Galileo’s Dream. Kim Stanley Robinson

that he held onto her upper arm as if to the trunk of a grapevine. She did not seem to mind, and it helped him to keep up with her. After a while he found himself helplessly making various erotic calculations having to do with her obvious strength (the box she carried looked heavy), calculations that caused his eyes to widen and his heart to pound. It was a little hard to believe she was human.

      ‘You are well named,’ he murmured.

      ‘Thank you,’ she said. ‘We name ourselves when we are young, at our rite of passage. That was a long time ago.’

      When they reached the far side of the little temple she paused. He let go of her arm. From here they had a view down the shattered sulphurous side of the great volcano they stood on, a view immensely tall, and so broad in extent that he could see a distinct curvature to the horizon, and at least a dozen smaller volcanoes, some of them steaming, others blasting great white geysers into the black sky.

      Hera waved at the awesome prospect in a proprietary way. ‘This is Ra Patera, the biggest massif on Io. Io is what you call Moon One, the innermost of the big four. Ra Patera is far taller than the tallest mountains on Earth, bigger even than the biggest mountain on Mars. We are looking down the eastern flank toward Mazda Catena, that rupture crack in the side of the shield, down there steaming.’ She pointed. ‘Ra was the ancient Egyptian sun god, Mazda the Babylonian sun god.’

      Galileo recalled the spotted surface of the sun as seen on the paper put under the telescope’s eyepiece. ‘It looks as if burnt by the sun, though we are so far from it. As hot as Hell.’

      ‘It is hot. In many places if you walked on the surface you would sink right into the rock. But the heat comes from inside Io, not from the sun. The whole moon flexes in the tidal stresses between Jupiter and Europa.’

      ‘Tides?’ Galileo said, thinking he had misunderstood. ‘But surely there are no oceans here.’

      ‘By tides we mean the pull a body has on all the others around it. Every mass pulls everything else toward it, that’s just the way it is. The bigger the mass, the bigger the pull. So, Jupiter pulls us one way, and the other moons pull other ways. Mostly Europa, being so close.’ She grimaced expressively. ‘We are caught between Jove and Europa. And all the pulls combine to warp Io continuously, first one way then another. We are therefore a hot world. Thirty times hotter than Earth, I have heard, and almost entirely molten, except for a very thin skin, and thicker islands of hardened magma like the one we stand on. The entire mass of Io has melted and been erupted onto its surface many times over.’