As far as she could see the ridge continued, a band of dull grey like a wall across the world, merging at last with the horizon.

Rosenberg came up to her, breathing hard, leaning against his traces. “Magnificent, isn’t it?” Rosenberg’s voice was odd: light, fragile. And his stance seemed awkward; he seemed to be leaning too far into the traces. She tried to look into Rosenberg’s helmet. But his visor was obscured by an orange reflection from the hazy sky, frost, a smear of tholin. Rosenberg was the doctor; Benacerraf had been focusing on her own minor ailments, and trusting Rosenberg to look after himself.

Maybe he wasn’t.

She looked up dubiously at the slope they would have to climb. “Magnificent. Absolutely. Let’s go on.”

She led the way once more.

It got easier, despite the slope. It was like climbing Othrys. The gumbo got gradually thinner and less clinging — though less supportive of the sled — until at last her snowshoes were clattering on bone-hard ice, and the sled’s base was scraping with an ominous grind across the slope.

She stopped again, and waited for Rosenberg to catch up. Even in such a short distance he seemed to have fallen a long way behind, and it took him some minutes to make up the ground.

“Time for the great changeover, Rosenberg,” she said.

His breath was a noisy rasp, and he didn’t attempt to speak; he just let himself out of his harness and began to unpick the knots on his sled cover.

They had to fix their aluminum runners to the bases of the sleds. Benacerraf found it difficult to handle the big wing nuts with her gloved hands. The first time she twisted too hard, and the thread of the bolt sheared right off, coming away in a spiral twist.

Rosenberg dug out a spare for her. “Take it easy,” he said. “Steel is brittle at these temperatures, remember.”

Next she fitted her skis to her feet. These were just slats of hull metal, with opened-out overshoes fixed to them for boots.

After stowing the snowshoes and lashing up the sleds, they resumed again.

It took her a few hundred yards to get used to the skis, and the half-sliding action they required. There was a lot of work for her knees and ankles, against the stiffness of her suit. Soon her joints were aching, and muscles on the back of her calves were announcing their existence with pulses of stiffness and pain.

But when she settled into a new rhythm, she seemed to make faster and easier progress than before. To haul the sled she was able to lean steadily into her traces, rather than having to yank at the sled as she’d had to over the gumbo, and that smoothed out the pains in the pressure points on her shoulders, hips and waist.

This haul was never going to be easy, but it was, she conceded, a relief to be free of the clinging of the gumbo.

For a while she felt almost exhilarated.

She reached the mouth of a wide, shallow gully.

In the white light of her helmet lamp, the gully walls were blue-grey, and there was a scattering of loose ice on the floor.

The slope further up was undulating — it was gathering itself into a series of huge, frozen waves — and the gully, although it got steeper and more narrow towards the end, seemed to offer the easiest route forward.

She looked back. A way below, Rosenberg’s gumbo-streaked suit and the yellow-grey canvas on his sled were easily visible against the orange-grey ice.

She pushed up into the gully.

She could feel loose granules crunch beneath her weight. But it didn’t get any easier to pull the sled; the friction actually increased, and she felt the granules grind beneath her skis.

She bent down, stiffly, and scooped up a handful of the ice granules. They were hard and round, nothing like the snow of Earth. On the surface they were loose, but a little deeper they stuck together — presumably thanks to a surface layer of organics — to form pebble-sized chunks that she could crush in her hand. What the geologists called duricrust, she thought.

She took a look at the runners on her sled. There were fine grooves scratched into the runners’ base by the ice crystals. She knew that at normal human temperatures, sleds and skates worked by melting a fine layer of water at the top of the ice, and then sliding across, lubricated by the water, with almost no friction. Here, these small, hard crystals wouldn’t melt under the pressure of her runners; they were like grains of sand, and what she was doing was more like dragging a sled across a desert.

She felt an unreasonable, crushing disappointment. They just got no breaks with the conditions here.

As her climb up the gully wore on, the gradient increased in severity. Soon she was free of the clinging granules, but now, to her irritation, the ice grew too slick. She just couldn’t win. Sometimes her skis slipped backwards at every step, and the only way she could proceed was to tack back and forth at forty-five degrees to the slope, which added a lot of extra distance to the whole.

Even so, soon she had risen so far that when she looked back the ground was hidden in the haze. And still the slope continued above her, eroded and ancient, up into the orange mists above. She got her head down and climbed on. She forced any thoughts of the future — even the pleasure of getting her boots off, or the distance they still had to cross — out of her head.

At last, she reached the top of the steepening gully. The landscape opened out before her. She seemed to have reached a plateau.

The ice at her feet was jumbled and cracked. And when she looked ahead, she saw a sprawling mass of ice, locked in suspended animation. Waves of ice, which must have been a hundred feet tall, reared up, caught in the instant of crashing against each other. Huge open chasms showed dark against the grey-white mass. There was noise here, too: deep groans resounded from the belly of the ice, pulsing back and forth across the broken landscape. Each ice wave was carved, sometimes into elaborate shapes, with fluted channels and sharp crests. The giant shapes marched to the close horizon, so big they were visible as they receded over the curve of the world, like ships sailing over a frozen sea.

A layer of methane cloud, dark and threatening, lay like a lid over the shattered icescape, obscuring the haze and merging with the ice at the close horizon into a complex band of grey, black and orange.

After some minutes, Rosenberg came staggering up the gully after her. He leaned against his sled, breath rattling, and stared out at the ice sea, which was reflected in his visor.

“Pressure ice,” he said. “Paula, I think this whole continent is a giant magma extrusion, distorting one whole side of the moon. It’s like the Tharsis Bulge on Mars. Maybe such features are common on small worlds like this… And all this ice is flowing slowly outwards and downwards from the magma extrusion, a huge, continent-sized viscous relaxation.”

Benacerraf looked around with new understanding. The pressure ridges were ice waves, magnified by Titan’s low gravity, frozen in time. She shivered, feeling dwarfed in space and time. If she could accelerate her perception — if she could live for a million years — she would see the ice flowing thickly away from Rosenberg’s magma mound, like warm icing off a wedding cake.

After a couple more minutes they pushed on, Benacerraf leading again.

She tried to select a route which would take them threading between the worst of the pressure ridges.

The waves took a variety of forms. Some of them were sharply defined ridges, some of them rounded hummocks; some took still more exotic shapes — rounded boulders, even torpedo shapes, forms out of nightmares, mounted on eroded, fragile-looking pillars that looked unable to hold up all that mass, low gravity or not.

Ways through the ridges were winding and uneven. She tried at first to use her skis, but the paths were too narrow and twisting, and the skis just got in the way. She took them off and stowed them in her sled. The paths were covered besides by an uneven layer of loose granules, difficult to judge; sometimes the granules crunched beneath her boots, taking her weight before bottoming out, but sometimes she would find her heel thudding against ice as hard as rock, concealed by a quarter-inch of gritty granules. Her sores and blisters chafed. Her sled bumped and rattled over the surface, every step a jarring uncertainty, and her harness dragged over her shoulders and waist, burning her. She found herself growing nostalgic for the miles of compliant, sticky gumbo.