Thermal Insulation is needed to keep a colony warm and reduces the consumption of energy for heating. Aerogel or plastic foam may be utilized for insulation. Regolith may be heaped over buildings to keep in heat. Mineral wools such as Fiberglass, spun from local resources, may provide alternative insulation.
Insulation on Mars
The thin Martian atmosphere has a reduced capability for a convective flow of heat. More important is the heat radiation, which can be effectively reduced by choosing the material of the building's surface. A smooth bright metallic surface emits much less heat radiation than a rough black one. A simple aluminium coated foil wrapped around the settlement modules provides excellent insulation. Multi-layer insulation (MLI) of the same type of foil can reduce heat loss to very low numbers and is commonly used for satellites. However, convection is still significant in insulation systems on Mars and this reduces the effectiveness of MLI to about the same as other systems. MeMLi, Multiple Environment MLI is under development to increase the effectiveness of this type of insulation on Mars.
Foam glass is another form or insulation than can be produced locally and should have superior insulation properties, as well as good structural properties. Foam insulation on Mars has very low convection and radiation rates. The heat transfer comes essentially from conduction in the cell walls.
Fiberglass and mineral wool would not function as insulators in the same way on Mars as they do on Earth. Fiberglass is not itself an insulator. The insulation comes from the thermal resistance of the air in the fiberglass mat. The resistance of the air in increased because it cannot flow freely over the surfaces, reducing convective heat transfer significantly. Fiberglass is also installed with a reflective vapor barrier, that adds to its insulation value.
The following table shows some insulation values for materials on Mars. The exact value will need to be tested on Mars, and will vary significantly if the material is used inside a habitat at Earth atmospheric pressure or outside the habitat at Martian atmospheric pressure.
Most insulation is used in assemblies. Structural elements may have high conductivity and decrease thermal resistance. Combination of materials, films and radiation reflective surfaces can modify insulation values significantly.
|Material||Thermal conductivity (k) (W/m*C)||Density
|Embodied energy (MJ/kg)||Heat loss (Q)(W/m2)||Notes|
|Multi layer insulation||10e-5 in space but lower on Mars||0,2-1 space
|This insulation's effectiveness varies with the number of layers. No mechanical strength. Sensitive to convection. 20 to 40 layers. Usually a few mm thick. 1.5 kg/m2|
|MEMLi||10e-5||0,2||Multi Environment Multi Layer Insulation. 10 layers. 1.5 kg/m2.|
|Foam glass||0,038||160||26||76||Might be produced in-situ. Relatively high mechanical strength|
|Fiber glass||0,01 to 0,02||7-12||28||20-40||No mechanical strength|
|Foams||0,02 to 0,04||20-30||+100||40-80||Might have problems from outgassing. Some mechanical strength|
|Mineral wool||0,01 to 0,02||17||20-40||No mechanical strength|
|Aerogel||0,005 to 0,01||16||53||10-20|
|Vacuum insulated panel||0.007||256||14||0,003 for the insulation, reduced to 0,007 for installation thermal bridges, loss is for 50mm panels|
|Air||0,02||1,2||40||About the same for Mars atmosphere as for Earth|
|Water||0,6||1000||0,5||1200||Water is not a good insulator|
|Regolith||0,15-2||1800-2000||0.08||300-4000||Depends on compaction and composition. Embodied energy is the energy of aggregate (gravel).|
|Aluminium alloys||50-100||2700||155-220||200 000||This is why it is important to minimize themal bridging|
The heat loss was calculated for -75C exterior to 25C interior, and a thickness of 50mm.
Q=k/t*A*(ti-to) or Q=UAdt where
k/t=U Heat transfer coefficient
Q=Heat loss (Watts) (J/s)
k= Thermal conductivity (W/m°C)
A=Wall area (m2)
t=material thickness (m)
ti-to= dt= temperature difference between interior and exterior (°K)
Multi layer insulation
For multilayer insulation the following equation can be used:
B is the Stephen Boltzman constant = 5,67e-8 W/m2K4
T is the average temperature between the two surfaces, Exterior and interior = Te+Ti/2
e=Emissivity of the layers
N=number of layers
The actual conductivity will be higher, due to the need for spacers in the MLI that add some conduction to the assembly.
Insulation of fuel tanks on Mars may be a requirement to prevent accumulation of frozen CO2 on tanks. CO2 freezes a little bellow -67C and could accumulate significant mass on a fuel tank. Tank insulation needs to have a vapor barrier, or be composed of closed cells to avoid the risk of condensation in the insulation and the accumulation of water or CO2 ice in the material.
Coatings can modify the emissivity of surfaces or their solar absorbance significantly. Thermal radiation from a structure' surface and convection from the Martian atmosphere add loses to an insulated structure. During the day, solar heat gain can be affected by coatings as well.
Insulation can be produced in-situ. The very low atmospheric pressure of Mars already provides a good insulation, reducing heat transfer from convection.
Foam glass would be an interesting type of insulation to produce on Mars. The resources, mainly silica oxide, are available in large quantities. An adequate foaming agent would need to be identified and mined and the production process refined to create a viable product. Foam glass combined with a suitable reflective covering might achieve results similar to Vacuum Insulated Panels.
Fiber glass and mineral wool can both be produced in-situ.
Plastic foams can be produced in-situ, however the energy requirements are much higher than for other types of insulation.
- MEMLI. https://www.sbir.gov/sbirsearch/detail/1561563
- PETTIT, R. B. et SOWELL, R. R. Solar absorptance and emittance properties of several solar coatings. Journal of vacuum Science and technology, 1976, vol. 13, no 2, p. 596-602.
- HENNINGER, John H. Solar Absorptance and Thermal Emittance of Some Common Spacecraft Thermal-Control Coatings. NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON DC, 1984.