Abstract

Water ice and other volatiles are vital in sustaining human settlement in space. Hydrogen and oxygen extracted from water by hydrogen-oxide reactions can be used as propellants on short-range interplanetary missions in the inner Solar System prior to developing more advanced means of space travel involving ion and nuclear propulsion systems for long-range interplanetary missions to follow. With the exception of Venus which has a mean surface temperature of 900°F beneath a thick carbon-dioxide atmosphere, all planets in the inner Solar System possess water-ice resources in varying forms and quantities.

Water ice and other volatiles occur in polar areas on Mercury and the Moon, the two major airless bodies in the inner Solar System. As a consequence of their low obliquity (<2°), polar areas on Mercury and the Moon contain a large number of permanently shadowed, topographically low areas in crater floors. These permanently shadowed areas are cold traps for volatiles that accumulated over the past one to two billion years (1 to 2 Ga) from impacts from volatile-rich comets and asteroids. Recent measurements from DIVINER Lunar Radiometer Experiment, one of seven instruments on board the Lunar Reconnaissance Orbiter (LRO) which was launched in June 2009, indicate temperatures less than 40 degrees above absolute zero in the floors of some craters near the Moon’s South Pole.

Evidence for water ice at the Moon’s poles is based on at least four main lines of evidence, including (1) polarized radar signatures, first detected by the Clementine probe in 1994 and later measured in refined detail by the Mini-SAR synthetic aperture radar on the Chandrayaan-1 probe in 2009, (2) neutron scattering signatures that indicate hydrogen, detected by the Lunar Prospector Mission in 1999 and confirmed by subsequent missions, (3) spectral reflectance data, imaged by the Moon Mineralogy Mapper (M³) on the Chandrayaan-1 mission, and (4) detection of hydroxyl (OH^-) ions from ultraviolet emission spectra in a dust- and ice-plume generated from impact of the upper stage of the LRO Centaur rocket at more than 5600 mi/hr (>9000 km/hr) into a permanently shadowed area in Cabeus A crater near the South Pole in October 2009. Radar reflectivity signatures indicate that lunar ice does not occur in extensive sheets at the surface, but rather in disseminated form in the shallow (<40-cm) regolith. Approximately 600 million metric tons of ice exists in the region of the Moon’s North Pole. This amount of ice could yield sufficient hydrogen and oxygen for daily launches of a space shuttle for 2,200 years.

Martian water-ice resources far exceed those on the Moon and Mercury. Water ice occurs in abundance on Mars in polar ice caps, shallow permafrost, and in layered terrain adjacent to the poles. Martian permafrost, which holds more water ice than the poles, occurs in a wide variety of forms, including tropical mountain glaciers that form debris aprons, collapse structures, polygonal terrain, and pingoes with morphologies similar to those of terrestrial periglacial features. Subsurface ice on Mars has an areal distribution exceeding 20 million km², whereas the polar caps, although 2.7 and 3.1 km thick at the North and South Poles, respectively, each encompass an area <1 million km². The northern polar cap is composed mainly of water ice and overlain by carbon dioxide frost that sublimates during spring. It contains numerous thin dust layers that record global cycles of dust storms and long-term (ca. 100,000-yr) climatic cycles related to major changes in Martian obliquity.