Building Harlequin’s Moon by Larry Niven, Brenda Cooper Page B

edge of the crater that cupped the Hammered Sea. The horizon was almost a flat line of water—the tallest edges of the far crater wall peeked above the sea like teeth, jagged and far away. He loved this place. It was a wild machine, much more controlled than it looked, often surprising.
    The rim was unstable. Ten degrees away from them, inside the crater wall, a rock worked loose and fell. Ursula pointed, her finger shaking. He watched it bounce slowly, exaggeratedly, down the jagged incline and splash into the bright water below; a tiny ball, graceful in the low gravity. “That was beautiful,” Ursula said.
    “Those rocks,” Gabriel said, “are the size of the planting machines you hate to drive.”
    Ursula’s eyes widened and she stepped back, losing her balance and falling onto the soft powdered rock that coveredthe rim of the crater. A fog of dust rose around her, changing the color of her skin.
    Gabriel laughed. Harry laughed too, standing at the edge, toes lined up with the end of a rock. In a few moments, Ursula’s laugh followed theirs, a nervous trill. She moved away from the rim.
    Rachel leaned forward, eyes pinned to the sea. She pointed toward water stains below them. “Does the water really get that high?”
    Ali smiled. “Yes, Rachel. Just watch. Gabriel had to make the crater taller and thicker twice just to contain the force of the water and tides.”
    “You made this? I thought you oversaw the planting.”
    “I do oversee the planting. But first I designed Selene. Someone had to bring water in for the plants.”
    “Duh,” Harry said.
    “So you just . . . made . . . the Hammered Sea? All by yourself?” Rachel was eyeing the far side, looking between Gabriel and the huge sea in front of her.
    Ali laughed again, louder, her head thrown back. “I wouldn’t underestimate Gabriel if I were you, Rachel.”
    Gabriel tried to look stern. It had been
hard
to shape this sea. He was proud enough to babble some. “I had help, of course. Mostly from a really smart program named Astronaut.”
    Ali glared at him. Her parents had died on Jupiter Station when the AI that ran it lost interest. Ali didn’t like AIs.
    “Ali helped too.” He sighed. Time to teach. They only knew part of the story. “There wasn’t any Selene when we came here. There was an oversized gas giant planet, Harlequin, and almost a hundred moons. We picked a big moon for a foundation. It had no spin—no day or night with reference to Harlequin. We made that, hitting it over and over in the same place, from the same angle, making tilt and spin. Days and seasons. Building Selene. Then we—went—cold—for a long time, to let the whole system stabilize and cool.
    “We woke up to a pockmarked ball covered in regolith. There was a little ice, a few small pockets of underground water, and the beginnings of an atmosphere, but humans need lots of easy-to-reach water, and thick atmosphere to shield us from space. So we brought in comets. Then we went cold again. Then we brought in more comets. The comets gave us the water you see here.”
    “So the water is from space?” Rachel asked.
    “Isn’t everything? This deep sea is the motor that drives Selene’s hydrology. We need this much water for the humidity to grow tropical plants rather than cactus. But we had to contain most of the water to limit the effect of the tides, which are worse here than on Earth, because of Harlequin.” He pointed at the gas giant, which hung just off center in the sky. “Gravity pulls the water toward Harlequin. If we didn’t contain it, Selene would be flooded with every high tide. We could have made hundreds of small seas instead of what we did make; the Hammered Sea and Erika’s Folly, but they would have been harder to manage. Oh, lots of craters have a little water in them, but over half our water is right here.” He paused, and said, “Besides, I wanted to make a real sea.” That made him feel giddy still: to build a sea because he wanted to!