change, and he finally turned back toward the ship. He was startled to find Ingersoll just behind him, though he certainly shouldn't have expected to hear him coming. 

	"Hello, Milt," he greeted the geologist. "Does that seem an adequate replica of their growing conditions? You said they were on dust when you found them." 

	"That's right. I don't suppose the dishes will make any difference. Why did you have covers on them, before?" 

	"The general idea is to keep foreign spores from settling in a culture. I was reasonably careful about that, and of course there won't be too many drifting around in the ship anyway —they'd have been cycled through the purifying plant too many times by now. I suppose that spores from the algae in the plant itself might be loose, but I don't think the danger's very great. Anyway, if your specimens have been contaminated, they're getting well sterilized now." 

	"How's that?" 

-

	Imbriano gestured around them. "This environment. Temperature and pressure would combine to dry out any Earthly life form in minutes. Creatures which formed spores might have time to do so, but the spores would die of ultra-violet irradiation quickly enough—no Terrestrial life has natural immunity, as far as I know. Those of us who can take it do so by virtue of a relatively opaque protecting layer of dead tissue. That's one thing which interests me enormously about your plants—they must obviously have some other protection, or else a genuine immunity to ultra-violet light. That's why I want to grow more of them. There aren't enough now to spare for experiment. They're amazing enough things as it is." 

	"How come?" Neither Ingersoll's voice, nor the face which could be seen inside the helmet, seemed unduly perturbed by the information which the doctor was deliberately providing. 

	"How come? Because even though they're adapted to the moon, they survived the pressure and oxygen concentration inside the ship. They were definitely alive when I examined them in there microscopically." 

	"Hmm. That is funny, now that you mention it. How do you account for it?" 

	"I don't yet. With more information, I suppose ideas will suggest themselves. I'll bring one of these dishes in just before the shadow of that peak reaches us, half a day or so from now, and leave the other one out to cool down in the dark. I'll settle on when to bring it in after I've examined the first one. That seems like a sensible program?" 

	"I'd say so. Let me know what you find out, will you? I'm a bit curious—after all, I found the things."

	"Don't worry. It will be remembered to your credit." The doctor wondered whether he had worded that answer badly, but Ingersoll gave no evidence of thinking the remark at all odd. He turned with Imbriano and started back toward the ship. 

	"Finished your work?" the doctor asked. 

	"Not yet. Can't stay in a suit forever, though. It'll be nice, to get back to a place where they can spare air for smoking." 

	Imbriano chuckled. "It isn't that we can't spare it, but that the algae in the 'fresher are too sensitive to tobacco smoke. If you really want fame, breed a variety with comparable photosynthetic efficiency which can stand a few impurities of that sort. The submarine boys will probably give you an honorary commission." The conversation broke off here, as climbing the ladder to the air lock took too much of a man's attention for other matters to intrude. 

-

	The two reached the main deck together, so there was no opportunity for those already there to ask the questions they would have liked; but the doctor made the general situation clear easily enough. 

	"We put the dishes out in the sun, and I'll bring in the first one just before the shadow gets here. Until then, I guess there's nothing to be done." 

	"Listen to him!" groaned one of the men. "Nothing to be done! Whoever planned this junket accounted for every minute of every man's time—except, of course, that of the good old M. D. I see him sitting around a good deal." 

	"You don't look too occupied yourself, Tick," retorted Imbriano. "That chair you're in seems pretty comfortable." This remark left him wide open, since all the "chairs" were bucket-seats fastened firmly to the frame of the rocket. The crewman ignored the opportunity, however. 

	"I'm sitting;" he said, "because it's easier than standing while my suit tanks get charged. I brought in a trailer load of specimens half an hour ago. Al and someone else immediately refuelled the tractor and took it out again with a different trailer. As soon as my suit is ready and I've had a chance to digest the sandwich I just ate—I'll get into my suit again and, with such help as I can get from anyone whose time isn't planned, I'll unload and catalogue the said specimens. If I should finish that before it's time to sleep ..." 

	"All right, you've made your point. I'll help with your cataloguing, if it doesn't take any more knowledge of mineralogy than I possess, and if no one develops a cold I have to treat in the meantime." 

-

	"Who's been sick so far? It's disgusting, how some people get paid for their vacations. I'll use your help. It doesn't take any brains." 

	The conversation wandered from that point, and both talk and labor bore little relation to the Ingersoll discovery for some hours afterward. Most of the time, the people were outside; all the work, or practically all of it, lay there. Even the physical measurements which did not actually demand sam ples of the moon were usually better made away from the metal of the hull. One man always remained aboard, as a safety measure, but this duty was taken in turn. 

	Tractors and trailers came and went; the trailer system permitted almost continuous use of the powered vehicles. The trailers were light affairs, having three pairs of very low-pressure balloon tires, with interchangeable bodies. They could be used for hauling equipment or specimens of virtually any sort; and of course at least one always carried "fuel"—working fluid for their nuclear turbines. 

	Theoretically, one tank of the fluid should last indefinitely, since the turbine exhaust was condensed and recycled; practically, there were always losses—the fluid was ordinary water, which was decomposed quite rapidly in the reactor. Also, occasional use of "emergency power" demanded a cycling rate greater than the condensers could always handle, since they could only get rid of heat by radiation. At such times automatic valves opened the condensers briefly to "outside", and fluid would be lost. One trailer tank could usually be counted on for three or four hundred miles of ordinary travel, but no one took the figure too much for granted. 

	There were pairs of investigators radiating in all directions about the crater. The central peak was receiving particular attention; it was one of the highest on the moon, a peculiarity of Moretus, and central peaks in general were still being used as ammunition in the perpetual fight between the meteoriticists and the endogenecists over the question of Lunar crater origin. A topographic map of the crater, with five-foot contour intervals and complete geological information on what underlay the contours, was the group's aim; while the mapping itself would not be done on the site, a fantastic amount of measuring had to be. 

	The photographic technicians had hardly been seen since the landing; they had been eating and sleeping in their laboratory, which had been set up in one of the used fuel tanks away from the ship. 

	As a result, not even Jack Imbriano gave a thought to the lichen specimens, or even to his ugly suspicion about Ingersoll, for a good many hours. When he did, the recollection was forced on him; the shadow of the mile-and-a-half-high central peak was nearing the pillar of the rocket, and most of the teams were coming in—the first time since the start of the project that so many had been in together. Recalling his plan for the plant specimens, the doctor suited up and went after them himself—he was not going to let anyone else touch them. 

	Unfortunately, he was a trifle late. It was a little hard to identify the remains of the Petri dishes and plants in the layer of dust where they had been left, and which had subsequently been traversed by the treads of one of the tractors. 

II

	Imbriano stood and thought. True, he had not put up a flag, or issued any other general warning to the crews about his little experiment; that he had to admit. On the other hand, the spot was unusually close to the ship, and the changing of trailers was usually accomplished in one area a little distance away. It was not impossible—for an objective mind, it would not even have been unlikely—for a tractor to cross the spot, but Imbriano was suspicious. He raked through the dust once more, seeing a few fragments of plastic glint in the sunlight, but found nothing clearly recognizable as part of one of the plants; and with a frown behind the face plate of his helmet he turned and headed rapidly for the ladder. 

	On the main deck, six of the ten members of the expedition were waiting when he arrived. Most of them were unconcerned, enjoying one of the rare periods of relaxation—Tick Wesley had not been exaggerating about the constant occupation of the group. The missing three were a pair of petrologists who were "chasing" the shadow, trying to get measurements of any spalling effect from the quick cooling and heating as it passed, and the stratigrapher, Milton Ingersoll. Kinchen was watching the hatch, evidently for the doctor's arrival; and the whole group fell silent at the expression on the newcomer's face. 

	"What's the matter, Doc? Someone catch cold and put you to work?" Detzel, fuel system expert who doubled as tractor operator while not in flight, put the question. Though only a few of the group had heard the doctor's suspicions about the life discovery, he did not take time to explain in de tail, but addressed Kinchen directly. 

	"The specimens I had out are gone. Someone drove a tractor over the site." 

	"Accidentally?" 

	"I wouldn't know. I'm afraid I didn't mark it." He went to the port overlooking the site of his misfortune, and pointed down to the tracks, clearly visible in the dust. "Does anyone here remember crossing that area—making those particular tracks—in the last twelve hours? Judging by their loneliness, it's only happened once. I should think you'd remember." 

-

	The rest of the group crowded around the port, and one by one denied having driven over that spot. All of them were certain; all were able to describe their work of the last half day in sufficient detail to show that their memories were trustworthy. As the evidence came in, Imbriano glanced more and more grimly at Kinchen.

	"I think Milt will have to do some explaining," he said at last. "He knew that I put the stuff there—saw me do it, and talked to me about it. Where is he now?" 

	"I'd still go easy on demanding explanations, Doc," the leader answered. "Remember, it's his own discovery you're accusing him of destroying, to put it at the very least. What you're really claiming, I don't like even to think. I admit that sort of thing has happened, but I still can't believe that Milt could possibly be so—well, unbalanced, as to try it. Will you please be careful if you must discuss it with him? Or better, let me do it?" 

	Imbriano frowned. For a moment, he was on the verge of asking whether that were an order, but he was adult enough to realize that the question would not make matters any better. 

	"All right, Ray," he said. "Please try to find out though. This business has wasted enough of our time already." There was a faint chuckle phrase "our time," and the doctor started to whirl around with a hot remark on his lips; but once again he got the better of his emotions, and said nothing. Kinchen tried to fill the awkward gap. 

	"Why don't you put out a couple of more plates, while Milt's away? He won't know anything about it, and you can find some spot a little farther from the ship where accidents won't happen." 

	"All right." The doctor stepped across the deck to the table which was considered his private domain, and then spun to face the others, fury showing plainly on his face. 

	"Unless someone has a really original sense of humor, there's been another accident," he remarked, keeping his voice under much better control than his features. "The dishes with the lichens are gone. I'll be as objective as I can, to keep our good commander happy, so I'll start by saying—this is far too serious for a joke, practical or otherwise. Did anyone borrow, or otherwise remove, from my table, here, six Petri dishes? Each containing some rather crumbly—looking bits of lichen?" There were no answers for a moment, then a collection of negatives. Imbriano looked at the commander. "How about it?" 

-

	Kinchen was extremely uncomfortable. He had been uncomfortable ever since the doctor had first hinted at the possibility of a Piltdown on Ingersoll's part. There was no point in delaying the issue by asking questions about opportunity; Ingersoll had served his turn on watch, alone in the ship, for more than an hour since the dishes had been set out. He could have done it. Why he should have was not quite so obvious. The astronomer thought for a moment, wishing as he did so that he been able to come as an astronomer rather than leader of men, which had never pretended to be. He finally began asking questions. 

	"How many of you heard directly from Milt of his discovery of plant life?" was his first question. The doctor started to say something, but closed his mouth again. Kinchen glanced at him. "I'm not changing the subject, or postponing the issue, Doc," he added quietly. "How many, please?" Four hands went up. 

	"How about you, Al? Did you hear about it at all?" Kinchen asked the only one who had not responded—the doctor had made no move, but the answer was already known in his case. 

	"Bill told me," Detzel answered. "I was asleep when Milt came in. I had the impression he was telling everyone, and had just missed me by chance." The commander nodded. 

	"So we all knew it," he said slowly. "Then Milton knew about Doc's test, since Doc carefully told him. And he knew, furthermore, that the test would show up any Terrestrial organisms. If he were actually trying to pull a Piltdown, what would he do?" 

	"Destroy the evidence, first of all!" answered the doctor promptly. Kinchen looked at him thoughtfully. 

	"What good would that do?" he asked. "We all knew of the discovery. If we knew it was faked, then ..." 

	"But, in a way, we don't know—or, at least, you refuse to admit that it's proved. And you're right, of course. With the specimens gone, there's no proof. We could never even make the charge." 

-

	"If that were all, I'd be quite relieved," Kinchen replied. "However, if he had really done this, and then destroyed the specimens, the fact would be bound to come out among us almost immediately. Either he'd make no more mention of the discovery, which would be a confession in, itself, or ..." 

	"Or he'd be as surprised and disappointed as anyone at the disappearance of the specimens, and insist that some enemy had done it to ruin his reputation. And how would we prove differently?" cut in Imbriano. Several pairs of eyes met as their owners considered this aspect of the matter. 

	The commander was silent for some moments. "I must admit I hope that's what happens," he said at length. 

	"Why, for goodness' sake?" snapped the doctor. 

	"Because then I will simply send two or three pairs of searchers to the area where he claims to have made the find, and really cover it. If we find more similar specimens, well and good. Milt's charge will have some stuffing—but personally I'd be inclined to keep the matter quiet. If we don't, then we just keep quiet about the whole thing, and Milt is deprived of discovery rights. He can submit his report, but he'll be taking his chances on belief, of course. What's happened to the specimens is certainly unbelievable. That would get the whole thing out of my hands, where I'd much prefer it to be. If, on the other hand, he's sufficiently unbalanced to feel that he's given himself away completely to us—this is now assuming that he's really guilty—I see two courses of action open to him." 

	"And those are?" 

	"To kill himself, literally or figuratively—that is, actually destroy himself, or go back to Earth with no reputation, which I for one would find trouble doing—or kill us." The last phrase came so abruptly that no one grasped it completely for several seconds. Then there was a babble of voices. 

	"He couldn't" was the concensus which made itself most clearly heard after the first few seconds. With that comforting thought, the noise died down; but Kinchen shook his head slowly. 

-

	"You're wrong. He could. Any one of us could. Have you really failed to grasp how completely each of us has been depending on the others for his life? Each of us has been alone in the ship time and again. Each of us has been in complete charge of food, drink, air, and the transportation back to Earth. You know as well as I that one man could fly this bucket home. Take-off orders are already in the tape, the only variables of noticeable magnitude are due to libration, and those are small enough to be handled by remote control from the computers on Earth—as they were planned to be handled. Your need for me ended when we touched down here. This machine could be started for home at any minute, by any man, and make it." 

	This point was digested in an even deadlier silence. This time no one looked at anybody else. 

	"I think that's one possibility we'd better dispose of right now." The quiet voice which broke the silence was that of Tick Wesley. "There are three obvious means of getting rid of us, granting that he wanted to. The food, the drink, and the air. Let's check them. Doc, you'd better find whether any of your drugs are missing." 

	"That won't take long," Imbriano answered. "Just a moment. You might as well hold off on the other checks. If there's nothing missing, there's not much he can have done to food or drink." 

	The check of his medical supplies took a scant five minutes, and was encouraging. "All accounted for," he said at last. "Better check the air plant, though I don't see what he could do about that without involving himself in the result." 

-

	Detzel and Wesley examined the intricate little pump-and-tank assembly—more intricate than seemed necessary at first, since it had to bubble air into water and get it out again in free fall as well as with weight to keep the liquid separate—but could find nothing. The lights were sound, the circuitry intact, the algae healthy. They returned with this news to the others. 

	"Then as far as we know, Milt is sincere," Kinchen said with visible relief. "And I can't believe he'd be idiotic enough to leave without taking care of us in some way, after what Doc told him ..." Several of the others were shaking their heads; and he remembered. "That's right. There's still the path of straight denial open to him. But that's all right—it's the one I'd like best to have him take. Frankly, I'll be happy as long as there's reasonable chance of his innocence, no matter what unpleasant possibility that will imply about someone else. Let's forget this for the moment and eat. The shadow will be past in a few hours—we're pretty close to its tip—and there's a lot of work to be done." 

	"Ben and Hans are coming in with their tractor," someone called from one of the ports. "Better get food ready for them, too. They'll be hungry." 

	"All right." Frake, whose turn it was to get the meal, disappeared toward the galley, several decks below the air lock level. 

	"I still would like to know where Milt is and what he's doing," remarked Imbriano. "I thought it was customary to check with someone—no matter who—before going out, in the interest of safety. 

	Kinchen shrugged. "He didn't, but he's gone. That is, unless a gremlin made off with one of the tractors. He didn't tell us on the other trip, either, remember. I nearly had heart failure when he didn't turn up for fifty hours and I didn't have the slightest notion which way to search. I suppose he'll be back with another discovery." The doctor glanced at him, but made no comment on this closing speech. Perhaps he might have, but he had no chance. 

-

	A voice came echoing up from the lower levels. 

	"Commander! Doc! Everyone! Come here!" The voice was that of Frake, and there was quite a jam at the hatch before the six men who rushed for it got themselves sorted out. Imbriano was first out of the tangle, Kinchen last. By the time the commander reached the galley deck, everyone else was staring at what Frake had to show. This, as it turned out, was practically nothing—a fact of some interest, since it should have been their food supply. 

	"We're—we're cleaned out!" Frake said. "There isn't a day's grub left, for the lot of us. How, and where, did it go?" 

	"Search the ship!" was Kinchen's instant order. 

	"That will be a waste of time," predicted he doctor. "He could have moved it out with no trouble at all. Instrument and data containers have been going in and out the airlock in a steady stream, practically all the time. None of us would notice the details of anyone else's gear, any more than we notice in particular when someone takes off with a tractor to do his part of the job. We've been too busy to pay attention to other people." There was no humor at the "We" this time. 

	"Make the search, anyway," the commander repeated. "Everyone but Doc, and Al." The others scattered, their faces serious; the two who remained with the astronomer were even grimmer.

	"What is it, sir?" asked the engineer, when they were alone. "You wanted me for some special reason." 

	"Yes, Al. Taking our food was pointless, unless something else was done, too. Remember we could get to Earth in a hundred hours. Check the power plant—every cubic centimeter of it that's not too hot to be touched. I'll bet you find something before the rest do," he added rather grimly. Detzel nodded, and disappeared downward. Kinchen turned to Imbriano, and eyed him thoughtfully. 

	"As , you say, Doc, I'm a hard man to convince—or didn't you quite get around to saying it? No matter. You seem to be right. Now we'll have to figure out where he is, catch him ..." 

	"Why catch him?" 

	"I'm sure it will turn out he's taken some essential part of our flight equipment with him, to prevent our simply heading back for Earth and leaving him behind. I'll admit he may be unbalanced, but I still can't picture him as a moron. Wait and see—there's not too much point chasing him until we know what we're looking for." 

III

	Very little happened in the next hour. The two men who had been seen approaching came in, and were told of the state of affairs. They had nothing to contribute; they had seen neither Ingersoll nor the missing tractor. No trace was found of the missing food. 

	Neither of these facts surprised the commander in the least. One which aid, however, was Detzel's failure to find anything whatever wrong with the reactor or any of its auxiliary gear. So far as he could tell, they could have strapped in and left the moon on ten minutes' notice. Kinchen was slightly tempted to do it, but his eternal uncertainty kept him from acting. He thought for a while, then ordered the group to make a check on which trailers, and what kinds, had gone with the tractor presumably containing Ingersoll. 

	This was accomplished quickly enough, and the conclusion reached that the fellow must have made off with what amounted to a freight train. Four of the heavy-duty trailers had disappeared, in addition to the extra "fuel" carrier. It was easy to see where the food must have gone. It was less easy to see what, other than abandoning the man on moon, was to be done about it. The group gathered around Kinchen, hoping he'd come up with a decision but quite willing to express ideas of their own if asked. The commander did his own deciding, this time. 

	"We give twenty-four hours to a search for Milt, with the object of bringing him back if at all possible. We have just one tractor for the purpose. Those who don't go on the search will wind up their various jobs as well as they can without long distance transportation. Volunteers for the search?" 

	"I'll go!" Imbriano said emphatically. "I'll probably be needed, anyway." 

	"Maybe—though I hadn't heard you were a psychiatrist. You're probaby right about going, though. Let's see ..." he glanced over the raised hands. "Al and Bill, you go with Dr. Imbriano. Do your best to catch Milt without hurting him. It seems important to me that we find out whether this has been caused by something about the moon, whether or not you care about Milt himself. Try not to get yourselves hurt, and for Pete's sake don't get both tractors crippled a hundred miles from here. There must be a limit to how far a man can walk in a space suit, even on the moon, but I'd rather not collect data on just what it is right now. Al, before you could you turn up the heat a trifle? This ship is getting positively chilly." 

	"It's been that way for some time," Frake remarked, "but I didn't like to say anything." 

	"What do you expect, in the shadow of a mountain on the moon?" Imbriano asked, with a slight trace of superiority in his tone. 

	"I'd expect to be cold," Frake said calmly, "but your crack seems irrelevant. We've been in shadow only about ten minutes, and I've been cold longer than that. Maybe it was psychological." 

	"Save it!" snapped Kinchen. "Al, run up the main thermostat as I asked. Then get suited up with Doc and Bill and get going." 

-

	Twenty minutes later, the tractor was rolling. There were two clues to follow; occasional tracks in. the dust, and the likelihood that Ingersoll would take his former course, which he had mapped and reported—truthfully, they hoped. 

	For some time, at least, the two sources of evidence agreed. It seemed likely that the fugitive would be forced to travel slowly, since he was carrying a long train of trailers. These would not only be a heavy load for his turbine, but might also prove a maneuvering problem if he got into any tight spots. If this proved not to be true, catching the fellow would probably be impossible; he had quite evidently taken an extra supply of turbine juice, using for the purpose the only spare carrier adapted for the stuff. If the pursuers did not sight him before reaching their range limit, they were out of luck. 

	Sighting the other vehicle was also likely to be a problem. In full sunlight, of course, the metal would glint and be recognizable over vast distances; but in shadows, where the only illumination was reflected light from the surrounding peaks, the problem was different. They carried a snooper—an infra-red viewer intended to help map the crater in terms of equilibrium-temperature variations as a clue to dust depth and petrological differences, but its field was narrow. Detzel used it on every deep shadow they passed, while Frake drove and Imbriano used his eyes; but no sign of the other tractor appeared, except occasional tread marks. 

-

	They were heading south and a trifle east (not the selenographer's east, but left of south) toward a spot where small crater breaks Moretus' southern rim. Here, according to Ingersoll's report, he had found a pass out of the walled plain which was possible for the tractors. The pursuers reached the area in a reasonable time, and found no difficulty in tracing the path, though there was no way of being sure, whether the tracks had been left on the original trip or only a few hours before. The driving was hard on the nerves; grades were steep along the way, and steeper to either side. They eventually reached the top, skirted the five-mile crater, made a last radio check with the ship, and were about to break line-of-sight contact with their friends when Kinchen suddenly interrupted Wesley's routine acknowledgement of their call. 

	"Al!" his voice came through clearly, with no attempt to cover its owner's anxiety. "We've found what was done to the ship. You may have to come back—listen. The upper manual safeties and the main tank were both opened—we can't tell when—and left that way. We don't know how much water we lost from evaporation, and we can't get the valves closed. Any ideas?" 

	Detzel matched the microphone from the doctor, who had been handling communications. 

	"The tanks were completely full, initially. We never touched them on landing. With those valves wide, the water would have boiled—we should have felt the vibration if we were in the ship. It must have been done while Ingersoll was on watch. Boiling water would spatter into the vents, and perhaps outside them, and as the evaporation pulled heat from it it would freeze. The valves are probably jammed with ice. 

	"You may not have lost much from the tanks, since a layer of ice would have formed sooner or later on the surface and cut down the evaporation rate. That must be what made the ship so cold—evaporation into a vacuum. I should think you could free the valves by melting the ice—you may have to do some improvising with electric heaters, but it shouldn't be difficult. When you get the valves shut, keep the main thermostat up the way I left it. When the ship temperature really starts to climb, the ice inside the tanks will have melted and you can reset it to make the place comfortable. With liquid in the tanks, you can compute the amount of juice from the reading of any of the static pressure gauges—preferably Number One, the lowest. There's a table in my kit for turning pressure readings into quantity for that tank under various acceleration conditions. We'd better go on, it seems to me. Whether or not there's enough juice left to get us home doesn't make much difference in what we can do about it." 

	Imbriano interrupted. "Why go on, though? Ingersoll must have been raving mad to pull that trick. It would doom him as surely as it does us, if too much water really boiled from the tank. He's probably driven himself over a cliff or opened his cab with his helmet off by this time, anyway!" 

-

	No one in the cab really heard Kinchen's answer to this. It came through, but it came through mixed with another voice. It was a dry, clear voice, enunciated so perfectly that the words were plain even mixed with those of Kinchen, and clear enough to permit the mocking overtones to be grasped. All three listeners got every word of it; none of them could remember afterward what Kinchen had been saying at the same moment. 

	"That sounds like our good doctor!" the mocking voice came. "The doctor who knows so much. The doctor' who shouldn't really have come to moon at all, since he knows much about it—knows it hasn't any life, and knows it hasn't any water. Such a smart fellow! And he feels sure I've killed myself, so that I won't have to starve on the moon like the others, because of course that dope Ingersoll could never find anything on the moon to replace water lost from the tanks! Oh, no!" 

	"Tell me, Dr. Imbriano, how, to you manage to live with your own brilliance? Doesn't t overwhelm you at times? Of course, you're right about one thing—you ought to go back. You won't get to water with the fuel you have. I can wait, wait until you're gone, and fuel up my tractor and come back, and refill the ship's tank, too. And I can take off for Earth with a very sick group of friends, and they just might die en route, and be jettisoned in space, so no one could ever tell just what they died of. And maybe they were a little crazy, because they destroyed my life specimens—don't you think that's a reasonable chain of events, you self-righteous, pompous, know-it- all? Don't you?" Ingersoll's voice fell silent, and the men in the cab looked at each other. 

	"He's really gone!" muttered Detzel. "Plant life— which I could and did swallow —but now water, which I certainly can't!"

	His attention was attracted by Kinchen's voice, asking why the tractor had stopped broadcasting. Evidently Ingersoll's waves were not reaching the ship, which was hardly surprising. Detzel extended the microphone to the doctor, so that he could explain what had happened, but Imbriano shook his head impatiently. He was obviously bothered by something, and didn't want his thoughts interrupted, so Detzel himself explained to the commander. Kinchen listened silently. 

	"If he's really out of range, you might as well come back," he said when the engineer had finished. "I wish those fellows who gave us all the tests before takeoff had been able to pick that up. We've lost one man, may lose nine more, and the project itself can't possibly be completed now. All that's over and above the fact that I liked Ingersoll." 

-

	Detzel was about to acknowledge the order when the doctor held up a hand imperiously. 

	"Wait!" he exclaimed. "Can he possibly be out of range of the tractor yet, if we can hear him on the radio?" 

	"It's hard to be sure, without knowing how far from a straight line the ground will force us to go, but I'd say it was unlikely. Why?" 

	"Beacuse we'll have to get him—have to. He's not crazy the way you think. I'm no psychologist, I admit, but I think I know what's wrong, and it's my fault. Sure, he's a bit paranoid—but I rode him too hard. If anything pushed him over the edge into this nonsense, it was the way I treated him—you could read that, in the way he was talking just now. I'm the one, he's down on, and—well, let's not go into it. We've got to get him." 

	"I can't see it," retorted Frake. "What difference does the cause make? Even if you feel guilty, and want to rescue him, what difference does it make if he's killed us all? I don't blame you, but ..." 

	"That's not it—at least, not all of it. Sure, I feel pretty rotten about what I've done to Milt, but that's not the whole story. He's not raving mad. He wants revenge on me. How can he get it unless he's telling the truth about the water?" 

-

	There was a moment of silence; then Detzel spoke. 

	"Either you're speaking from knowledge 'way outside your field, or you're filling in a graph with a lot of guesswork, or you're nuttier than Ingersoll," he remarked. "Just how do you get the notion of water on the moon? Every part of the blasted rock ball gets above the boiling point of water, or even what the boiling point would be at sea level on Earth. And the moon can't hold any gas with a molecular weight of less than about sixty. Hydrate minerals like gypsum form from the evaporation of salt solutions, and if the moon ever had seas I'll drink an equivalent quantity as soon as it's proved." 

	"Never mind the cosmology," snapped Imbriano. "It's irrelevant. Ingersoll, remember, is a geologist. I don't think he's a very good one, and it's my own fault that I didn't keep that to myself. But he's not a complete dope and I never said he was. He claims, indirectly, that he's found water. He should be competent to know whether he has or not. If you don't want to stay on the moon to be discovered by the next expedition, then get back to the controls and start us along that trail once more. Ingersoll may be really crazy, but I'm betting he isn't. Give me the mike." 

	The engineer obeyed, muttering something about "wishful thinking," and started up the turbine. Imbriano called the commander. 

	"We're not coming back gas just yet," he said. "I can't explain why over the radio. Expect us when you hear from us." He snapped the microphone onto its hook with a gesture of finality, and settled an back into his seat with an expression on his face which prevented either of the others from speaking. The tractor nosed its way along the small crater rim and began to switchback down into the incredibly broken country between Moretus and Short. The trail was clear enough, here; most of the ground was not only too rough for a tractor but too steep for dust, and everywhere a vehicle could go there was enough dust to take its tracks. More than once the marks showed multiple; evidently Ingersoll was retracing his earlier path. 

-

	For some fifteen miles projectile distance, which the torturous way made into more like forty, they followed westward between Moretus and Short. Then the trail led up the outer slopes of a ten-mile crater which overlapped the northern rim of Short, and down a terrifying ridge where the two merged out onto the somewhat smoother floor of the latter. The trail was more difficult to see here, but the drivers were catching on to the logic Ingersoll seemed to have used in finding the passes; and between this and the occasional tracks, they were able to follow almost straight across the thirty-mile walled plain of Short to another intruding pit on its southern rim. They sloped up along the latter, and eventually emerged on the eastern brink of Newton. They were perhaps ninety miles from the ship in a straight line, but had ridden considerably more than twice that distance. 

	The scene below them was something Earth could not offer, and even the moon would have had trouble in equalling. Newton comes the closest of any ringed plain of its size to having the entire floor visible from one of the walls. Usually the far side is well below the horizon; but Newton is deep. The men were not at the highest point of the rim; that was nearby, a four-and-a-half-mile peak more impressive than any mountain of Earth, since the four and a half miles was above the nearby plain rather than a sea several hundred miles away. Even from the point where the tractor was parked, the drop to the central plain was stomach-wrenching—something better than twice the depth of Arizona's Grand Canyon. 

	A little ahead of them, the wall curved in and descended toward and even beyond the center of the ring, almost as though Newton were two partly-fused craters. It seemed likely that the trail they were following would go down this way; the fugitive had certainly come this way before, and it seemed unlikely that he would have resisted the temptation to make the descent along what looked like a God-given path. 

-

	North and south the walls curved westward, finally swinging back together and meeting some seventy miles away. Inside, they alternated stretches of appalling steepness with what amounted to broad terraces; on the far side, the lowest of these could just barely be seen above the bulge of the moon's curvature. The curve itself showed plainly on the floor of Newton, though even allowing for this the "plain" was far from level. The northern half seemed deeper than the southern, carrying on to some extent the impression of two merged craters; much of the deeper floor was invisible in the shadow of the north rim, the sun being less than fifteen degrees above the northern "horizon." It was less than a day past local noon. 

	"This is a bad place to park if we don't want Milt to know we're coming," remarked Detzel after absorbing the scenery for some minutes. "This metal buggy must be gleaming all over the crater. If he's anywhere inside, he must know we're here already." 

	Imbriano didn't answer directly. He was scanning every dark patch he could see within Newton's ring with the infra-red viewer, and the northern part of the floor was a lot to cover with the narrow-field instrument. "I should think that even a man in a space suit would radiate visibly against that background," he muttered. "It's cold. Not a flicker on the screen, at any gain this thing can take. Any metal reflection in the sunlight areas?" 

	"Nothing so far." Both the other men spoke together. 

	Frake added, "You want a spell on that snooper?" 

	"All right." Imbriano removed his face from the visor, and handed the gear forward. For some time there was little sound as Frake very slowly and methodically scanned the impenetrable darkness be1ow. Then he stopped, and played with the gain control for a moment or two. 

	"That should be it," he said. "It's about the right temperature for a condenser radiator. I can't see any motion, but he's a long way off—forty miles, I'd guess, though it's hard to be sure when we can't see the bottom contour. He could be on a hill a lot closer." 

	"Where?" both the others asked simultaneously. 

-

	"See that peak just coming up into sunlight on the floor, just below another on the far rim? There. It's warm enough to show on the screen. Now, swing the viewer to the right slowly—just a couple of degrees—that's it; you should have him." 

	"There's a spot on the screen, all right," Imbriano admitted. "I can't read these colors well enough to judge temperature, but you should know this gadget better than I. If you say it's the right temperature, it must be Milt. I can't imagine any other source of warmth down there. Let's go." 

	"Which way?" 

	"Keep along the trail. I know it takes us farther away from that radiation source, but I can't see diving straight down hill toward it" 

	Detzel nodded, started the turbine again, and sent the vehicle crawling forward. As they had expected, the trail led out onto the spur which merged into the floor miles across the plain. It was impossible to follow rapidly; on the original trip, Ingersoll must have been amazingly lucky to find the way down in the time he had, been away. It turned out that the trail reached the floor well before the buttress did, switching down the north side so they were able to keep the radiation source in sight nearly to the bottom. On the floor itself, of course, the curve of the moon put the other machine below the horizon. 

	The trail now, led almost straight toward the northern shadows; the sun crawled visibly toward the scarp miles above as they advanced. 

	"We're going to need lights here," remarked Frake. "There's reflection from the peaks, all right, but I wouldn't trust it to keep us out of a crack." 

	Detzel grunted agreement; Imbriano was silent. A faint memory was crawling up into his consciousness. He kept sweeping the darkness ahead of them, hoping the other tractor would show on the screen; but the minutes crawled by with nothing appearing 

-

	The sun vanished at last. The ground about them could just be seen in the light reflected from the ring of peaks, but as Frake had predicted, the lights of the tractor were needed. If the other vehicle were still in shadow, it must be using lights too; but of course these would be almost impossible to see unless pointed straight at the pursuers. Imbriano kept the viewer in use. 

	The ground, when they firs entered the shadow, was the typical, dark, dusty lunar plain. At first, they saw an occasional track; then they must have wandered a little off the line, for no more of these appeared. When Detzel finally pointed this out, and asked the doctor which way to go, Imbriano answered, "As you are. Keep angling west, and toward the north rim. That's about the direction to the spot where he was, and there's something else I want to see, anyway." 

	"You won't see much with these lights," replied the driver. "You'd better wait until the sun gets here. It looks as though we might be waiting, anyway; turbine juice is running low. We're about to the halfway mark on the gauge, and there's a big hill to climb the way back." Imbriano smiled, seemed about to speak, but didn't. 

	Then, slowly, the ground changed. Its color under the lights was paler, as though more feldspar were showing in the predominantly basaltic rock, and the doctor began to nod slowly. At last the surface seemed almost white. 

	"Bear a little to the left—five degrees or so," he said abruptly. Detzel obeyed without asking why, and silence fell again for another ten minutes. Then something appeared on the ground ahead. 

	"Tracks!" exclaimed Erake, the first to see them. "We've found the trail again!" 

	"I thought we'd be pretty sure to cross it," Imbriano said quietly, "and of course, it would show up well here." 

	"Why of course? Because the dust is so light-colored? I'm surprised it's deep enough, on this flat surface. The trail looks almost like marks in snow." 

	"Uh-huh." Imbriano drawled the answer in a manner which would not have been tolerated even in a child actor, but the tone got his hearers' attention. They whirled in their seats to face him. 

	"Are you implying it really is snow?" gasped Detzel. 

-

	"Eyes front, driver. I am too much of an ignoramus to dare imply anything. I think I owe Milt Ingersoll a profound apology, though. If one of you will switch on the radio, I'll try to make it. He might be close enough for diffraction to get him even if he isn't quite line-of-sight from here." 

	"Wait a minute." Detzel made no move toward the radio. "I don't care what the stuff out there looks like. If it has a boiling point much below that of feldspar, I'll melt and drink it. You know as well as I that even ice has a respectable vapor pressure near its freezing point, and when the sun gets on his stuff it's a darned sight hotter than the freezing point of ice." 

	"Minor catch, Al. When does the sun get on it?" 

	"Why—in the daytime, of course. It ..." 

	"I hate to be a party popper, but isn't it daytime right now, on this part of the moon? Correct me if I'm wrong." 

	Detzel whistled gently. "You're right. Some of this shadow would get light when the sun was farther east or west, but most of it, right against the wall particularly—but wait. What about seasonal changes?" 

	"On the moon? With its axis about one degree from the perpendicular to its heliocentric orbit? Sorry. I don't know how permanent that axial orientation is—with all the perturbations there must be—but I'll bet it hasn't wandered very far from its present line since the moon's rotation matched its geocentric revolution. Some of this area may have been dark for only a few thousand or a few million years, but right in against the cliffs it's been more like two or three billion, I expect" 

	"I see what Milt didn't like about you. You're too darned right. All right, I concede, drink the stuff. But wait a minute. Granting that it could stay here, how did it get here: I don't buy rain, springs, frost, dew, rivers, or any other normal way." 

	"You'd better not drink it. I expect it's ice only by courtesy. I wouldn't be surprise if a good healthy lacing of ammonia and perhaps methane were there; as well as water. As far as how goes, I don't really know. But as a working guess, the moon must have passed through quite a few comet tails in the last couple of billion years." 

	"But comet tails are thin—a ton to the million miles of length, or something like that…"

	"Two billion years is a long time. But I don't insist on that. I haven't tried to work it out quantitatively; and wouldn't be able to get an answer if I did try. Maybe the solar system went through a nebula or something—I don't know. I just say there's something like snow out there, and Ingersoll seems to have convinced himself that's what it is, judging by his remarks a few hours ago. That's why I say—give me the radio. I want to apologize to him." Detzel obeyed in dazed silence, and Imbriano sent a call pulsing out over the crater floor, but there was no answer. He stopped after a few minutes, judging that he either wasn't being heard or was being snubbed, and they kept on along the trail. 

IV

	Perhaps an hour later, after several more unanswered calls, they reached a spot where something seemed to have happened. There was a dark patch of irregular shape in the "snow." The white deposit was now some half an inch deep on the plain; but here it seemed to have been cleared away. The edges of the bare region were sharp and well defined, though irregular. The men all reached the same conclusion at the same time; they had all shovelled too many snowy driveways to be fooled here. 

	"He scraped the stuff up to put in his tank!" exclaimed Frake. "That's what he meant about water, all right—though he'll spend a good long time getting up enough to make much impression on the ship's tank, I should think. But hadn't we better do the same? Our own fluid gauge is reading lower than I really like, at this distance from Moretus." 

	"How about it, Al?" asked Imbriano. "Suppose this stuff is largely ammonia and/or methane? What would happen if we used it in the tractor?" 

	"Either one is all right so far as straight theory goes," Detzel replied carefully. "They're both low-boiling, low molecular weight compounds which would operate perfectly well in a turbine. I'm just afraid they might be a little too low boiling. That would cut down of efficiency, and at our working temperature their vapor pressures might be too much for our tank." 

	"I was afraid of that. Is there any way we can make sure, safely?" 

	"I should think so. There are safety valves on the tanks —after all, even water is apt to get pretty hot if the tractor stands in the sun for long. The regular relief valves might keep things safe, but I could ease off their springs a bit to make them safer. If we don't put too much of the stuff in at once, we might get away with it. After all, Ingersoll seems to have." 

-

	"He seems to have loaded the stuff. We don't know that he got away with it," responded the doctor dryly. "I suggest, Al, that we quietly put one pinch of the stuff in the tank and see what happens—in fact, could we draw a bucket or can or something of water from the tank and put our pinch of snow in that, at some distance from the tractor? I admit I'd be happier that way." 

	"I guess a cup of water would last long enough for that. We'll try, anyway." The three men donned their helmets, pumped a reasonable fraction of the cab's air into the low-pressure economy tank, and opened up. Detzel found a paper drinking cup and stepped out, making his way around to the trailer which carried the, fluid tank. There he bent, held the cup under a stop-cock, and quickly opened and, closed the latter. Water squirted out violently; it was warm enough to have a vapor pressure of several centimeters of mercury. The stream of liquid hit the cup and splashed, but enough remained inside to be useful. Detzel grimaced behind his face plate.

	"Offends my economical soul," he remarked, staring at the bubbling, frothing liquid. 

	"You'll be wasting more if you don't get moving," retorted Frake. "Get some of the snow in before everything boils away." 

	Detzel obeyed. He took a small scraper from its place on the side of the trailer and walked over to the edge of the clear area. He set the cup on the ground where the men could see it; Frake was holding the beam of a flashlight on the scene. He picked up a bit of the snowy material on the end of the scraper, and tipped it into the cup. 

	The results were spectacular; as Imbriano said a moment later, "Water holds quite a bit of latent heat, doesn't it?" The contents of the cup fountained skyward and failed to return, fading into invisible vapor before the moon's feeble gravity could do much about it. The cup itself was intact, but the fact was rather surprising to the witnesses. 

	"I don't think any valves made will take that, or let the tank take it," Detzel remarked "I'm afraid we'll have to depend on what's still in the tank to get us back to the ship." 

-

	"What?" Even Imbriano was startled to hear the dry voice of Ingersoll in his headset once more. "What? Can't the brilliant doctor solve such a simple problem? Even when he just mentioned the answer? But of course, you have a slight disadvantage. You have only one fuel tank, haven't you? I very carelessly brought the spare with me. It was empty when I filled it—with snow, friends—no water. No stored heat to speak of. I've packed the snow into it, and we'll just let it melt very slowly, and the methane can evaporate quietly through the valves, and the ammonia stay in solution if it wants ... 

	"I'll tell you what, good doctor: why don't you just dump all your water out of that tank? Then in a little while it will be cool enough to take the snow safely, and you can go back to starve with your friends—for you can't catch me, can you? I have two tanks, and that makes the big difference, doesn't it? I'm going, by the way, and—I'm sure you can see me with your instruments, but you can't follow. You don't dare go any way but back to Moretus, do you? Of course, I'm not going far either— I'm not going to take this tank out into sunlight for a while—but you don't dare even chase me around in circles, do you? Fuel is getting a little short." 

	He broke off as abruptly as he had started. The drivers looked at the doctor. He shrugged invisibly in his suit, and led the way back inside the cab. There, with air once more about them and their helmets off, Frake finally spoke up. 

	"Well? Was he right?" He was looking at Imbriano as he spoke. 

	"I'm not the engineer," the doctor said wearily. "So far as I can see, he is perfectly right. Personally, I'm optimistic about the fuel in the ship's tanks. I don't think we could possibly have lost much before the ice layer formed. But that doesn't make me any happier about Ingersoll." 

	"Maybe we'd better tell him about the ice stopping the evaporation," suggested Frake. 

	"You do it. He certainly wouldn't believe me," the doctor replied wearily. Frake took the microphone. 

-

	He called Ingersoll's name several times, without answer; then he told about the freezing in the tank, sure that the other was listening. He ended with an air of frankness. 

	"I admit we don't know there's enough to get us home," he said, "but you know I'm talking sense when I say there's a good chance of it. If you want to take that chance, just stay where you are and watch. You can probably see the takeoff from here. You' know about when it will be— you can guess how long it will take us to get back. We're starting now. You can stay or come, as you please." 

	He hung up the microphone and Detzel started the tractor out toward the sunlight, slanting back toward the foot of the trail leading down from the rim. Imbriano rode with head turned over his shoulder, in the general direction that he believed the other vehicle to be. There was sound from the radio. 

	But it was Detzel who saw the other machine, and called their attention to it. It was parallelling their course, half a mile to the north, and gradually pulling ahead of them. It was just barely visible; almost all that could be seen was scattered light from its lenses, and the streak of illumination stretching over the ground ahead of it. Detzel took the microphone.

	"Glad you're coming, Milt," he called. "Want to lead? You must know this road enough better than we do, so you can go faster safely." There was a brief pause. 

	"All right. Pull over this way, and fall in behind me." The voice had lost all trace of emotion. Detzel slanted obediently to the left, and relaxed a trifle—he had been giving close thought to the problem of navigation. Imbriano did not; and it was just as well. 

-

	They were a scant hundred yards from the other machine, and were just about able to make it out in the light now reflected from the mountains, when Detzel's attention was jerked back to full operational level. With a turn that threatened to snap the couplings of its trailers, Ingersoll's tractor was whipping around; its lights glared directly into their eyes, and Imbriano and Frake ducked instinctively. Fortunately, Detzel's reactions were of a more constructive nature; he wrenched their own vehicle to the right, and managed to avoid the first charge. 

	"Get your helmets on!" he snapped to the others. "Then take the wheel, Bill, while I do mine. If he even grazes us there'll be no air in this cab!" 

	"We can outrun him. He's pulling a bigger load," the doctor pointed out as he fitted his helmet in place. 

	"We could on the straight—but we're not sure we can go straight. If anyone knows the crevasses around here, it's Ingersoll, not me." 

	"Even he shouldn't know them too well. He can't have spent all his time exploring cracks," Frake put in optimistically. 

	"He doesn't have to know them at all to have a big advantage," snapped Detzel. "The sad fact is that we're going first. If we can keep going, he can. We can keep ahead just as long as I don't have to detour." 

	"Head out into the sunlight!" cried Imbriano. "He won't dare take that trailer of snow out there. It would boil too fast." 

	"We don't know what he'd dare. It's a metal tank, and would take a while to heat up. And if he's willing to risk his own life in a collision, he can't be very rational anyway. I'm already on the way toward sunlight, in case you hadn't noticed." 

	"Put on more juice! He's catching up!" called Frake. Detzel tried, but the turbine was already whirling at its safe limit. 

	"Something's wrong. Our trailer must be dragging," he snapped. "We didn't take time to service it properly before we set out on this junket." 

	"That's not it. I can see now. The back right tire is flat. Either it picked a gruesome time to hit something sharp, or Milt nicked it on that first pass." 

-

	"If we can't outrun him, we'll have to outmaneuver him," grunted Detzel. "We should still be able to make tighter turns than he can, tire or no tire. Tell me when he's about twenty yards back." 

	"He's closer than that already, I'd say, though it's hard to be sure with the lights right in my eyes." Detzel's answer was another twist to the right. At the same moment Imbriano started the economy pump since they all had their helmets sealed by this time. Neither of the others noticed. Detzel would probably have objected to the waste of power if he had. 

	The turn was almost, but not quite, successful. The other machine grazed the rear of the trailer, some projection on it ripping their other back tire. Fortunately, the fuel tank front made the trailer's center of gravity a trifle ahead of middle pair of wheels, so it didn't settle too badly on back ones except under acceleration; but the additional flatting of the middle tires added quite a bit of drag. 

	For a moment, it looked as though Detzel might be overcoming this disadvantage. He held his turn, and the other train was unable to match it, as he had hoped. Slowly he drew ahead; then he was parallel, going the other way; then drawing up behind as he lapped Ingersoll. Then they were traveling only a yard or two away from the back trailer of the other machine, and matching its angular speed. As they reached this point, Imbriano opened the door by his seat and swung out. 

-

	For a moment, neither of the others noticed. By the time they did, he was climbing across the back of the cab and almost within reach of Ingersoll's rear trailer. He reached, but couldn't quite make it. 

	"Closer, Al," he snapped. The others heard his voice, didn't for a moment realize where he was since the suit radios gave little indication of distance, and Detzel obeyed the without asking why. Then Frake looked back, discovered the doctor missing, and after, added a moment located him. 

	"Doc! You idiot!" he cried. The call distracted Detzel, but fortunately not enough to disturb his driving. "What's the matter?" he asked without taking his eyes from the other 

	"Doc's climbing onto Milt's trailer! He's nuts!" 

-

	"Shut up, stupid!" Imbriano's voice came. "Well, never mind. It's too late now." Frake had forgotten that they were now using the suit radios, and Ingersoll could hear anything they said. The doctor, with secrecy at an end, addressed the geologist directly. 

	"Here I am, Milt. Right on your rear trailer. Any ideas about how to run into me now? You might as well leave the other tractor alone. Getting it won't get me, will it?" 

	The answer that came back was unprintable, except for the concluding sentence: "Anyone who helps you needs squashing, too." The larger train swerved away and slowed down, trying to bring Detzel ahead, but the engineer was alert and held his position to the other's right rear. 

	Imbriano, holding firmly to the body of the trailer, spoke again. "Don't waste too much fuel, Milt. You may find you don't have much to spare, after all." He began to crawl forward along the train as he finished speaking. The bodies of the vehicle were mostly empty—they never knew why Ingersoll had taken so many—and the spare tank containing the snow was bolted to the front of the second one in line. The tank on the first was, of course, actually in service. 

	Reaching dangerously around the snow tank, Imbriano found the pin of the coupling which connected the trailer to the one in front, and pulled. 

	He was unable to move it; there was too much tension on the coupling as long as the tractor was pulling. There were several cases on the front trailer, however—probably the missing food—which prevented Ingersoll from seeing what the doctor was doing; and this uncertainty led the geologist to solve the other's problem for him. 

	Thinking that Imbriano was damaging his precious reserve tank, Ingersoll began randomly braking and accelerating in an effort to shake him off. This was nearly successful, but it also enabled the doctor to work the pin free after a few cycles, since each time the push changed to a pull or vice versa there was an instant when it was loose. At last he got it out, and had the satisfaction of seeing the tractor and front trailer bound away from him as Igersoll applied power once more.

-

	The geologist realized instantly what had happened, cut around in as tight circle as he could to bring his lights on the trailers and Imbriano, and stopped. He evidently wasn't ready to come out; it was too dark to see inside his cab—especially past lights—but the pause suggested that he was helmeting up and pumping back his air. Imbriano assumed that he was preparing to come out anyway, and thought of a delaying move. 

	"Just a minute, Milt—don't come out yet. If I see your door open, you'll see this stop-cock do the same thing. How about it?" Imbriano had his gloved hand on the bottom tank drain.

	For a moment there was silence. Then, "Go ahead and open it. Here I come!" 

	The doctor couldn't see the cab door open beyond the lights, but he wasn't looking anyway. He carefully opened the stopcock and sprang back, expecting a jet of vapor comparable to the one from the cup not long before: He was watching for it so anxiously that he almost didn't see Ingersoll coming, for the watching job took no longer than he had expected. Nothing happened. 

	Fortunately for the doctor, Ingersoll had seen the whole thing, and he came to a stop beside the trailer and laughed. "Smart boy, Doc. I suppose you expected the stuff to boil right out and leave me stranded didn't you? You didn't remember that the tank has never been in the sun since it was filled; and it had no water in it, and had been out of the sunlight long enough to cool down even before it was filled. Where did you expect the energy to come from? Or doesn't the medical profession believe in conservation of energy? Why, you little ..." his language became profane and irrelevant once more, and he made a leap in Imbriano's direction. 

	The doctor had plenty of time to get out of the way; and his own leap took him out of the direct beams of the headlights, so that for a moment he effectively vanished. Ingersoll started to follow; then a flash of reason crossed his mind, and he headed back for the cab of his own tractor. He got the idea more quickly than any of the others, and made it with plenty of time. He had left the turbine idling, so there was no delay in starting, and neither the doctor nor Frake, who had also leaped from their tractor the moment Detzel brought it to a halt, had a chance to get aboard Ingersoll's. 

	"Get back with Al!" called Imbriano. "Get back in the tractor, and keep it out of the way. I'm safe enough. Maybe he'll cool down enough to reason with after he's made a few passes at me. Unless he's taught that machine to jump, he'll never catch a man on foot with it!" 

	Frake agreed, though his words were nearly drowned in another flood of language from Ingersoll. Imbriano was promptly given the opportunity of proving his claim that he could keep out of the way of a tractor. 

-

	His imagination supplied the thunderous turbine whine which the lunar vacuum could not transmit. Some sound, but not much, came through tracks, ground, and feet; but practically, the chase might have been recorded on an old silent film. Frake, later, claimed he was surprised not to see subtitles; but his sense of humor was not very subtle. 

	Imbriano was not feeling humorous at all. He was able to dodge, all, right, but it was not very easy, and he was afraid of leaping too far. A bad landing could be disastrous, since not very much has to go wrong with a space suit to kill its occupant. After a few passes which would have won very little applause in a Spanish bull-ring but were quite as exciting for Imbriano as he wished, it occurred to him that Ingersoll might be a little slower if the dodging were being done around his precious reserve tank. Accordingly, the doctor made his, next leap or two in this direction, and began playing tag around the stranded trailers. 

	He was still hoping that Ingersoll might cool down and be reasonable; but there was no sign of such an event, and he couldn't think of anything to say that might have a calming effect. Throughout the affair, he had been worried by the feeling of guilt he had expressed earlier, and the we may have slowed him down—certainly some of his escapes were narrower than they needed to be. 

	Then a different feeling began to take hold of him. However reasonable Ingersoll's initial resentment may have been, this grimly-determined effort to repay unpleasantness and discourtesy with murder was going a little too far. Imbriano's sympathy and guilt-feeling began to give way to resentment and anger; his temper, never outstandingly good, was wearing thin. He was thinking, now, in terms of force rather than persuasion. But that did him little good; granted that a man on foot could keep from being harmed by the man in a tractor, there seemed nothing whatever he could do on the offensive. Certainly Imbriano could think of nothing. He kept as close as he could to the stranded trailer, answered the questions of Detzel and Frake as reassuringly as his breath permitted, and kept moving. He didn't get onto the trailer itself; later he convinced himself without much trouble, that his own subconscious kept him off. 

-

	The end of the contest was, in one way, something of an anticlimax. Imbriano had thought of nothing brilliant; Frake and Detzel had made no contribution; and Ingersoll had shown no sign of giving up and when the whole situation was changed—instantly and without warning. 

	The doctor had suffered his closest shave yet, just barely escaping the charging treads, and had ducked around the front end of the train to its right side. Ingersoll made his closest turn thus far, cutting a trifle left to get his single attached trailer clear and then swinging around so as almost to graze the front of the motionless one. There was no collision; Detzel had his lights on the scene at the moment, and he, Frake, and Imbriano himself were all certain that nothing solid touched the stranded vehicle. Imbriano, who was actually touching it at the time, was sure he would have felt the impact. 

	Nevertheless, something happened. It was not an explosion —at least, not exactly so. The tank which had been filled with "snow" opened almost deliberately, and sprayed over everything in front of it a furiously boiling, dense, misty vapor which glowed a bright blue-green, dazzling even against the background of the brilliantly-sunlit mountains. It covered Ingersoll's cab completely; and blinded by the featureless glare, he brought his machine to a stop. That was enough for Detzel, who had been waiting for any sort of opportunity. He hurled his own tractor toward the other, angled it across Ingersoll's front so that the geologist was cramped between Detzel's tractor and the detached trailers. His own trailer, still attached, prevented him from backing without making a "cut" which his front end was not free to do. Ingersoll, or rather his machine, was pinned completely. Getting the man himself, at odds of three to one with the one under a steering wheel, was not too difficult. 

-

	"I hope they can straighten him out on Earth," Imbriano said soberly to Kinchen a dozen hours later. "He's way beyond me. He had made a real discovery there in Newton— he must have made it on the first trip, to have planned the second as he did. Instead of reporting it, and getting all the credit he seems to have wanted so, badly, he pulls this incredibly complex trick. It's like a kid who's daydreamed all the details of a party he's going to attend, and flies into a tantrum when the facts don't follow his imagined program, I think Milt planned the plant discovery before we ever left Earth —he must have, to have brought the lichens with him—and wasn't quick enough on the uptake to throw the game aside when he made the real discovery. Life moved too fast for him. 

	"Of course, it moved too fast for me, too. I still can't see what happened to his tank back there. As far as I can see, he was perfectly right about the snow still being soil and there not being enough energy to do anything." 

	"You surprise me," grunted Kinchen. 

	"Why?" asked the doctor.

	"Your admitting that you don't know." Imbriano flushed, started an angry retort, then calmed down. 

	"Don't rub it in, Chief. I feel enough of a heel already. I suppose it was that which helped push Milt as far as he went. I don't say I'll stop because habits are hard to break, but I'll try. What did happen to the snow, though?" 

	"I don't know, either, the astronomer replied. "It will take analysis to make sure. I think, though, that your suggestion about the snow collecting from space—nebular material, comet's tails, or what have you—is probably right. But it isn't—or a lot of it isn't—nice plain water, ammonia, and methane. 

-

	"There's a lot of radiation in space, and a lot of innocent molecules floating around there get knocked apart. What you have left is radicals—highly-reactive fragments of molecules: NH, OH, C2, CH2, and so on. I suppose equilibrium temperature there in Newton's permanent shadow can't be more than twenty or or -thirty degrees absolute, so the radicals were "frozen"—held below even their very low activation temperature. I'm a little surprised you were able to run the tractors over the stuff safely—but I suppose the treads were pretty cold by the time you got there. 

	"As for what finally touched off that tank, my guess would be the exhaust from Milt's safety valves. You say he was running the machine full blast for several minutes, and. even in that environment it wouldn't take what water he had left very long to heat up—after all, it must have been more than half gone by then anyway." 

	"It was," confirmed Detzel. "We transferred it to our own tank, and didn't manage to fill up even then. Without it, we'd have walked the last fifty miles back here." 

	"Well, that's my hypothesis, then. I'm glad we don't have to salvage some of that snow for the ship, though I suppose we could get away with it—add it a tiny bit at a time and let it react. The products would be useable enough. They'd be largely the water, ammonia, and methane Milt thought they were. That cleans up practically everything, I guess." 

	"Practically?" Imbriano was curious. 

	Kinchen looked at him narrowly. "Just how sure are you that the plants Ingersoll discovered are Terrestrial, and that he was faking the find?" 

	Imbriano hesitated before answering. 

	"I know what I think, but I've done enough damage broadcasting it already," he said at last. "I wish some of those specimens had been saved, and I certainly wish I'd had a chance to see what exposure to moon conditions did to those I put out. If they'd survived, or even formed viable spores ... " 

	"They'd have been quite radical, wouldn't they?" asked Frake. 

	He wondered why he was sent to look for more lichens. 

The End