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of ice below prevents any really spectacular mountains from forming.
    I'm not sure I'm getting this across. You see, ice is as hard and stable as any other rock at the local temperature of 100 degrees Kelvin—I'm not as good at figures as geography, but that's an easy one to remember, one hundred degrees Celsius above absolute zero—but at the local noon (which of course has no relation to the Earth time we use inside the domes), it can be fifty degrees warmer, and deep down below the pressure can heat it some too, so in the course of millions of years that ice does soften and flow a little. This planet has been around for four billion years or so, so the flow obliterates the extreme features. Result: shallow, rounded craters standing, as it were, shoulder to shoulder, rim to rim, and one inside another, and overlapping: This world is made of craters, and none of them are any effort to navigate. There are no cutting edges on Callisto. You might say the features of the surface have been eroded by water: not water coursing over from above, as on Earth, but slowly squeezing out from below.
    That's why we were moving along so well in our wheeled vehicle; there was very little natural obstruction.
    The sky was more interesting. This was night, on the surface as well as inside the dome, but Jupiter was full, and his baleful light flooded the rolling rills of Valhalla. Jupiter was anything but dull, with his violently contrasting bands of atmosphere and the various gaseous eyes staring at us. Surely Jove was watching our puny efforts with disdain—but he was our destination. I was, of course, sorry to leave my home world, for all my experience was invested here on Callisto and all my prior hopes for success had been defined by the Halfcal culture and hierarchy. But I knew that in those bands of turbulent color on the Jovian Planet was opportunity as vast as Jupiter himself. We would certainly be better off there; we would no longer be peasants, there!
    I looked directly up, trying to see the other gravity lens, the one above us, close. Such lenses don't just fix the gravity inside the domes, they govern the light we receive. This can be hard for people who don't reside on moons to understand, so I'll try to make it simple: Light is affected by gravity, technically the curvature of space that we call gravity, so a lens that bends gravity waves also bends light waves.
    Properly formed, a large gravity lens can be used to focus the light of the distant sun on a smaller area, making it proportionally more intense. There's a lot of energy in light, as a magnifying glass can demonstrate when used to set fire to things. Since the sun's light is much less intense out here at Jupiter's orbit than it is at Earth's orbit, we need to focus it to match what our bodies and our plants are used to.
    We are all transplants from Earth, really, though we may have been born or seeded here; a few centuries can't erase a few billion years of evolution.
    So above each dome-city is a huge gravity lens that is twenty-seven times the area of the dome, and the lens focuses the wan sunlight to that amount, and it shines in through the dome's transparent roof to light and warm the city, exactly as would be the case on Earth. Well, not exactly; Earth's copious atmosphere filters out many deleterious aspects of the radiation, so our twenty-seven-times-concentrated sunlight would burn us if we took it straight. But the material of the dome is designed to filter out the harmful radiation, substituting for the missing depth of atmosphere, and so the net effect is similar.
    The same is true for the agricultural domes; they are literally greenhouses. This is convenient to do on an airless planet, since nobody lives outside to complain about being deprived of sunlight. Naturally the focused brilliance at the dome is at the expense of the twenty-seven-times-as-large area around it, which receives very little light. We had not noticed any difference because we pedaled