But I am ahead of my story. This takes us into the engineering side of ecology. Ganymede was bare rock and ice before we came along, cold as could be, and no atmosphere to speak of—just traces of ammonia and methane. So the first thing to do was to give it an atmosphere men could breathe.

The material was there—ice. Apply enough power, bust up the water molecule into hydrogen and oxygen. The hydrogen goes up—naturally—and the oxygen sits on the surface where you can breathe it. That went on for more than fifty years.

Any idea how much power it takes to give a planet the size of Ganymede three pressure-pounds of oxygen all over its surface?

Three pressure-pounds per square inch means nine mass pounds, because Ganymede has only one third the surface gravitation of Earth. That means you have to start with nine pounds of ice for every square inch of Ganymede—and that ice is cold to start with, better than two hundred degrees below zero Fahrenheit

First you warm it to die freezing point, then you melt it, then you dissociate the water molecule into oxygen and hydrogen—not in the ordinary laboratory way by electrolysis, but by extreme heat in a mass converter. The result is three pressure pounds of oxygen and hydrogen mix for that square inch. It's not an explosive mixture, because the hydrogen, being light, sits on top and the boundary layer is too near to being a vacuum to maintain burning.

But to carry out this breakdown takes power and plenty of it—65,000 Btus for each square inch of surface, or for each nine pounds of ice, whichever way you like it. That adds up; Ganymede may be a small planet but it has 135,000,000,000,000,000 square inches of surface. Multiply that by 65,000 Btus for each square inch, then convert British thermal units to ergs and you get:

92,500,000,000,000,000,000,000,000,000,000 ergs.

Ninety-two-and-a-half million billon quadrillion ergs! That figure is such a beauty that I wrote it down in my diary and showed it to George.

He wasn't impressed. George said that all figures were the same size and nobody but a dimwit is impressed by strings of zeroes. He made me work out what the figure meant in terms of mass-energy, by the good old E = MC² formula, since mass-energy converters were used to give Ganymede its atmosphere.

By Einstein's law, one gram mass equals 9x10²⁰ ergs, so that fancy long figure works out to be 1.03x10¹¹ grams of energy, or 113,200 tons. It was ice, mostly, that they converted into energy, some of the same ice that was being turned into atmosphere—though probably some country rock crept in along with the ice. A mass converter will eat anything.

Let's say it was all ice; that amounts to a cube of ice a hundred and sixty feet on an edge. That was a number I felt I could understand.

I showed my answer to George and he still was not impressed. He said I ought to be able to understand one figure just as easily as the other, that both meant the same thing, and both figures were the same size.

Don't get the idea that Ganymede's atmosphere was made from a cube of ice 160 feet on a side; that was just the mass which had to be converted to energy to turn the trick. The mass of ice which was changed to oxygen and hydrogen would, if converted back into ice, cover the entire planet more than twenty feet deep —like the ice cap that used to cover Greenland.

George says all that proves is that there was a lot of ice on Ganymede to start with and that if we hadn't had mass converters we could never have colonized it. Sometimes I think engineers get so matter of fact that they miss a lot of the juice in life.

With three pressure-pounds of oxygen on Ganymede and the heat trap in place and the place warmed up so that blood wouldn't freeze in your veins colonists could move in and move around without wearing space suits and without living in pressure chambers. The atmosphere project didn't stop, however. In the first place, since Ganymede has a low escape speed, only 1.8 miles per second compared with Earth's 7 m/s, the new atmosphere would gradually bleed off to outer space, especially the hydrogen, and would be lost— in a million years or so. In the second place, nitrogen was needed.

We don't need nitrogen to breathe and ordinarily we don't think much about it. But it takes nitrogen to make protein—muscle. Most plants take it out of the ground; some plants, like clover and alfalfa and beans, take it out of the air as well and put it back into the ground. Ganymede's soil was rich in nitrogen; the original scanty atmosphere was partly ammonia—but the day would come when we would have to put the nitrogen back in that we were taking out. So the atmosphere project was now turned to making nitrogen.

This wasn't as simple as breaking up water; it called for converting stable isotope oxygen-16 into stable isotope nitrogen-14, an energy consuming reaction probably impossible in nature—or so the book said—and long considered theoretically impossible. I hadn't had any nucleonics beyond high school physics, so I skipped the equations. The real point was, it could be done, in the proper sort of a mass-energy converter, and Ganymede would have nitrogen in her atmosphere by the time her fields were exhausted and had to be replenished.

Carbon dioxide was no problem; there was dry ice as well as water ice on Ganymede and it had evaporated into the atmosphere long before the first homesteader staked out a claim.

Not that you can start farming with oxygen, carbon dioxide, and a stretch of land. That land was dead. Dead as Christopher Columbus. Bare rock, sterile, no life of any sort—and there never had been any life in it. It's a far piece from dead rock to rich, warm, black soil crawling with bacteria and earthworms, the sort of soil you have to have to make a crop.

It was the job of the homesteaders to make the soil.

See how involved it gets? Clover, bees, nitrogen, escape speed, power, plant-animal balance, gas laws, compound interest laws, meteorology—a mathematical ecologist has to think of everything and think of it ahead of time. Ecology is explosive; what seems like a minor and harmless invasion can change the whole balance. Everybody has heard of the English sparrow. There was the Australian jack rabbit, too, that darn near ate a continent out of house and home. And the Caribbean mongoose that killed the chickens it was supposed to protect. And the African snail that almost ruined the Pacific west coast before they found a parasite to kill it.

You take a harmless, useful insect, plant, or animal to Ganymede and neglect to bring along its natural enemies and after a couple of seasons you'll wish you had imported bubonic plague instead.

But that was the chief ecologist's worry; a farmer's job was engineering agronomy—making the soil and then growing things in it.

That meant taking whatever you came to—granite boulders melted out of the ice, frozen lava flows, pumice, sand, ancient hardrock—and busting it up into little pieces, grinding the top layers to sand, pulverizing the top few inches to flour, and finally infecting the topmost part with a bit of Mother Earth herself–then nursing what you had to keep it alive and make it spread. It wasn't easy.

But it was interesting. I forgot all about my original notion of boning up on the subject just to pass a merit badge test. I asked around and found out where I could see the various stages going on and went out and had a look for myself. I spent most of one light phase just looking.

When I got back to town I found that George had been looking for me. "Where in blazes have you been?" he wanted to know.

"Oh, just out and around," I told him, "seeing how the 'steaders do things."

He wanted to know where I had slept and how I had managed to eat? "Bill, it's all very well to study for your merit badges but that's no reason to turn into a tramp," he objected. "I guess I have neglected you lately—I'm sorry." He stopped and thought for a moment, then went on, "I think you had better enter school here. It's true they haven't much for you, but it would be better than running around at loose ends."