Turnbull poured himself a second glass of sherry, took a sip, and rolled it carefully over his tongue.
The Centaurus Mystery. That's what the explorers had called it back in 2041, nearly a century and a half before, when they'd found the great city on one of the planets of the Alpha Centaurus system. Man's first interstellar trip had taken nearly five years at sublight velocities, and bing!—right off the bat, they'd found something that made interstellar travel worthwhile, even though they'd found no planet in the Alpha Centaurus system that was really habitable for man.
They'd seen it from space—a huge domed city gleaming like a great gem from the center of the huge desert that covered most of the planet. The planet itself was Marslike—flat and arid over most of its surface, with a thin atmosphere high in CO_2 and very short on oxygen. The city showed up very well through the cloudless air.
From the very beginning, it had been obvious that whoever or whatever had built that city had not evolved on the planet where it had been built. Nothing more complex than the lichens had ever evolved there, as thousands of drillings into the crust of the planet had shown.
Certainly nothing of near-humanoid construction could ever have come into being on that planet without leaving some trace of themselves or their genetic forebears except for that single huge city.
How long the city had been there was anyone's guess. A thousand years? A million? There was no way of telling. It had been sealed tightly, so none of the sand that blew across the planet's surface could get in. It had been set on a high plateau of rock, far enough above the desert level to keep it from being buried, and the transparent dome was made of an aluminum oxide glass that was hard enough to resist the slight erosion of its surface that might have been caused by the gentle, thin winds dashing microscopic particles of sand against its smooth surface.
Inside, the dry air had preserved nearly every artifact, leaving them as they had been when the city was deserted by its inhabitants at an unknown time in the past.
That's right—deserted. There were no signs of any remains of living things. They'd all simply packed up and left, leaving everything behind.
Dating by the radiocarbon method was useless. Some of the carbon compounds in the various artifacts showed a faint trace of radiocarbon, others showed none. But since the method depends on a knowledge of the amount of nitrogen in the atmosphere of the planet of origin, the rate of bombardment of that atmosphere by high-velocity particles, and several other factors, the information on the radioactivity of the specimens meant nothing. There was also the likelihood that the carbon in the various polymer resins came from oil or coal, and fossil carbon is useless for radio-dating.
Nor did any of the more modern methods show any greater success.