She stepped over the crack in the ice and hauled the sled across.

She came to more leads of open water, slick with hydrocarbons, opening and refreezing. Some of them were too wide to risk crossing, and she had to detour, tracing up. and down between the leads. In some places the ice was so thin it was spongy and creaking, and if she stepped too close to an edge it would crumble away into the open water. She found a lead that was closing, its edges grinding noisily together. Where the two plates met, the ice was cracking, its sharp sounds ricocheting out across the emptiness, echoing from the iron-hard ice.

The ice field stretched on; ridges and plates pushed out of the plain like pieces of gigantic, abandoned furniture. From all around her rang out the aching, grinding noise of moving ice, crackling like the shock waves from a Shuttle launch. The noises came together in great waves, punctuated by godlike silences.

As she penetrated the field of frozen-over ejecta, the visibility opened out, the pervading gloom of Titan’s orange sky lifting a little. Thin methane clouds, dark and tangled, blew ahead of her, obscuring the tall orange sky. Perhaps the relative warmth of the water was clearing the air of some of the organic haze.

At last she came to a place where the broken layer of methane clouds, ahead of her, grew still darker. The darkness — near to black — seemed to begin in a sharp discontinuity, almost a straight line, scraped across the sky.

She smiled. Rosenberg had warned her to expect this.

It was a water sky.

There must be a wide stretch of open water, no more than a few miles away, reflecting darkly from the low methane clouds of Titan.

She pitched her camp on a large plate of ice, hundreds of yards from any open leads. The air was so warm that she was able to strip off the outer layers of her suit. It felt like a great luxury, as she rubbed handfuls of half-melted ice over her bruised skin to clean herself.

She drank her fill of cool comet water.

That night, as she lay huddled against the tent’s plastic wall, she listened to the muffled groaning of the thin ice beneath her. She imagined the slow swell of the comet water, the big underground waves travelling back and forth across the ejecta sheet.

At any moment this plate could crack, pitching her into the cold water, suit and all. But somehow that wasn’t a frightening prospect. She was, after all, made of water. Water was home.

She slept, without dreaming, as well as she had done since Rosenberg’s death.

She went through her waking ritual for the last time.

She breakfasted on dried strawberries, crackers, and tiny, sweet lettuce leaves from the CELSS farm. She took a final dump, into an empty plastic food bag, and cleaned herself thoroughly.

She blew her nose on a fragment of parachute fabric. It was the last time she’d be able to do that, even.

There was a last time for everything, she thought: not just the grand actions, but the small, human things. It all counted.

She pulled on her suit. She tucked her little packet of photos inside her suit, over her heart. She sealed her helmet and gloves, and turned the switch that powered up her PLSS. She heard the familiar high-pitched whine of the pumps and fans, the cool hiss of the oxygen blowing over her face.

She packed away what she could: her food and waste bags, the power cell. Soon, the tent was as neat as she’d left Discovery.

She pushed her way out of the tent’s cramped little airlock. Outside, standing on the thin, grinding ice, she tucked Rosenberg’s canister of spores under her arm, to keep it as warm as possible.

She looked around her little outpost. The half-empty sled stood on the ice, its parachute-fabric cover loosely knotted over it. The tent, closed up, was compact and neat.

She fixed her Hasselblad to the S-band antenna stand, and lined it up so it framed the tent and sled. She checked that the antenna was still aligned correctly on Cassini; it was possible the drifting of the pack ice during her sleep had pushed it off its line.

Feeling self-conscious, she went to stand in the camera’s field of view. Standing there before her little camp, in her grimy, battered, much-repaired EVA suit, she held up her canister of spores, while the camera fired image after image up to Cassini.

She hated these Armstrong poses. But maybe, she thought, this one was justified. After all, if Rosenberg was right, with this one act she might be shaping the future of a new biosphere.

These might be the most important photographs ever taken.

She wondered whether to smile or not.

Her residual sense of orderliness made her walk around the camp once more, checking everything was intact and stowed away.

Then she turned and strode off, across the ice, towards the water.

A wind began to pick up, blowing off the broken ground in front of her, hard and piercing; she found she relished its resistance.

She could feel her packet of photographs, a hard rectangle pressed against her chest by the suit.

She felt as if she was discarding her life, in huge layers: first Earth itself, shrunken to a pinprick of light by the huge distance she’d travelled; then Tartarus Base, with its painfully assembled and repaired life-sustaining gadgets; and at last even the trappings of her own little encampment out here on the water ice. Now, she was left with nothing but her body, and the battered suit that was its last protection.

The leads began to widen and interconnect.

Soon the ice was broken up into isolated islands, some only a few feet across, separated by channels of grey, scummy water. Ahead, fragmented ice stretched in a loose mosaic. She could see the open water ahead of her, a dark band encroaching from the horizon, flecked with loose ice floes.

She pressed on, climbing over the narrower channels, taking care to stick to the larger ice floes. But the ice was fragmenting rapidly. Soon, even the biggest floes were unstable beneath her feet.

She couldn’t go any further. This would have to do.

She kicked off her skis, and stacked them neatly to one side. She wouldn’t be needing them any more. She took a last sip of orange juice, from the worn plastic nipple inside her helmet.

She walked to the edge of the ice. She took Rosenberg’s canister of spores, and dipped both her gloved hands in the water. The cold of the water was a thrilling shock, easily penetrating the feeble resistance of the gloves’ heating elements.

Under the water, she opened Rosenberg’s canister, and shook out the spores, scattering them as widely as she could.

When the canister packet was empty she withdrew it, shook it clear of ice, and tucked it neatly into a sample pocket, buttoning closed the flap.

Then she stood straight. She looked around at the haze-drenched world around her: the cramped, close horizon, the scattered darkness of the methane clouds above, the shattered ice landscape, with that band of free water, just out of reach.

She reached up and snapped the switch on her chest that shut down her PLSS.

The sound of pumps and fans died immediately. The air stopped washing over her face, and felt thicker, more stale. The cold of Titan dug into her flesh through the pattern of heating elements. And she could hear the moan of the wind, a remote bass tone, and the deep crackling of the ice sea, emerging from all over the landscape.

It was the first time she’d heard the music of Titan, unmasked by the man-made noises of her equipment.

She walked forward, across this icy beach.

Before she could reach the edge of the floe, the ice crumbled under her weight.

There was a moment of falling — extended by Titan’s low gravity — long enough for a small stab of terror to dig into her consciousness. But then her feet and legs hit the thick, oily surface of the water. The meniscus rushed up her body, its cold mass enclosing, and joined over her head.