The information brought back by the Apollo astronauts finally resolved the conflicting speculations concerning the Moon’s nature and origins that had been mooted by generations of Earth-bound observers. Soon after the Solar System was formed, 4,500 million years ago, give or take a few, the Moon became molten to a considerable depth, possibly halfway to the center; the heat was generated by the release of gravitational energy as the Moon continued to accumulate. During the cooling that followed, the heavier, iron-bearing minerals sank toward the interior, while the less dense, aluminum-rich ones floated to the surface to form the highland crust. Continual bombardment by meteorites stirred up the mixture and complicated the process to some degree but by 4,300 million years ago the formation of the crust was virtually complete. The bombardment continued until 3,900 million years ago, by which time most of the familiar surface features already existed. From then until 3,200 million years ago, basaltic lavas flowed from the interior, induced in some places by remelting due to concentrations of radioactive heat sources below the surface, to fill in the impact basins and create the darker maria. The crust continued cooling to greater depths until molten material could no longer penetrate. Thereafter, all remained unchanging through the ages. Occasionally an additional impact crater appeared and falling dust gradually eroded the top millimeter of surface, but essentially, the Moon became a dead planet.
This history came from detailed observations and limited explorations of Nearside. Orbital observations of Farside suggested that much of the same story applied there also, and since this sequence was consistent with existing theory, nobody doubted its validity for many years after Apollo. Of course, details remained to be added, but the broad picture was convincingly clear. However, when man returned to the Moon in strength and to stay, ground exploration of Farside threw up a completely different and totally unexpected story.
Although the surface of Farside looked much the same as Nearside to the distant observer, it proved at the microscopic level to have undergone something radically different in its history. Furthermore, as bases, launch sites, communications installations, and all the other paraphernalia that accompanied man wherever he went, began proliferating on Nearside, the methodical surface coverage that this entailed produced oddities there, too.
All the experiments performed on the rock samples brought back from the eight sites explored before the mid-seventies gave consistent results supporting the orthodox theories. When the number of sites grew to thousands, by far the majority of additional data confirmed them-but some curious exceptions were noted, exceptions which seemed to indicate that some of the features on Nearside ought, rightfully, to be on Farside.
None of the explanations hazarded were really conclusive. This made little difference to the executives and officers of UNSA, since by that time the pattern of Lunar activity had progressed from that of pure scientific research to one of intense engineering operations. Only the academic fraternity of a few universities found time to ponder and correspond on the spectral inconsistencies between dust samples. So for many years the well-documented problem of "lunar hemispheric anomalies" remained filed, along with a million and one other items, in the "Awaiting Explanation" drawer of science.
A methodical review of the current state of knowledge in any branch of science that might have a bearing on the Lunarian problem was a routine part of Group L’s business. Anything to do with the Moon was, naturally, high on the list of things to check up on, and soon the group had amassed enough information to start a small library on the subject. Two junior physicists, who didn’t duck quickly enough when Hunt was giving out assignments, were charged with the Herculean task of sifting through all this data. It took some time for them to get around to the topic of hemispheric anomalies. When they did, they found reports of a series of dating experiments performed some years previously by a nucleologist named Kronski at the Max Planck Institute in Berlin. The data that appeared in those reports caused the two physicists to drop everything and seek out Hunt immediately.
After a long discussion, Hunt made a vi-phone call to a Dr. Saul Steinfield of the Department of Physics of the University of Nebraska, who specialized in Lunar phenomena. As a consequence of that call, Hunt made arrangements for the deputy head of Group L to take charge for a few days, and he flew north to Omaha early the next morning. Steinfleld’s secretary met Hunt at the airport, and within an hour Hunt was standing in one of the physics department laboratories, contemplating a three-foot-diameter model of the Moon.
"The crust isn’t evenly distributed," Steinfield said, waving toward the model. "It’s a lot thicker on Farside than on Nearside-something that has been known for a long time, ever since the first artificial satellites were hung around the Moon in the nineteen sixties. The center of mass is about two kilometers away from the geometric center."
"And there’s no obvious reason," Hunt mused.
Steinfleld’s flailing arm continued to describe wild circles around the sphere in front of them. "There’s no reason for the crust to solidify a lot thicker on one side, sure, but that doesn’t really matter, because that’s not the way it happened. The material that makes up the Farside surface is much younger than anything anybody ever believed existed on the Moon in any quantity up until about, ah, thirty or so years back-one hell of a lot younger! But you know that-that’s why you’re here."
"You don’t mean it was formed recently," Hunt stated.
Steinfield shook his head vigorously from side to side, causing the two tufts of white hair that jutted from the sides of his otherwise smooth head to wave about in a frenzy. "No. We can tell that it’s about as old as the rest of the Solar System. What I mean is-it hasn’t been where it is very long."
He caught Hunt’s shoulder and half turned him to face a wall chart showing a sectional view through the Lunar center. "You can see it on this. The red shell is the original outer crust going right around-it’s roughly circular, as you’d expect. On Farside-here-this blue stuff sits on top of it and wasn’t added very long ago."
"On top of what used to be the surface."
"Exactly. Somebody dumped a couple of billion tons of junk down on the old crust-but only on this side."
"And that’s been verified pretty conclusively?" Hunt asked, just to be doubly sure.
"Yeah… yeah. Enough bore holes and shafts have been sunk all over Farside to tell us pretty closely where the old surface was. I’ll show you something over here…" A major section of the far wall comprised nothing but rows of small metal drawers, each with its own neatly lettered label, extending from floor to ceiling. Steinfield walked across the room, and stooped to scan the labels, at the same time mumbling to himself semi-intelligibly. With a sudden "That’s it!" he pounced on one of the drawers, opened it, and returned bearing a closed glass container about the size of a small pickle jar. It contained a coarse piece of a light gray rocky substance that glittered faintly in places, mounted on a wire support.
"This is a fairly common KREEP basalt from Farside. It-"
"‘Creep’?"
"Rich in potassium-that is, K-rare earth elements, and phosphorus: KREEP."
"Oh-I see."
"Compounds like this," Steinfield continued, "make up a lot of the highlands. This one solidified around 4.1 billion years ago. Now, by analyzing the isotope products produced by cosmic-ray exposure, we can tell how long it’s been lying on the surface. Again, the figure for this one comes out at about 4,100 million years."
Hunt looked slightly puzzled. "But that’s normal. It’s what you’d expect, isn’t it?"
"If it had been lying on the surface, yes. But this came from the bottom of a shaft over seven hundred feet deep! In other words, it was on the surface for all that time-then suddenly it’s seven hundred feet down." Steinfleld gestured toward the wall chart again. "As I said, we find the same thing all over Farside. We can estimate how far down the old surface used to be. Below it we find old rocks and structures that go way back, just like on Nearside; above it eve
rything’s a mess-the rock all got pounded up and lots of melting took place when the garbage came down, all the way up to what’s now the surface. It’s what you’d expect."
Hunt nodded his agreement. The energy released by that amount of mass being stopped dead in its tracks would have been phenomenal.
"And nobody knows where it came from?" he asked.
Steinfield repeated his head-shaking act. "Some people say that a big meteorite shower must have got in the way of the Moon. That may be true-it’s never been argued conclusively one way or the other. The composition of the garbage isn’t really like a lot of meteorites, though-it’s closer to the Moon itself. It’s as if they were made out of the same stuff-that’s why it looks the same from higher up. You have to look at the microstructure to see the things I’ve been talking about."
Hunt examined the specimen curiously for a while in silence. At length he laid it carefully on the top of one of the benches. Steinfield picked it up and returned it to its drawer.
"Okay," Hunt said as Steinfield rejoined him. "Now, what about the Farside surface?"
"Kronski and company."
"Yes-as we discussed yesterday."
"The Farside surface craters were made by the tail end of the garbage-dumping process, unlike the Nearside craters, which came from meteorite impacts oh… a few billion years back. In rock samples from around the rims of Farside craters we find that things like the activity levels of long half-life elements are very low-for instance, aluminum twenty-six and chlorine thirty-six; also the rates of absorption of hydrogen, helium, and inert gases from the Solar wind. Things like that tell us that those rocks haven’t been lying there very long; and since they got where they were by being thrown out of the craters, the craters haven’t been there very long, either." Steinfield made an exaggerated empty-handed gesture. "The rest you know. People like Kronski have done all the figuring and put them at around fifty thousand years old-yesterday!" He waited for a few seconds. "There must be a Lunarian connection somewhere. The number sounds like too much of a coincidence to me."
Hunt frowned for a while and studied the detail of the Farside hemisphere of the model. "And yet, you must have known about all this for years," he said, looking up. "Why the devil did you wait for us to call you?"
Steinfield showed his hands again and held the pose for a second or two. "Well, you UNSA people are pretty smart cookies. I figured you already knew about all this."
"We should have picked it up sooner, I admit," Hunt agreed. "But we’ve been rather busy."
"Guess so," Steinfield murmured. "Anyhow, there’s even more to it. I’ve told you all the consistent things. Now I’ll tell you some of the funny things…" He broke off as if just struck by a new thought. "I’ll tell you about the funny things in a second. How about a cup of coffee?"
"Great."
Steinfield lit a Bunsen burner, filled a large laboratory beaker from the nearest tap, and positioned it on a tripod over the flame. Then he squatted down to rummage in the cupboard beneath the bench and at last emerged triumphantly with two battered enamel mugs.
"First funny thing: The distribution of samples that we dig up on Farside that have a history of recent radioactive exposure doesn’t match the distribution or strength of the activity sources. There ought to be sources clustered in places where there aren’t."
"How about the meteorite storm including some, highly active meteorites?" Hunt suggested.
"No, won’t wash," Steinfield answered, looking along a shelf of glass jars and eventually selecting one that contained a reddish-brown powder and was labeled "Ferric Oxide." "If there were meteorites like that, bits of them should still be around. But the distribution of active elements in the garbage is pretty even-about normal for most rocks." He began spooning the powder into the mugs. Hunt inclined his head apprehensively in the direction of the jar.
"Coffee doesn’t seem to last long around here if you leave it lying around in coffee jars," Steinfield explained. He nodded toward a door that led into the room next-door and bore the sign "RESEARCH STUDENTS." Hunt nodded understandingly.
"Vaporized?" Hunt tried.
Again Steinfield shook his head.
"In that case they wouldn’t have been in proximity to the rock long enough to produce the effects observed." He opened another jar marked "Disodium Hydrogen Phosphate." "Sugar?"
"Second funny thing," Steinfield continued. "Heat balance. We know how much mass came down, and from the way it fell, we can figure its kinetic energy. We also know from statistical sampling how much energy needed to be dissipated to account for the melting and structural deformations; also, we know how much energy gets produced by underground radioactivity and where. Problem: The equations don’t balance; you’d need more energy to make what happened happen than there was available. So, where did the extra come from? The computer models of this are very complex and there could be errors in them, but that’s the way it looks right now."
Steinfield allowed Hunt to digest this while he picked up the beaker with a pair of tongs and proceeded to fill the mugs. Having safely completed this operation, he began filling his pipe, still silent.
"Any more?" Hunt asked at last, reaching for his own cigarette case.
Steinfield nodded affirmatively. "Nearside exceptions. Most of the Nearside craters fit with the classic model: old. However, there are some scattered around that don’t fit the pattern; cosmic-ray dating puts them at approximately the same age as those on Farside. The usual explanation is that some strays from the recent Farside bombardment overshot around to the Nearside…" He shrugged. "But there are peculiarities in some instances that don’t really support that."
"Like?"
"Like some of the glasses and breccia formations show heating patterns that aren’t consistent with recent impact… I’ll show you what I mean later."
Hunt turned this new information over in his mind as he lit a cigarette and sipped his drink. It tasted like coffee, anyway.
"And that’s the last funny thing?"
"Yep, that’s about the broad outline. No, wait a minute-last funny thing plus one. How come none of the meteorites in the shower hit Earth? Plenty of eroded remains of terrestrial meteorite craters have been identified and dated. All the computer simulations say that there should be a peak of abnormal activity at around this time, judging from how big the heap of crud that hit the Moon must have been. But there aren’t any signs of one, even allowing for the effects of the atmosphere."
Hunt and Steinfield spent the rest of that day and all of the next sifting through figures and research reports that went back many years. Hunt did not sleep at all during the following night, but smoked a pack of cigarettes and consumed a gallon of coffee while he stared at the walls of his hotel room and twisted the new information into every contortion his mind could devise.
Fifty thousand years ago the Lunarians were on the Moon. Where they came from didn’t really matter for the time being; that was another question. At about the same time an intense meteorite storm obliterated the Farside surface. Did the storm wipe out the Lunarians on the Moon? Possibly-but that wouldn’t have had any effect on them back on whatever planet they had come from. If all the UNSA people on Luna were wiped out, it wouldn’t make any lasting difference to Earth. So, what happened to the rest of the Lunarians? Why hadn’t anybody seen them since? Had something else happened to them that was more widespread than whatever happened on the Moon? Could the something else have caused the meteorite storm? Could a second something else have both caused the first and extinguished the Lunarians in other places? Perhaps there was no connection? Unlikely.
Then there were the inconsistencies that Steinfield had talked about… An absurd idea came from nowhere, which Hunt rejected impatiently. But as the night wore on, it kept coming back again with growing insistence. Over breakfast he decided that he had to know the story that lay below those billions of tons of rubble. There had to be some way of extracting enough information to reconstruct the characteristic
s of the surface just before the bombardment commenced. He put the question to Steinfield later on that morning, back in the lab.
Steinfield shook his head firmly. ‘We tried for over a year to make a picture like that. We had twelve programmers working on it. They got nowhere. It’s too much of a mess down there-all ploughed up. All you get is garbage."
"How about a partial picture?" Hunt persisted. Was there any way that a contour map could be calculated, showing just the distribution of radiation sources immediately prior to the bombardment?
"We tried that, too. You do get a degree of statistical clustering, yes. But there’s no way we could tell where each individual sample was when it got irradiated. They would have been thrown miles by the impacts; a lot of them would have been bounced all over the place by repeat impacts. Nobody ever built a computer that could unscramble all that entropy. You’re up against the second law of thermodynamics; if you ever built one, it wouldn’t be a computer at all-it would be a refrigerator."
"What about a chemical approach? What techniques are available that might reveal where the prebombardment craters were? Could their ‘ghosts’ still be detected a thousand feet down below the surface?"
"No way!"
"There has to be some way of reconstructing what the surface used to look like."
"Did you ever try reconstructing a cow from a truckload of hamburger?"
They talked about it for another two days and into the nights at Steinfield’s home and Hunt’s hotel. Hunt told Steinfield why he needed the information. Steinfield told Hunt he was crazy. Then one morning, back at the laboratory, Hunt exclaimed, "The Nearside exceptions!"
"Huh?"
"The Nearside craters that date from the time of the storm. Some of them could be right from the beginning of it."
"So?"
"They didn’t get buried like the first craters on Farside. They’re intact."
"Sure-but they won’t tell you anything new. They’re from recent impacts, same as everything that’s on the surface of Farside."
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