## 4 Discussion  
[PERSON] and [PERSON] (2023) presented NASA's overview of ISRU [62] and [PERSON] and [PERSON] (2023) presented an update on NASA ISRU plans, priorities, and activities at the same meeting [63]. Because NASA is currently in the process of a major program to send humans to the Moon, the main NASA focus is presently on lunar ISRU as opposed to Mars ISRU. At first glance, this makes sense. NASA is engaged in a major undertaking to exploit the Moon, while a human mission to Mars seems likely to be at least several decades in the future. If lunar ISRU were equally feasible to Mars ISRU, and if lunar ISRU produced comparable return on investment as Mars ISRU, then NASA's focus on lunar ISRU would be fully justified. However, ascent from the Moon requires considerably less propellant than ascent from Mars, delivery of propellants to the Moon from Earth is far simpler and less demanding of resources than delivery to Mars, and most important of all, resources on the Moon are far more difficult to access than resources on Mars. From a long-term point of view, lunar resources might be viewed as a large-scale source of propellants and oxygen in cis-lunar space to fuel all deep space missions in the future. But for the short term, producing propellants for ascent from the Moon in relatively small quantity may require more effort than it is worth. [PERSON] (2018) analyzed various options for short-term lunar ISRU, and he concluded that they did not provide adequate return on investment, while Mars ISRU produced a substantial return on investment [9]. NASA's plan for lunar ISRU appears to look beyond the early human landings on Mars to a futuristic scenario where large-scale mining, moving, and processing of regolith play a major role in supplying propellants to cis-lunar space [64]. This encompasses processing equatorial regolith to extract oxygen from oxide, as well as processing polar ice to obtain H\({}_{2}\)O from which H\({}_{2}\) and O\({}_{2}\) can be produced by electrolysis. There are some problems with this. First, the lunar ice lies in deep crevices in lunar polar areas where mining would be energy-intensive and technically difficult. There is no plausible power source available. The ice, once mined, must be transported a considerable distance upward to a plateau where horizontal solar power is available. The NASA ISRU plan conceives that if lunar ISRU is fully developed, it will provide an important starting point for Mars ISRU as provided by the slogan: \"Moon to Mars Forward ISRU\". Therefore, recent activity in Mars ISRU has assumed that indigenous water will be available on Mars. [PERSON] et al. (2016) presented options for mining presumed Martian ice with NASA funds [65]. It seems likely that early human missions to Mars will land at an equatorial site where prospects for available near-surface water are very poor. Although the possibility of ice down to 30 degrees north latitude has been raised [66], this remains very unlikely. This study combined data from several sources of direct and indirect evidence of near-surface water into a suggestive map of where such deposits might lie, and there is no direct data. [PERSON] et al. (2021) considered options for acquiring and processing regolith of various types on Mars [65]. [PERSON] et al. (2016) [66] and [PERSON] et al. (2019) [67] also investigated access to water on Mars. [PERSON] et al. (2016) investigated the effectiveness of using microwaves to release hydrated H\({}_{2}\)O from minerals at low temperature. Favorable results were obtained [68]. But large-scale mining of regolith, delivering regolith to solid processors, and carrying out input and output of regolith to processors autonomously seems likely to be relegated to later missions to Mars, well after the first human landings. Alternatively, if a futuristic human mission were to land at a high latitude on Mars where near-surface water is available, the processes for collecting water would be quite different than in the dark crevices on the Moon.