The ascent and emplacement of basaltic magma on the earth and moon is modeled by the application of geological and physical observations and constraints. Relatively simple mathematical models of the motion of gas/liquid mixtures are shown to be adequate in the treatment of basaltic eruptions, provided that allowance is made for the coalescence of gas bubbles and that realistic geological and petrochemical constraints are applied to the numerical values of variables. Because gas exsolution from magmas on the earth and moon commonly occur at depths of less than 2 km, it is generally convenient to consider separately the rise of bubble-free magmatic liquid at depth in a planetary crust and the more complex motions occurring near the surface with gas exsolution.

http://www.planetary.brown.edu/pdfs/262.pdf

Ascent and eruption of basaltic magma on the earth and moon

As with all science, our continually developing concepts of lunar evolution are firmly tied to both new types of observations and the integration of these observations to the known pool of data. This process invigorates the intellectual foundation on which old models are tested and new concepts are built. Just as the application of new observational tools to lunar science in 1610 (Galileo’s telescope) and 1840 (photography) yielded breakthroughs concerning the true nature of the lunar surface, the computational and technological advances highlighted by the Apollo and post-Apollo missions and associated scientific investigations provided a new view of the thermal and magmatic evolution of the Moon.

### 1.1. Pre-Apollo view of the thermal and magmatic evolution of the Moon

Many of the early views of the Moon manifested in mythology and art throughout the world were primarily tied to lunar and terrestrial cycles and the relationships between the Sun and the Moon. Prophetically, myths involving the lunar deities Mwuetsi from Zimbabwe and Coyolxauhqui from Mexico told of rather violent or catastrophic events in which the Moon was expunged from the Earth. Numerous ancient scientific observations were made about the nature of the Moon ranging from those uncovered in early Neolithic sites that correctly identified mare Crisium and mare Humorum to the insights made by Greek philosophers such as Anaxagoras (ca. 500-428 B.C.) and Democritus (ca. 460-370 B.C.), who attached terrestrial analogues to its character (stone, mountains).

With the advent of the telescope (1610) and photography (1840) as scientific tools for lunar exploration, semiquantitative data could be collected that would provide an intellectual foundation for scientific interpretation. Initially, modern terrestrial geological analogs were extended to the Moon (lunar highlands, volcanic craters, seas). Combined with the rigors of computational modeling, these observational data were extended to predict the original thermal state of the Moon and its thermal and magmatic history. Its proximity to the Earth …

Thermal and Magmatic Evolution of the Moon

In situ chemical analyses of Martian soil by the Viking lander indicate mafic to ultramafic source rocks, consistent with both remote sensing data indicating the presence of pyroxene and olivine and with petrologic modeling which suggests that Martian lavas are iron-rich and ultramafic. Photogeological analysis of the Martian surface reveals two types of volcanic morphology: (1) central volcanoes, developed by continued and prolonged eruption from a point source vent; and (2) volcanic plains, recognized by mare ridges and flow lobes. When these volcanic morphologies are combined with relative age data, a volcanic history may be derived that is consistent with a moonlike thermal history involving a lithosphere of increasing thickness with time which gradually suppresses the volcanism.

Volcanism on Mars

The surface of the Moon is a critical boundary that shapes our understanding of the Moon as a whole. All geologic mapping and remote sensing techniques utilize only the outermost portion of the Moon. Before leaving the Moon for study in our laboratories, all lunar samples that have been studied existed at or very near the surface. With the exception of the deeply probing geophysical techniques, our understanding of the interior of the Moon is derived from surficial, but not superficial, information, coupled with boundary of the lunar crust, it is the lower boundary layer of the tenuous lunar atmosphere and constitutes both a source and a sink for atmospheric gases. The surface is also where the Moon interacts with the space environment, causing changes in the physical nature of lunar materials, and provides a laboratory for the study of processes that occur on all airless bodies.

The data obtained remotely by the Galileo, Clementine, and Lunar Prospector missions, as well as data derived from lunar meteorites, have resulted in major changes to our understanding of global distributions of chemistry and rocks. This chapter summarizes the current understanding of this critical interface, the surface of the Moon, in its role as the lower boundary of the lunar atmosphere, the upper boundary of the crust, and the window through which we view, through remote sensing, the composition of the crust and the history of the Moon. In this post-Lunar Prospector time, the view of the Moon has changed, lending new perspectives to lunar samples and lunar processes. But the New View will likely remain in flux as we continue to digest the results from these recent space missions and move forward to a new era of lunar exploration.

Despite the freshness of our perspective, this is an important moment to capture, …

Understanding the Lunar Surface and Space-Moon Interactions

The chronology of lunar volcanism is based on radiometric ages determined from Apollo and Luna landing site samples, regional stratigraphic relationships, and crater degradation and size-frequency distribution data for units largely defined prior to the end of the Apollo program. Here we report on new crater size-frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins (Australe, Tranquillitatis, Humboldtianum, Humorum, Serenitatis, and Imbrium). Crater size-frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the Moon. Compared to crater degradation ages, crater size-frequency ages, performed on spectrally defined units, offer significant improvements in accuracy. Our investigation showed that (1) in the investigated basins, lunar volcanism was active for at least 1.5–2.0 b.y., starting at about 3.9–4.0 b.y. and ceasing at ∼2.0 b.y., (2) most basalts erupted during the late Imbrian Period at about 3.6–3.8 b.y., (3) significantly fewer basalts were emplaced during the Eratosthenian Period, (4) basalts of Copernican age were not found in any of the investigated basins, (5) lunar basin-filling volcanism probably started within ∼100 m.y. after the formation of the individual basins. We also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins. We found that (1) in all pre-Nectarian basins (Australe and Tranquillitatis) as well as in the Humboldtianum basin, which is of Nectarian age, the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3.6–3.8 b.y., (2) in the younger basins (Humorum, Serenitatis, and Imbrium) a second peak at 3.3–3.5 b.y. is observed, (3) basalt eruptions younger than 2.6 b.y. occur only intermittently, and (4) in the youngest basins, Serenitatis and Imbrium, we see an extended period of active basin-filling volcanism (1.5–1.6 b.y.) which is 500 m.y. longer than in the Australe and Humorum and even ∼1.0 b.y. longer than in Tranquillitatis and Humboldtianum.

Ages of mare basalts on the lunar nearside

The paper attempts a synthesis of the major-element chemistry, petrography, mineral chemistry, and crystal chemistry of the mare basalts returned by Apollo and Luna missions A classification of the mare basalts based on major-element chemistry is given, and textural sequences within each major-element group are identified The mineral chemistry and crystal chemistry of each mineral group are considered within the framework of the major-element groups and the textural sequences The various classes of models for the origin of the mare basalts and the nature of their source regions are discussed in the context of the major- and trace-element chemistries and experimental investigations

A. E. Bence

Papers

Mare basalts: Crystal chemistry, mineralogy, and petrology