Water on the moon?

If the planets formed at falling temperatures with volatile substances accreting last, the low abundance of lead, bismuth, indium, and thallium in lunar rocks implies an initial water content of no more than 370 grams per square centimeter, and probably much less. The depletion of volatile substances might be expected a priori if the moon accreted as an original satellite of the earth.     

Detection of a nonuniform distribution of polonium-210 on the moon with the apollo 16 alpha particle spectrometer

The polonium-210 activity of the lunar surface is significantly larger than the activity of its progenitor radon-222. This result establishes unequivocally that radon emanation from the present-day moon varies considerably within the 21-year half-life of lead-210, the parent nuclide of polonium-210. There are large variations and well-localized enhancements in polonium-210 activity over much of the moon's surface.     

Of time and the moon

Considerable information concerning lunar chronology has been obtained by the study of rocks and soil returned by the Apollo 11 and Apollo 12 missions. It has been shown that at the time the moon, earth, and solar system were formed, approximately 4.6 approximately 10(9) years ago, a severe chemical fractionation took place, resulting in depletion of relatively volatile elements such as Rb and Pb from the sources of the lunar rocks studied. It is very likely that much of this material was lost to interplanetary space, although some of the loss may be associated with internal chemical differentiation of the moon. It has also been shown that igneous processes have enriched some regions of the moon in lithophile elements such as Rb, U, and Ba, very early in lunar history, within 100 million years of its formation. Subsequent igneous and metamorphic activity occurred over a long period of time; mare volcanism of the Apollo 11 and Apollo 12 sites occurred at distinctly different times, 3.6 approximately 10(9) and 3.3 approximately 10(9) years ago, respectively. Consequently, lunar magmatism and remanent magnetism cannot be explained in terms of a unique event, such as a close approach to the earth at a time of lunar capture. It is likely that these phenomena will require explanation in terms of internal lunar processes, operative to a considerable depth in the moon, over a long period of time. These data, together with the low present internal temperatures of the moon, inferred from measurements of lunar electrical conductivity, impose severe constraints on acceptable thermal histories of the moon. Progress is being made toward understanding lunar surface properties by use of the effects of particle bombardment of the lunar surface (solar wind, solar flare particles, galactic cosmic rays). It has been shown that the rate of micrometeorite erosion is very low (angstroms per year) and that lunar rocks and soil have been within approximately a meter of the lunar surface for hundreds of millions of years. Future work will require sampling distinctly different regions of the moon in order to provide data concerning other important lunar events, such as the time of formation of the highland regions and of the mare basins, and of the extent to which lunar volcanism has persisted subsequent to the first third of lunar history. This work will require a sufficient number of Apollo landings, and any further cancellation of Apollo missions will jeopardize this unique opportunity to study the development of a planetary body from its beginning. Such a study is fundamental to our understanding of the earth and other planets.     

Meteoroid storms detected on the moon

Seismometers on the moon have detected several brief periods of enhanc ed meteoroid-impact activity, believed to represent encounters of the moon with "c louds" of objec ts in the kilogram range. The latest and most active encounter, in June 1975, is interpreted as a meteoroid c(loud of diameter 0.1 astronomical unit and total mass 10(l3) to 10(14) grams.     

Planetary science. Nitrogen on the moon

The nitrogen isotopic compositions seen in lunar soils have long been a mystery to planetary scientists. As Becker discusses in his Perspective, new techniques, such as the depth profiling of mineral grains reported by Hashizume et al., are now shedding some light on the matter, allowing some theories to be excluded. Nevertheless, the relative role of the solar wind and other processes remains hotly debated.     

Astrophysics from the moon

The surface of the moon would be an excellent location for astronomical telescopes, and, if a lunar base were to be established, the construction and maintenance of instruments would become feasible. The prospects are reviewed, with particular attention given to large optical aperturesynthesis instruments analogous to the Very Large Array of the National Radio Astronomy Observatory. Typical parameters for a particular system are presented.     

The origin of the moon

The origin of the moon is considered within the theory of formation of the terrestrial planets by accumulation of planetesimals. The theory predicts the occurrence of giant impacts, suggesting that the moon formed after a roughly Mars-sized body impacted on the protoearth. The impact blasted portions of the protoearth and the impacting body into geocentric orbit, forming a prelunar disk from which the moon later accreted. Although other mechanisms for formation of the moon appear to be dynamically impossible or implausible, fundamental questions must be answered before a giant impact origin can be considered both possible and probable.     

Water on the moon and a new nondimensional number

A nondimensional number called the Jeffreys number, which represents the ratio of the Reynolds number to the Froude number, is useful in geophysical problems related to the motion of viscous masses under gravity. The Jetireys number is used to show that it is impossible for the lunar maria to be underlain by a layer of material 1 kilometer thick having the plastic properties of ice.