P42A-01
Constraints on Deep Moonquake Focal Mechanisms Through Analyses of Tidal Stress
Changes in tidal stress within the lunar interior, induced by Earth's varying relative position, may influence deep moonquake activity, as suggested by the monthly periodicities observed in event occurrence times. In addition, the typically large S- to P-wave arrival amplitude ratios observed on deep moonquake seismograms indicate the occurrence of shear failure at depth, despite the prohibitive temperature and pressure conditions. Transformational faulting, in which shear failure is induced by mineral phase changes, clearly occurs at depth within the Earth and may also occur within the Moon. We investigate the relationship between tidal stress and deep moonquake occurrence by searching for a linear combination of the normal and shear components of tidal stress that best approximates a constant value, when evaluated at the times of moonquakes from each of 39 different moonquake clusters. We compute the stresses, resolved onto a suite of possible failure planes, and determine which orientation performs best. While our failure criterion adequately describes deep moonquakes for some clusters, we see three categories of additional complexity: (1) For some clusters, the best-fitting linear combination of shear and normal stress is not strongly dependent on plane orientation, suggesting that the process responsible for generating moonquakes has two components: one that is dependent on plane orientation, and another that is independent of orientation. (2) Some clusters have best-fitting linear combinations of stresses for which the relative contribution of normal stress is larger, which may be indicative of transformational faulting since the phenomenon is known to originate as anti-cracks that form perpendicular to the maximum compressive stress. (3) For some clusters, the fault plane can be better constrained when the failure criterion is expanded to include the influence of shear and normal stress rates. This rate-dependence may reflect a delay between the stress state that marks the onset of anti-crack formation and the eventual coalescence of anti-cracks that leads to failure.
P42A-02
Insight Into Lunar Crustal Magnetization by Joint Analysis of Gravity and Magnetic Field Data
Among the most important clues to understanding the early geologic evolution of a planet is the pattern of crustal remanent magnetization. We study the processes that magnetized the lunar crust by applying a method that has already been used to study the magnetized crust of Mars. The approach involves a least squares inversion of both the gravity and magnetic field data sets and a joint analysis of the results. The use of multiple data sets reduces the inherent non-uniqueness of the inversions. The density and magnetization distributions and their correlation together with geologic and topographic data are used to infer the processes that modified the crust, e.g., magnetization or demagnetization by crustal processes. The principal concentrations of magnetic field anomalies with the strongest magnitudes are on the far side of the Moon antipodal to the Crisium, Serenitatis, Imbrium, and Orientale impact basins. Isolated near side magnetic anomalies have been mapped at Reiner Gamma, Rima Sirsalis, and the craters Descartes and Airy. Richmond and Hood (2008) have mapped previously unidentified magnetic anomalies near the craters Abel, Hartwig and Stöfler, inside the Crisium and Moscoviense basins, and near the Snellius and Rheito crater chains. Proposed sources of the lunar crustal magnetization include the solar wind magnetic field, the geomagnetic field, transient magnetic fields produced by impacts, and a lunar dynamo. Many of the magnetic anomalies have geologic and/or albedo features, that may be related to the magnetization mechanism. Analyses of the magnetic anomalies at Mare Crisium and Mare Moscoviense may provide insight into the possible existence of a former core dynamo if the anomalies within the basins are produced by crustal thermoremanent magnetization. The furrowed terrane at Mare Crisium and the albedo feature at Mare Moscoviense have been proposed to be due to seismic effects or ejecta materials from the Orientale and Humorum impacts, respectively. It is possible that the processes that formed these features may also have magnetized the crustal rocks. Abel crater is interesting because an albedo feature has not been mapped in this region and it would therefore provide a test of the association between albedo and magnetization. We will discuss the reults of the gravity and magnetic analyses of the above features.
P42A-03
Producing Martian Lithologies with Geophysically-Constrained Martian Mantle Compositions
The Martian meteorites, rocks measured by the Mars Exploration Rovers (MER) and lithologies detected by orbital assets represent a diversity of igneous rocks that collectively provide insight into the formation and evolution of Mars. Experimental studies aimed at reproducing the observed igneous lithologies have met with varying degrees of success [e.g., 1,2,3], No study has yet been able to reproduce both Martian meteorite parent magmas and the basalts measured by MER at Gusev Crater [e.g., 1,3]. We attempted a different approach to successfully reproducing Martian igneous lithologies by using geophysical constraints on Martian bulk Fe (wt.%), Fe/Si and mantle Mg# [4,5] to identify mixtures of chondrite compositions that formed plausible Martian mantle compositions. We identified two candidate chondrite mixtures for Mars, CM+L and H+L. We synthesized the CM+L and H+L compositions from oxide, carbonate and phosphate powders and fixed them at an oxygen fugacity below the magnetite-wüstite buffer (MW-1). We conducted experiments at 2 GPa (corresponding to ~150 km in the Martian mantle) between 1300-1600 °C for 4-48 hours in the end-loaded piston cylinder apparatus at the Geophysical Laboratory. Thusfar, we have also conducted experiments at 4 GPa (corresponding to ~320 km in the Martian mantle) between 1425-1475 °C for 210-240 minutes in a Walker-type multi-anvil apparatus at the Geophysical Laboratory. We utilized an 18/11 (octahedron edge length/truncated edge length, in mm) assembly. In both assembly types, the sample was contained within a graphite capsule welded into a Pt tube. We analyzed the experiment products in electron probes at either the Geophysical Laboratory or Arizona State University. Fe and Mg contents of olivine, orthopyroxene and melt were used to assess the attainment of equilibrium for each run product. No significant difference exists between the CM+L and H+L experiment products. The near-solidus phase assemblage of the 2-GPa experiments is ol+opx+cpx. Melts at 2 GPa have MgO, FeO, and Mg# values that either overlap those of Martian meteorite parent melts or are capable of reproducing Martian meteorite parent melt compositions through low-pressure olivine fractionation. The 2- GPa melts do not, however, have CaO/Al2O3 values that intersect those of the Martian meteorite parent magmas. This finding mirrors the inability of previous studies [e.g., 1] to form the Martian meteorites. However, the 2-GPa products can lead to Gusev-like basalts via a two-step process. 20-25% melting yields basalt compositions from which subsequent low pressure olivine fractionation leads to basalts with MgO, FeO, CaO and Al2O3 contents and Mg# and CaO/Al2O3 values like those of the Gusev basalts. The near-solidus phase assemblage of the 4-GPa experiments is ol+opx+cpx+garnet. The melt composition resulting from ~20% melting of the CM+L mantle composition has MgO, FeO, CaO and Al2O3 contents and Mg# and CaO/Al2O3 values that fall among Martian meteorite parent magma compositions. Thus, the geophysically-constrained mantle compositions are capable of producing melts with Gusev and Martian meteorite parent magma affinities by simply shifting the pressure of melting. [1] Bertka C.M. and Holloway J.R. (1994) CMP 115, 313-322. [2] Agee C.B. and Draper D.S. (2005) LPSC XXXVI, #1434. [3] Monders A. et al. (2007) MaPS, 42, 131-148. [4] Bertka C.M. and Fei Y. (1998) Science, 281, 1838-1840. [5] Bertka C.M. and Fei Y. (1998) EPSL, 157:79-88.
P42A-04
Layered mantle convection and magma production on present-day Mars
Mantle convection and plume decompression melting have been used to explain volcanism on present-day Mars in previous studies. Most of these studies assume single layer convection in Martian mantle. However, layered Martian mantle has been suggested by models of magma ocean and mantle overturn. According to these models, a dense layer of up to several hundred km may exist at the bottom of Martian mantle. The possible effects of layered mantle on early Martian volcanic and magnetic dynamo histories were explored before. In this study, we explore how the layered martian mantle affects mantle convection and magma production on present-day Mars. Layered mantle convection is inefficient, resulting in a reduction in the heat flow out of the core and a higher temperature at the core-mantle boundary relative to models with a single convecting layer. The low core heat flux may contribute to the absence of a magnetic dynamo on Mars, in agreement with observations. The magma production rates in models with one and two convective layers are similar, provided that the temperature at the interface between the convective layers in the two layer system is the same as the temperature at the core-mantle boundary in the single layer system. Thus, with layered convection, it still holds that the recent volcanism rate on Mars implies a thermal Rayleigh number of slightly less than 107 on present-day Mars. Our models also show that the deep, dense layer can be stable for geologically long periods of time and that the topography on the interface between the two mantle layers is quite small for plausible values of the density difference between the layers.
P42A-05
Density variations between the southern and northern hemisphere of Mars: Implications from gravimetric inversion
One remarkable feature of Mars is the crustal dichotomy which divides the surface into an old southern highland hemisphere rising several kilometers above the zero level and a superficially younger northern lowlands hemisphere well below the datum. Whether this crustal dichotomy is also reflected in composition, density, and thickness is not known -- although, a crustal thickness variation is generally suggested based on the assumption of a constant crustal density. We used gravimetric methods to place constraints on the maximum crustal density of the southern highlands. Gravimetric methods are ambiguous with a noted trade- off between the crustal thickness and density. However, combining two different methods, the geoid- topography ratio and Bouguer inversion, helps to constrain a maximum density of the crust for regions that show a homogeneous unit with respect to lateral density variations and compensation state. For the Martian Noachian southern highlands a combination of these methods suggests a maximum crustal density of 3020 ± 70 kgm-3, assuming a single-layer crustal structure. Two-layer crustal structures show similar results: the upper crustal density is also limited to ~ 3000 kgm-3, but a denser uniformly thick lower crust is possible. The obtained results together with the findings on crustal densities (and composition) of other regions on Mars are consistent with various scenarios of crustal evolution: 1) A 'temporal' evolution in the densities with low densities of the ancient crust and comparatively higher densities of the young (Amazonian-era) volcanic material. Such a 'temporal' increase may result from different formation mechanisms or possibly from a change in composition of the basaltic magmas over time. 2) The density variation is already manifested in the early evolution during the formation of the crustal dichotomy, i.e. the Noachian crust of the northern lowlands has a different density than the Noachian southern highland crust. The ancient northern hemisphere might have a higher density than the crust of the ancient southern hemisphere, assuming the high density crust of the Elysium region being representative for the entire Northern lowlands. If correct, this also suggests a much lower curst-mantle undulation as generally assumed. 3) As in case 2, the density variation is already manifested in the early evolution during the formation of the crustal dichotomy. However, in contrast to case 2, the ancient northern hemisphere has a lower density than the crust of the ancient southern hemisphere as suggested by TES data. The spectra can be interpreted as basalt in the southern hemisphere and andesite in the northern hemisphere. A consequence of that density variation is a stronger crust-mantle undulation than assumed by Bouguer inversion with constant crust density. In the subsequent evolution of dichotomy formation and bulk crust formation, the volcanism in Elysium and Tharsis becomes more enriched in iron and therefore shows an increasing density.
P42A-06
Radar Sounding of Phobos by MARSIS
We report preliminary results from the first observations of Phobos ever performed by an orbiting radar sounder. The MARSIS experiment on board ESA's Mars Express is a synthetic-aperture, orbital sounding radar that works by transmitting a low-frequency radar pulse that is capable of penetrating below the surface of a planetary body, and is reflected by any dielectric discontinuity present in the subsurface. MARSIS is optimized for deep penetration, having detected echoes down to a depth of 3.7 km over the South Polar Layered Deposits, and is capable of transmitting at four different bands between 1.3 MHz and 5.5 MHz, with a 1 MHz bandwidth. MARSIS has observed Phobos in the course of nine flybys so far. The latest flyby took place on July 28, 2008, when Mars Express reached a distance of only 96 km from Phobos. During these flybys MARSIS successfully collected several thousand echoes. Planning and commanding observations for these flybys was potentially risky for the instrument, and required the development of a special operation mode, including the storage of the unprocessed echoes in the internal mass memory. MARSIS successfully detected echoes from the surface of Phobos, and subsequent echoes that could be either coming from surface features away from nadir or be real subsurface features. The nature of secondary echoes will be analyzed, and implications for the internal structure of Phobos will be discussed. Results from these observations will have a profound impact on current space mission studies in which radar sounders are proposed to probe the interior of asteroids, such as in NASA's Deep Interior and in ESA's ISHTAR.
P42A-07
The oxygen isotopic composition of captured solar wind: first results from the Genesis mission
Oxygen is the major constituent of rocky planets and the third most abundant element comprising the Sun, yet the solar oxygen isotopic composition has remained essentially unknown. One reason is that the usual appeal to primitive meteorites does not work because oxygen is isotopically distinct in all different classes of meteorites. The cause of this premier "isotopic anomaly" (first discovered in 1973) has been variously ascribed to nucleosynthetic input, e.g. from a nearby supernova, or to exotic isotope-selective chemistry in the solar nebula, e.g. based on molecular symmetry or UV photolysis. Knowledge of the average starting composition of the solar system, which is preserved in the Sun, would provide a baseline from which one could interpret the oxygen isotopic compositions of planetary materials. To this end, NASA flew the Genesis Mission to capture samples of the solar wind (SW) in ultra-pure target materials and return them to Earth for laboratory analysis. At UCLA, we have designed and constructed a hybrid secondary ion and accelerator mass spectrometer (SIMS/AMS), called the "MegaSIMS", specifically to tackle the unique analytical challenges posed by the Genesis samples: dilute elemental concentrations, limited sample material, and close proximity of likely surface contamination to the implanted solar wind ions. Three years after the crash- landing of the sample return capsule in the Utah desert, we have succeeded in making oxygen isotopic measurements on SW captured in a SiC target from the Genesis SW concentrator. Our preliminary data indicate that the Sun is enriched in 16O by at least 5% relative to Earth and meteorites. Implications for planetary science will be discussed.
P42A-08
First Results on Kr and Xe Abundances in the Bulk Solar Wind Measured in Silicon Targets exposed on GENESIS
The solar wind (SW) Kr and Xe elemental and isotopic composition is one of the primary objectives of the GENESIS mission. Solar Kr and Xe abundances cannot be analyzed insitu in the present-day SW due to their low abundances nor can they be measured in the solar photosphere due to the lack of suitable spectral lines. Thus, solar data have been exclusively derived from SW-irradiated regolith samples. Here we present first results on bulk SW Kr and Xe abundances, as well as selected isotope ratios, from captured SW returned by GENESIS. Five aliquot analyses were done by UV laser ablation from Si targets (rastered areas are between 10 and 50mm2). Measured SW fluences (atoms/cm2) are 2.97(4)E+10 36Ar, 1.22(6)E+7 84Kr and 1.4(2)E+6 132Xe. The measured 86Kr/84Kr of 0.3035(36) is in good agreement with SW- Kr obtained from lunar regoliths. The same is true for 129Xe/132Xe of 1.043(25). Our preliminary Kr and Xe elemental abundances are in fair agreement with earlier values derived from lunar soils, although the Genesis SW 36Ar/84Kr of 2390(150) is 30% larger than the lunar value, presumed to be representative for the solar wind in the last 100 Ma. The GENESIS 84Kr/132Xe of 8.2(1.5) is within 10% of the value derived from relatively recently irradiated lunar soils. The understanding of fractionation processes in the SW is important to finally deduce solar abundances for noble gases (and other elements) from SW data. Former investigations showed that Kr and Xe are enriched in the SW relative to Ar and solar abundances. This fractionation process operates upon ionization of SW particles and affects mainly the elemental composition of the SW. However, adopting the recently strongly reduced solar Ne and Ar abundances at constant Kr and Xe would now question this fractionation model for Kr and Xe. We reassess models of fractionation processes in the light of the modified solar abundances and also compare the GENESIS Ar, Kr and Xe data with current estimates of solar abundances.