She was forced to scale some of the ridges.

They were exactly like frozen waves, a hundred feet tall or more. She tacked at a shallow angle to reach the top of each ridge. At the top she turned so that the sled went ahead of her as she slid down the slope of the far side. Then it was time to clamber painfully up to the top of the next ridge. She was like an insect, she thought, struggling over the meniscus of some giant pond.

She had no crampons or ski-sticks; she had to paw at the surface with her gloves and the sides of her skis to gain leverage. Soon her knees and elbows were bruised, and her fingers and toes ached. Sometimes her sled slid sideways and pulled her back down into a trough.

She paused at a crest. The ice was bare and blue-grey. Gritty granules lay in the hollows. The ice here was polished, and when she ran a gloved hand over it, it felt as smooth as glass, hard as concrete. The wave had been scoured out by gritty granules, and then polished to a sheen by fine aerosol dust.

When she looked back, she could see Rosenberg toiling through the valleys between the waves. The great ridges thrust upwards all around him, dwarfing him, and his helmet lamp splashed little puddles of yellow light against the shimmering walls around him. Sometimes he would pass a clearer patch of blue ice, and his light would penetrate the bulk of the waves; Benacerraf would see the beam glimmering within the bulk of a wave, scattering and sparkling from complex fissure patterns within the ice, an arc of Earth light illuminating these giant, dead, silent fairy castles.

Rosenberg stopped, several times, and took samples, scrapings of the eroded surfaces. He photographed the wave shapes. He even measured the angles of the frozen crests. His voice was weak, but Benacerraf could hear his enthusiasm as he found the opportunity to do a little science. “So beautiful… Benacerraf, each of these waves might be a million years old. And as the wind wears away at them, it’s exposing ice billions of years older than that — ice older than life on Earth… so beautiful…”

She found a new hazard.

She had to skirt huge crevasses; they looked to be hundreds of feet deep, with walls of a clean Earth-like blue where her helmet lamp shone on them. As the ice flowed out of the heart of Cronos, it was splitting along gigantic faults.

The crevasses parallel to the flow weren’t difficult to handle, as they pointed the path she wanted to take, towards the heart of the continent. But in some places, where the ice was compressed as it flowed, the crevasses ran transverse to the flow. She had to take wide detours to reach a narrowing of each crevasse, so that she could straddle them with heir skis.

In the most difficult country there was a mix of transverse and parallel crevasses, presumably because of some distortion of the flow. The crevasses intersected, cracking the ice into gigantic, parallel pillars, some of which had tumbled and shattered, so it was as if she was picking her way across the smashed sidewalk of some giant, ruined city.

She kept a weather eye on Rosenberg; his progress was slow, but he was plodding along in her wake, his head down.

After some hours of this, she found a place to camp. It was at the hollow between two giant pressure waves, a patch of regolith granules not much larger than the area of their two sleds. They had to anchor the tent to the sleds, because their metal pegs would not drive into the ice layer.

All around, as far as Benacerraf could see in the orange-brown light, there were pressure mounds and cracks. Their little encampment was like a small boat, she thought, lost in a giant sea.

Before they could crawl into the tent, a wind came up, blasting through the valley as if through a wind tunnel.

This was the seventh night, some fifty miles from Tartarus, and they were getting into a routine when they established camp.

Benacerraf hadn’t managed to take a dump that day as she walked — which was the preferred way, because then at the end of the day they just had to dig the crap out of their diapers, and some semblance of privacy was maintained. But now she could feel pressure building inside her. She suspected she was coming down with some kind of diarrhea. It was probably the antibiotics she was taking.

“Sorry, Rosenberg,” she said in advance.

He was piping water into today’s ration bags. He looked at her, his eyes glassy, and shrugged.

She opened her suit as wide as she could. She stood up and bent over in the tent’s cramped confines, with the open front of her suit close to the heater. She fumbled to get an old ration bag inside the suit. She found her butt. The skin of her buttocks felt flaccid, the flesh depleted of fat. It was, she thought, an old woman’s ass. She clamped the bag as best she could over her butt, and let go.

The crap emerged as a hard, hot spray, accompanied by an explosive fart. She tried to catch it all in the bag, but it wasn’t easy, and she could feel excrement splashing her hands, sleeves and legs.

The smell, erupting from the interior of her suit, was moist and pungent. My own contribution to Titan’s methane layer, she thought.

She closed up the bag and pulled it out, and her first priority was to close up her suit, trying to trap whatever warmth was left. Then she swathed the bag of sludgy excrement in a couple of other bags, wiped her hands on the back of her legs — the stuff would freeze off there tomorrow anyhow — and lodged the bag in the corner of the tent, with her piss bag.

She huddled closer to Rosenberg and the fire, shivering, her arms wrapped around herself.

Rosenberg was working at the cooking, but slowly. His left hand had got frostbitten a few days before, when damp had gotten inside his glove, and the cold of the ice ridges to which he had to cling had found a route to his fingers. Now, three of his main finger blisters had burst, the dead skin falling away to reveal raw stumps, like uncooked meat.

“Rosenberg, that looks like agony. You want me to take over?”

“No,” he said. His face was thin, the flesh disturbingly slack; his cheeks seemed to descend in folds over the corners of his mouth. “It’s not so bad now the blisters have burst. Before, sometimes the fluid in them would freeze.”

“Ouch.”

“We both got problems. Here. Eat.”

She took her food packets; the warmth, cupped in her hands, was welcome.

The meal passed in uncompanionable silence.

During the last couple of stops, the sour thoughts she’d previously been able to leave outside the tent’s airlock had started to seep inside.

She’d come to loathe Rosenberg’s personal habits. The yellow stink of his urine bags. The icicle-like dribbles of snot and saliva and tears that formed on his spindly beard. The way the wounds of his hands wept over her food.

And she started to become obsessed with the fairness, or otherwise, of the way Rosenberg handled the food.

The business with the carrots was one thing. Benacerraf had tried again to eat the things, but failed. So he got to eat all the damn carrots. And now Rosenberg had developed other little habits. Like he would take her discarded soup bags, turn them inside out, and lick the inner surface clean of any residue, before stowing them for use later. It drove her crazy. She started to insist on a turn making the soup, so she could get to lick the bags.

The NASA rations, in their bags and tins, were easy to split fairly. But the stuff from the CELSS farm — the carrots, their crude bread, wheat, rice — had to be divided. And if they had to choose between two portions, Benacerraf became obsessed by the need to stop Rosenberg getting the bigger portions, every damn day.

They came up with ways to deal with it. They took turns working the food. If something had to be split, one would make the break as fairly as possible, and the other got the chance to choose. They would alternate that, day by day.