Lunar Poles Research Articles

Shadows at the Lunar Poles

Shadows at the Lunar Poles

Topography of the lunar poles from radar interferometry: a survey of cold trap locations.

Detailed topographic maps of the lunar poles have been obtained by Earth-based radar interferometry with the 3.5-centimeter wavelength Goldstone Solar System Radar. The interferometer provided maps 300 kilometers by 1000 kilometers of both polar regions at 150-meter spatial resolution and 50-meter height resolution. Using ray tracing, these digital elevation models were used to locate regions that are in permanent shadow from solar illumination and may harbor ice deposits. Estimates of the total extent of shadowed areas poleward of 87.5 degrees latitude are 1030 and 2550 square kilometers for the north and south poles, respectively.

Illumination conditions at the lunar South Pole

Recent remote sensing data strongly suggest that deposits of ice occur in permanently shadowed regions at the lunar poles. Clementine, by providing the first contiguous coverage of the Moon, has enabled the production of an illumination map of the south pole. This map shows the extent of the areas of permanent darkness during winter in this region as well as identifying places on the lunar surface that receive illumination for more than 50% of the lunar day. The permanently dark areas are prime candidates for locations of ice deposits while regions that are illuminated for anomalous periods are possible sites for lunar bases. In this paper we study in detail the illumination history during a lunar day of some of these sites. While we find no area that receives permanent illumination, we do find two areas, only 10 km apart, which collectively receive sunlight for over 98% of the time.

The Lunar Prospector gamma-ray and neutron spectrometers

Gamma-ray and neutron spectrometers (GRS and NS, respectively) are included in the payload complement of Lunar Prospector (LP) that is currently orbiting the Moon. Specific objectives of the GRS are to map abundances of O, Si, Fe, Ti, U, Th, K, and perhaps, Mg, Al, and Ca, to depths of about 20 cm. Those of the NS are to search for water ice to depths of about 50 cm near the lunar poles and to map regolith maturity. The designs of both spectrometers are described and their performance in both the laboratory and in lunar orbit are presented.

Gamma-ray spectrometer for Japanese lunar polar orbiter

We review the current status of the development of Gamma-Ray Spectrometer (GRS) for the Lunar mission SELENE. The GRS instrument will measure gamma-rays in the energy range from 100 keV to 9 MeV. The instrument is a high-purity Ge detector surrounded by BGO and plastic scintillators which are operated as an anticoincidence shield, and is cooled by a Stirling cycle cryocooler. The primary objective is to provide global maps of the lunar composition. Measurements are anticipated for Fe, Ti, U, Th, K, Si, Mg, Al, O, Ca and Na over the entire lunar surface. The abundance of water ice in the permanently shaded craters at both the lunar poles will be measured with this instrument.

Lunar Polar Ice: Implications for Lunar Development

The discovery of water ice at the lunar poles can spur lunar exploration and development and may be a factor in accelerating human exploration beyond the Earth-Moon system. Theoretical models predict the existence of ice in the cold traps, and the Clementine and Lunar Prospector missions have given evidence that ice is indeed present. The Lunar Prospector results suggest that the ice concentrations are low, consistent with a cometary source for ice, which is mixed into the regolith by meteorite impact. Initial concepts for extracting ice from the lunar cold traps, where extreme environmental conditions prevail, have been developed; however, more information on the precise location and physical properties of the ice deposits is required before an approach can be selected. The principal implications of lunar ice development could be: (1) cutting the cost of human exploration missions to the Moon by a factor of two, due to the availability of propellant for returning crews and materials to Earth (this could also accelerate the use of the Moon for scientific outposts and human exploration technology testbeds); and (b) creating an opportunity for export of lunar materials to space at a low cost relative to Earth-to-space transportation. The latter would require the development of a reusable, lunar-based transportation system, fueled on the Moon, and capable of delivering payloads to lunar and Earth orbits. If an export economy is initiated, it is important to consider from the outset the conservation of the lunar ice, which is a nonrenewable resource, and to develop alternatives to its use in transportation systems.

Fluxes of Fast and Epithermal Neutrons from Lunar Prospector: Evidence for Water Ice at the Lunar Poles

Maps of epithermal- and fast-neutron fluxes measured by Lunar Prospector were used to search for deposits enriched in hydrogen at both lunar poles. Depressions in epithermal fluxes were observed close to permanently shaded areas at both poles. The peak depression at the North Pole is 4.6 percent below the average epithermal flux intensity at lower latitudes, and that at the South Pole is 3.0 percent below the low-latitude average. No measurable depression in fast neutrons is seen at either pole. These data are consistent with deposits of hydrogen in the form of water ice that are covered by as much as 40 centimeters of desiccated regolith within permanently shaded craters near both poles.

Cheapest Mission Finds Moon's Frozen Water

Lunar Prospector, the cheapest of the NASA Discovery program9s faster, cheaper, better missions, last week proved itself the equal of more costly spacecraft. After only a month in orbit, the $63 million, five-instrument spacecraft gathered the first definitive proof of water on the moon. Not only does the detection of water frozen near the lunar poles bolster hopes for eventual colonization of the moon, it strengthens claims that Mercury--innermost and hottest of the planets--has polar water, too.

Arecibo Radar Mapping of the Lunar Poles: A Search for Ice Deposits

The Arecibo 12.6-centimeter wavelength radar system was used to image the polar regions of the moon at a resolution of 125 meters in a search for ice deposits in areas of possible permanent shadow from the sun. No areas greater than 1 square kilometer were found with high radar backscatter cross sections and high circular polarization ratios, properties suggestive of the presence of ice. A number of areas smaller than 1 square kilometer were found with these properties, but optical images from spacecraft missions have shown some of these features to be in sunlight. Arecibo radar images of Sinus Iridum at latitude 47°N also showed a number of small features with similar properties. The coincidence of some of these features with the radar-facing slopes of craters and their presence in sunlit areas suggests that very rough surfaces rather than ice deposits are responsible for their unusual radar properties.

Mercury radar studies and lunar comparisons

Results from radar observations of Mercury are reviewed, the used as a basis for comparison of Mercury with its closest geologic relative, the Moon. Both bodies show very similar quasispecular scattering properties, indicating surfaces which are, on average, rougher (on scales of meters) and more porous than those of Venus and Mars. As with the Moon, some of the more prominent radar features on Mercury are associated with fresh, rayed craters. Mercury also has some prominent radar-bright features in the unimaged hemisphere which show no obvious impact structure; these features may come from smooth plains and possibly even (in one case) a large shield volcano. The Mercurian smooth plains show the quasispecular smoothness and down-bowed topography characteristic of lunar maria, providing support for the idea that the smooth plains and maria are geologic analogues. Permanently shaded craters at the north and south poles of Mercury show extremely bright radar features consistent with volume scattering from ice deposits. This discovery has helped to revive the old idea that ice may exist at the lunar poles. Recent radar searches for this lunar ice have proved either negative or inconclusive.

Lunar magnetic fields: Implications for utilization and resource extraction

Although the Moon possesses no large‐scale intrinsic magnetic field, locally strong crustal fields exist which must be considered in assessing lunar utilization scenarios. On the basis of available data, orbital anomalies most probably imply the presence of subsurface concentrations of metallic iron in the single‐domain size range. Such concentrations may indicate larger iron volume fractions than are characteristic of sampled materials, allowing more efficient extraction of metal than is now considered likely using nominal regolith compositions. Most other implications relate to the deflection of interplanetary and magnetospheric ions by crustal fields. Numerical simulations are employed to show that solar wind ion deflection by strong lunar magnetic anomalies can produce local increases, as well as decreases, in the implantation rate of solar wind hydrogen. The largest concentrations of implanted solar wind volatiles may therefore be found in local areas near strong anomalies. If water ice exists in permanently shadowed regions of the lunar poles, locally strong magnetic fields can shield these volatiles from sputter erosion losses by interplanetary and magnetospheric ion fluxes. Model simulations indicate that the ability of magnetic anomalies to shield the surface from incident ions increases with the angle of incidence and hence for most particle sources, with selenographic latitude. Finally, the possibility that relatively strong anomalies can provide significant protection of men and materials against major solar flare particle events is examined and found to be unlikely.

Exospheric transport restrictions on water ice in lunar polar traps

There is little doubt that at least 1017 g of water has accreted on the moon as a result of the reduction of ferric iron at the regolith surface by solar wind protons, the vaporization of chondrites, and perhaps comet impacts. Lacking an efficient escape mechanism, most of this water (or its progeny) is probably on the moon now. If the water were to have migrated to permanently shaded cold traps near the lunar poles, ice deposits with densities greater than 1000 g cm−2 would cover the traps, providing accessible resources. However, exospheric transport considerations suggest that the actual amount of water ice in the cold traps is probably too small to be of practical interest. The alternative is global assimilation of most of the water into the regolith, a process that must account for about 30 micromoles of water per gram of soil.

Lunar neutron leakage fluxes as a function of composition and hydrogen content

Areas of the Moon that might have enhanced concentrations of hydrogen are discussed. The fluxes of thermal, epithermal, and fast neutrons escaping from the lunar surface were calculated for eight different compositions and several hydrogen contents. These flux ratios depend on composition and are very sensitive to the hydrogen content. Their measurement using flight‐proven techniques from lunar orbit should allow detection and identification of hydrogen residing in either permanently shaded cold traps near the lunar poles or mature regoliths. Knowledge of composition, especially of Fe and Ti, improves the thresholds for hydrogen detection and also could allow identification of Gd and Sm.

Secular effects of oceanic tidal dissipation on the Moon's orbit and the Earth's rotation

The earth and moon are considered as a two‐body system in gravitational isolation from the sun and other planets. The lunar orbit is taken as circular, and the solid earth is assumed to be a rigid sphere (with no tidal deformation) so that there are no precessional torques other than those arising from the tidally deformed ocean. Numerical solutions to Laplace's tidal equations, with dissipation by linear bottom friction, are used to obtain ocean‐wide distributions of tidal amplitude for two idealized continentalities: a single circular continent (spherical cap) centered at the north pole, and a single spherical cap centered on the equator. Calculations are made for two different values of the frictional resistance coefficient, thus giving rise to four sets of solutions. The computed tidal amplitudes are used to calculate the oceanic tidal torque, which in turn is used to integrate the orbital equations backward in time for solution of the two‐body problem. The Coriolis parameter and the tidal frequencies change with time, thus requiring that the tidal equations be solved several times during the course of each orbital integration. In this manner, the earth's rotational and the moon's orbital histories are determined on a geologic time scale for each of the four models. The calculated position of the lunar orbit at 4.5 billion years ago is found to range from 38 to 53 earth radii in the four models and corresponds to a sidereal month of 330 to 550 hours. The sidereal day would have been 12 to 18 hours, with a relative inclination of 3° to 22° between the terrestrial and lunar poles. These results are in sharp contrast to those from previous studies of the earth‐moon system, most of which indicated a Roche limit approach of the two bodies roughly 1 to 2 billion years ago and presented therefore a time scale difficulty in theories of lunar origin. This contrast arises mainly from the fact that previous modelers avoided solution of Laplace's tidal equations by prescribing a constant frictional phase lag angle between the angular position of the moon and the major axis of the second‐degree harmonic of the tidally deformed surface of the earth. The amount of phase lag was established from the present astronomically deduced rate of tidal dissipation, but this precluded dynamic variations in tidal torque over geologic time, which are critical for determination of the orbital time scale. The present rate of oceanic tidal dissipation is evidently anomalously high because of the near resonance of the oceanic response in both frequency and shape to the tidal forcing. For the earth and moon to have evolved to their present separation from a distance of less than about 35 earth radii solely on the basis of oceanic tidal dissipation would have required the M2 response to have been near resonance for a significant portion of geologic history, a rather implausible scenario. The calculations reported here show that, on the contrary, frictional coupling between the earth and moon was much weaker than at present throughout most of the orbital history, and consequently the moon's closest approach could not have been as small as 35 earth radii. Although idealized continentalities were used in these calculations, the general conclusion of a weaker frictional coupling in the past is believed to be relatively insensitive to the continental configuration. This is because at faster paleorotation, the semidiurnal tidal frequencies would be resonant only with smaller‐scale normal modes of the ocean rather than with global scale modes as at present. The time scale difficulty heretofore present in models of orbital evolution is thereby eliminated. It should be noted that the orbital history developed here supports the theory of binary planetary accretion for lunar origin as opposed to fission or tidal capture.

Geophysical parameters of the Earth‐Moon System

Doppler tracking data from Lunar Orbiter 4 have been combined with laser ranging data from lunar retroreflectors to yield a number of geophysical and geodetic parameters for the earth and moon. This joint solution gives values of (1) the lunar principal polar moment C/MR2 = 0.3905 ± 0.0023, (2) GME = 398600.461 ± 0.026 km3/s2, and (3) an earth/moon mass ratio at 81.300587±0.000049. Also determined are the harmonics of a complete lunar gravity field through degree and order 5, the obliquity of the lunar pole, selenocentric coordinates of the lunar retroreflectors, geocentric coordinates of the McDonald Observatory, and the lunar secular acceleration. The lunar potential Love number is weakly determined at 0.022±0.013, and a surprisingly large dissipation of rotational energy is inferred, though either solid body tidal dissipation or liquid core mantle interactions could be causes.

Ice in the lunar polar regions

The idea that ice and other trapped volatiles exist in permanently shadowed regions near the lunar poles was proposed by Watson, Murray, and Brown [1961]. It is reexamined in the present paper, in the light of the vast increase of our lunar knowledge. The stability of the traps and the trapping mechanism are verified. Four potential sources of lunar H2O, (1) solar wind reduction of Fe in the regolith, (2) H2O‐containing meteoroids, (3) cometary impact, and (4) (the least certain) degassing of the interior, can supply amounts of trapped H2O estimated in the range of 1016–1017 g. Two important destructive mechanisms have been identified: photodissociation of H2O molecules adsorbed on the sunlit surface and sputtering or decomposition of trapped H2O by solar wind particles. The effect of impact gardening is mainly protective. The question of the presence of H2O in the traps remains open; it can be settled by experiment.

Latitude effects in lunar thermal evolution

A simple analytical model is developed from which we have calculated the temperature throughout the lunar interior resulting from internal heat sources and the imposition of surface temperature boundary conditions. The surface temperature is determined almost entirely by the balance of solar heating and surface reradiation; as a consequence this temperature is latitude dependent, decreasing towards the lunar poles. The internal solution shows that the latitude effect exists almost undiminished to great depths within the Moon. It is suggested that this dependence on latitude may have a significant effect on the Moon’s thermal evolution. Using the liquefaction model the high concentration of lunar maria at low latitudes may be explained.