Based on previous in situ measurements, the Martian atmosphere has been determined to be composed predominantly of carbon dioxide (95 percent), with nitrogen, argon, and oxygen (all nontoxic) present in abundances greater than 0.1 percent (Owen, 1992). There is a small amount of toxic carbon monoxide (0.07 percent), as well as traces of ozone up to 0.2 ppm. Scientists have used ultraviolet and infrared remote sensing techniques to search for a variety of candidate trace gases in the Martian atmosphere. No organic molecules or toxic gases, such as those containing N, S, P, or Cl, have been detected down to limits of 0.01 ppm (Owen, 1992). Even if a habitat were vented completely to the Martian atmospheric pressure of 0.6 kPa (6 mbar) and then refilled with the habitat's breathable mixture at 100 kPa (1 bar), the dilution factor would be over 160. In this scenario, the astronauts would be exposed to less than 0.6 percent carbon dioxide, 4 ppm of carbon monoxide, and 1 ppb ozone by volume. All of these amounts are well below the current NASA standard for these toxic gases. In addition, the atmospheric revitalization systems on spacecraft include systems for removing carbon dioxide and contaminants. The committee expects that the same capabilities would be provided in a human habitat on Mars. In addition, any highly reactive species, such as hydroxide radicals or other highly oxidizing species, created by photochemical processes in the Martian atmosphere by ultraviolet radiation would quickly evolve to less-hazardous chemical forms upon coming into contact with habitat airlock surfaces. Thus, sufficient knowledge is already available to ascertain that the Martian atmosphere does not pose a toxic risk for astronauts, and no further characterization is required. Long-term oxidizing effects on materials continuously exposed externally is a separate problem, as discussed earlier in this chapter.
If NASA chooses to measure the oxidation properties of the Martian atmosphere, the committee recom-mends&—as it did with respect to measuring the oxidation properties of soil and airborne dust&—that this measurement be done on the surface of Mars rather than via a sample return. The committee has the same concerns&—that is, that the oxidants might dissipate during a sample return transfer unless the sample is maintained in near-Martian conditions during transit. If NASA chooses to measure the oxidation characteristics on Mars, the committee recommends exposing a variety of materials, such as space suit material, to the Martian atmosphere and to assess the effects of superoxidants or other radicals on the materials.
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