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precipice. Holding the rope loosely in both hands, I raised my chin, surveying the surface one last time.
    It is this moment that I cherish the most. Not the climb itself, or even the moment of triumph, but this moment, the moment just before. The canyon was still a mystery -- unexplored, unseen, unknown. When I do reach a destination, even when I'm the first, I'm always a little saddened. There would be one less new thing to see.
    "Remember," I said, "stay behind me on the rope. I touch the bottom first."
    I turned away from the others, looked down into the black abyss, and stepped over the rim.

    For the first several hundred meters, the rising sun illuminated the canyon's wall. From above, it appeared to be a sheer cliff, but inside, it blossomed into three dimensions. Bulges, ledges, sharp angles impossible in higher
g
. Mottled mixtures of white and gray, ice and rock, just like the surface.
    Ten minutes in, the light faded to reflections, then disappeared. The canyon was now pitch black. I turned up my headlamp and lightly squeezed the rope, slowing my descent to a steady two meters per second.
    Wil and Katherine were descending smoothly, thirty meters up rope. Skillfully, I had to admit.
    They were a husband and wife team, "Firsters," like me, but for all the wrong reasons. They'd been the first to summit Olympus Mons on foot, the first to surf a comet's tail, the first to ride a geyser on Enceladus. Stunts, not exploration. For Wil and Katherine, it wasn't about seeing new things, it was about being seen.
    They were the last people I wanted to climb with, but I couldn't afford a trip to Uranus alone. So we made a deal. I let them in on the expedition, and they agreed to let me explore the canyon floor first, alone. I had little doubt they would try to break that agreement.
    It wasn't Wil and Katherine who worried me now, though, it was Shelley. She lagged hundreds of meters up rope, her headlamp swinging wildly each time she adjusted speed. Every few hundred meters, she'd pause to inspect the wall and take mass spec data. Every kilometer, she'd stop her descent entirely to chisel a rock sample.
    I reached the top ledge on schedule, followed by Wil and Katherine. I stayed on the line, blocking their path, until they'd unclipped from the rope. They moved to the far side of the ledge where they sat, feet suspended over the edge, talking on their private frequency.
    Finally, fifteen minutes later, Shelley arrived. She unclipped, walked directly to the canyon wall, knelt down, and began chipping away at a rock.
    I walked up behind her and activated a private channel.
    "You can hammer during breaks," I said, "but don't stop your descent. We're already twenty minutes behind schedule."
    Shelley stopped chiseling. Her shoulders rose and fell with a heavy sigh. "Lance, do you even know why I'm here? Why I'm risking my life in some uncharted canyon, with you of all people? Have you even thought about that?"
    There were several ways to answer that, so I just stayed silent.
    "Take a look at these two rocks. Tell me what you see."
    She'd laid out two samples. One white, the other gray.
    I said, "One is mostly ice. The other some kind of basalt. Just like every other rock on this moon."
    "Exactly! That's what's so remarkable."
    I knew what she was getting at, of course, why geologists were so obsessed with Miranda. I just didn't care. I was interested in the canyon's present, not its past.
    "Do you remember that time we hiked up Ophir Peak? When I spotted Uranus in my scope and told you all about the mystery of Miranda?"
    I remembered Ophir Peak. A small mountain overlooking Mars' Candor Chasma. Spectacular views. I'd spent months exploring the hidden nooks and caves of that canyon, before the mining bots moved in.
    "Well," Shelley said, "nothing's changed. It's still a mystery. We've now imaged every moon in the solar system, and they all show some kind of recognizable geology -- young rocks on top of old, or at least patterns. But