| by Carlton C. Allen Lockheed Martin Engineering and Sciences Co. |  |
To date, humans venturing into spacehave relied exclusively on equipment and supplies carried fromEarth. This strategy is certainly appropriate for operationsin Earth orbit, or for stays of a few days on the surface of theMoon. However, the ability to effectively utilize local resources,to "live off the land," will prove vital for long termhuman habitation of the Moon and planets.
Our research has focused on the extractionof oxygen, a key example of in-situ resource utilization whichwill directly support early human presence on the Moon. Thisis because one of the largest elements in any rocket is the oxygenrequired to burn the fuel. Locally-produced oxygen for rocketpropulsion promises by far the greatest cost and mass saving ofany in-situ resource.
The samples returned by six Apolloand three Luna missions contain no free oxygen, nor any water,ice, or water-bearing minerals. All lunar rock and soil do, however,contain approximately 45 wt% oxygen, combined with metals or nonmetalsto form oxides. This oxygen can be extracted if thermal, electrical,or chemical energy is invested to break the chemical bonds. Overtwenty different methods have been proposed for oxygen extractionon the Moon.
The reduction processes, particularlythose which use hydrogen as the reducing agent, are the most technologicallymature. Oxygen which is chemically bound to iron in lunar mineralsand glasses can be extracted by heating the material to temperaturesabove 900°C and exposing it to hydrogen gas. The basic equationis:
This process results in release ofthe oxygen as water vapor. The vapor must be sepa-rated fromthe excess hydrogen and other gases and electrolyzed. The resultingoxygen is then condensed to liquid and stored. Experiments usingsamples of lunar ilmenite, basalt, soil, and volcanic glass havedemonstrated the required conditions and efficiency of this process.
Ilmenite -Most early work on lunar resources has focused on the mineralilmenite (FeTiO3) as the feedstock for oxygen production. Thismineral is easily reduced, and oxygen yields of 8-10 wt% (massof oxygen per mass of ilmenite) may be achievable. Ilmenite occursin abundances as high as 25 wt% in some lunar basalts. Maximumyields calculated as mass of oxygen per mass of rock thus rangefrom 2-2.5 wt%.
Basalt - Previousoxygen production experiments utilized lunar basalt 70035 whichwas crushed but not otherwise beneficiated. The sample produced2.93 wt% oxygen in a 1050°C hydrogen reduction experiment. Of the minerals in this rock, the most oxygen was extracted fromilmenite, with lesser amounts from olivine and pyroxene.
Soil - Oxygencan be produced from a wide range of unprocessed lunar soils,including those which contain little or no ilmenite. Figure 1shows oxygen yield measured for four lunar soils which were reactedwith hydrogen at 1050°C. The rate of oxygen extraction washighest during the first 30 minutes, but continued throughouteach 3 hour experiments. Oxygen yield from lunar soils is stronglycorrelated with initial iron content (Figure 2). The dominantiron-bearing phases in lunar soil are ilmenite, olivine, pyroxene,and glass. Each of these phases is a source of oxygen. Ilmeniteand iron-rich glass react most rapidly and completely. Olivineis less reactive. Pyroxene is the least reactive iron-bearingphase in lunar soil.
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Figure 1. Oxygen release with time for lunar samples 74220 (volcanic class), 71131 (mare soil), 12032 (mare soil), and 62241 (highland soil), reacted in hydrogen at 1050°C. |
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Figure 2. Oxygen yield as a function of initial iron content for lunar soils reacted in hydrogen at 1050°C. |
Volcanic Glass - Theoptimum feedstock for a lunar oxygen production process may bevolcanic glass. At least 25 distinct glass compositions havebeen identified in the Apollo sample collection. The iron-richspecies promise particularly high oxygen yields. The depositsampled by the Apollo 17 astronauts (sample 74220) is uniformlyfine-grained and friable, offering a feedstock which reacts rapidlyand can be used with little or no processing prior to oxygen extraction. Complete reduction of the total FeO content of this iron-richglass is equivalent to an oxygen yield of 5 wt%. Over 80% ofthis yield was achieved after 3 hours at a temperature of 1050°C(Figure 1).
The production of oxygen from lunarmaterials is now a reality. Oxygen release by means of hydrogenreduction has been demonstrated in the laboratory with samplesof lunar basalt, soil, and volcanic glass. Yields from soilsare predictable, based solely on each sample's iron abundance. The reactions are rapid, with most of the release occurring ina few tens of minutes. All of the major iron-bearing phases inlunar soil release oxygen, though with differing degrees of efficiency. These data can support the design of an oxygen production plantat a future lunar base.
For more details see:
- Allen, C. C., R. V. Morris, and D.S. McKay 1996. Oxygen Extraction from Lunar Soils and PyroclasticGlass. J. Geophys. Res. 101. 26,085-26,095.
