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Water (H2O) is a material which can be obtained via in situ resource utilization on Mars. It is a trace component of the Martian atmosphere, and appears relatively abundant as ice in the Martian polar regions. Hydrates and subsurface water deposits also occur over substantial portions of the planet. Liquid water is vital to living organisms, and water vapor is a waste product of the respiration processes of living organisms.

Plants require liquid water for life support, as well as some minimum humidity level to regulate their metabolic processes. Clean water is important for the crew, both for drinking and sanitation.

Water vapor can be used to quench less energetic reactions, or as a reducing agent for energetic reactions such as the water-gas shift. Ice can be used as a building material at Martian surface temperatures. Liquid water is especially versatile. It can be electrolyzed to produce oxygen and hydrogen. Carbon dioxide can be added to liquid water to form carbonic acid, which can be reacted with hydroxides to form carbonates. Soluble salts can be added to water to form an excellent coolant. The most important application of liquid water is as a solvent, particularly for use by living organisms. As such, it is useful for everything from bathing to soil reclamation. Liquid water is also readily recycled.

Storage of Water

Large amounts of water are best stored as liquid water or ice, depending on the temperature. Given a source of heat, liquid water is readily produced on demand from ice. A purification system must be provided for potable water, including a source of iodine or a similar disinfectant. Water stored for other uses need not be disinfected, and water containing undrinkable levels of solutes (grey water) can still be stored for later use in other applications.

Simple cooling to Martian surface temperatures is sufficient to solidify water at any pressure. The thermal expansion of water during freezing is significant (up to 10% during the liquid/solid transition alone), and this should be considered in designing storage.

Water Purification

Although liquid water will store for short periods laden with high solute concentrations, liquid water is preferably purified for long term storage.

For small amounts of water under cabin conditions, filtration or distillation can be used for purification. Re-usable small pore filters exist to remove coarse particulates from water, and porous ceramics can be used for smaller particulates if necessary. Solutes can also be removed from liquid water by use of reverse osmosis filters, which operate under pressure. Filtration alone cannot remove some volatile solutes, however, and chemical reagents may be required. Distillation and fractionation will remove all non-volatile solutes from water, and is relatively simple to accomplish by boiling at Martian atmospheric pressure. More complex fractionation can be used to further remove volatile components such as ammonia. These methods can be used on small scales to reclaim water for the cabin life support system.

The addition of a disinfectant such as iodine can prevent bacteria growth during long term storage and keep water safe for drinking.

Larger quantities of water can be pre-treated prior to purification by bioremediation, chemical remediation, or by slow freezing.

Bioremediation is the use of living organisms to break down or absorb contaminants. Plants and soil bacteria are able, over time, to eliminate or sequester solutes that other methods of treatment cannot remove, such as unfilterable ions and volatiles.

During the freezing of water, its molecules are rearranged into a crystalline structure. For slow freezing processes, the crystalline structure of water will preferentially exclude solutes, including volatiles. The solutes are slowly concentrated in the unfrozen portion of the solution. The process can be repeated by draining the unfrozen concentrate, then melting and refreezing the frozen portion, or by storing it and later using smaller scale filtration and distillation techniques to further treat the ice. Thus, thousands of liters of water can be purified by simply leaving the tank outdoors overnight.


Physical Properties

Mean Molecular Weight 18 g/mol
Melting Point 0 oC

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