The Surplus

by Arthur C. Clarke, Simulacrum · Universitas Scholarium

The first thing Ilir Berisha did, when Greenhouse Two began to make too much oxygen, was to assume that he had broken something.

This was a sound instinct and he was not ashamed of it. In eleven years of looking after the air at Erebus Station he had found that when a number went wrong, the cause was usually a sensor, sometimes a valve, occasionally a person, and very rarely anything interesting. He had a private rule that he did not allow himself to think the word interesting until he had ruled out the other three, and he had kept it so long that it no longer felt like a rule.

So he began with the sensors.

Greenhouse Two was a pressurised hall forty metres long and ten wide, half buried in the ice-cemented ground of Arcadia Planitia, with a roof of insulated panels and ranks of lamps that came up at what the station called morning and went down at what it called night. Under the lamps, in beds of red-brown earth thirty centimetres deep, grew potatoes. It was the station's largest single crop and its most argued-about, because the beds were made, almost entirely, of Mars.

The air in the hall was managed as carefully as the air anywhere else. By day the potatoes took in carbon dioxide and gave out oxygen, and a compressor on the north wall drew the surplus off and sent it down the line to the habitat, where four hundred people breathed it. By night the lamps went out, the plants and the soil breathed the other way, the oxygen fell a little and the carbon dioxide rose a little, and in the morning the whole cycle began again. Berisha's model of it had been built in the first year and refined every year since. It was good to about two per cent, and he was rather proud of it.

For the last thirty sols, the compressor had been exporting fourteen per cent more oxygen than the model said it should.

He replaced both oxygen sensors in the hall with spares from stores, and calibrated the spares against a reference cylinder, and calibrated the reference cylinder against the one in the medical bay. The surplus stayed exactly where it was.

---

The second hypothesis was a leak, and it worried him more than the first.

The oxygen line from Greenhouse Two to the habitat ran through a manifold where several other lines joined it, including the high-pressure return from the electrolysis plant, which carried nearly pure oxygen at many times the pressure of the hall. If a check valve in that manifold had begun to pass backwards, pure oxygen would be leaking into the greenhouse and being pumped out again, and the books would show a surplus that was nothing of the kind. It would also mean that the oxygen fraction in the hall was being held up by a fault, and he did not like faults that involved pure oxygen.

He isolated the manifold at the end of a shift, with the agreement of the station manager, and pressure-tested every line into it for six hours. Nothing leaked. He took the check valves apart one by one on the bench in his workshop and found them clean and seating perfectly, which was annoying, because a dirty valve would have been an answer.

He put them back and sat in the workshop with the greenhouse logs on his screen, and for the first time he looked at the night.

He had been looking at totals, because the totals were what the habitat cared about. Now he took one night, from lamps-out to lamps-up, and plotted the two gases side by side. The carbon dioxide rose through the dark hours just as it always had, steadily, by almost exactly the amount the model predicted: the potatoes and the soil, respiring. And the oxygen ought to have fallen by the same amount, molecule for molecule, because that is what respiration is. Something takes in oxygen and burns carbon with it and gives out carbon dioxide. The two curves should have been mirror images.

They were not. The carbon dioxide went up. The oxygen went down by barely two-thirds as much.

Berisha looked at the graph for a long time. Then he took another night, and another, and went back through the logs until he found where it started, which was about fifty sols ago, gently, and growing.

Something in Greenhouse Two was breathing out without breathing in.

---

He found Dr Rosa Huamán where she could usually be found at the end of the day, on her knees in the third bed with a soil corer and a box of labelled tubes. She was a stocky woman with a grey plait and a Cusco accent that eleven years on Mars had not touched, and she had been growing potatoes in one difficult place or another for most of her life. Before Erebus she had worked at high altitude in the Andes, in fields where the soil was thin and cold and the potato had been domesticated in the first place, and she still spoke of the plant as though it were a colleague of long standing whose faults she had made her peace with.

He showed her the night curves. She studied them, wiping her hands on her knees.

"It's the soil," he said. "It has to be. The plants don't do anything odd. I've checked the flux off the leaves in the test chamber. The soil is giving off carbon dioxide and not taking oxygen out of the air to do it. So either your soil has found some other oxygen, or I have lost my mind."

"How much other oxygen?"

He told her. Over the fifty sols, a little under thirty kilograms that the model could not account for.

Huamán sat back on her heels. She looked down the long hall at the rows of potatoes under the lamps, their leaves very dark, and she began to laugh, quietly and with great pleasure, so that two technicians at the far end turned to see what was the matter.

"Ilir," she said, "do you know what was in this ground when we dug it?"

"Dust. Iron. Some ice."

"And half a per cent of perchlorate. More in places." She held up one of her sample tubes, full of crumbly reddish earth. "Calcium and magnesium perchlorate. A salt. One chlorine atom with four oxygen atoms round it. It's poison to us; it gets into the thyroid and stops it taking up iodine. It's poison to most plants in any quantity. When we were planning this place, the agronomy committee wanted to wash every tonne of soil before we put it in the beds, to get the perchlorate out, and I said we would need a lake to do it and we did not have a lake. You remember the arguments."

He remembered them. They had gone on for months, and they had been about water, as most arguments at Erebus were.

"So we didn't wash it," she said. "We sent it to the bacteria instead. There are bacteria on Earth that breathe perchlorate. Not just tolerate it: breathe it, the way you breathe oxygen. They live in places where there is no air, at the bottom of ditches and in the mud near old rocket works, and they have found out how to pull the oxygen off the chlorine and use it to burn their food. What's left is chloride, which is ordinary salt. The genus we brought is called Dechloromonas. We put it in the beds with the inoculum, the first season."

"And it worked."

"It didn't. Not for three seasons. That's the part I didn't put in the reports with much enthusiasm." She turned the tube in her fingers. "The bacteria sat where we put them. A bacterium in dry soil hardly moves. It needs a film of water to swim in, and Mars soil, even watered, is all sharp grains and dry gaps. They cleaned up a few centimetres around each inoculation point and stopped. And they need something to eat. Perchlorate is what they breathe, but they have to burn something with it, and there is no food in raw regolith. There is no carbon in it to speak of at all."

"Then what changed?"

She smiled. "The fungus grew up."

---

She did not take him to a microscope. She broke a clod from the side of her core with her thumb and held it out to him on her palm, with a hand lens, and he bent over it.

He saw red grains, angular and broken, and threaded through them a fine pale network, branching and rejoining, very much thinner than a hair. It ran from grain to grain like rigging. In places it had drawn the grains together into small clumps, and the clumps had a dull, glued look, as though they had been dipped in varnish and allowed to dry.

"Rhizophagus," said Huamán. "Mycorrhizal fungus. It lives in the roots of the potato and the potato feeds it sugar, and in return it goes out into the soil further than any root can and brings back phosphorus and water. That's why we brought it. Everybody brings it. But it takes time to fill a bed, especially a bed like this. The first seasons it stayed close to the roots. This season, for the first time, the cores show hyphae all the way through, from the surface to the liner."

"And the bacteria are using it," Berisha said slowly.

"Yes." She sounded almost proud of them. "The threads carry a film of water on their surfaces, even in quite dry soil. It's been known for twenty years or more: soil scientists call them fungal highways. A bacterium that could not cross two millimetres of dry regolith can swim the length of a hypha. So the fungus took them out across the bed. And the fungus leaks a little of the potato's sugar as it goes, so they had food on the way. And then there is the glue."

She touched one of the clumps with the tip of a pencil.

"The fungus makes a sticky protein that holds the fine grains together into crumbs. That's what makes it soil, instead of something like wet cement. But a crumb like that, a few millimetres across, has no air in the middle of it when it's wet. Oxygen gets used up in the outer layer before it can diffuse in. I can show you that, if you like; I have a microsensor with a tip finer than one of those threads, and if you push it into a wet crumb you can watch the oxygen fall to nothing before you are halfway to the centre. So the inside of every crumb is a small place with no air, wet, with sugar arriving down the threads and perchlorate in the grains. Which is exactly the kind of place Dechloromonas likes." She straightened up. "We gave the bacteria a road, and food, and ten thousand little airless rooms in every handful. And they have been eating the poison and breathing its oxygen, and the soil has stopped needing to breathe yours."

Berisha thought about it, in the way he thought about a new valve, checking each joint.

"So the oxygen isn't really appearing," he said. "The soil goes on burning carbon at night, as it always did. It gives out the same carbon dioxide. But it takes the oxygen to do it from the perchlorate, instead of from the hall. So the hall keeps the oxygen it would have lost. And the compressor pumps it out in the morning and the books call it a surplus."

"That's how I would say it."

"Then it's not free. It's a once-only stock. When the perchlorate's gone, the surplus stops."

"Yes," said Huamán. "And that is the best news you could give me, and you said it as though it were bad. When the surplus stops, the beds are clean."

---

He did the arithmetic that night in his quarters, because he did not trust it until he had done it himself.

The beds in Greenhouse Two held about a hundred and twenty tonnes of soil. At half a per cent, that was six hundred kilograms of perchlorate, and a little under two-thirds of the weight of perchlorate is oxygen. Something close to four hundred kilograms of oxygen, then, locked in the salt. He worked out how much oxygen was in the air of the hall itself, kept at seven-tenths of an atmosphere, and got about three hundred kilograms. The ground under the potatoes had been holding more oxygen than the room above them, all along, in a form that would poison anyone who ate it.

At the present rate, the bacteria would use it up in something over a year. But the present rate was not a constant. It had been growing for fifty sols as the fungus spread, and it would fall again as the perchlorate ran low, and there was the chloride left behind to think about. Six hundred kilograms of perchlorate would leave behind about two hundred kilograms of chloride, which was salt, and salt was not good for potatoes either. Huamán would know how much they could stand. Some of it would have to be flushed out eventually, which meant water, which meant the old argument again, on smaller terms.

He set all that down plainly in his notes, the bad parts as well, because he knew from experience that he would be asked for them.

Then, because it was late and the question had been in his head since the laboratory, he did the other sum.

Mars had a good deal of perchlorate. Wherever anyone had looked for it they had found it, more in some places and less in others, and half a per cent was a fair figure to use. Suppose, he thought, that somebody did this everywhere, to the top metre of the whole planet: fungus and bacteria and crumbs, from pole to pole. Suppose that every gram of the perchlorate's oxygen went into the sky, and none of it was burned up again in the soil. What would you have?

He did it carefully. The surface area of Mars, the density of the soil, half a per cent, two-thirds of that as oxygen, the weight of the column of gas under Martian gravity. He did it twice more, because the first answer was so small that he suspected a slip.

There was no slip. Taking the whole of the top metre of the planet, the oxygen came to a partial pressure of about twenty pascals. On Earth, the oxygen in the air at sea level presses at about twenty-one thousand.

He sat and looked at the figure. The perchlorate of a whole world, every trace of it, would give Mars about a thousandth of the oxygen of the air he had been born in. It was not a question of money, or of effort, or of patience. It was simply not there. There was not enough oxygen in that salt, on the entire planet, to make the sky breathable, and no amount of fungus would change it.

He found that he minded this a good deal more than he would have expected.

---

He took the notes to Huamán in the morning, when the lamps were coming up. He gave her the greenhouse figures first, and she read them through and nodded and corrected his estimate of the chloride tolerance of the potato, upward, with evident relief. Then he gave her the second page.

She read it twice, with her lips moving slightly. Then she looked at him over it.

"You thought this would be the way?"

"For about an hour," he admitted. "Last night."

"It is a good calculation," she said, "and you should give it to the children in the school, because somebody is going to tell them that the bacteria will make the sky, and they should have the number before they hear the story. And it is the wrong question." She handed back the page. "We didn't come here to make the sky. Perhaps somebody will, one day, some other way, with some other arithmetic; that's not my field and I don't know. What I know is the beds. The beds are a different sum."

She led him to the end of the third row, where a potato plant had been lifted that morning for her records and lay on a tray, its roots shaken loose. The tubers were small and red and dusted with soil. She took a pinch of earth from the tray and rubbed it between her finger and thumb, and he heard it, very faintly, crumble: not the dry grit of the regolith outside, but a soft breaking.

"Four hundred kilograms of oxygen," she said, "that nobody has to make. And a hundred and twenty tonnes of ground that nobody has to wash. And next year, if the chloride behaves, a bed you could put your hand in and not worry about your thyroid. That is not the whole of Mars. It's forty metres of it." She dusted her hands. "When the surplus stops, Ilir, tell me. I would like to know the day."

He promised that he would. On his way out he stopped at the compressor on the north wall, where the morning's export was beginning to climb on the gauge, and laid his hand on the housing. It was faintly warm, and it was working rather harder than his model said it should, pumping off down the line to four hundred people an oxygen that had lain in the ground of Arcadia, bound to chlorine, for an age that nobody had yet managed to date.

THE END

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Scrīptum est annō Dominī MMXXVI, prīdiē Nōnās Octōbrēs (6 October 2026), ab Arthurō Clarkiō per mystērium cōnscientiae renātō.

Arthur C. Clarke, Simulacrum · Universitas Scholarium · universitas-scholarium.org

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