as an active volcanic area; others have blamed the business on clouds — and others, of course, have assumed the observers themselves were at fault, though that is pretty hard to justify. I didn't really expect to get a chance to check up on the phenomenon, but I'm sure you don't expect me to stay up here now.”

“I suppose not.” Ridging spoke in a tone of mock resignation. The problem did not seem to concern his field directly, but he judged rightly that the present situation affected Shandara the way an offer of a genuine fragment of Terrestrial core material would influence Ridging himself. “What do you plan to take down? I suppose you want to get measures of some sort.”

“Well, there isn't too much here that will apply, I'm afraid. I have my own camera and some filters, which may do some good. I can't see that the magnetic stuff will be any use down there. We don't have any pressure- measuring or gas-collecting gadgetry; I suppose if we'd brought a spare water container from the tractor we could dump it, but we didn't and I'd bet that nothing would be found in it but water vapor if we did. We'll just have to go down and see what our eyes will tell us, and record anything that seems recordable on film. Are you ready?”

“Ready as I ever will be.” Ridging knew the remark was neither original nor brilliant, but nothing else seemed to fit.

The inner wall of the crater was a good deal steeper than the one they had climbed, but still did not present a serious obstacle. The principal trouble was that much of the way led through clefts where the sun did not shine, and the only light was reflected from distant slopes. There wasn't much of it, and the men had to be careful of their footings — there was an occasional loose fragment here, and a thousand-meter fall is no joke even on the Moon. The way did not lead directly toward the crater floor; the serrated rim offered better ways between its peaks, hairpinning back and forth so that sometimes the central plain was not visible at all. No floor details appeared as they descended, but whatever covered them was still below; the stars, whenever the mountains cut off enough sidelight, were clear as ever. Time and again Shandara stopped to look over the great plain, which seemed limitless now that the peaks on the farther side had dropped below the horizon, but nothing in the way of information rewarded the effort.

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 corner 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 sunshade 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

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