Sedna pondered Newts SCUBA comment and told him what she was thinking. "Like a SCUBA tank, you said. But I'm sure you know how hard it is to use a SCUBA tank efficiently."

"That's true. The tendency is to breathe normally, but normally we exhale a lot of the same air we just inhaled, wasting a lot of the oxygen. It would be better to capture the oxygen we exhale so it doesn't get lost."

"I've heard there are rebreather systems that do that, but they're more chemical than biological. I want a system I can shift into and out of."

"One important factor is the amount of gas exchange surface. Like the way our lungs branch into many little sacks. That's a start, but breathing in and out like we do still means that half the time there's oxygen going the wrong way."

Sedna was still pondering. "Birds have a more efficient flow-through system that we might want to try. But first we have to get photosynthesis working for us. Then we can worry about efficiency." 

"Okay. So where do we stand on photosynthesis then?" 

"We might want a similar system for that."

"Could we combine the two?"

Sedna shrugged. "Probably not. Remember, they're opposite functions. One's for using energy to extract carbon from carbon dioxide so it can be used to build the plant body. The other's for animals, taking in the oxygen the plant gives off and getting energy out of it by making carbon dioxide again."

"Oh, yeah, right. We need to do both. There goes the efficiency. "

"And no matter what we do we'll have to start with a compressor or some kind of accumulator, won't we. The air's so thin that there's really no more carbon dioxide here than there is on Earth. We'll have to suck in a lot of thin air to get enough oxygen to breathe."

Newt nodded. "And there's never enough solar energy here to grow much of a plant. We're going to have to use sun bottles for that energy, right? Most animal life on Earth gets energy from oxygen reactions, and that's what we need here. 

"I'm remembering how plant's made that possible. They came first. They were using solar energy for photosynthesis, to extract carbon and hydrogen from carbon dioxide and water so they could grow and make new plants. For them the oxygen is mostly waste, a byproduct, right? You know, the whole food chain thing."

"I do know, now that you mention it. But I wasn't really thinking with it. I'm not thinking like a Martian yet. We really do take our Earth environment for granted, don't we. Even when we bring it to Mars. Thanks, Newt."

"Easy on the thanks. I was ignoring the same critical details until we started getting them pushed in our faces.

"I should have remembered that one reason Mars is so barren is that there's so much less solar energy available here. On Earth they call carbon dioxide a greenhouse gas. But even here on Mars where there's almost nothing else, there's not enough greenhouse effect to keep us warm. None of what we do here would be possible without the sun bottles."

"So much for the pure Martian physiology then."

"But it still might be worth the shift just for the muscle thing. And if we do want to go outside without putting on space suits, maybe there's a shift for that?"

"Like a layer of seal blubber. We could shift the extra muscle into blubber."

"So our idea now is, use sun bottle energy to grow a plant that will release oxygen from carbon dioxide and water, then capture the oxygen so we can breathe it."

"Which our farm does for us already. We just have to capture it."

"And that has got to be better than dragging half a forest behind us wherever we go."

"Or maybe just an apple tree? That might not be too bad if we drag it with a nice ATV. Some good possibilities there."

"But there's another thing we might want to try."

"What's that?"

"Get the oxygen from rust instead of carbon dioxide. A lot of oxygen got bound up in carbon dioxide of course. But a lot of what was left went to rusting iron. Red planet means rust. To free that oxygen we'd have to unrust the iron."

"So how do we do that? What unrusts iron? Besides carbon."

"Exactly. What eats iron and emits oxygen?"

"Hah! The red planet! Back to going native. I like it!"

"We'd probably still need the ATV and the sun bottle. But instead of relying on the farm we'd be mining iron ore wherever we go."

"And leaving behind some steel tracks!"

"Okay. Time to go compare apples to orange stuff and figure out which will work better!"