“It’s not a question of what you allow or don’t. You have no authority on Io. This is our world.”

“This is nobody’s world! It’s a world of exiles and renegades, as you well know, being chief among them. My own group has taken refuge here.”

Hera said, “We let people live here who will, but we’ve been here the longest, and so we decide what happens here.” She went to Galileo’s side, and her friends moved as a group to stand between the two of them and the stranger.

Hera said to Galileo. “Welcome to Io. I was with you when they made their dive into the ocean of Europa. Do you remember that?”

“Not quite,” Galileo said uncertainly. Blue depths; a sound like a cry …

With a disgusted glance at Ganymede, she said, “Ganymede’s use of amnestics is crude, very much of a piece with the rest of his actions. I can perhaps return some of your memories to you later. But first I think it may be best to explain the situation to you a bit. Ganymede has not told you the full story. And some of what he’s told you is not true.”

She picked up the pewter box from the ground, and kept it in her arms as she led him away from the expostulating Ganymede and the group surrounding him. Despite Ganymede’s objections, Galileo followed her, interested to hear what she might say. He already knew that she was going to get what she wanted, no matter what. He had seen willful women before.

SHE WAS AT LEAST A HAND TALLER than he was, maybe a head taller. Walking uncertainly at her side, bouncing up and down, he had to grasp her arm to keep from falling. He let go when his feet were under him, then almost fell and had to grab her again. After that, he held on to her upper arm as if to the trunk of a grapevine. She did not seem to mind, and it helped him to keep up with her. After a while he found himself helplessly making various erotic calculations having to do with her obvious strength (the box she carried looked heavy)—calculations that caused his eyes to widen and his heart to pound. It was a little hard to believe she was human.

“You are well named,” he murmured.

“Thank you,” she said. “We name ourselves when we are young, at our rite of passage. That was a long time ago.”

When they reached the far arc of the little temple, she paused. He let go of her arm. From here they had a view down the shattered sulphurous side of the great volcano they stood on—a view immensely tall, and so broad in extent that he could see a distinct curvature to the horizon, and at least a dozen smaller volcanoes, some of them steaming, others blasting great white geysers into the black sky.

Hera waved at the awesome prospect in a proprietary way. “This is Ra Patera, the biggest massif on Io. Io is what you call Moon One, the innermost of the big four. Ra Patera is far taller than the tallest mountains on Earth, bigger even than the biggest mountain on Mars. We are looking down the eastern flank toward Mazda Catena, that steaming crack in the side of the shield.” She pointed. “Ra was the ancient Egyptian sun god, Mazda the Babylonian sun god.”

Galileo recalled the spotted surface of the sun as seen on the paper put under the telescope’s eyepiece. “It looks as if burnt by the sun, though we are so far from it. As hot as Hell.”

“It is hot. In many places, if you walked on the surface you would sink right into the rock. But the heat comes from inside, not from the sun. The whole moon flexes in the tidal stresses between Jupiter and Europa.”

“Tides?” Galileo said, thinking he had misunderstood. “But surely there are no oceans here.”

“By tides we mean the pull a body has on all the others around it. Every mass pulls everything else toward it, that’s just the way it is. The bigger the mass, the bigger the pull. So, Jupiter pulls us one way, and the other moons pull other ways. Mostly Europa, being so close.” She grimaced expressively. “We are caught between Jove and Europa. And all the pulls combine to warp Io continuously, first one way then another. We are therefore a hot world. Thirty times hotter than Earth, I have heard, and almost entirely molten, except for a very thin skin, and thicker islands of hardened magma like the one we stand on. The entire mass of Io has melted and been erupted onto its surface many times over.”

Galileo struggled to imagine a world regurgitating itself, molten rock flowing inside to outside, then sinking down to be melted and thrown up again.

“There isn’t a single drop of water left,” Hera went on, “nor any of the other light and volatile elements you are used to on Earth.”

“What is it made of, then?”

“Silicates, mostly. A kind of rock, mostly melted. And a lot of sulphur. That’s the lightest element not to have been burnt off, and being the lightest, it tends not to sink but to froth on the surface, as you see.”

“Yes. It looks like burnt sulphur.” He had seen pots of the stuff, bubbling in an alembic. He sniffed, but smelled nothing.

“Mostly sulphur, yes, or sulphur salts and sulphur oxides. Here we are near the triple point for sulphur, so it vaporizes when it erupts out of the interior, literally explodes on exposure to the vacuum. It can shoot out of a geyser and land more than fifty miles away.”

“I don’t understand,” Galileo confessed.

“I know.” She gave him a glance. “You are brave to admit it. Although very few people really understand.”

“I’ve noticed that.”

“Yes. Well, I’m not the one to tell you the details of the physics or chemistry involved. But I can tell you more about what you have seen here, and the person who brought you here. And why he and his group are acting as they are.”

“I would appreciate that very much,” Galileo said politely. It was always good to have potential alternative sources of patronage; sometimes one could then balance them, or pit them against each other, or otherwise use them to create a differential advantage, a leverage. “You said they brought me to Europa, and we descended into its ocean—it must be a very different world from here, I must say!—and they were hoping to stop others from descending, because that is a forbidden place. But we had something happen. Some kind of encounter. I almost remember; it was like a waking dream. I seem to recall we were somehow … hailed. By something living in the ocean. There was a noise, like wolves howling.”

“There was. Very good. I’m not surprised you remember it, despite the amnestics they gave you. Abreactions fire across the blocked areas by way of similar memories, so being here helps you to recall your previous visits.”

“Visits?”

“What I am surprised at is that Ganymede took you along on that incursion. It may be that he did not know the timing of the Europans’ descent, and had to include you in something that was not meant for you.”

“Ah.”

“I do know he’s been telling you that his group has brought you to our time to advise them on a matter of fundamental importance.”

“It seemed unlikely,” Galileo said with an unconvincing show of modesty.

She smiled briefly. “According to Ganymede, you are the first scientist, and as such, one of the most important people in history. Nevertheless, to ask your advice was not his reason to bring you here.”

“Then what was?”

She shrugged expressively, like a Tuscan would have. “Possibly he felt your presence would help him defend his actions on Europa. No one else on the council wanted to take the responsibility of interfering with the Europans. Ganymede took the position that what they were proposing was a dangerous contamination of a crucial study zone, so that stopping them would be the best scientific practice, and also the safest for humanity. He brought you forward in a prolepsis that he hoped would support that position.”

“Why should my presence matter?” Galileo wondered.

“I don’t know,” she admitted, frowning as she looked at him. “He’s created so many more analepses than anyone else that it’s hard to get a fix on what he is up to. I wonder if he mainly brings you here to change you, to cause you to do what he wants you to do back in your time. Even with the amnestics blocking your conscious memory, you are still changed here. Then again, when he has you here he flaunts his rashness with the entangler, and thus hopes to scare the council. Or perhaps he thinks you bolster his authority, as you are the first scientist. The patron saint of scientists, you might say. Or of Ganymede’s cult, anyway.”