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Io, the innermost moon of Jupiter, as big as Luna. The yellow slag world, awesome upchucking of a moon’s guts, regurgitation over and over until everything more volatile than sulfur has long since burned off. Sulfur, sulfur everywhere, and nary a place to stand. Four hundred live volcanoes bursting through the slag like angry boils, geysering sulfur dioxide hundreds of kilometers into the air. A moon with an interior hotter than Earth’s-and try putting your hand in front of the steam coming out of the volcanic vent on Nea Kameni, in the caldera of Santorini, to feel just how hot Earth is; it looks like the steam on your stove top, but you will quickly find it is three times hotter. Even though you snatch your hand away instantly, your skin will blister. And Io’s interior is thirty times hotter than that.

It looks it. A hellworld, flexed hugely in the immense tidal pull between Jupiter and Europa, almost torn apart. That’s gravity at work. Then also Jupiter’s radiation field is so vast and so strong that Io sizzles inside it; even Deinococcus radiodurans perishes in it. Nothing lives on Io.

Except humans, and the little suite of biota they carry everywhere they go. For it is possible to find islands of hard rock in the highlands of the enormous volcanoes, and bore into that rock, and hide a little station. A cube to hold Wang’s qube. Everything there must be triply protected, first by physical walls, then by a magnetic field strong enough to counteract Jupiter’s radiation; but this field itself would be enough to kill, so inside that field a Faraday cage is necessary, to protect you from your protection.

Descend in a blue magnetic aurora, a fire of electrons. Below, the moon spreads from a ball to a plain to a tumultuous mountainscape of overlapping volcanoes, the bulky cones hard to spot in all the overlapping swaths of yellow on tan on white on black on brick on bronze, swaths of every burnt color, but most of all, yellow. Here and there scattered rings of black or red or white reveal active vents, pouring out the guts of the interior in irregular circles around the vents; but most of the patches are much less regular, and taken altogether, the surface is a jumble that cannot be resolved by the eye into a topography. It is what it looks like, a molten world, a world on fire. The names humans have applied are redundant. Fire gods, thunder gods, lightning and volcano gods, every combustible deity, from Agni the Hindu god of fire to Volund the German blacksmith of the gods: all these names attempt to humanize the moon, but fail. Io is not a human place. The hard crust on its surface, cooled only by contact with the chill vacuum of space, is so thin that in many places it would not support a standing person. Some early explorers found this out the hard way: walking too far away from their lander, they plunged through the sulfurous ground into red-hot lava and disappeared.

We think that because we live on cooler planets and moons, we live on safer ground than that. But it is not so.

SWAN AND WANG

The station on Io holding Wang’s qube and its support team was located high on the flank of Ra Patera, one of the biggest mountains in the solar system. As the ferry descended, the tilt of Ra’s broad apron came to seem barely off the horizontal. The ferry dropped into a gap in a concrete pad, and a roof then closed over them; from then on they were mostly underground. Everything they could see of the moon on the station’s various viewscreens, and through the little windows in the station’s little conning tower, was part of the apron of Ra.

There were several people in the station’s bridge, up in the conning tower. None of them looked up at Swan and Wahram, nor at Wang when he came in.

Wang Wei turned out to be a round person with an innocuous manner. A real principal investigator, as Mqaret would have said: one of the system’s foremost experts on qubes. Sometimes such individuals were sovereign to quite remarkable little Ruritanias. Swan wondered if Alex had been right in her notion that the solar system’s balkanization was a deliberate but unconscious human reaction to qubes, some kind of resistance to their incipient power.

Wang greeted Swan and Wahram, and with a quick “Ah, thank you,” took the envelope from Alex that Swan offered to him. It appeared he had already known about it. He read the letter in it, then plugged the tab that fell from it into the nearest desk. He stared at the desk console for a long time, reading carefully, using a forefinger to keep his place.

“So sorry to lose Alex,” he said to Swan finally. “Condolences from the heart. She was the hub of our little wheel, and now we spin off like broken spokes.”

Surprised at this, Swan said, “She told me in her note to me that I should come to you. The messages were left for me in her study. A kind of contingency plan, I guess. And part of it was this envelope for you.”