## 5 Conclusions  
ISRU has the instant appeal that it appears to be more efficient to produce resources on the Moon or Mars than to bring them from Earth. In both cases, the predominant initial target would be to produce ascent propellants in situ, particularly oxygen. The return on investment for any ISRU process depends on the amount of oxygen produced, the availability of feedstock, the practicality of the proposed process, and the relative cost of transporting the resources from Earth. Prior to the advent of MOXIE in 2014, the history of Mars ISRU development was sporadic and intermittent, and always at low TRL. In 2014, NASA took a bold, imaginative, unprecedented step to fund a major project in Mars ISRU (\"MOXIE\"), in which an oxygen production plant based on SOEC was developed, and finally demonstrated on Mars in 2022 and 2023. While MOXIE leaves behind it a wealth of accomplishments, there remains the need to close remaining gaps with additional laboratory work. The MOXIE Team has unique capability in electrolysis of CO2 and created a world-class laboratory for testing devices. NASA is currently focused on lunar ISRU because NASA is embarked on a major program to exploit the Moon. There is less interest in supporting Mars ISRU since a human mission to Mars seems to be decades away.  
The current NASA interest in Mars ISRU is focused on distantly future missions when indigenous H2O might be available. The first human-handed missions on Mars will most likely process atmosphere only for ISRU. Mars ISRU based on processing the atmosphere has an excellent return on investment and has already been demonstrated on Mars, whereas lunar ISRU has at best dubious return on investment, and has yet to be demonstrated to be feasible. Solid oxide technology for electrolysis and fuel cells has been under development worldwide for some thirty years for terrestrial applications [28]. Over the past thirty years, SOEC technology was widely supported across the world, and by DOE in the US. The foundation laid down at Oxeon Energy LLC by DOE funding over the years prior to 2014 provided the technology background that enabled the implementation of SOEC of CO2 on Mars in the MOXIE Project beginning in 2014. It makes a great deal of sense for NASA to continue to fund solid oxide electrolysis relevant to Mars ISRU because the NASA work would be highly leveraged by relevant technology advances from the far greater investment outside NASA in solid oxide technology for terrestrial applications. In this paper, we have placed the relatively mature MOXIE technology advance and solid oxide electrolysis in general in perspective to the historical evolution of low TRL Mars ISRU technology. While visionaries imagine futuristic applications of Mars ISRU, extension of MOXIE appears to be the most practical near-term application of Mars ISRU. Section 3.6 provides six steps for advancing the solid-oxide technology of MOXIE to a state of readiness for mission application.  
Conceptualization, D.R.; methodology, D.R.; investigation, D.R. and V.J.I.; writing--original draft preparation, D.R. and V.J.I.; writing--review and editing, D.R. and V.J.I.; visualization, D.R. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The authors declare no conflicts of interest.