The Leak and the Fill

The balloon was fat but not fat enough, and the helium tank had gone quiet again.

"It's leaking," Maya said. She had her ear against the valve. "Not the balloon. The tank. Listen."

Soren listened. A thin hiss, so soft it was almost the wind. "That's the fitting. Helium's the worst for that. Smallest atom except hydrogen. It finds every gap."

"Then how does anything hold it?"

"Nothing really does. Not forever." He tightened the fitting with the little wrench, and the hiss got quieter but did not stop. "Helium sneaks through rubber. Through metal, if you wait long enough. It's why balloons go sad overnight."

Maya sat back on the cold roof gravel and looked at the balloon, which was straining upward like it wanted to be somewhere else. "So we're never going to fill it all the way. It leaks out as fast as we pump it in."

"Not that fast."

"But some."

"Some," Soren agreed.

She was quiet, watching the balloon tug. Then she said the thing she said when a small annoying fact had grown teeth. "Where does it go? The helium. When it gets out."

"Up. It's lighter than air."

"Up to where?"

Soren opened his mouth to say the sky, and stopped, because the sky is not an answer, the sky is a place you have to keep following. "Up past the air, I guess. If nothing catches it."

"Does anything catch it?"

He thought about the tank, hissing patiently in his hands. He thought about how the whole point of a fitting was to hold a thing that did not want to be held. "The Earth catches it. Gravity. That's the fitting. That's the whole tank."

Maya turned and looked at him. "But helium sneaks through everything. You just said. Smallest atom except hydrogen."

"Yeah."

"So it sneaks through gravity too. A little. The fast ones."

Soren felt the roof get bigger under him. He put the wrench down so he could think with both hands empty. "Up high, the air's hot from sunlight. Hot means the atoms move fast. And the lightest ones move fastest, the hydrogen and the helium. If one's going fast enough, near the top, where there's almost nothing to bump it back down."

"It gets out," Maya said. "The planet's leaking." She said it the way she'd said it about the tank, except now her voice had gone up at the end, delighted and a little alarmed. "The whole thing. Same as this. Same as the balloon overnight."

"It has to be. Same rule. There's no fitting tight enough for hydrogen and helium. Not the tank, not the Earth."

"How much?"

"I don't know how much."

"Guess."

Soren looked up. The launch clouds were coming in from the west, orange underneath from the streetlights. "A lot. It's a big planet. But it's been leaking for billions of years and there's still air." He frowned. "That's the part that doesn't work. If it leaks and never stops, and it's had four and a half billion years, the tank should be empty by now. We should be standing on the Moon. No air at all."

They both looked at the balloon. It had not gone flat. It was, in fact, still tugging.

"So something's filling it," Maya said. "While it leaks. Something's got a hose on the other side."

"Where would the hose be?"

Maya did the thing she did, which was answer before she knew how she knew. "Down. And out."

"Down and out."

"The tank refills from somewhere. Ours doesn't, ours just empties, that's why it's sad. But the Earth's tank isn't sad after four billion years. So it's got two hoses we don't." She counted them off on cold fingers. "One down. One out."

Soren picked the wrench back up, not to use it, just to hold something. "Down is inside the planet. Volcanoes." He said it slowly, testing whether it broke. It didn't break. "Every eruption, that's gas coming out of the rock. Steam and carbon dioxide and stuff that was locked in the inside. The inside's been breathing out this whole time. Refilling from below."

"And out?"

"Out is—" He stopped. He looked up again, at the sky that was not an answer, and this time he kept following it. "Rocks. Stuff falls in. Meteors. All the little ones you never see, burning up. They're bringing mass in. Every night. Tons of it."

Maya was on her feet now. "So the sky's leaking out the top and raining in the top at the same time. And the ground's breathing up from the bottom. All three, all the time." She spread her arms like she was trying to hold the whole roof. "And it almost balances. It has to almost balance, or we wouldn't be here to fill a stupid balloon."

"Almost," Soren said. "Not exactly. The leak and the fill aren't the same tap. There's no reason they'd match perfectly."

"So it's close. Really close. On purpose but not on purpose." She laughed, the alarmed delighted laugh. "Every breath you take has been part of the leak. Some of the air up there right now is packing its bags. And some of it fell in from space last Tuesday."

Soren wrote that down. His hand moved fast in the cold: the leak and the fill, two taps, almost. Volcano up. Meteor down. Helium out the top forever.

"Read it back," Maya said.

"The planet is a tank with a slow leak and two hoses, and nobody's watching the gauge, and it's held for four billion years anyway."

"That's the part that gets me," Maya said. "Nobody's watching the gauge."

They finished the fill. The balloon rose fat and tugging, and Soren felt the string go tight, felt the whole thing wanting up, up, past the roof, past the clouds, past where the fitting of the planet could hold it.

"Let it go?" Maya asked.

"Not yet. The sensor's not on."

But she was already watching the top of the balloon, where the helium was, right now, in absolute silence, beginning to find its way out through the skin. Beginning the trip it was always going to make. She put her hand flat under the balloon and felt it pull, patient and certain, toward the leak at the top of the world.