For weeks the two scientists debated the mystery of the abrupt disappearance of the native Minervan land dwellers. They ruled out physical catastrophe on the assumption that anything of that kind would have destroyed the terrestoid types as well. The same conclusion applied to climatic cataclysm.
For a while they considered the possibility of an epidemic caused by microorganisms imported with the immigrant animals, one against which the native species enjoyed no inherited, in-built immunity. In the end they dismissed this idea as unlikely on two counts; first, an epidemic sufficiently virulent in its effects to wipe out each and every species of what must have numbered millions, was hard to imagine; second, all information received so far from Ganymede suggested that the Ganymeans had been considerably farther ahead in technical knowledge than either the Lunarians or mankind—surely they could never have made such a blunder.
A variation on this theme supposed that germ warfare had broken out, escalated, and got out of control. Both the previous objections carried less weight when viewed in this context; in the end, this explanation was accepted as possible. That left only one other possibility: some kind of chemical change in the Minervan atmosphere to which the native species hadn’t been capable of adapting to but the terrestoids had. But what?
While the pros and cons of these alternatives were still being evaluated on Jupiter Five, the laser link to Earth brought details of a new row that had broken out in Navcomms. A faction of Pure Earthists had produced calculations showing that the Lunarians could never have survived on Minerva at all, let alone flourished there; at that distance from the Sun it would simply have been too cold. They also insisted that water could never have existed on the surface in a liquid state and held this fact as proof that wherever the world shown on Charlie’s maps had been, it couldn’t have been anywhere near the Asteroids.
Against this attack the various camps of Minervaists concluded a hasty alliance and opened counterfire with calculations of their own, which invoked the greenhouse effect of atmospheric carbon dioxide to show that a substantially higher temperature could have been sustained. They demonstrated further that the percentage of carbon dioxide required to produce the mean temperature that they had already estimated by other means was precisely the figure arrived at by Professor Schorn in his deduction of the composition of the Minervan atmosphere from an analysis of Charlie’s cell metabolism and respiratory system. The land mine that finally demolished the Pure Earthist position was Schorn’s later pronouncement that Charlie exhibited several physiological signs implying adaptation to an abnormally high level of carbon dioxide.
Their curiosity stimulated by all this sudden interest in the amount of carbon dioxide in the Minervan atmosphere, Hunt and Danchekker devised a separate experiment of their own. Combining Hunt’s mathematical skill with Danchekker’s knowledge of quantitative molecular biology, they developed a computer model of generalized Minervan microchemical behavior potentials, based on data derived from the native fish. It took them over three months to perfect. Then they applied to the model a series of mathematical operators that simulated the effects of different chemical agents in the environment. When he viewed the results on the screen in one of the console rooms Danchekker’s conclusion was quite definite: "Any air-breathing life form that evolved from the same primitive ancestors as this fish and inherited the same fundamental system of microchemistry, would be extremely susceptible to a family of toxins that includes carbon dioxide—far more so than the majority of terrestrial species."
For once, everything added up. About twenty-five million years ago, the concentration of carbon dioxide in the atmosphere of Minerva apparently increased suddenly, possibly through some natural cause that had liberated the gas from chemical combination in rocks, or possibly as a result of something the Ganymeans had done. This could also explain why the Ganymeans had brought in all the animals. Perhaps their prime objective had been to redress the balance by covering the planet with carbon-dioxide-absorbing, oxygen-producing terrestrial green plants; the animals had been included simply to preserve a balanced ecology in which the plants could survive. The attempt failed. The native life succumbed, and the more highly resistant immigrants flourished and spread out over a whole new world denuded of alien competition. Nobody knew for sure that it had been so on Minerva. Possibly nobody ever would.
And nobody knew what had become of the Ganymeans. Perhaps they had perished along with their cousins. Perhaps, when their efforts proved futile, they had abandoned Minerva to its new inhabitants and left the Solar System completely to find a new home elsewhere. Hunt hoped so. For some strange reason he had developed an inexplicable affection for this mysterious race. In one of the Lunarian texts he had come across a verse that began: "Far away among the stars, where the Giants of old now live. . ." He hoped it was true.
And so, quite suddenly, at least one chapter in the early history of Minerva had been cleared up. Everything now pointed to the Lunarians and their civilization as having developed on Minerva and not on Earth. It explained the failure of Schorn’s early attempt to fix the length of the day in Hunt’s calendar by calculating Charlie’s natural periods of sleep and wakefulness. The ancestors of the Lunarians had arrived from Earth carrying a deeply rooted metabolic rhythm evolved around a twenty-four-hour cycle. During the twenty-five million years that followed, some of the more flexible biological processes in their descendants adapted successfully to the thirty-five-hour day of Minerva, while others changed only partially. By Charlie’s time, all the Lunarians’ physiological clocks had gotten hopelessly out of synchronization; no wonder Schorn’s results made no sense. But the puzzling numbers in Charlie’s notebook still remained to be accounted for.
In Houston, Caldwell read Hunt and Danchekker’s joint report with deep satisfaction. He had realized long before that to achieve results, the abilities of the two scientists would have to be combined and focused on the problem at hand instead of being dissipated fruitlessly in the friction of personal incompatibility. How could he manipulate into being a situation in which the things they had in common outweighed their differences? Well, what did they have in common? Starting with the simplest and most obvious thing—they were both human beings from planet Earth. So where would this fundamental truth come to totally overshadow anything else? Where but on the barren wastes of the Moon or a hundred million miles out in the emptiness of space? Everything seemed to be working out better than he had dared hope.
"It’s like I always said," Lyn Garland stated coyly when Hunt’s assistant showed her a copy of the report. "Gregg’s a genius with people."
The arrival in Ganymede orbit of the seven ships from Earth was a big moment for the Jupiter Four veterans, especially those whose tour of duty was approaching an end and who could now look forward to going home soon. In the weeks to come, as the complex program of maneuvering supplies and equipment between the ships and the surface installations unfolded, the scene above Ganymede would become as chaotic as that above Luna had been during departure preparations. The two command ships would remain standing off ten miles apart for the next two months. Then Jupiter Four, accompanied by two of the recently arrived freighters, would move out to take up station over Callisto and begin expanding the pilot base already set up there. Jupiter Five would remain at Ganymede until joined by Saturn Two, which was at that time undergoing final countdown for Lunar lift-out and due to arrive in five months. After rendezvous above Ganymede, one of the two ships (exactly which was yet to be decided) would set course for the ringed planet, on the farthest large-scale manned probe yet attempted.
The long-haul sailing days of Jupiter Four were over. Too slow by the standards of the latest designs, it would probably be stripped down to become a permanent orbiting base over Callisto. After a few years it would suffer the ignoble end of being dismantled and cannibalized for surface constructions.
With all the hustle and traffic congestion that erupted in the skies over Ganymede, it was three days before the time came for the group of UNSA scientists to be ferried to the surface. After months of getting used to the pattern of life and the company aboard the ship, Hunt felt a twinge of nostalgia as he packed his belongings in his cabin and stood in line waiting to board the Vega moored alongside in the cavernous midships docking bay. It was probably the last he would see of the inside of this immense city of metal alloys; when he returned to Earth, it would be aboard one of the small, fast cruisers ferried out with the mission.
An hour later Jupiter Five, festooned in a web of astronautic engineering, was shrinking rapidly on the cabin display in the Vega. Then the picture changed suddenly and the sinister frosty countenance of Ganymede came swelling up toward them.
Hunt sat on the edge of his bunk inside a Spartan room in number-three barrack block of Ganymede Main Base and methodically transferred the contents of his kit bag into the aluminum locker beside him. The air-extractor grill above the door was noisy. The air drawn in through the vents set into the lower walls was warm, and tainted with the smell of engine oil. The steel floor plates vibrated to the hum of heavy machinery somewhere below. Propped up against a pillow on the bunk opposite, Danchekker was browsing through a folder full of facsimiled notes and color illustrations and chattering excitedly like a schoolboy on Christmas Eve.
"Just think of it, Vic, another day and we’ll be there. Animals that actually walked the Earth twenty-five million years ago! Any biologist would give his right arm for an experience like this." He held up the folder. "Look at that. I do believe it to be a perfectly preserved example of Trilophodon—a four-tusked Miocene mammoth over fifteen feet high. Can you imagine anything more exciting than that?"
Hunt scowled sourly across the room at the collection of pin-ups adorning the far wall, bequeathed by an earlier UNSA occupant.
"Frankly, yes," he muttered. "But equipped rather differently than a bloody Trilophodon."
"Eh? What’s that you said?" Danchekker blinked uncomprehendingly through his spectacles.
Hunt reached for his cigarette case. "It doesn’t matter, Chris," he sighed.