Tidal Moon

BOB AMHERST shivered a little despite the heated interior of the autobus, but grinned none the less as he made out the frosty towers of Hydropole. He was always glad to return to the polar city, if only for the pleasure of staring up at buildings piled story upon story like those of his native Syracuse on a gray planet some half a billion miles sunward. Hydropole, south polar city of Jupiter's third major moon, Ganymede, was a chilly town at all seasons with its thirty degree Fahrenheit mean, and its variation of only ten degrees. But it was certainly the only settlement on the satellite that was worthy of the title of city.
Amherst had served four terrestrial years on the watery planet as collector for Cree, Inc., moving from town to town gathering the precious medicinal moss, to take it finally to Hydropole, the rocket port, for transshipment to Earth.
He was one of the hundreds of such collectors for the giant company, each with his own route, each picking his own way from town to town, riding his hipp (the sea-horse of Ganymede, Hippocampus Catamiti) through the wild torrents of the afterfloods, past mountains whose locked valleys were apt to spill countless millions of tons of water upon him with no warning save the crash of the bursting mountain walls.
Only in Hydropole was there safety. Situated on the south pole, it escaped the great wash of water which, due to the strong gravitational pull of Jupiter, every three months encircled the tiny moon...