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Comments below this answer to the question Which is easier to build on mars per square kilometer; greenhouse windows or photovoltaics/LEDs? discuss the possibility of using plastic instead of glass for transparent greenhouse roofs on Mars in square kilometer quantities for a contributing role in food production there.

If one is shipping then from Earth, mass is important. The densities of common transparent glass and plastic materials useful for windows are around 2.4 and 1.2 g/cm^3, so you could ship twice as many panes of plastic as glass if the dimensions were equal.

But they won't be. The completed window assembly must support some pressure differential, and insulate against an infrared radiative heat loss to space through the thin Martian atmosphere in order to keep plants at their ideal temperature for maximum growth. At night you can use opaque insulating covers, but while the windows are letting in sunlight, you're going to be radiating into space. If you don't believe me, point an IR thermometer up into a blue sky during the day, but away from the sun. It will register c-c-c-cold!

Even in the UV received on Earth at sea level, many plastics will age in direct sunlight. The UV breaks the weaker of the organic bonds in the plastic. Results are solarization, crazing, and structural weakening. In the much stronger solar UV flux on the surface of Mars, this will be accelerated.

Are there any known transparent plastic candidates for an optimized "Mars plastic" that could replace glass as 15-year greenhouse windows on Mars? Would they be strong enough so that the total shipping weight per square kilometer would be lower than that for panes of optimized "Mars glass"?


below: "Lexan window after 7 years" (on Earth, of course), from Lexan vs. Plexiglass.

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below $\times$3: "This Plexiglass port is 38 years old." (on Earth, of course), from Lexan vs. Plexiglass.

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    $\begingroup$ An UV blocking filter above the plastic window may reduce the aging of the plastic. But the filter itself may age too. $\endgroup$ – Uwe Jun 20 '17 at 9:35
  • $\begingroup$ @Uwe sunscreen lotion for windows? :) Patent it now! er... no better patent it later and keep it a secret for now. Seventeen years is not very long. $\endgroup$ – uhoh Jun 20 '17 at 9:39
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    $\begingroup$ If you visit the chat my calculations show the ballpark of UV degradation on Mars is similar, or lower than that on Earth. UV irradiance is roughly twice that on Earth (no atmosphere but weaker sunlight), meanwhile oxygen, which is essential to the degradation, would be present only on the inside of the greenhouse (halving the affected area) and likely at lower pressure and lower partial pressure (further reducing the effect). $\endgroup$ – SF. Jun 20 '17 at 14:08
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    $\begingroup$ Also, plastics can be recycled and remade in a process less "industrial" than required by glass. (no need for high-temperature furnaces). OTOH, ISRU production of glass is more viable due to plastic requiring a plenty of hydrogen, which is not readily available on Mars. Silica is abundant, and processing it into glass is quite straightforward (if energy intensive). $\endgroup$ – SF. Jun 20 '17 at 14:11
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    $\begingroup$ Why should it be a single glass or plastic panel? Windows of armoured cars are multiple layers of glass and plastic. For a stable and light weight sunroof, a multilayer construction with honeycomb elements should be considered. Such a sunroof should not be perforated by small meteorites. An explosive decompression of the greenhouse may kill a gardener. To prevent the plastic layers from aging by UV, the top layer should be an UV blocking filter. But replacing a damaged window should be possible without evacuating the whole greenhouse. $\endgroup$ – Uwe Jun 20 '17 at 20:26
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In practise, a composite approach would be used, combining the strengths of different materials.

The required layers can be summarized as:

  • A bladder which contains the atmosphere
  • An open weave restraint (webbing) to reduce membrane stress in the bladder
  • A UV, abrasion and micrometeorite shield

The general construction technique (sans shield) is similiar to that used for super pressure balloons such as from Project Loon: Super Pressure Balloon from Project Loon.

The bladder needs to be transparent and have good tensile strength, the restraint should have excellent tensile strength. The tensile strength of polyethylene is only something like 30MPa whereas materials like Kevlar can have a tensile strength of around 3600MPa. A denser restraint results in reduced overall mass by taking up more of the stress with the strong restraint layer, but at the expense of reduced transparency.

The shield needs to be transparent, block UV, reasonably abrasion resistant and have a high probability of protecting the membrane from micrometeorites, these are not as much of a threat on Mars as they are in deep space, those which don't disintegrate during atmospheric entry will be travelling at terminal velocity, less than 1km/s. These could still be capable of puncturing a thin membrane and for a greenhouse with an area measured in hundreds of m^2 micrometeorite strikes are inevitable, though simply repairing any punctures would be a viable strategy provided the membrane is tear-resistant.

Additionally one of the layers should have a radiant barrier (low-e coating) the purpose of which is to trap infrared energy and maintain warmer night time temperatures. This typically consists of an exceedingly thin coating of metal such as silver, these barriers are fragile and degraded by oxygen so the coating should be on the outside of the membrane or the inside of the shield so it is protected from both dust abrasion and oxygen. The mass of this coating is negligible.

One study, linked below, estimates the mass of the bladder, restraint and shield: 1.22kg/m^3 for membrane, 0.66kg/m^2 for restraint

For an overall mass of around 3kg/m^2. This is not that much lighter than window glass, but a typical window is only designed to withstand wind loads of ~0.5kPa, not the approximate 30kPa of an inflatable greenhouse.

Some useful PDFs are linked below:

Inflatable Transparent Structures for Mars Greenhouse Applications (pdf)
Engineering concepts for inflatable Mars surface greenhouses (pdf)

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  • $\begingroup$ Interesting, thanks for the discussion and links. I don't see anything about a silver or metal film in those papers, and I don't see how that would function as a thermal barrier while also remaining thin enough to server as a window for a greenhouse. Is it possible to elaborate? $\endgroup$ – uhoh Dec 2 '18 at 20:22
  • $\begingroup$ I added a little to the answer, but it's basically your typical low-e coating that reflects far infrared (and maybe near infrared) so visible light can get in and infrared is trapped, this is particularly relevant to maintaining warm night time temperatures. Some greenhouse concepts involve an insulative cover that is deployed at night, which could fulfill this role. Also all greenhouses would likely require at least some active heating, if there's an excess of waste heat to be dumped into the greenhouse then there's less/no need to trap infrared. $\endgroup$ – Blake Walsh Dec 3 '18 at 9:22
  • $\begingroup$ Thanks again! I haven't heard of such a low emissivity coating that is made of silver and transparent in the visible but reflects far infrared, but as long as the plasma frequency is somewhere between vis and thermal IR it should work. I'll do some more reading about it. $\endgroup$ – uhoh Dec 3 '18 at 9:50
  • $\begingroup$ @uhoh it's really no different to the gold coating on spacesuit helmets, it's transparent because it's so thin, but the longer wavelength photons interact strongly with the metal atoms (like how microwaves can be blocked by a metal mesh) $\endgroup$ – Blake Walsh Dec 3 '18 at 13:16
  • $\begingroup$ This is interesting! Instead of working this out in comments, I've made more space for this: How does the thin gold film in the glass of spacesuit helmets block thermal IR but transmit visible? What's the physics? $\endgroup$ – uhoh Dec 3 '18 at 13:25

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