Throughout the millennia of the history of human explorations, water has always been the most essential resource that enabled humans to go further, settle and prosper on new lands. Our own biology forces to take that quest for water as we venture away from the home watery planet and attempt to land humans on other planets for long stays.
Water is in extreme abundance in the solar system from Mars to the moons of Jupiter and from asteroids to the rings of Saturn. Liquid water however is not the norm and Earth remains a special place in what is called the habitable zone. Owing to the low temperatures and pressures at many planetary locations, ice is the most ubiquitous form of water as many spacecraft have confirmed its abundance in shadowed craters on the Moon, over the polar regions of Mars, in asteroids, comets and the worlds orbiting Jupiter and Saturn. The Jovian moon Europa is covered with thick water ice hiding a suspected liquid ocean and Enceladus orbits Saturn while spewing water ice into space. Water is also critical in ISRU to electrolyze it to obtain hydrogen and oxygen. These elements can be used in chemical reactors processing soils or other gases and as fuel and oxidizer in propulsion engines.
Remote sensing image of water ice coverage in southern polar region of the Moon.
The color scale indicate the depth at which stable ice is expected. (Image credit: NASA)
Volatile compounds are those that can be separated from the regolith by simply applying heat. Water and many other volatiles can be found together in comets, asteroids, Jovian moons and the polar craters of our Moon. Recent discoveries made by scientists using the Lunar Reconnaissance Orbiter (LRO) suggest the possibility of thin deposits of water ice that may make up ~20% of the surface of the regolith floor of crater Shackleton, a permanently shadowed crater in the southern polar region. The observational limits of the laser backscatter measurement by LRO only suggest a proposed thickness of a micrometer, which the authors say could be greater. In shadowed polar craters of the Moon like Cabeus where temperatures average less than 40K (-233°C or -388°F),
LCROSS detected water vapor and ice to an average of 5.6%(±2.9%) of the material ejected by impact (Table 1).
Table 1. Summary of total water vapor and ice and ejecta dust detected by the NIR instrument on LCROSS during impact inside the Cabeus lunar crater. (Source: A. Colaprete, 2010.)
It also identified a very diverse repository of volatiles among which the most abundant are hydrogen sulfide (H2S), ammonia (NH3), sulfur dioxide (SO2). Our knowledge of the presence of these compounds is still very limited as shown is Table 2 and can only be improved by direct measurements on the lunar surface.
Table 2. Abundances of volatile compounds detected in the ejecta plume of LCROSS impact with Cabeus crater. The uncertainty in each derived abundance is shown in parenthesis (e.g., for H2O: 5.1 (1.4)E19 = 5.1 ± 1.4x1019 molecules per cm2). (Source: A. Colaprete, 2010.)
This critical task is assigned to the RESOLVE prospector mission that will quantify water and the volatile compounds in the polar lunar regions. Colaprete et al. (2010) point out that the relative abundance of volatiles on the Moon is greater than that found in comets due to its origin in the early stages of the formation of the solar system.
Although the data contains a large amount of uncertainty, the presence of hydrocarbons such as ethylene (C2H4), methane (CH4) and methanol (CH3OH) on the Moon and in comets and asteroids is of great importance for ISRU; these compounds represent the foundation blocks of the chemistry of many polymers, resins, synthetic fabrics and plastics that we use everyday. The harvesting of such compounds in space in sufficient quantities is a major challenge yet unsolved although compounds such as methane and ethane are very abundant on worlds like Titan, where hydrocarbon lakes have been discovered.
This image acquired by the Mars Express High-Resolution Stereo Camera in 2010 shows a part of the northern polar region of Mars at the northern hemisphere summer solstice. After warmer temperatures evaporated carbon dioxide ice, water ice remains on the surface. (Photo credit: ESA/DLR/FU Berlin)
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Constraints on the volatile distribution within Shackleton crater at the lunar south pole, M. T. Zuber, J. W. Head, D. E. Smith, G. A. Neumann, E. Mazarico, M. H. Torrence, O. Aharonson, A. R. Tye, C. I. Fassett, M. A. Rosenburg, and H. J. Melosh, Nature 486, 378–381 (21 June 2012).
The presence and stability of ground ice in the southern hemisphere of Mars, J. T. Mellon, W. C. Feldman, T. H. Prettyman, Icarus 169 (2): 324–340 (2003).
The lakes of Titan, E. R. Stofan, C. Elachi, J. I. Lunine, R. D. Lorenz, B. Stiles, K. L. Mitchell, S. Ostro, L. Soderblom, et al., Nature 445 (1): 61–64 (2007).