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Apparently, Lunar regolith will melt if you expose it to moderate power microwave energy.

Based on the accepted answer to the referenced question, it has to do with the presence of iron in the regolith.

Since iron is known to be a major part of the Martian regolith, do we expect that Martian soil will also melt easily by microwave radiation?

It seems like making "Martian bricks" by microwaving soil would be very useful. One of the "waste products" of this process would presumably be steam from any water-ice that was present in the soil before you processed it - and you'd get bricks that could be used for building, of course.

Can Martian soil be easily melted with microwave radiation?

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    $\begingroup$ Whether or not it can, the presence of perchlorates in Martian regolith means that it's probably a bad idea to use a standard microwave, in a human habitat at least, so you'd probably want a purpose-built one--at which point the context for "easily" changes a bit. $\endgroup$
    – called2voyage ♦
    Commented Mar 3, 2023 at 19:59
  • $\begingroup$ @called2voyage - microwave is just EM radiation. You could make a pretty cheap phased array and let the waste escape to the Martian atmosphere. I'm mostly interested in the potential for in-situ building materials, sinceI think the "metal and glass bubble" habitats of science fiction are... largely going to remain in fiction. They'll be too heavy to transport, especially if we can make Mars-crete with just a couple solar panels, a phased array, and a robot smart enough to operate a shovel. ;-) $\endgroup$
    – codeMonkey
    Commented Mar 3, 2023 at 20:18
  • $\begingroup$ I'm planning on asking a second question about the challenges we'd face recovering the water in a couple of days, assuming this question is well received. $\endgroup$
    – codeMonkey
    Commented Mar 3, 2023 at 20:20
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    $\begingroup$ Easily is a relative term @codeMonkey $\endgroup$
    – Starship
    Commented Mar 3, 2023 at 22:28
  • $\begingroup$ If you're trying to make bricks, would you neccessarily be melting regolith rather than some other less energy-intensive method? $\endgroup$
    – ikrase
    Commented Mar 4, 2023 at 5:23

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Answer: Not really. Martian regolith may be heated with microwaves. Considering this “easy” is a stretch. The two most common components of regolith are certainly not “easily” heated with microwaves.

According to https://labs.seprosystems.com/what-happens-when-you-microwave-rocks ,

“Some rocks, like those made of silica, are microwave transparent. And so the microwaves will pass through … For other materials that are highly magnetic, the materials are too conductive and microwaves will be reflected. However, materials with semi-conducting properties, such as sulfides and different metal oxides, heat very well in response to microwaves. Because ores are typically composed of different phases with different microwave properties, some phases heat while others do not.” Blockquote

Martian regolith is mostly composed of silica (it’s concentration is divided by 10 in the graph below) enter image description here

https://en.wikipedia.org/wiki/Martian_soil

So, the major component of regolith is silica, which is transparent to microwaves. Bad news for a Martian microwave smelter. Impurities may improve absorption above that of pure silica material.

The next must common component is ferrous oxide. Ferrous oxide absorbs microwaves poorly until heated to 550-1100*C

Bottom line: the minerals which make up the bulk of Martian regolith are poor absorbers of microwaves. A work-around would be to heat the regolith “conventionally” with direct solar thermal energy before turning on the microwave oven. This would bring the ferrous oxide up to a temperature at which it could absorb the microwaves. But if you need to build a solar thermal smelter, why not let it do the whole job? Solar panels, energy storage and microwave generators seem overly complex if direct solar thermal can do the job.

Addendum: This article on lunar regolith explains that FeO has been converted to metallic (reduced) Fe in the top few mm of the Lunar regolith. This was done by protons in solar radiation. This Fe is what allows Lunar regolith to absorb microwaves. Martian regolith would be exposed to lower levels of radiation and Martian wind may agitate the surface enough that significant levels of metallic Fe does not accumulate without first oxidizing in the scant atmosphere. Although there is very little oxygen in Mars atmosphere, there seems to be enough to oxidize surface minerals.

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    $\begingroup$ Its useful to remember the difference between melting and smelting. Smelting is the process of chemically reducing oxidized ore using fossil fuels, hydrogen or electricity. Melting is a phase change, but does not achieve reduction. Melting regolith will not produce metallic iron. $\endgroup$
    – Woody
    Commented Mar 4, 2023 at 14:20
  • $\begingroup$ I worried that exposed to radiation was key to the Lunar regolith's high microwave absorption. It appears that fear was founded. $\endgroup$
    – codeMonkey
    Commented Mar 6, 2023 at 16:14
  • $\begingroup$ Now I'm getting greedy - Alumina appears to be the third most common material, what's the microwave absorption profile for that? The "microwave rocks" link said metal oxides heat well, but my google-fu is mostly returning stories about aluminum foil (Al, not Al2O3) in the kitchen microwave. $\endgroup$
    – codeMonkey
    Commented Mar 6, 2023 at 16:24
  • $\begingroup$ @codeMonkey ... calm yourself, Grasshopper. It is easier for a camel to pass through the eye of a needle than to reduce a group IIIA metal oxide. That's why aluminum is smelted by electrolytic reduction. Melting does not =smelting. $\endgroup$
    – Woody
    Commented Mar 6, 2023 at 16:38
  • $\begingroup$ There are proposals to process aluminum oxide from regolith via electrolysis, further reading here. $\endgroup$
    – Cadence
    Commented Mar 6, 2023 at 16:50
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Leapfrogging from the comment by Erin Anne and the answer by Woody.

Using raw, unprocessed Martian regolith will most likely not be successful as Woody states in his answer. However, with preprocessing of the regolith to remove the unwanted silicates and concentrate the desired iron based and sulfide based minerals it appears a product that could be melted by microwaves could be produced. Experimentation would be required.

Firstly, regolith is not of uniform size distribution, it contains material that will range in size from a speck of dust to pebbles and boulders. Use a screen to remove the large items, picking a number, say anything above 10 mm.

Everything which is sized below 10 mm is crushed and then ground to produce a powder of size, picking a number, 1 mm.

Using a batch of cyclones, in series and the atmospheric air as the transporation medium, each cyclone can take the heavier underflow from the previous cyclone and concentrate the heavier minerals and thus remove the lighter silicates in a staged process.

The final product will have a larger contraction of iron and sulfides than the unprocessed regolith. It can then undergo additional processing, if required, and if possible, to remove other unwanted minerals. The end product can then be zapped with microwaves to bake a brick.

enter image description here

Other equipment that could be trialed, with or without cyclones, are Reichert Cones or Gravity Spirals, but I envisage cyclone would most likely be more successful.

Alternatively, if what you want is magnetic then magnetic separation techniques could be used to separate the magnetic minerals from the non magnetic ones using devices similar to what is used in mineral sands processing and the processing of some rare earth minerals.

The simplest methods, used to separate magnetic mineral sands uses a magnetic roller at the end of a conveyor belt. The magnetic mineral stay close to the roller and the belt and fall off close to the roller. The non magnetic minerals fly off the end of the conveyor belt onto a pile farther away from the roller.

enter image description here

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  • $\begingroup$ Cyclonic separators don't work in a hard vacuum and would have difficulty coping with the medium-vacuum of Martian atmosphere. $\endgroup$
    – Woody
    Commented Mar 4, 2023 at 14:02
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    $\begingroup$ Ferrous oxide (FeO, the form of iron in regolith) is antiferromagnetic, unlike Ferric oxide (Fe2O3) and Magnetite (Fe3O4). I suspect it cannot be separated magnetically $\endgroup$
    – Woody
    Commented Mar 4, 2023 at 14:21
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It should be possible to melt Martian regolith by applying enough energy, however is this really the best use of limited electrical resources?

It would take a good deal of experimentation to refine, but I suggest a much better method would be to separate the regolith by particle size and the re-blend an appropriate mix of those particle sizes and then then apply high pressure to form a brick. This process could be aided by the addition of small quantities of water or other waste products to help the binding process.

It has been reported that some of these bricks made using simulate regolith are fairly strong and could be used for a range of purposes such as outer radiation protection for a habitat, blast screens and landing pads

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