The primary constituents of the Martian atmosphere are carbon dioxide (CO2), nitrogen (N2) and argon (Ar), whereas Earth’s are mainly N2, oxygen (O2) and Ar. Therefore, all the elements required to make breathable, Earth-like air are available in Martian air.
A Mars habitat may include inflatable extensions to significantly increase habitat volume beyond the size of the initial spacecraft. If the Hab is landed along with the MAV during a pre-deployment phase, it will be remotely activated from Earth, initiating the ISAP system, and causing the inflatable extensions to inflate. If the Hab is sent one launch window earlier than the crew, we will have about 20 months to inflate the Hab and test its other systems before the crew leave Earth. In order to inflate the extensions it will be preferable to make the necessary air using ISRU technology rather than bring air from Earth, which would require tanks of compressed O2 and N2.
The mass of the proposed ISAP system is likely to be less than the mass of full O2 and N2 tanks. More importantly, having the capability to manufacture breathable air from local resources will be a useful advantage for the mission, providing increased safety and reducing limitations on the mission. Locally manufactured air can compensate for air losses due to leaks, airlock cycling, atmosphere scrubbing, refilling of O2 and N2 tanks in pressurised vehicles and marssuits, punctures if they occur, and other potential causes.
The DRA proposes separating N2 and potentially also Ar from the Martian atmosphere for use as a buffer gas. However, it is actually much cheaper and easier to use the gas that remains after CO2 and dust is removed from Martian air, as this is mostly N2 and Ar, both of which are perfectly acceptable buffer gases. The Martian atmosphere may also contain undesirable toxic gases such as ozone (O3), carbon monoxide (CO), and nitric oxide (NO), that exceed safe limits (as specified in JSC 20584: Spacecraft Maximum Allowable Concentrations for Airborne Contaminants). However, these can be relatively easily scrubbed out using ordinary COTS catalytic converters, and this process is far simpler and energetically cheaper than trying to separate out pure N2 from the air. (Note that NO is not actually toxic, but when combined with O2 it rapidly oxidises to form NO2, which is.)
Step 1: Mars atmosphere is drawn into the ISAP system through a dust filter.
Step 2: Water is removed from the gas mix via zeolite adsorption. The captured water is stored in the Hab’s water tank and used to replace recycling losses, and for O2 production.
Step 3: Water is separated into H2 and O2 via electrolysis. The O2 is stored for habitat atmosphere.
Step 4: Microchannel adsorption or cryogenic separation (CO2 freezing) is used to separate CO2 (about 96%) from the gas mix.
Step 5: The CO2 is reacted with H2 via the reverse water gas shift (RWGS) reaction:
CO2 + H2 → CO + H2O
The H2O produced is returned to the water tank. The CO may be stored for use in fuel cells, or it may simply be vented.
The gas mix that remains after CO2 is removed is mostly N2 and Ar, with trace amounts of various gases, which may include neon (Ne), krypton (Kr), xenon (Xe), O3, CO, NO, methane (CH4), hydrogen peroxide (H2O2), and sulphur dioxide (SO2).
Step 6: This gas mixture is first passed through an ozone scrubber, which reduces the O3 to O2.
Step 7: The resultant gas mixture is then passed through an ordinary automobile 3-way catalytic converter, which converts any CO and NO into CO2 and N2.
Step 8: The result is a safe buffer gas comprised mostly of N2 and Ar, with small amounts of O2 and CO2, and traces of Ne, Xe and Kr. This mixture can be combined with additional O2 to provide breathing gas for the Hab. The CO2 level in this gas mixture is slightly higher than the proposed upper limit for the habitat atmosphere, but this excess will be removed by the ECLSS.
The DRA describes several methods for making O2 from CO2, including:
- SOCE (Solid Oxide CO2 Electrolysis)
- Sabatier reaction
- RWGS reaction
SOCE requires considerable energy, whereas the other two options require H2. Fortunately, H2 is available because the Hab will contain H2O for the crew, which makes SOCE’s less appealing than the other two alternatives.
RWGS, which produces CO and H2O, is preferred over the Sabatier reaction, which produces CH4 and H2O, because in RWGS all the H2 is converted to H2O, which is a valuable resource in its own right, and from which H2 can be easily recovered via electrolysis. Using the Sabatier reaction would either consume H2, or the H2 would have to be recovered from the CH4. In theory CH4 could be used in a fuel cell to provide additional energy to the Hab. Combustion of the CH4 would produce H2O, which could be captured. However, it remains to be seen if fuel cells will be relevant for the Hab, and this process would add a layer of complexity and inefficiency to H2 recovery. Choosing RWGS eliminates the need for any method to recover H2 from methane. An effective method for separating H2O from the gas stream existing the RWGS chamber is to adsorb it with zeolite 3A, as in step 2.
The DRA specifies that H2 be brought from Earth in order to make water for the crew; however, in the Blue Dragon architecture H2O is obtained from the atmosphere, and potentially also from the ground. Research into extraction of H2O from the Martian atmosphere (Grover et al, 1998; Williams et al, 1995) has shown how 3.3kg of H2O per day (enough to replace losses through life support regenerative processes) could potentially be obtained from the Martian atmosphere using adsorption into zeolite 3A. This idea has been incorporated into the ISAP process above, indicating that H2O necessary for the RWGS reaction need not necessarily come from the Hab’s supplies. The amount of H2O obtainable from the atmosphere may be small, but this doesn’t matter because predeploying the Hab allows plenty of time to collect the necessary H2O and make the air, and the H2 is recycled anyway.
More research needs to be done into this system. We need to investigate efficient gas separation techniques, the rate at which air can be manufactured, the unit’s mass, volume and energy requirements, and how it integrates with the ECLSS.