The Wrong Kind of Hot

The photograph had been printed wrong. That was what Soren thought at first.

He held it under the desk lamp and tilted it. The surface of Io looked like a pizza someone had dropped -- orange and yellow and brown, patched with white, with a dark smear near the top that the caption said was a volcanic plume caught mid-eruption. The plume was frozen in the image but you could feel the speed of it. Material blasted three hundred kilometers above the surface, the caption said. Three hundred.

Soren set the photo down and wrote the number in his notebook.

Maya was already three boxes ahead, pulling out photos and stacking them by date. She had a system. He had never fully understood it, but it worked.

"This one's weird," she said, not looking up.

Soren moved to her side of the table. She was holding a thermal image, false-colored so that hot areas glowed white and cold areas showed deep blue. Io was lit up like something was burning inside it. Dozens of white points scattered across the surface. Volcanic hotspots, each one a mountain of fire.

"So it's volcanic," Soren said. "We knew that."

"No." Maya put the thermal image next to a regular photograph. "Look at the size of it. And then think about what's inside it."

He looked. Io was roughly the size of Earth's moon. Small. Rocky. Old, the way all the moons out there were old.

"It should be cold," he said.

"It should be completely cold. The moon is cold. Mars is almost cold." Maya set both images down. "Things that size run out of heat. They just do."

Soren understood what she was pointing at. When rocky worlds form, they start hot, built from collisions and radioactive elements decaying in their cores. But small worlds lose that heat fast. They don't have enough mass to hold it. The moon had volcanoes once. Now it had nothing. Io was the same size as the moon.

Io had over four hundred active volcanoes.

"So where's the heat coming from," he said. It wasn't a question. It was him thinking out loud.

"That's the part," Maya said.

Soren went back to the caption on the first photograph. He read it again, more slowly. It mentioned Jupiter. It mentioned the other moons. It mentioned something called tidal flexing, but it was only one line and the line used the word simply in a way that made Soren trust it less, not more.

He found the reference binder on the shelf above the boxes. Dr. Vasquez had assembled it for the campaign, thick and overstuffed, and she had left a sticky note on the front that said do not reorganize this I know where everything is. He did not reorganize it. He paged through it carefully.

There was a diagram. Jupiter enormous in the center, Io small and orbiting close. But Io's orbit was not perfectly circular, because the gravity of the other moons, Europa and Ganymede, kept tugging it into a slight oval. So Io was sometimes closer to Jupiter and sometimes farther. And as it moved, Jupiter's gravity pulled at it differently, stretching it and releasing it, stretching and releasing, over and over, on every single orbit.

"Like squeezing a handball," Soren said aloud.

"What?"

He brought the binder to Maya's side of the table. He put his finger on the diagram. "The moons keep Io's orbit slightly off-circle. So Jupiter's gravity isn't pulling at a constant distance. It's pulling harder, then softer, harder, softer. And that squeezing --"

"Makes heat," Maya said. "Like bending a wire back and forth."

"Like bending a wire back and forth. Exactly like that." Soren paused. "Io is volcanically active because Jupiter is squeezing it. Not because of anything inside Io. Because of something outside it."

Maya went very still.

Soren recognized that stillness. He waited.

"Europa," she said.

He had not gotten there yet. He went back to the binder and found Europa's section. Europa was farther from Jupiter than Io, but the same mechanics applied, the same orbital pull from the neighboring moons, the same gravitational flexing. Except Europa had a thick shell of ice. And under the ice, the binder said, there was almost certainly a liquid ocean. Kept liquid not by the sun, which was too far away to matter much. Not by radioactive heat. By the same squeezing. By Jupiter.

He read that paragraph three times.

Then he sat down on the floor because the stool was too far away and this felt like a sitting-down kind of moment.

"There might be an ocean," he said, "that exists only because of the shape of an orbit."

Maya sat down too, on the other side of the table, her back against the cabinet.

"We look for life," Maya said slowly, "near stars. In the warm zone. The place where liquid water can exist because of sunlight."

"Yes."

"But if gravity can make that heat --" She stopped.

"Anywhere there's a big enough gravity well and the right orbital mechanics," Soren said, "you could have liquid water. It doesn't have to be near a star at all."

Neither of them said the next part. The galaxy had hundreds of billions of planets. Around those planets, moons. Most of those moons cold and dark and far from any sun. But if the moons were in the right orbital relationship, if something large enough was pulling at them, squeezing and releasing, squeezing and releasing --

Soren looked down at what he'd written in his notebook. The number three hundred, from the height of Io's volcanic plume. Just that number, sitting on the page.

Down the hall, the planetarium projector clicked and powered off. The ventilation hummed.

Maya picked up the thermal image of Io and held it above her head, looking up at all those white points of heat against the cold blue, and her arm stayed raised, perfectly still, the photograph glowing under the desk lamp like something that was trying to tell them it was only the beginning of the list.