## 3 Results  
### Overview of Mars ISRU Technologies  
The NASA Mars Architecture Team (MAT) describes the latest NASA concept for the first human mission to Mars, involving a very short round-trip duration, with only a 30-sol surface stay for two crew, minimal surface infrastructure, and ascent propellants brought from Earth [10]. This contrasts with previous architectures that showed benefits for a long stay with ISRU [5]. We believe that a long stay mission will be implemented using Mars ISRU, whether in the first landing, or possibly only in secondary landings. The main feedstocks for processing on Mars are the atmosphere, ice imbedded in the regolith at suitably high latitudes, and regolith containing hydrated minerals. The main component of the Martian atmosphere is CO\({}_{2}\), with smaller amounts of N\({}_{2}\) and Ar, and traces of other gases (Table 1). Figure 1: Simplified timeline of highlights from the history of Mars ISRU, relevant NASA studies, and relevant events in relevant terrestrial technology. Red bar highlights demonstration on Mars. Our concept is that the initial human landing on Mars would be limited to the simple ISRU based on atmosphere only, which is immediately accessible without the challenge of remote, autonomous mining of regolith, transporting regolith, and processing regolith. In a second generation of missions, it might be possible to include processing regolith, either for hydrated minerals or for imbedded ice. There exist many Mars ISRU concepts, ranging from very likely practical, to likely second generation, to imaginative, futuristic ideas that might be pursued in the 22 nd century or beyond. Here, we focus on ISRU processing for the first- and second-generation missions. It seems likely that the early application of Mars ISRU will be restricted to highly feasible atmospheric processing for production of ascent propellants and life support. The main product would be oxygen. Less likely, the inert gases in the atmosphere might be captured and the nitrogen might be used as a diluent or as a feedstock for processing. The second-generation missions in the campaign are likely to include processing atmosphere and water, obtained either from ice deposits, or hydrated minerals in the regolith. Several hydrated minerals on Mars have a high H\({}_{2}\)O content (10%) and subsurface ice is abundant, albeit at various depths at higher latitudes [1; 11]. Availability of water would enable a range of possible chemical engineering processes involving carbon, hydrogen, and oxygen, leading to a variety of potential products (Figure 2). \begin{table}
\begin{tabular}{c c} \hline \hline
**Component** & **\%** \\ \hline CO\({}_{2}\) & 95 \\ Ar & 1.6 \\ N\({}_{2}\) &  
The abundant CO\({}_{2}\) can be converted into O\({}_{2}\) and CO by SOEC conversion or CO and H\({}_{2}\)O by reverse water-gas shift reaction (RWGS). The use of H\({}_{2}\) in the RWGS process is internal. All the H\({}_{2}\) is recycled. The O\({}_{2}\) can then be used as an ascent propellant and for breathing, and if N\({}_{2}\) is recovered, it can be processed, or used as a buffer gas. A significant amount of CO is produced, and in the likely simplest case, vented. If the SOEC process is utilized, there is no need to separate trace components from the CO\({}_{2}\) in the Mars atmosphere since the inert gases simply pass through the electrolyzer. However, if cryogenic compression is used, separation is easily accomplished [11]. Studies of the RWGS process appear to have utilized pure CO\({}_{2}\), so it is not clear whether pure CO\({}_{2}\) is needed. Separation would be needed to acquire the N\({}_{2}\) in the Mars atmosphere. Processing of nitrogen is likely to be delayed until the second generation of landings [12]. In second-generation applications of Mars ISRU, where indigenous H\({}_{2}\)O and CO\({}_{2}\) are both acquired, the use of CO as a feedstock can be further pursued along several avenues.