"Sulfur is triboelectric. Sledge picks up charges. Burton's brain is intact.

Language is data. Radio is medium. Am machine."

"I don't believe you."

Trudge, drag, trudge, drag. The world doesn't stop for strangeness. Just because she'd gone loopy enough to think that Io was alive and a machine and talking to her, didn't mean that Martha could stop walking. She had promises to keep, and miles to go before she slept. And speaking of sleep, it was time for another fast refresher–just a quarter-hit–of speed.

Wow. Let's go!

As she walked, she continued to carry on a dialogue with her hallucination or delusion or whatever it was. It was too boring otherwise.

Boring, and a tiny bit terrifying.

So she asked, "If you're a machine, then what is your function? Why were you made?"

"To know you. To love you. And to serve you."

Martha blinked. Then, remembering Burton's long reminiscences on her Catholic girlhood, she laughed. That was a paraphrase of the answer to the first question in the old Baltimore Catechism: Why did God make man? "If I keep on listening to you, I'm going to come down with delusions of grandeur."

"You are. Creator. Of machine."

"Not me."

She walked on without saying anything for a time. Then, because the silence was beginning to get to her again, "When was it I supposedly created you?"

"So many a million of ages have gone. To the making of man. Alfred, Lord Tennyson."

"That wasn't me, then. I'm only twenty-seven. You're obviously thinking of somebody else."

"It was. Mobile. Intelligent. Organic. Life. You are. Mobile. Intelligent. Organic.

Life."

Something moved in the distance. Martha looked up, astounded. A horse. Pallid and ghostly white, it galloped soundlessly across the plains, tail and mane flying.

She squeezed her eyes tight and shook her head. When she opened her eyes again, the horse was gone. A hallucination. Like the voice of Burton/Io. She'd been thinking of ordering up another refresher of the meth, but now it seemed best to put it off as long as possible.

This was sad, though. Inflating Burton's memories until they were as large as Io.

Freud would have a few things to say about that. He'd say she was magnifying her friend to a godlike status in order to justify the fact that she'd never been able to compete one-on-one with Burton and win. He'd say she couldn't deal with the fact that some people were simply better at things than she was.

Trudge, drag, trudge, drag.

So, okay, yes, she had an ego problem. She was an overambitious, self-centered bitch. So what? It had gotten her this far, where a more reasonable attitude would have left her back in the slums of greater Levittown. Making do with an eight-byten room with bathroom rights and a job as a dental assistant. Kelp and talapia every night, and rabbit on Sunday. The hell with that. She was alive and Burton wasn't–by any rational standard that made her the winner.

"Are you. Listening?"

"Not really, no."

She topped yet another rise. And stopped dead. Down below was a dark expanse of molten sulfur. It stretched, wide and black, across the streaked orange plains. A

lake. Her helmet readouts ran a thermal topography from the negative 230°F at her feet to 65°F at the edge of the lava flow. Nice and balmy. The molten sulfur itself, of course, existed at higher ambient temperatures.

It lay dead in her way.

They'd named it Lake Styx.

Martha spent half an hour muttering over her topo maps, trying to figure out how she'd gone so far astray. Not that it wasn't obvious. All that stumbling around.

Little errors that she'd made, adding up. A tendency to favor one leg over the other. It had been an iffy thing from the beginning, trying to navigate by dead reckoning.

Finally, though, it was obvious. Here she was. On the shores of Lake Styx. Not all that far off course after all. Three miles, maybe, tops.

Despair filled her.

They'd named the lake during their first loop through the Galilean system, what the engineers had called the "mapping run." It was one of the largest features they'd seen that wasn't already on the maps from satellite probes or Earth-based reconnaissance. Hols had thought it might be a new phenomenon–a lake that had achieved its current size within the past ten years or so. Burton had thought it would be fun to check it out. And Martha hadn't cared, so long as she wasn't left behind. So they'd added the lake to their itinerary.

She had been so transparently eager to be in on the first landing, so afraid that she'd be left behind, that when she suggested they match fingers, odd man out, for who stayed, both Burton and Hols had laughed. "I'll play mother," Hols had said magnanimously, "for the first landing. Burton for Ganymede and then you for Europa. Fair enough?" And ruffled her hair.

She'd been so relieved, and so grateful, and so humiliated too. It was ironic. Now it looked like Hols–who would never have gotten so far off course as to go down the wrong side of the Styx–wasn't going to get to touch rock at all. Not this expedition.

"Stupid, stupid, stupid," Martha muttered, though she didn't know if she were condemning Hols or Burton or herself. Lake Styx was horseshoe-shaped and twelve miles long. And she was standing right at the inner toe of the horseshoe.

There was no way she could retrace her steps back around the lake and still get to the lander before her air ran out. The lake was dense enough that she could almost swim across it, if it weren't for the viscosity of the sulfur, which would coat her heat radiators and burn out her suit in no time flat. And the heat of the liquid. And whatever internal flows and undertows it might have. As it was, the experience would be like drowning in molasses. Slow and sticky.

She sat down and began to cry.

After a time she began to build up her nerve to grope for the snap-coupling to her airpack. There was a safety for it, but among those familiar with the rig it was an open secret that if you held the safety down with your thumb and yanked suddenly on the coupling, the whole thing would come undone, emptying the suit in less than a second. The gesture was so distinctive that hot young astronauts-intraining would mime it when one of their number said something particularly stupid. It was called the suicide flick.

There were worse ways of dying.

"Will build. Bridge. Have enough. Fine control of. Physical processes. To build.

Bridge."

"Yeah, right, very nice, you do that," Martha said absently. If you can't be polite to your own hallucinations ... She didn't bother finishing the thought. Little crawly things were creeping about on the surface of her skin. Best to ignore them.

"Wait. Here. Rest. Now."

She said nothing but only sat, not resting. Building up her courage. Thinking about everything and nothing. Clutching her knees and rocking back and forth.

Eventually, without meaning to, she fell asleep.

"Wake. Up. Wake. Up. Wake. Up."

"Uhh?"

Martha struggled up into awareness. Something was happening before her, out on the lake. Physical processes were at work. Things were moving.

As she watched, the white crust at the edge of the dark lake bulged outward, shooting out crystals, extending. Lacy as a snowflake. Pale as frost. Reaching across the molten blackness. Until there was a narrow white bridge stretching all the way to the far shore.

"You must. Wait," Io said. "Ten minutes and. You can. Walk across. It. With ease."

"Son of a bitch!" Martha murmured. "I'm sane."

In wondering silence, she crossed the bridge that Io had enchanted across the dark lake. Once or twice the surface felt a little mushy underfoot, but it always held.

It was an exalting experience. Like passing over from Death into Life.

At the far side of the Styx, the pyroclastic plains rose gently toward a distant horizon. She stared up yet another long, crystal-flower-covered slope. Two in one day. What were the odds against that?