Murdoch smiled inwardly. He'd known the walls would be solid; he'd only wanted to check the alien's watchfulness. Now he knew that there was more to it than just the robot, and that the voice was standard wherever it came from.
Not that the information helped any.
He walked back to the middle of the building and went through the door across the lobby. In that half of the building were a library, a gymnasium and what was evidently a Solar System museum. There was nothing new to him in the museum. Though there were useful tables and data in the library, he was too tense to study. The gymnasium he'd use later.
He went outside, walking gingerly on the gravel. The rear of the building was a featureless semi-circle, the lawns and hedges unvaried. He took deep breaths of the air perfumed by flowers.
He jumped at a sudden buzz near his elbow. A bee circled up from a blossom and headed for the top of the building to disappear over the edge. Murdoch considered jumping for a hold and hauling himself up to the top of the building to see if there were hives there, but decided not to risk the aliens' displeasure. He realized now that he'd been hearing the bees all the time without recognizing it, and was annoyed at himself for not being more alert. He paid more attention now, and saw that there were other insects too; ants and a variety of beetles. There were no birds, mammals, or reptiles that he could see.
He parted the hedge and leaned close to the clear wall, shading the surface with his hands to see into the ice. There were a few rocks in sight. He found one neatly sliced in two by the force-field, or whatever it was, showing a trail of striations in the ice above it where it had slowly settled. On Ganymede, the rate of sink of a cool rock would be very slow in the ice.
Far back in the dimness he could see a few vague objects that might have been large rocks or ships. There were some other things with vaguely suggestive shapes, like long-eroded artifacts. Nothing that couldn't have been the normal fall-in from space.
He went to the front of the building again and stood for a while, looking at the graveled other half of the place. He couldn't see any insects there, and not a blade of grass. He approached the barrier and leaned against it, to see how it felt. It was rigid, but didn't feel glass-hard. Rather it had a very slight surface softness, so he could press a fingernail in a fraction of a millimeter.
He remembered that on Earth bees would blunder into a glass pane, and looked around to see if they hit the barrier. They didn't. An inch or so from it, they turned in the air and avoided it. Neither could he see any insects crawling on the invisible surface. He pressed his face closer, and noticed again the odd reluctance he'd felt when crossing on the way in.