Russ shrugged. "Maybe. We'll find out at the next stop. And, incidentally, that's not going to be on Jupiter itself, but on its moon Callisto. We've traced the line of distortions."
"That's good news," said Burl. "I had the feeling you were worried about Jupiter. The planet's so huge it would have meant real trouble trying to land. The books say its atmosphere is thick, unbreathable, and moves in gale velocity around it."
Russ nodded. "With Jupiter almost 89,000 miles in diameter, it would have been a tough problem to maneuver outside this ship ... in fact, impossible, not to mention the fact that the atmosphere, mostly ammonia and other frigid gases, moves in several independent belts. However, Callisto should be okay."
"That's something we know about our opponents, anyway," said Burl, "They must have physical limitations enough like ours to rule out places where we couldn't move, either."
Boulton showed no further effects from his experience. In time, the Magellan drew near Jupiter. Callisto, its fifth satellite outward, moved about the mighty planet at a distance of 1,170,000 miles. It was a large satellite as they go, 3,220 miles in diameter, larger, in fact, than Mercury. But, as Russ explained, it was a queer place in its own fashion.
For despite its size, Callisto was apparently not a solid body as we think of it. Its density totaled only a little more than that of water, its mass half that of the Earth's Moon—a notoriously porous body.
They bore down on Callisto, matching their speed to its, and swung close to its surface. It had almost no atmosphere, just a thin layer of the heavier gases. It was a belted world, without clearly defined continents or surface markings. Its equatorial zone was one vast, featureless belt of darkish-gray. Its temperate zones were white, with patches of yellow here and there. But its poles were gray again.
"The satellite's like a huge ball of thin mud that's never hardened," said Burl as they studied the strange terrain.
"The equator's the softest—it seems to be a river of muddy water, hundreds of miles wide—only it can't be water. Probably semisolidified gases holding dust and grains of matter in suspension," said Russ. "The temperate zones are the same stuff, only colder, and therefore more stable. A thin crust of frozen gases over a planet-wide ocean of semiliquid substance."
"The Sun-tap station's on the southern pole," said Burl. "That must be solid."