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’?"