## 2 The Lunar South Pole
### Lunar Volatiles  
The lunar south pole was certainly not indicated as the best human and robotic lunar base for its extreme environment but for its peculiarity of harboring ice and other volatiles; water ice may indeed be allowed to accumulate in permanently shadowed regions on airless bodies with a small tilt of the rotation axes in the inner solar system, as also happens for Mercury and Ceres [15]. The lunar surface is covered by a fragmental layer of rubbles (regolith) exposed to hydrogen as a proton; from the space environment, the solar wind implants protons into the lunar surface, which could be retained as molecular hydrogen, hydroxyl or molecular water. Another important contribution in the presence of water comes from water-bearing meteorites, asteroids and comets. But why is the presence of water and other volatiles so substantial? Water is a fundamental resource because it can be processed to make propellant, radiation-shielding and life-supporting consumables. One of the most efficient mixtures in a liquid propulsion system is based on hydrogen as fuel and oxygen as an oxidizer; currently, the average cost per kilogram to launch materials from the Earth's surface to the LEO is roughly $35,000, which increases dramatically for escaping the Earth's gravity. Hence, it becomes clear how cost-effective it would be to manufacture rocket propellant on the lunar surface. The physical form of water is not unique at grain-size scales (also known as the particle size, which is the average size of a sediment); there are several forms such as ice, frost, hydrated minerals, adsorbed molecules and soil mixtures that can constitute a key variable in the distribution [16]. Also, for this reason, the detection of lunar polar volatiles and their ensuing compositional analyses are not trivial; to achieve an exhaustive insight, it is necessary to cross-reference data from different detection techniques (imaging, spectroscopy, etc.) so as to compensate for the limitations of each. The primary indicator of lunar water ice is hydrogen rather than oxygen, which is notably abundant on the Moon occurring in silicate, oxide minerals and glasses in all lunar material while hydrogen occurrence is limited due to the result of the solar wind exposure [6]. Clementine first and then the Lunar Prospector registered initial markers of an anomalously high hydrogen level at the poles; the leading explanation fell on the presence of water ice and volatile deposits accumulated as ejecta from lunar impacts. Figure 3: Elemental abundances of iron, potassium, thorium and titanium in the lunar south pole detected with the Lunar Prospector instruments, adapted from [13]. About ten years later, further important contributions came in. Near-infrared reflectance spectra from the Moon Mineralogy Mapper instrument detected surface-exposed water ice through the concurrent presence of three absorption ranges and the comparison of laboratory spectra of pure water frost presenting strong \"blue\" spectral continuum slopes (reflectance decreases with increasing wavelength) whereas typical lunar spectra exhibit an opposite \"reddening\" effect [15]. Neutron spectroscopy (a spectroscopic method of measuring atomic and magnetic motions from the kinetic energy of emitted neutrons) is more sensitive to the presence of H and is not limited to a surface analysis but it can survey the lunar soil up to a meter in depth compared to reflectance spectroscopy; the Lunar Energetic Neutron Detector (LEND) on board the LRO probe has measured an enhanced hydrogen concentration around the south pole, estimating a range value of 0.3-0.5 wt% water-equivalent hydrogen (WEH: the percentage (by weight) of water that the subsurface material would contain if all of the detected hydrogen was present in the form of H\({}_{2}\)O) within the uppermost meter of the surface in PSRs [17], as shown in Figure 4. The LEND data are also able to provide latitude dependence graphs, which demonstrate H abundance at a latitude poleward of 79\({}^{\circ}\)[18].