by Hal Clement
It was the last few hundred meters of descent that began to furnish something of interest. Shandara was picking his way down an unusually uninviting bit of slope when Ridging, who had already negotiated it, spoke up sharply.
“Shan! Look at the stars over the northern horizon! Isn’t there some sort of haze? The sky around them looks a bit lighter.” The other paused and looked.
“You’re right. But how could that be? There couldn’t suddenly be enough air at this level—gases don’t behave that way. Van Maanen’s star might have an atmosphere twenty meters deep, but the moon doesn’t and never could have.”
“There’s something between us and the sky.”
“That I admit; but I still say it isn’t gas. Maybe dust—”
“What would hold it up? Dust is just as impossible as air.”
“I don’t know. The floor’s only a few yards down—let’s not stand here guessing.” They resumed their descent.
The crater floor was fairly level, and sharply distinguished from the inner slope of the crater wall. Something had certainly filled, partly at least, the vast pit after the original explosion; but neither man was disposed to renew the argument about the origin of Lunar craters just then. They scrambled down the remaining few yards of the journey and stopped where they were, silently.
There was something blocking vision; the horizon was no longer visible, nor could the stars be seen for a few degrees above where it should have been. Neither man would have had the slightest doubt about the nature of the obscuring matter had he been on Earth; it bore every resemblance to dust. It had to be dust.
But it couldn’t be. Granted that dust can be fine enough to remain suspended for weeks or months in Earth’s atmosphere when a volcano like Krakatoa hurls a few cubic miles of it aloft, the moon had not enough gas molecules around it to interfere with the trajectory of a healthy virus particle—and no seismometer in the last four weeks had registered crustal activity even approaching the scale of vulcanism. There was nothing on the moon to throw the dust up, and even less to keep it there.
“Meteor splash?” Shandara made the suggestion hesitantly, fully aware that while a meteor might raise dust it could never keep it aloft. Ridging did not bother to answer, and his friend did not repeat the suggestion.
The sky straight overhead seemed clear as ever; whatever the absorbing material was it apparently took more than the few feet above them to show much effect. That could not be right, though, Ridging reflected, if this stuff was responsible for hiding the features which should have been visible from the crater rim. Maybe it was thicker farther in. If so, they’d better go on—there might be some chance of collecting samples after all.
He put this to Shandara, who agreed; and the two started out across the hundred kilometer plain.
The surface was fairly smooth, though a pattern of minute cracks suggestive of the joints formed in cooling basalt covered it almost completely. These were not wide enough even to constitute a tripping danger, and the men ignored them for the time being, though Ridging made a mental note to get a sample of the rock if he could detach one.
The obscuration did thicken as they progressed, and by the time they had gone half a dozen kilometers it was difficult to see the crater wall behind them. Looking up, they saw that all but the brighter stars had faded from view even when the men shaded their eyes from the sunlit rock around them.
“Maybe gas is coming from these cracks, carrying dust up with it?” Shandara was no geologist, but had an imagination. He had also read most of the serious articles which had ever been published about the moon.
“We could check. If that were the case, it should be possible to see currents coming from them; the dust would be thicker just above a crack than a few centimeters away. If we had something light, like a piece of paper, it might be picked up.”
“Worth trying. We have the map,” Shandara pointed out. “That should do for paper; the plastic is thin enough.” Ridging agreed. With some difficulty—spacesuit gloves were not designed for that purpose—he tore a tiny comer off the sheet on which the map was printed, knelt down, and held the fragment over one of the numerous cracks. It showed no tendency to flutter in his grasp, and when he let go it dropped as rapidly as anything ever did on the moon, to lie quietly directly across the crack he had been testing. He tried to pick it up, but could not get a grip on it with his stiff gloves.
“That one didn’t seem to pan out,” he remarked, standing up once more.
“Maybe the paper was too heavy—this stuff must be awfully fine—or else it’s coming from only a few of the cracks.”
“Possibly; but I don’t think it’s practical to try them all. It would be smarter to figure some way to get a sample of this stuff, and let people with better lab facilities figure out what it is and what holds it off the surface.”
“I’ve been trying to think of a way to do that. If we laid the map out on the ground, some of the material might settle on it.”
“Worth trying. If it does, though, we’ll have another question—why does it settle there and yet remain suspended long enough to do what is being done? We’ve been more than an hour coming down the slope, and I’ll bet your astronomical friends of the past have reported obscurations longer lasting even than that.”
“They have. Well, even if it does raise more problems it’s worth trying. Spread out the map, and we’ll wait a few minutes.” Ridging obeyed; then, to keep the score even, came up with an idea of his own.
“Why don’t you lay your camera on the ground pointing up and make a couple of time exposures of the stars? You could repeat them after we get back in the clear, and maybe get some data on the obscuring power of this material.”
“Good enough.” Shandara removed the camera from its case, clipped a sun shade over its lens, and looked up to find a section of sky with a good selection of stars. As usual, he had to shield his eyes both from sunlight and from the glare of the nearby hills; but even then he did not seem satisfied.
“This stuff is getting thicker, I think,” he said. “It’s scattering enough light so that it’s hard to see any stars at all—harder than it was a few minutes ago, I’d say,” Ridging imitated his maneuver, and agreed.
“That’s worth recording, too,” he pointed out. “Better stay here a while and get several shots at different times.” He looked down again. “It certainly is getting thicker. I’m having trouble seeing you, now.”
Human instincts being what they are, the solution to the mystery followed automatically and immediately. A man who fails, for any reason, to see as clearly as he expects usually rubs his eyes—if he can get at them. A man wearing goggles or a space helmet may just possibly control this impulse, but he follows the practically identical one of wiping the panes through which he looks. Ridging did not have a handkerchief within reach, of course, and the gauntlet of a spacesuit is not one of the best windshield wipers imaginable; but without giving a single thought to the action, he wiped his face plate with his gauntlet.
Had there been no results he would not have been surprised; he had no reason to expect any. He would probably have dismissed the matter, perhaps with a faint hope that his companion might not have noticed the futile gesture. However, there were results. Very marked ones.
The points where the plastic of the gauntlet actually touched the face plate were few; but they left trails all the way across—opaque trails. Surprised and still not thinking, Ridging repeated the gesture in an automatic effort to wipe the smears of whatever it was from his helmet; he only made matters worse. He did not quite cover the supposedly transparent area with glove trails—but in the few seconds after he got control of his hand the streaks spread and merged until nothing whatever was visible. He was not quite in darkness; sunlight penetrated the obscuring layer, but he could not see any details.
“Shan!” The cry contained almost a note of panic. “I can’t see at all. Something’s covering my helmet!” The cartographer straightened up from his camera and turned towa
rd his friend.
“How come? You look all right from here. I can’t see too clearly, though—”
Reflexes are wonderful. It took about five seconds to blind Shandara as thoroughly as Ridging. He couldn’t even find his camera to close the shutter.
“You know,” said Ridging thoughtfully after two or three minutes of heavy silence, “we should have been able to figure all this out without coming down here.”
“Why?”
“Oh, it’s plain as anything—”
“Nothing, and I mean nothing, is plain right now.”
“I suppose a map maker would joke while he was surveying Gehenna. Look, Shan, we have reason to believe there’s a magnetic storm going on, which strongly suggests charged particles from the sun. We are standing, for practical purposes, on the moon’s south magnetic pole. Most level parts of the moon are covered with dust—but we walked over bare rock from the foot of the rim to here. Don’t those items add up to something?”
“Not to me.”
“Well, then, add the fact that electrical attraction and repulsion are inverse square forces like gravity, but involve a vastly bigger proportionality constant.”
“If you’re talking about scale I know all about it, but you still don’t paint me a picture.”
“All right. There are, at a guess, protons coming from the sun. They are reaching the moon’s surface here—virtually all of them, since the moon has a magnetic field but no atmosphere. The surface material is one of the lousiest imaginable electrical conductors, so the dust normally on the surface picks up and keeps a charge. And what, dear student, happens to particles carrying like electrical charges?”
“They are repelled from each other.”
“Head of the class. And if a hundred-kilometer circle with a rim a couple of kilos high is charged all over, what happens to the dust lying on it?”
Shandara did not answer; the question was too obviously rhetorical. He thought for a moment or two, instead, then asked, “How about our face plates?”
Ridging shrugged—a rather useless gesture, but the time for fighting bad habits had passed some minutes before.
“Bad luck. Whenever two materials rub against each other, electrons come loose. Remember your rubber-and-cat-fur demonstrations in grade school. Unless the materials are of identical electronic make-up, which for practical purposes means unless they are the same substance, one of them will hang onto the electrons a little—or a lot—better than the other, so one will have a negative net charge and the other a positive one. It’s our misfortune that the difference between the plastic in our face plates and that in the rest of the suits is the wrong way; when we rubbed the two, the face plates picked up a charge opposite to that of the surrounding dust—probably negative, since I suppose the dust is positive and a transparent material should have a good grip on its electrons.
“Then the rest of our suits, and the gloves we wiped with in particular, ought to be clean.”
“Ought to be. I’d like nothing better than a chance to check the point.”
“Well, the old cat’s fur didn’t stay charged very long, as I remember. How long will it take this to leak off, do you think?”
“Why should it leak off at all?”
“What? Why, I should think—Hm-m-m.” Shandara was silent for a moment. “Water is pretty wonderful stuff, isn’t it?”
“Yep. And air has its uses, too.”
“Then we’re . . . Ridge, we’ve got to do something. Our air will last indefinitely, but you still can’t stay in a spacesuit too long.”
“I agree that we should do something; I just haven’t figured out what. Incidentally, just how sure are you that our air will last? The windows of the regenerators are made, as far as I know, of the same plastic our face plates are. What’ll you bet you’re not using emergency oxygen right now?”
“I don’t know—I haven’t checked the gauges.”
“I’ll say you haven’t. You won’t, either; they’re outside your helmet.”
“But if we’re on emergency now, we could hardly get back to the tractor starting this minute. We’ve got to get going.”
“Which way?”
“Toward the rim!”
“Be specific, son. Just which way is that? And please don’t point; it’s rude, and I can’t see you anyway.”
“All right, don’t rub it in. But Ridge, what can we do?”
“While this stuff is on our helmets, and possibly our air windows, nothing. We couldn’t climb even if we knew which way the hills were. The only thing which will do us the least good is to get this dust off us; and that will do the trick. As my mathematical friends would say, it is necessary and sufficient.”
“All right, I’ll go along with that. We know that the material the suits are made of is worse than useless for wiping, but wiping and electrical discharge seem to be the only methods possible. What do we have which by any stretch of the imagination might do either job?”
“What is your camera case made of?” asked Ridging.
“As far as I know, same as the suits. It’s a regular clip-on carrier, the sort that came with the suits—remember Tazewell’s remarks about the dividends AirTight must have paid when they sold the suits to the Project? It reminded me of the old days when you had to buy a lot of accessories with your automobile whether you wanted them or not—”
“All right, you’ve made your point. The case is the same plastic. It would be a pretty poor wiper anyway; it’s a box rather than a bag, as I remember. What else is there?”
The silence following this question was rather lengthy. The sad fact is that spacesuits don’t have outside pockets for handkerchiefs. It did occur to Ridging after a time that he was carrying a set of geological specimen bags; but when he finally did think of these and took one out to use as a wiper, the unfortunate fact developed that it, too, left the wrong charge on the face plate of his helmet. He could see the clear, smooth plastic of the bag as it passed across the plate, but the dust collected so fast behind it that he saw nothing of his surroundings. He reflected ruefully that the charge to be removed was now greater than ever. He also thought of using the map, until he remembered that he had put it on the ground and could never find it by touch.
“I never thought,” Shandara remarked after another lengthy silence, “that I’d ever miss a damp rag so badly. Blast it, Ridge, there must be something”
“Why? We’ve both been thinking without any result that I can see. Don’t tell me you’re one of those fellows who think there’s an answer to every problem.”
“I am. It may not be the answer we want, but there is one. Come on, Ridge, you’re the physicist; I’m just a high-priced picture-copier. Whatever answer there is, you’re going to have to furnish it: all my ideas deal with maps, and we’ve done about all we can with those at the moment.”
“Hm-m-m. The more I think, the more I remember that there isn’t enough fuel on the moon to get a rescue tractor out here, even if anyone knew we were in trouble and could make the trip in time. Still—wait a minute; you said something just then. What was it?”
“I said all my ideas dealt with maps, but—”
“No; before that.”
“I don’t recall, unless it was that crack about damp rags, which we don’t have.”
“That was it. That’s it, Shan; we don’t have any rags, but we do have water”
“Yes—inside our spacesuits. Which of us opens up to save the other?”
“Neither one. Be sensible. You know as well as I do that the amount of water in a closed system containing a living person is constantly increasing; we produce it, oxidizing hydrogen in the food we eat. The suits have driers in the air cycler or we couldn’t last two hours in them.”
“That’s right; but how do you get the water out? You can’t open your air system.”
“You can shut it off, and the check valve will keep air in your suit—remember, there’s always the chance someone will have to change emergency tanks. It’ll be a j
ob, because we won’t be able to see what we’re doing, and working by touch through spacesuit gauntlets will be awkward as anything I’ve ever done. Still, I don’t see anything else.”
“That means you’ll have to work on my suit, then, since I don’t know what to do after the line is disconnected. How long can I last before you reconnect? And what do you do, anyway? You don’t mean there’s a reservoir of liquid water there, do you?”
“No, it’s a calcium chloride drier; and it should be fairly moist by now—You’ve been in the suit for several hours. It’s in several sections, and I can take out one and leave you the others, so you won’t suffer from its lack. The air in your suit should do you for four or five minutes, and if I can’t make the disconnection and disassembly in that time I can’t do it at all. Still, it’s your suit, and if I do make a mistake it’s your life; do you want to take the chance?”
“What have I to lose? Besides, you always were a pretty good mechanic—or if you weren’t, please don’t tell me. Get to work.”
“All right.”
As it happened, the job was not started right away, for there was the minor problem of finding Shandara to be solved first. The two men had been perhaps five yards apart when their face plates were first blanked out, but neither could now be sure that he hadn’t moved in the meantime, or at least shifted around to face a new direction. After some discussion of the problem, it was agreed that Shandara should stand still, while Ridging walked in what he hoped was the right direction for what he hoped was five yards, and then start from wherever he found himself to quarter the area as well as he could by length of stride. He would have to guess at his turns, since even the sun no longer could penetrate the layer of dust on the helmets.
