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At least 163 records · Page 9
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NASA NTRS2015

Lunar Magma Ocean Crystallization: Constraints from Fractional Crystallization Experiments

The currently accepted paradigm of lunar formation is that of accretion from the ejecta of a giant impact, followed by crystallization of a global scale magma ocean. This model accounts for the formation of the anorthosite highlands crust, which is globally distributed and old, and the formation of the younger mare basalts which are derived from a source region that has experienced plagioclase extraction. Several attempts at modelling the crystallization of such a lunar magma ocean (LMO) have been made, but our ever-increasing knowledge of the lunar samples and surface have raised as many questions as these models have answered. Geodynamic models of lunar accretion suggest that shortly following accretion the bulk of the lunar mass was hot, likely at least above the solidus]. Models of LMO crystallization that assume a deep magma ocean are therefore geodynamically favorable, but they have been difficult to reconcile with a thick plagioclase-rich crust. A refractory element enriched bulk composition, a shallow magma ocean, or a combination of the two have been suggested as a way to produce enough plagioclase to account for the assumed thickness of the crust. Recently however, geophysical data from the GRAIL mission have indicated that the lunar anorthositic crust is not as thick as was initially estimated, which allows for both a deeper magma ocean and a bulk composition more similar to the terrestrial upper mantle. We report on experimental simulations of the fractional crystallization of a deep (approximately 100km) LMO with a terrestrial upper mantle-like (LPUM) bulk composition. Our experimental results will help to define the composition of the lunar crust and mantle cumulates, and allow us to consider important questions such as source regions of the mare basalts and Mg-suite, the role of mantle overturn after magma ocean crystallization and the nature of KREEP

Rapp, J. F.↗
NASA NTRS1991

Relationships among basaltic lunar meteorites

During the past two years four meteorites of dominantly mare basalt composition were identified in the Japanese and US Antarctic collections. Basalts represent a much higher proportion of the lunar meteorites than is expected from photogeologic mapping of mare and highland regions. Also, the basaltic lunar meteorites are all described as VLT mare basalt, which is a relatively uncommon type among returned lunar samples. The significance of the basaltic meteorites to the understanding of the lunar crust depends on the evaluation of possible relationships among the individual meteorites. None of the specimens are paired meteorites. They differ from each other in petrography and composition. It is important to determine whether they might be paired ejecta which were ejected from the same mare region by the same impact. The question of paired ejecta must be addressed using a combination of exposure histories and petrographic/compositional characteristics. It is possible that the basaltic lunar meteorites are paired ejecta from the same region of the Moon. However, the relationships among them are more complicated than the basaltic breccias being simply brecciated mare gabbros.

Lindstrom, Marilyn M.↗
NASA NTRS1977

Basis for interpretation regarding the ages of the Serenitatis, Imbrium and Orientale events

One of the most important objectives of lunar study is to relate the lunar sample data to important lunar events. This paper utilizes as the basis of interpretation consideration of the following: (1) photogeologic data, (2) the choice of a cratering model, (3) estimates of temperature of impact ejecta and shock-induced heating, (4) petrologic data of lunar breccias and their thermal and shock history, and (5) meaningful age measurements. Both the author's interpretations and alternative views are discussed. The age of the Serenitatis event is not yet known. The interpreted age of the Imbrium event is between 3.90 and 3.84 Ga. The age of the Orientale event is 3.84 Ga.

Chao, E. C. T.↗
NASA NTRS1985

The scaling of complex craters

The empirical relation between the transient crater diameter (Dg) and final crater diameter (Dr) of complex craters and basins is estimated using cumulative terrace widths, central uplift diameters, continuous ejecta radii, and transient crater reconstructions determined from lunar and terrestrial impact structures. The ratio Dg/Dr is a power law function of Dr, decreasing uniformly from unity at the diameter of the simple-complex crater morphology transition to about 0.5 for large multiring basins like Imbrium on the moon. The empirical constants in the Dg/Dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater, and hence the extent of collapse, is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock. The continuity of the Dg/Dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences.

Croft, S. K.↗
NASA NTRS2006

Mars Express Science Overview

This viewgraph presentation provides a brief and more detailed overview of the Mars Express Science mission as well as results from the Mars Express instruments and the High Resolution Camera.

ejecta↗
NASA NTRS2010

South Pole Hydrogen Distribution for Present Lunar Conditions: Implications for Past Impacts

It has been known since the Lunar Prospector mission that the poles of the Moon evidently harbor enhanced concentrations of hydrogen [1,2]. The physical and chemical form of the hydrogen has been much debated. Using imagery from Clementine it was possible to roughly estimate permanently-shadowed regions (PSRs), and to perform image reconstructions of the Lunar Prospector epithermal neutron flux maps [3,4]. The hydrogen concentrations resulting from these reconstructions were consistent with a few weight percent water ice in selected locations. With the LCROSS impact, we now know that hydrogen in the form of ice does exist in lunar polar cold traps [5]. Armed with this information, and new data from LRO/Diviner, we can examine whether the pre-sent-day distribution of hydrogen in the form of water ice is consistent with a past large impact that delivered a large mass of volatiles to the lunar surface. These volatiles, mixed with solid impact ejecta, would then be lost from locations having high mean temperatures but would otherwise remain trapped in locations with sufficiently low mean annual temperatures [6]. The time scales for loss would depend on the location-dependent temperatures as well as impact history.

Elphic, R. C.↗
NASA NTRS1978

Radial thickness variations of Orientale basin ejecta

Moore et al. (1974) measure the thickness of Orientale basin ejecta on the basis of filling of individual prebasin craters and a depth-diameter relation for fresh lunar craters. In the reported investigation the concept of filling of preexisting craters with basin ejecta is utilized somewhat differently to ascertain Orientale basin ejecta thicknesses and volume from the Cordillera ring with a radius of 450 km out to almost 2 radii. Briefly, the approach is to assume a reasonable geometric model for the form of Orientale ejecta, calculate how many pre-Orientale craters would be destroyed by the deposition of the ejecta, and match the model to Orientale crater statistics. The results of the investigation show that a radial ejecta thickness function can be derived from crater statistics.

Cordell, B. M.↗
NASA NTRS1975

Experimental hypervelocity impact into quartz sand - Distribution and shock metamorphism of ejecta

Results are presented for vertical impacts of 0.3-g cylindrical plastic projectiles into noncohesive quartz sand in which vertical and horizontal reference strate were employed by using layers of colored sand. The impacts were performed at velocities of 5.9-6.9 km/sec with a vertical gun ballistic range. The craters, 30-33 cm in diameter, reveal a radial decay of the ejecta mass per unit area with a power of -2.8 to -3.5. Material displaced from the upper 15% of the crater depth d is represented within the whole ejecta blanked, material from deeper than 28% of d is deposited inside 2 crater radii, and no material from deeper than 33% of d was ejected beyond the crater rim. Shock-metamorphosed particles (glassy agglutinates, cataclastic breccias, and comminuted quartz) amount to some 4% of the total displaced mass and indicate progressive zones of decay of shock intensity from a peak pressure of 300 kbar. The shock-metamorphosed particles and the shock-induced change in the grain size distribution of ejected samples have close analogies to the basic characteristics of the lunar regolith. Possible applications to regolith formation and to ejecta formations of large-scale impact craters are discussed.

Stoeffler, D.↗
NASA NTRS1979

Ejecta emplacement of the Martian impact crater Bamburg

The paper examines the ejecta emplacement of the Martian impact crater Bamburg. During ejecta emplacement, secondary crater formation preceeded the deposition of mobile surface flows, which are overrun by more viscous flows characterized by longitudinal grooves and transverse ridges. Many areas of flat terrain on the ejecta deposits may indicate that the sediment-laden melt water percolated out of the volatile-rich ejecta and the crater rim. The number of secondary craters associated with Bamburg is less than one third the value for lunar and Mercurian craters of equivalent size; this is attributed to preferential destruction of ejecta blocks sufficiently large to form secondary craters or to the subsequent burial of such craters.

Mouginis-Mark, P. J.↗
NASA NTRS2007

Modeling Lunar Radar Scattering from Icy Regoliths

The Apollo 15, 16, and 17 core tubes show that the uppermost few meters of the lunar regolith are interlaced layers of a fine grained powders and blocky crater ejecta. The layers of crater ejecta have dielectric constants in the range of 7-9 while the fine-grained powders has dielectric constant on the order of 2.7. These differences in dielectric constant, in turn, create radar reflections that are both refracted and reflected back through the space-regolith interface. Note that for a dielectric constant of 2.7 for the lunar regolith, radio waves incident on the lunar surface at the angle of 30-degrees from the normal will propagate in the regolith at an angle of 18-degrees. At the limb, radio waves incident on the lunar surface at an angle near 90-degrees from the normal will propagate in the regolith at an angle of about 37-degrees. These angles are within the range where radar backscatter is in the quasi-specular regime. When these buried crater ejecta layers are modeled using Hagfors' formulation (Hagfors,1963), echo powers match the behavior observed for average lunar backscatter at centimeter wavelengths for higher (30 to 90) angles of incidence. In addition, Hagfors et al. (1965) conducted an experiment where the Moon was illuminated at 23-cm wavelength with circular polarization and the differences were observed in orthogonal linear polarizations. Modeling of these observations and assuming again that the buried crater ejecta scatter in a quasi-specular manner, echo differences in horizontal and vertical linear polarizations are in good agreement with the observations.

Moon↗
NASA NTRS1997

Constraints on the Magnitude of Vertical and Lateral Mass Transport on the Moon

The role of vertical and lateral mass transport of crustal materials on the observed patterns of lunar surface composition, and the effects on our understanding of the geologic evolution of the planet, have been the subject of much debate in the lunar science community. The primary consensus that emerged from analyses of these processes in the 1970's and 1980's was that vertical and lateral mixing through impact gardening was a relatively inefficient process, and not likely to have contributed significantly to compositional units and variations on the Moon. The supporting evidence for this view is that unit boundaries (e.g. mare-highland contacts, contacts between mare color units) are still apparently quite distinct and sharp despite several aeons of impact activity, and cores from the Apollo landing sites did not show any evidence of widespread homogenization of the surface composition, nor distinct compositional gradients across geologic boundaries. In addition, modeling of vertical and lateral transport generally showed that the effects on composition should be confined to horizontal scales of about a kilometer and vertical scales of a meter. The problem with this consensus is that there is ample contradictory evidence. The fundamental discovery of Wood et al. (1970) was made possible by significant horizontal transport of highland material to the center of Mare Tranquillitatis. The continuous and discontinuous ejecta from the crater Copernicus has clearly influenced the surface composition of a large area of the lunar maria, while rays and ejecta from many highland craters are easily recognized in and around the nearside maria. Despite this contrary evidence, there have been few detailed studies to quantify the amount and rate of material redistribution through impact processes (a notable exception is reported in the paper by Pieters et al, 1985), largely because data adequate to critically analyze this process were lacking. However, the multispectral images acquired by the Galileo and Clementine missions now permit the investigation of this process.

Mustard, John F.↗
NASA NTRS2002

Lunar lava tube radiation safety analysis

For many years it has been suggested that lava tubes on the Moon could provide an ideal location for a manned lunar base, by providing shelter from various natural hazards, such as cosmic radiation, meteorites, micrometeoroids, and impact crater ejecta, and also providing a natural environmental control, with a nearly constant temperature, unlike that of the lunar surface showing extreme variation in its diurnal cycle. An analysis of radiation safety issues on lunar lava tubes has been performed by considering radiation from galactic cosmic rays (GCR) and Solar Particle Events (SPE) interacting with the lunar surface, modeled as a regolith layer and rock. The chemical composition has been chosen as typical of the lunar regions where the largest number of lava tube candidates are found. Particles have been transported all through the regolith and the rock, and received particles flux and doses have been calculated. The radiation safety of lunar lava tubes environments has been demonstrated.

Non-programmatic↗
NASA NTRSDate not supplied

Understanding the Impact of High-Velocity Dust Due to Lunar Landings

Evidence from earlier landings by Surveyor and Apollo have provided ample evidence regarding the dangers of high-velocity ejecta. These dangers include the generation of dust that settle on equipment, posing a hazard to equipment fol-lowing landing, the sandblasting of objects near the landing site, and the spead of high-velocity dust over the entire lunar surface and in lunar orbit that may remain for long periods of time. Recent work has provided valuable insight into the behavior and nature of the dust as a function of lander size, but further work is needed to gain a better understanding of the initial trajectories of this high- velocity dust.

lunar↗
NASA NTRS2009

The Lunar Regolith

A thick layer of regolith, fragmental and unconsolidated rock material, covers the entire lunar surface. This layer is the result of the continuous impact of meteoroids large and small and the steady bombardment of charged particles from the sun and stars. The regolith is generally about 4-5 m thick in mare regions and 10-15 m in highland areas (McKay et al., 1991) and contains all sizes of material from large boulders to sub-micron dust particles. Below the regolith is a region of large blocks of material, large-scale ejecta and brecciated bedrock, often referred to as the "megaregolith". Lunar soil is a term often used interchangeably with regolith, however, soil is defined as the subcentimeter fraction of the regolith (in practice though, soil generally refers to the submillimeter fraction of the regolith). Lunar dust has been defined in many ways by different researchers, but generally refers to only the very finest fractions of the soil, less than approx.10 or 20 microns. Lunar soil can be a misleading term, as lunar "soil" bears little in common with terrestrial soils. Lunar soil contains no organic matter and is not formed through biologic or chemical means as terrestrial soils are, but strictly through mechanical comminution from meteoroids and interaction with the solar wind and other energetic particles. Lunar soils are also not exposed to the wind and water that shapes the Earth. As a consequence, in contrast to terrestrial soils, lunar soils are not sorted in any way, by size, shape, or chemistry. Finally, without wind and water to wear down the edges, lunar soil grains tend to be sharp with fresh fractured surfaces.

Noble, Sarah↗
NASA NTRS1977

Crater clusters and light mantle at the Apollo 17 site - A result of secondary impact from Tycho

The morphologies of Tycho secondary craters and their ejecta deposits were studied using full-moon, Lunar-Orbiter, and Apollo panoramic photographs. These data were compared with similar data for the secondary craters and light mantle of the Apollo 17 landing site. The results indicate that (1) the central crater cluster and the light mantle can be attributed to Tycho, (2) the dominant mechanism for emplacement of the light mantle was impact by secondary craters that threw material across the valley floor, and (3) level sheets of material may be emplaced locally by secondary impact. Analysis of returned samples confirms that secondary impacts rework mostly local material.

Lucchitta, B. K.↗
NASA NTRS1985

Geologic processes on Venus: An update

Studies of Venera 15 and 16 radar image and altimetry data and reevaluation of Pioneer Venus and earlier Venera data have greatly expanded the perception of the variety and complexity of geologic processes on Venus. PV data have discriminated four highland regions (each different in geomorphic appearance), a large upland rolling plains region, and smaller areas of lowland plains. Two highland volcanic centers were identified that may be presently active, as suggested by their geomorphologic appearance combined with positive gravity anomalies, lightning strike clusters, and a change in SO2 content in the upper atmosphere. Geochemical data obtained by the Venera landers have indicated that one upland area and nearby rolling plains are composed of volcanic rocks, probably basalts or syenites. New Venera radar images of the Ishtar Terra region show folded and/or faulted linear terrain and associated volcanic features that may have been deformed by both compressional and extensional forces. Lowland surfaces resemble the mare basaltic lava flows that fill basins on the Moon, Mars and Earth. Ubiquitous crater like forms may be of either volcanic or impact origin; the origin of similar lunar features was determined by the character of their ejecta deposits.

Masursky, H.↗
NASA NTRS1988

Meteorites on Mars

Four types of meteoritic material should be found on Mars: (1) micrometeorites, many of which will survive atmospheric entry unmelted, which should fall relatively uniformly over the planet's surface, (2) ablation products from larger meteorites which ablate, break up and burn up in the Mars atmosphere, (3) debris from large, crater forming objects, which, by analogy to terrestrial and lunar impact events, will be concentrated in the crater ejecta blankets (except for rare, large events, such as the proposed C-T event on earth, which can distribute debris on a planetary scale), and (4) debris from the early, intense bombardment, which, in many areas of the planet, may now be incorporated into rocks by geologic processes subsequent to the intense bombardment era. To estimate the extent of meteoritic addition to indigenous Martian material, the meteoritic flux on Mars must be known. It is estimated that the overall flux is twice that for the Moon and 1.33 that for Earth. For small particles, whose orbital evolution is dominated by Poynting Robertson drag, the flux at Mars can be estimated from the Earth flux. The smaller Martian gravitational enhancement as well as the decrease in the spatial density of interplanetary dust with increasing heliocentric distance should reduce the flux of small particles at Mars to about 0.33 times the flux at Earth. Because of the smaller planetary cross-section the total infalling mass at Mars is then estimated to be 0.09 time the infalling mass in the micrometeorite size range at Earth.

Flynn, G. J.↗