How would you make steel on Mars?

For most of history, steel was made using charcoal. You created a pile alternating charcoal and iron bars, and after the fire went out, there was a layer of steel on both sides of each iron bar. Beating them with hammers would break that layer loose. Then you worked just that part, heating and folding and hammering, and eventually you had your sword, or whatever. All of that was done with charcoal. The process didn't produce consistent steel, which is why swords (orr whatnot) varied quite a lot. It wasn't until the 18th century that they started using coal, or actually using coke. (Coke is to coal as charcoal is to wood.)

The process of turning iron ore (iron oxide) into steel (iron with some carbon in it) is to reduce the oxide with a reducing agent. This means letting the oxygen in the oxide form a bond with something of higher affinity than iron. Carbon is used because it works, it is available naturally, in pure form, and it is abundant; but otherwise, you can use electricity (ala aluminum reduction) or other agents (sulfur for example). The tricky thing is you must find this agent in pure form (non-oxide) or you will have to purify it from its oxide... which would be harder to do than purify iron from iron oxide in the first place. Carbon oxide also has a nice property that it is a gas, so it escape the reaction site and leave the iron behind; and any un-reactive carbon left in the iron end up strengthening it (steel alloy). So, carbon, in general, is pretty good. Carbon, as an element, is abundant in the universe. It is even more abundant than oxygen or iron. Our body, our food and anything living contain a lot of carbon. So, it's pretty hard to imagine a place where you can't find carbon. If the question is only that there is no pure carbon available naturally in the form of coke, coal or oil, there will always be char-coal from the biomass. Were I inhabit the world you describe, my solution would be to convert the biomass into carbon, then use the tried and true method of iron oxide reduction using carbon instead of inventing something new. That said, you can read up on the aluminum electrolysis process; it's less economically feasible, but it's would work for iron too. The process does use a carbon electrode, but the reduction (lysis) is done by the electricity.

http://www.youtube.com/watch?v=8tt7RG3UR4c

Well, yes, solar power definitely has the potential to create the heat for the necessary reactions to take place. But what are the reactions?

Honestly, I don't know. But I don't see why you would use any different reaction than you do here on earth with traditional methods. I am not a chemist, but I do remember balancing chemical equations in my college chem class, and they always looked like: [reactant 1] + [reactant 2] + ... + [reactant N] + heat = [product 1] + [product 2] + ... + [product N] They never said "at earth's gravity", or "presupposing that 'heat' is generated in a particular way". So, I concede that I don't know how to smelt steel from iron ore, but I don't see why your location or choice of fuel would be significant. If you need specific materials, then you need those materials, but if you have them, then I don't see the problem. If instead the question is "given a lack of certain materials on Mars, what other processes would result in similar metals to steel?" that's a different but also interesting question. Also, giant solar reflectors are cool.

If you're talking about doing this on Mars, forget about solar. Sunlight is only about 40% as bright as on Earth.

There are metal reducing anaerobic bacteria out there, such as http://en.wikipedia.org/wiki/Geobacter metallireducens, which reduce iron. Not all the way to metal that I've heard, but I know of no reason they couldn't with the help of a little judicious genetic engineering, if necessary. They do need hydrocarbon substrates, but those could be supplied by rock eating bacteria on a place like Mars pretty easily, I'd think. The oxygen could be released to the atmosphere as a contribution to terraforming, if your little bacterial communities were to be self-sustaining enough to spread across the planet on their own.

The reduction is sunlight at mars orbit isn't a problem. You just build a bigger reflector. There are several solar thermal systems in use in the world currently that certainly reach the tempature necessary for steel. See http://en.wikipedia.org/wiki/Nevada_Solar_One for more details on this. Of course you still have the problems of making the mirrors for a low tech society, assuming that is what you are talking about but that problem is not insurmountable, just not the way things evolved here and an array of parabolic reflectors is non obvious solution to the problem unlike just builidng a bigger fire IS an obvious solution. The other problem is iron metal smelting appears to be an accidental discovery of building a big fire and using iron bearing rocks for the fire ring to produce iron. For making steel the carbon content of pig iron is actually reduced from the smelted state, so using a non carbon heat source can actually solve some problems. Pure iron is really quite soft and not as useful as bronze for tools (really). Most of what we call iron is either mild steel or pig iron and the carbon makes the iron much harder but more brittle (this is why cast iron shatters-pretty high carbon content). Read the wikipedia entry on iron smelting (and you probably already have). The gravity is used to seperate different elements that are not wanted in the molten iron for modern blast furnaces. Before the industrial revolution, iron was often made without ever getting to a molten state. Bog iron is something you should look into for alternative sources and methods that would be accesible to a society without fossil fuel resources but with an active biosphere. I think a blacksmith using a solar collector would be fascinating as opposed to a charcoal or coal powered forge. The techniques would likely be different and the final result would probably be of much higher quality and more repeatable, unlike metal smelting on earth before the industrial revolution. Those burning fossil fuels can introduce a lot of unwanted contaminants to the iron alloy and really screw up your quality. But you would need a plausible source for the solar reflectors...maybe polished native copper? or some kind of glass with a copper or gold or silver backing (all these metals occur in the elemental state, although rarely).

You also need a solar tracking system for your solar collector. The sun moves in the sky, even on Mars. So you've got this humongous mirror, 2.5 times what it would need to be on Earth. Since the gravity on Mars is 38% of Earth, it weighs about the same as the Earth mirror. But the momentum doesn't change, and moving something that large in order to track the sun really isn't all that easy. This is not something you'd be doing with medieval technology and a horse or two.