Effective January 1, 2011, LPI seminars will be held on Fridays.
LPI seminars are held from 3:30–4:30 p.m. in the Lecture Hall at USRA, 3600 Bay Area Boulevard, Houston, Texas. Refreshments are served at 4:30 p.m. For more information, please contact Georgiana Kramer (phone: 281-486-2141; e-mail: email@example.com) or Patricia Craig (phone: 281-486-2144; e-mail: firstname.lastname@example.org). A map of the Clear Lake area is available here. This schedule is subject to revision.
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The intense photochemistry that takes place in Titan’s dense atmosphere, mainly composed of N2 and CH4, leads to the production of complex organic molecules and to the subsequent formation of aerosols in suspension in the atmosphere. The CAPS and INMS instruments onboard Cassini have shown that these aerosols are not only formed in the neutral lower stratosphere but also at higher altitudes in the ionosphere, where an active chemistry between ion and neutral species occurs. To try and understand this chemistry, we use a dusty plasma experiment that simulates the atmospheric reactivity on Titan. This experiment, called PAMPRE, uses a radio frequency capacitively coupled plasma discharge, produced in a continuous gas flow, to induce the chemistry between N2 and CH4. More complex molecules are formed in the plasma and lead to the production of solid particles, analogues of Titan’s aerosols, called tholins. In the study presented here, different nitrogen-methane gas mixtures (from 1% to 10% CH4) have been used to study the influence of methane on the chemistry and on the subsequent production of tholins. From in situ mass spectrometry measurements, it has been observed that the methane concentration in the gas phase during tholin production (plasma steady state) is lower than the injected methane concentration. By varying the initial methane concentration, it has been found that tholins could be produced in methane steady state concentrations similar to Titan’s atmospheric conditions (~1.5% CH4) when injecting an initial CH4 concentration of ~5%. The tholin mass production rate has been quantified as a function of the initial methane concentration. The production was found to be the most efficient for a steady state CH4 concentration in agreement with Titan’s atmospheric CH4 concentrations. The carbon gas to solid conversion rate has also been inferred for each gas mixture: the tholin carbon integration efficiency decreases when the initial CH4 concentration in the gas mixture is increased. Here we highlight, from the mass production rate measurements, two competitive chemical regimes controlling the tholin production efficiency: an efficient growth process which is proportional to the methane consumption, and an inhibiting process which opposes the growth process and dominates it for initial methane concentrations higher than ~5%. To explain these two opposite effects, we propose two mechanisms: one involving HCN patterns in the tholins for the growth process, and one involving the increasing amount of atomic hydrogen in the plasma as well as the increase in aliphatic contributions in the tholins for the inhibiting process. This study highlights new routes for understanding the chemical growth of the organic aerosols in Titan’s atmosphere.
We now know that primordial ice exists in at least three distinct Solar system reservoirs; the Oort cloud, the Kuiper belt and the asteroid main-belt. Continuing efforts to determine the nature of the ice and its distribution are important for several scientific reasons. First, the mere existence of the ice sets a limit to the degree of thermal processing of the objects in which it is found, and therefore constrains geophysical models of thermal evolution of ice rich bodies. Second, water ice, if in the amorphous form, can trap other volatiles from the protoplanetary disk of the Sun at high abundance. Their subsequent release upon crystallization can perhaps explain the anomalous activity observed in many comets and is a source of energy, since crystallization is exothermic. Third, water and other volatiles on the terrestrial planets seem likely to have been delivered, in part, from the ice reservoirs. The comets and ice-rich asteroids therefore may hold the key to understanding the origin of the oceans and atmosphere. In this talk, I will aim to provide a broad overview of our current knowledge (and lack of knowledge) of the primordial ice reservoirs. I will emphasize links to the formation epoch and draw connections for those interested in the origin of the oceans and the atmosphere and in the thermal evolution of asteroids and comets.
A small amount of material from Jupiter-family comet Wild-2 was returned to Earth by NASA's Stardust mission in 2006 for study in the lab. The fact that these samples are bona fide material from a known comet that spent the majority of its life in the Kuiper Belt gives researchers the opportunity to test theories of comets, the solar nebula, and the origin of other astromaterials. A benefit of a sample-return mission is the ability to analyze samples in large, state-of-the-art, Earth-bound instruments. We have analyzed Stardust material in aerogel and aluminum foil with x-ray microprobe, photoemission electron microscope, TEM, and other instruments. I will present multi-instrument analyses of Stardust samples, the methods we use to analyze the same samples in different instruments, and the synthesis of the data into a description of the formation and history of the comet. Conversely, with samples from a known comet, we can use the same instrument to directly compare the Stardust samples with chondritic-porous interplanetary dust particles. These aircraft-collected particles are speculated to have originated in comets, a hypothesis which is now experimentally testable.
Mars has fluvial features over much of its surface which suggest that its climate was much wetter in the distant past, but there is considerable disagreement as to how warm the planet’s climate must have been to form them. Climate modelers (including this one) have had difficulty reproducing warm martian paleoclimates, in part because of Mars’ distance from the Sun, and in part because the Sun itself was less bright at that time. Other groups (e.g., Segura et al., Science, 2002) have suggested that Mars’ climate was warm only transiently, and that the fluvial features were formed by rainout of steam atmospheres created by large impact events. I will show that this latter hypothesis is unlikely, as the amount of water needed to carve the valleys was much larger than this model allows. Ways of augmenting the greenhouse effect of a dense CO2 early martian atmosphere will be discussed.
Pseudotachylite bodies in impact structures are dike-like and consist of angular and rounded wall-rock fragments enveloped by a microcrystalline and sporadically glassy matrix that crystallized from a melt. Knowledge of the formation of pseudotachylite bodies is important for understanding mechanics of complex crater formation. Most current hypotheses of pseudotachylite formation inherently assume that fragmentation and melt generation occur during a single process, either by (1) shock loading, (2) frictional shearing, or (3) decompression. Based on the structure and of pseudotachylite bodies and chemical composition of matrices at the Sudbury and Vredefort impact structures we show that these processes differ in time and space. We demonstrate that the cm- to km-scale bodies are effectively fragment- and melt-filled tension fractures that formed by differential rotation of target rock during cratering. Highly variable pseudotachylite characteristics can be accounted for by a single process, i.e., drainage of initially superheated impact melt into tension fractures of target rocks during late stages of crater formation.
Regoliths on small bodies represent valuable natural laboratories for evaluating various models of impact cratering processes since they may present crater structures or ejecta features that either do not form or are hidden on higher-gravity bodies like the Moon. Quantifying the extent to which impact processes generate and redistribute regoliths on small body surfaces is pivotal to the issue of how to relate meteoritical samples to their asteroidal parent bodies and a better understanding of the processes at work in these unique environments is crucial for designing technologies and techniques for future robotic and human exploration, resource utilization, and impact hazard mitigation. Ejecta blocks represent the coursest fraction of small body regoliths and are important, readily-visible 'tracer particles' for crater ejecta blanket units that may be linked back to specific source craters, thus yielding valuable information on physical properties and constraining various aspects of impact cratering in low-gravity environments.These blocks, launched from the surface of a small, rapidly-rotating, and highly-elongated and irregularly-shaped body, are subjected to a complex dynamical process. Dynamical models of reaccretion of impact ejecta on asteroids thus provide important and necessary tools for a detailed investigation of the distribution and morphology of blocks and finer regolith across their surfaces.
Decades of studies of porphyry-Cu deposits from around the world reveal intimate spatial and genetic connections among magma chamber cupolas, porphyry dikes, zones of alteration, and mineralized stockwork veins. Despite recognition of this close magmatic-hydrothermal relationship, much remains unknown about how the magmatic-hydrothermal connection really works. Trace element concentrations and complex SEM-cathodoluminescent (CL) textures preserved in magmatic and hydrothermal quartz and rutile provide a key to unraveling the complex evolution of porphyry-Cu deposits. Microprobe trace element analyses of Ti and Zr combined with mineral geothermometery, SEM-CL imaging, calculations of Ti diffusion in quartz, and SEM-electron backscatter diffraction (EBSD) mapping of quartz grain orientations offer remarkable insights into how the magmatic fluids generate hydrothermal veins. Calculated temperatures indicate that porphyry systems grow by discrete cycles of transitory high temperature dike intrusions, hydrofracturing, and vein formation. These processes occur on timescales significantly shorter than estimates for the overall time span of porphyry deposit formation, indicating that short-lived magmatic and hydrothermal injections occur repeatedly over a span of ~1 Ma to form a porphyry deposit such as Butte. Crystallographic EBSD maps of quartz combined with inferences from these thermal, temporal, and textural datasets imply that some groups of hydrothermal quartz veins form by remarkably seamless epitaxial nucleation and growth. This formation mechanism requires growth of quartz into open fractures rather than diffusive alteration of host rock. Combining multiple micro-analytical techniques provides a powerful means of investigating the complex evolution of porphyry systems.
The small particles strewn throughout the Solar System are produced by comets and asteroids and reveal the composition of those bodies as well as the dynamical processes by which they evolve. I will show recent work on the structure of the zodiacal light and interplanetary dust cloud as well as freshly produced dust from comets, including the 2007 explosion of comet Holmes.
Physico-chemical modeling is central to understand the important physical and chemical processes that operate in cometary atmospheres (comae). Photochemistry is a major source of ions and electrons that further initiate key gas-phase reactions, leading to the plethora of molecules and atoms seen in comets. The effects of photoelectrons that react via electron impact reactions are important to the overall ionization. Relevant physico-chemical processes are identified within a global modeling framework using reactive gas dynamics to understand observations and in situ measurements of comets and to provide valuable insights into the intrinsic properties of their nuclei; possibly shedding light on issues of comet formation (time and place) and matters of the prebiotic to biotic evolution of life. Details of these processes are presented in the collision-dominated, inner coma of comets; including thermodynamics (e.g., temperature and velocity structure) and photo- and gas-phase chemistry (e.g., composition, gas and electron energetics) throughout this inner region. Prior model results have successfully accounted for the comet Halley water-group composition , in situ measurements of the PEPE instrument onboard the Deep Space 1 Mission to comet Borrelly , S2 in comet Hyakutake , observations of C2, C3, CS, and NS in comet Hale-Bopp [4, 5] and HCN in comet Machholz . This extensive modeling effort to investigate these important cometary processes is highly relevant to ground-based observations of comets and past, on going, and future spacecraft missions to these primitive objects. References  H.U. Schmidt, R. Wegmann, W.F. Huebner, and D.C. Boice, Comput. Phys. Comm. 49, 17 (1988).  D.C. Boice and R. Wegmann, Adv. Space Res. 39, 407 (2007).  C. Reylé and D.C. Boice, Astrophys. J. 587, 464 (2003).  J. Helbert, H. Rauer, D. Boice, and W. Huebner, Astron. & Astrophys. 442, 1107 (2005).  M.V. Canaves, A.A. de Almeida, D.C. Boice, and G.C. Sanzovo, Adv. Space Res. 39, 451 (2007).  D.C. Boice & S. Martinez, “Modeling the Coma of Comet Machholz,” (oral), IAU Symposium 263, 3-7 August, Rio de Janeiro, Brazil, abstract #OS14-05 (2009).
Organic compounds are observed in star-forming regions of space. Such compounds were present during the formation of our Solar System, as they were incorporated and preserved in carbonaceous chondrite meteorites. Organic matter in carbonaceous chondrites exists as two components: compounds that are soluble in polar organic solvents, and insoluble organic matter (IOM), which is a kerogen-like, macromolecular material. Soluble organic compounds include those of pre-biotic interest, including amino and carboxylic acids. The Tagish Lake meteorite fell on January 18, 2000 onto the frozen surface of Tagish Lake in northern B.C. Samples of the meteorite were recovered within a week of the fall and kept frozen and untouched by hand. The meteorite is an ungrouped carbonaceous chondrite, containing 2.5 wt % organic carbon. The circumstances of its fall and retrieval provide a unique opportunity to study organics in the early Solar System; Tagish Lake is the world's most pristine meteorite. The Tagish Lake meteorite is heterogeneous, with a range of macroscopic characteristics, including differences in the proportions of matrix and chondrule-like objects, and mineralogy. Remarkably, these lithological differences are borne out in differences in the organic matter: IOM in different samples varies in terms of H/C and H isotopic composition over a range that encompasses several carbonaceous chondrite groups; soluble organics vary from sample to sample in abundance and type. Ongoing work on amino acids shows a larger complement in our samples than previously observed. These results suggest that aqueous alteration on the asteroid parent body has played a role in the modification (destruction and/or synthesis) of interstellar organic matter, and have significant implications for the mechanisms involved in the formation of pre-biotic compounds, and the delivery of such compounds to the early Earth and other planets.
Recent studies of variations in the Sm, Nd, & Ba isotopic compositions of bulk meteorites have yielded contradictory results, in terms of early planetary evolution. A key question is whether the observed variations in 142Nd are due to different contributions of nucleosynthetic (p-, s-, & r-process) components, or are caused by differing Sm/Nd ratios generated through planetary differentiation whilst 146Sm was alive. Whereas carbonaceous chondrites are deficient in p-process 144Sm, the 148Sm/154Sm & 150Nd/144Nd ratios of chondrites; eucrites; shergottites; the Moon; and the Earth are constant, indicating that the Solar Nebula possessed a uniform ratio of r/s nuclides. However, carbonaceous chondrites exhibit anomalies in 135Ba and 137Ba consistent with an excess in r-process nuclides. The Ba isotopic compositions of ordinary chondrites and eucrites are indistinguishable from that of the Earth. Thus the Ba, Sm, and Nd isotope ratios that are sensitive to variations in the r/s ratio are consistent, except those of carbonaceous chondrites, suggesting that whereas s- and r-process nuclides were homogeneously distributed in the inner Solar Nebula, radial isotopic heterogeneities did exist within the nebula. The issue of whether different planetary bodies accreted with the same bulk solar Sm/Nd ratio remains unresolved. If Earth is chondritic in its Sm/Nd composition, an early-formed reservoir must exist, but no geochemical evidence of this has yet been found. The eucrite parent body has chondritic Sm/Nd, whereas data from Mars and the Moon are less conclusive.
Natural impact craters occur in heterogeneous target materials. Rocks and ices on planetary surfaces are layered, faulted and fractured, and in general composed of parts with differing mechanical properties. Layering can lead to concentric (nested) crater morphology, but the effect of sub-vertical inhomogeneities in crater formation remains poorly understood. The presence of a dominating set of fractures may result in a polygonal crater shape drastically different from the highly idealized ”circular hole in the ground” –concept. Such polygonal craters have been known to exist on the Moon for at least a century, and also the association between orientations of the straight crater rim segments and surrounding tectonic structures has long since been established. The best-known example of a polygonal impact crater is the square-shaped Meteor Crater in Arizona. Similar structurally controlled craters occur on all types of bodies with rigid cratered crusts in the Solar System, regardless of the type of the target material. When a large enough population of polygonal craters is studied, their straight rim segments can be used as an additional tool for paleotectonic mapping of planetary surfaces. They are particularly useful in studying large-scale crustal structures – like the fracture zones surrounding lunar multi-ring basins – in highly cratered areas with only a few visible faults or ridges. Three different formation mechanisms (preferred excavation along fracture strike, and slumping or thrusting along fault planes) have been proposed, but the significance of each mechanism remains to be studied. Polygonal craters also seem to be most ubiquitous in a certain size range, which may reflect the different formation mechanisms, or target layering. In addition to crater shape, target structures affect the distribution of ejecta and – based on scarce experiments and hypotheses – possibly also the formation of rays. Thus, the study of polygonal impact craters can help understanding crater and ejecta formation in natural heterogeneous targets, the influence of impact basins on their surroundings, and the tectonic evolution of the Moon. Integrating Lunar Reconnaissance Orbiter, Clementine and Lunar Orbiter datasets offers great possibilities to address this issue.
On Earth, the Critical Zone ranges from the outer vegetation canopy to the lower limit of groundwater. In that zone, bedrock equilibrated at depth re-equilibrates to surficial conditions through chemical and physical processes that are mediated by biota (the “weathering engine”). Weathering processes are documented by the depth distributions in regolith of i) elements and minerals in the solid phases, ii) grain size, and iii) microbiota. Investigations of these gradients will be discussed for weathering systems on Earth. Such terrestrial interpretations can inform interpretations of observations from Mars. Data from Mars may also provide a window into the processes that may have occurred on the early Earth. Of particular interest, where organisms accelerate or inhibit reactions, these biotic effects may affect the observed gradients. Such effects might be useful as biosignatures, or “organosignatures”. For example, the weathered surface layers of two of the oldest-known basalt-derived paleosols—the Mount Roe (2.76 Ga) and the Hekpoort (2.25 Ga)—were considerably depleted in Fe and P but not in Al, consistent with the presence of organic ligands. Cu, retained in the Mount Roe paleosol but considerably mobilized in the Hekpoort paleosol, may furthermore document formation under an anoxic atmosphere and an oxic atmosphere, respectively. As we learn to read gradients in chemistry, mineralogy, and grain or corestone size, our understanding of the history of the surface of Earth and Mars will improve.
Presolar materials consist of both circumstellar grains that formed around evolved stars and in supernova ejecta, and interstellar or protosolar grains and organic matter that formed in cold molecular clouds. Both types of materials survived the collapse of the molecular cloud from which our solar system originated and were incorporated into the primitive meteorites, interplanetary dust particles (IDPs) and micrometeorites in which we find them today. Laboratory studies of these components and their host materials provide information about stellar and interstellar grain formation environments, and about conditions acting in the early solar nebula. Presolar silicates are the newest major addition to the presolar grain inventory. In addition to providing information about the circumstellar environments in which they formed, studies of these grains provide opportunities to investigate secondary processes taking place in the parent bodies in which they are found. H and N isotopic anomalies, thought to originate through low-temperature interstellar chemistry, are commonly observed in IDPs and some primitive meteorites. Despite the fact that these anomalies are thought to be hosted by organic matter, C isotopic anomalies are much less common. Recent work suggests that both primary and secondary processes are probably responsible for the low abundances observed.
Life is shaped by, and shapes, the flow of energy in the environment around it. Qualifying and quantifying the specifics of this relationship provides constraints on ecology and biogeochemistry, or their respective “astrobiological projections”, habitability and biosignatures. Environmental energetics are typically considered at a thermodynamic level, e.g., with reference to the availability of Gibbs energy. However, the power (energy per unit time) dimension of life’s relationship with energy offers significantly greater resolution in characterizing life’s dependence and imprint upon host environment energetics. The life-power relationship and its bearing on habitability and biosignatures will be considered at a conceptual level, and subsequently in specific application to the question of the habitability/ecology of serpentinizing systems for methanogenic microorganisms.
The thermal histories of terrestrial planets are investigated using two parameterized mantle convectioin models for either Earth like planets and planets with no active plate tectonics. Our models estimate the amount of volatile in mantle reservoir, and calculate the outgassing and regassing rates. The kinematic viscosity of the mantle is thus dynamically affected by the activation energy through a variable concentration in volatile. The rate of volatile exchanged between mantle and surface is calculated by balancing the amount of volatiles degassed in the atmosphere by volcanic and spreading related processes and the amount of volatiles recycled back in the mantle by the subduction process. The degassing effect depends on the intensity of convection and on the amount of volatile in a partial melt zone below the lithosphere. The regassing effect is dependent on the subduction rate and on the amount of volatile present on a serpentinized layer within the subducting slab. The optimum efficiency factors found are in the range of 0.01-0.06 for degrassing/regassing processes, in agreement with more recent estimates. An important effect of the volatile cycling process is a negative feedback loop that results in a general trend to adjust the mantle volatile content in time to a value set by the energy balance in the system. As a result, the initial amount of volatile in the mantle is rendered irrelevant for late stage of thermal evolution. In the case of no plate tectonics, the opposite effect takes place: initial volatilization plays an important role through entire evolution.
The MESSENGER spacecraft has completed three flybys of the planet Mercury, and is preparing for orbital investigation from March 2011 - March 2012. The spacecraft's instrument complement includes imagers to map spectral properties and morphology of the surface; an ultraviolet through infrared spectrometer to measure the surface and exosphere; X-ray, gamma-ray and neutron spectrometers to measure elemental composition of the surface; a laser altimeter to measure topography; and a magnetometer and particle detectors to characterize the magnetic field and magnetosphere. First results from the flybys have already begun to revolutionize our understanding of the planet. There is evidence for formation of a thin crust consisting of volcanic flows of differing composition, and locally pyroclastic volcanism has been important. Tectonics are concentrated in impact basins, but each basin exhibits a distinct style of deformation. The dominant style of tectonics is lobate scarps, which are more abundant than understood previously, suggesting greater amounts of global contraction of Mercury than had been recognized. The exosphere is highly time-variable, with large variations in abundances on the timescale of months. The iron content of the surface is surprisingly large, but apparently present mostly as non-silicate phases. This presentation will summarize MESSENGER's goals and objectives, what has been learned from the flybys, and the plan for the orbital mission.
When the asteroid 2008 TC3 impacted Earth on 7 October 2008 over Northern Sudan, the resulting meteorite fragments became the first to be collected from an asteroid observed on its collision course with Earth. Some 600 meteorites were collected and called Almahata Sitta, referring to the region of Sudan’s Nubian Desert in which the strewn field is located. Though initially classified as a ureilite, the fragments of Almahata Sitta show a wide range of physical properties and elemental compositions. Among these fragments, one sample (#25) was quickly classified as an H5 chondrite rather than a ureilite. Since this time, a variety of other lithologies likely associated with Almahata Sitta have been identified including fine- and coarse-grained ureilites, ordinary H chondrites, enstatite chondrites and sulfide-metal assemblages. In this study, we employ two-step laser-desorption laser-ionization mass spectrometry to analyze the bulk polycyclic aromatic hydrocarbon contents of a variety of Almahata Sitta samples including ureilitic and chondritic lithologies. These bulk studies show that Almahata Sitta meteorites lack the diversity of alkylation series of parent PAHs commonly observed in Murchison and other carbonaceous chondrites. Furthermore, spatial mapping of aromatic species across the surface of a fragment of sample #4 reveals heterogeneous distribution of PAHs and allows detection of a signal at m/z 181 believed to be the amino acid tyrosine. This peak is not observed in bulk measurements. Potential sources of terrestrial contamination are discussed and we conclude that sample #25 – an H5 chondrite – was included in asteroid 2008 TC3 as a foreign clast at the time it impacted Earth.
Thermal emission spectroscopy provides information about the mineralogical composition of the martian surface. I will review the strengths and limitations of the technique as well as previous global results. I will then discuss three examples of how high-resolution thermal emission spectroscopy has contributed information about various aspects of crustal evolution and surface alteration. Rock-dominated surfaces relatively enriched in olivine and pyroxene overlie degraded plains that are depleted in these minerals; these stratigraphically distinct units likely represent separate episodes of upper crust formation. High resolution thermal imagery allows for investigation of rock compositions and their relationships to the mobile surface sediment layer. In some areas, soils eroded from nearby bedrock are depleted in olivine relative to the source, suggesting that olivine degradation or dissolution may be an important aspect of soil formation processes. Lastly, crater ejecta blankets in Tyrrhena Terra are less mafic than surrounding target material. Based on geological context, the most likely explanation for this is that early surficial weathering altered the cratered plains to decrease plagioclase abundance. Less-altered subsurface materials were then later exposed by impact. The weathering regime necessary to produce this trend is not inconsistent with early, low water-rock ratio acidic weathering invoked from other studies.
I will discuss two topics relevant to impact processes in the inner Solar System. First, the Moon is being bombarded by the near-Earth objects population while circulating the Earth. The geocentric orbital motion induces a mild asymmetry in the cratering rate, with the apex of motion enhanced. A detailed orbital model was used to model lunar bombardment and show that there is a pole/equator depression of about 10%, and that the apex/antapex ratio is about 1.3 (the latter in agreement with counts of young lunar rayed craters). Secondly, we will consider the ramifications of NEO impacts on Mercury, which much launch impact ejecta into heliocentric orbit. Studies of the orbital evolution of this debris show that former estimates of mercurian meteorite delivery were too pessimistic. These simulations also have strong implications for the hypothesis that the proto-mercurian mantle was stripped by a giant impact and then disposed of into the Sun.
The upper mantle is comprised mostly of nominally anhydrous olivine and its polymorphs. These minerals can host significant amounts of hydrogen as defects in their crystal structures at high pressure. However, the amount of hydrogen in the upper mantle is poorly constrained. The presence of hydrogen affects many properties of nominally anhydrous minerals; such as electrical conductivity, transformation rates, strength, elasticity, seismic velocities, melting, and phase equillibria. Conceptual understanding of how hydrogen affects olivine is integral to understanding Earth. We have experimentally hydrated olivine samples and altered them at high pressure and temperature to simulate upper mantle processes. Hydrogen significantly enhances transformation rates of olivine into its higher pressure polymorphs, wadsleyite and ringwoodite. Growth rates determine the likelihood that a metastable wedge of olivine could persist into the mantle transition zone inside cold, subducting slabs. The metastable olivine wedge theory was postulated as a mechanism for triggering deep focus earthquakes (300-700 km depth), which are enigmatic because materials are expected to be ductile in this regime. The growth rate measurements presented in this talk imply that metastable olivine is incompatible with even sparse amounts of hydrogen (75 ppm-wt H2O). Confirmation of the existence or absence of metastable olivine wedges would constrain the physical and chemical conditions of their locations. For the second part of this talk deuterium-hydrogen interdiffusion results for olivine are presented. These experiments were conducted to simulate hydrogen self diffusion, where deuterium serves as a traceable species. Our diffusion coefficient measurements are used to help determine point defect chemistry in olivine. We also create an electrical conductivity model for hydrous olivine, based on the Nernst-Einstein relation. Comparison of this model to geophysical magnetotelluric data allows for first order calculations of the hydrogen contents in the upper mantle.
High field strength elements (HFSE) are important geochemical indicators in many geological settings, in particular in subduction zones. The HFSE signature of arc magmas is depleted relative to MORB, and the bulk silicate Earth is subchondritic with respect to Nb/Ta. Rutile is an important HFSE sequestering phase in many rocks, and residual rutile in subducted slabs is often invoked as the cause of the observed HFSE depletion in arc magmas. This assumed low solubility in slab-derived fluids is based on experiments at low temperatures, and some experiments at high P-T in pure H2O. However, large crystals of rutile are often observed in hydrothermal veins and ore deposits, suggesting that rutile is not insoluble in all fluids. Despite this apparent contradiction, very little experimental work on rutile solubility at elevated P-T in more geologically realistic fluids exists. This talk presents new data on the solubility of rutile in supercritical aqueous fluids and the partitioning of Nb and Ta in the system. The experimental data indicates that Ti mobility is greatly increased in chlorine-, and in particular fluorine-rich fluids, and thus fluid composition and rutile solubility are inherently linked. Moreover, analysis of experimental run products reveals that the saline fluids investigated are capable of strongly fractionating Nb from Ta. Calculated rutile/fluid partition coefficients indicate that Ta is much more mobile in saline fluids than previously thought, while Nb is strongly retained in rutile. This has implications for the processing of HFSE in subduction zones, as HFSE retention is not simply due to the low solubility of rutile. Rather it is a more complex problem of changing fluid composition on equilibration with different mineralogies. On slab dehydration, Nb retention in rutile is likely, while Ta is more easily mobilised in the fluid, producing a viable mechanism for Nb/Ta fractionation on a local scale. This mechanism also could produce rutile grains of variable Nb/Ta, as are observed in natural samples, and suggests that Nb/Ta budgets of whole-rock samples must be treated with some caution. This data has profound implications for the modelling of global HFSE cycles.
Conventionally Solar Activity is measured basing on the number of Sun Spots on the visible solar disk. Direct observations of sun spots go back to 1826 A. D. Attempts have been made to construct solar activity back to 1700 A.D. This talk deals with a new approach to measure solar activity in the past 35,000 years based on solar plasma emitted by the Sun. Fortunately there exists a veritable proxy for the solar plasma. Cosmic rays as they enter the heliosphere are strongly modulated by the solar plasma. This results in appreciable change in the cosmic ray flux in the near Earth environment which is inversely proportional to the solar activity. If cosmic ray flux can be accurately measured in the past, one has a direct measure of the solar activity in the past. We have proposed that the most direct and accurate method of measuring paleo-cosmic ray fluxes is to measure cosmic ray produced 14C in polar ice (Lal et al., Nature, 346, 350, 1990). It must be stressed that the cosmic ray flux in the polar region is unaffected by any changes in the geomagnetic field intensity. I present records of solar activity in two time frames: (i) during the past 35,000 years, and a higher resolution record (ii) in the past 1000 years.
High-pressure phase equilibrium experiments on Apollo 15 Group A, green glasses exhibit liquidus multiple-phase saturation points, with olivine and orthopyroxene, at 2.1 GPa; however the high-Mg# Group C green glasses have been shown to have a shallower liquidus multiple-phase saturation point at 1.5 GPa (Elkins-Tanton et al., 2003). Assimilation of overturned magma-ocean cumulates can explain this inverted cumulate Mg# profile, but the chemistry of the original melts is difficult to discern. Forward modeling of high-pressure melts of garnet-lherzolite and assimilation of evolved magma-ocean cumulate lithologies can reproduce the Apollo 15 Groups A and C major element compositions, and this melt-assimilant scenario provides an explanation for the presence of negative-Eu anomalies found in the Apollo 15 ultramafic glasses.
The Moon, with a preserved cratering, rock, and mineral record dating back to well before 4 Ga, holds the key to unlocking the ancient bombardment history of the Earth at the dawn of life, and can also inform us of the early biological potential of other rocky worlds in our solar system, such as Venus and Mars. In particular, a period dubbed the Late Heavy Bombardment (LHB) at ~3.9 Ga, which has been suggested based on analyses of lunar crustal rocks and impact melts, may have profoundly affected terrestrial life, the earliest evidence of which dates back to 3.83 Ga. To evaluate the thermal effects of the LHB, a simulation was designed consisting of: (i) a stochastic cratering model which populates the surface with craters within constraints derived from the lunar cratering record, the size/frequency distribution of the asteroid belt, and dynamical models; (ii) analytical expressions that calculate a temperature field for each model crater; and (iii) three-dimensional thermal models of lunar, terrestrial, and martian lithospheres, where craters are allowed to cool by conduction in the subsurface and radiation at the surface. This simulation can constrain the intensity of the LHB (or the post-accretionary bombardment) through comparisons to data derived from lunar samples, particularly from microprobe studies of lunar zircons and apatites. However, more data are needed. Lunar bombardment history can be further elucidated through detection of previously unrecognized impact basins, as well as age-dating the major known basins, both in-situ and through sample return missions.
Japanese asteroid sample return mission, Hayabusa, successfully landed on asteroid Itokawa and returned its sample capsule in June this year. Itokawa is an S-type asteroid, showing and LL chondrite like composition with low degree, developing space weathering. JAXA plans to launch Hayabusa 2 spacecraft to a C asteroid, 1999JU3 in 2014, which will return its sample in 2020. In my talk, I will review the results of Hayabusa mission and try to elaborate on what is expected from Hayabusa 2 mission, including the differences in space weathering effects between S- and C-type astroids.
Given observations of the Enceladus plumes and the icy surface from which they arise, we ask whether liquid water exists below the surface and at what depth it might occur. We have estimates of the power radiated by the surface, the power carried by the gas as latent heat, the velocity distribution and composition of the gas, and the velocity distribution, size distribution, and composition of the particles. With better observations and better data analysis techniques, we are constantly improving these estimates, and with better hydrodynamic models we are getting closer to answering the questions about liquid water. The talk will be a progress report and a preview of coming attractions.
1) Cleaning of lunar dust adhered to astronaut spacesuits is of critical importance for long-term lunar exploration. We are developing three kinds of cleaning systems that in-volve the use of electrostatic and magnetic forces. One of the systems employs an alter-nating electrostatic field that forms a barrier on the surface of fabrics. Two-phase rec-tangular voltage is applied to parallel wires stitched into the insulating fabric. Particles are flicked outwards from the fabric. It was demonstrated that more than 70% of the adhered dust can be cleaned by this system. The second system employs a combination of electrostatic separation and electrostatic transport. A high voltage is applied between a Mylar sheet positioned under the surface fabric and the electrodes, which contains holes. Because of the electrostatic force dust adhered to the fabric is captured by the holes of the plate electrode. The captured dust is transported by the traveling wave and transferred to a collecting bag. The observed cleaning rate was higher than 60%. The last system utilizes magnetic force based on the fact that lunar dust is magnetic. The de-vice consists of a shaft, stationary multi-pole magnetic roller, rotating sleeve, plate magnet, and collection bag. Magnetic lunar dust is attracted to the stationary magnetic roller and transported via the rotating sleeve by means of magnetic and frictional forces. The magnetic roller is designed such that a repulsive force acts on the particles at a cer-tain position. When the dust is transported to this position, particles are separated from the sleeve, and are attracted to the plate magnet facing the release position. The dust particles then gather in the collecting bag that covers the plate magnet. The advantages of the system are that it is very simple, and that it works without power consumption. The observed cleaning rate was about 50%.; 2) In order to realize a long-term lunar exploration, it is essential to develop a technology for transporting lunar soil and ice for in-situ resource utilization. We are developing a particle transport system that utilizes electrostatic traveling-waves. The conveyer con-sists of parallel electrodes printed on a plastic substrate. Four-phase rectangular voltage is applied to the electrodes to transport particles on the conveyer. Mechanical vibration was applied to the conveyer to transport particles more efficiently. The results of our investigation are as follows. (1) The observed transport rate in air was 13.5 g/min for a conveyer with a width of 100 mm. By performing numerical calculations based on the 3D distinct element method, we predicted that the system performance would improve in the high-vacuum and low-gravity environment on the moon. (2) Power consumption in this system is very less. It was only 10 W for a conveyer with an area of 1.0 m2. (3) Crashed ice mixed with lunar dust can be transported with this system. (4) We demon-strated an inclined and curved transport path as well as a flat and straight transport path. In addition, we demonstrated that transportation of particles through a tube and accu-mulation of scattered particles were also possible.; and 3) A unique cleaning system has been developed utilizing electrostatic force to remove lu-nar dust adhered to the mechanical parts, such as bearings and seals, used for lunar ex-ploration. A single-phase rectangular voltage is applied to parallel electrodes printed on a flexible substrate to remove the dust. More than 90% of adhered dust was repelled from the surface of the slightly inclined device in a vacuum, and the cleaning perform-ance of the system would be further improved in the low-gravity environment of the Moon. This technology is expected to increase the reliability of equipment used in long-term manned and unmanned activities on the lunar surface.
Numerical modeling is an important part of multi-disciplinary approach to impact cratering. Although basic equations are well-known and modern computer capabilities are almost unlimited, the results of numerical simulations do not correlate well with observations and laboratory experiments. It means that some important physical processes are not included into the models. As impact ejecta in terrestrial records are sparsely preserved, deposits of volcanic eruptions may play an important role in model improvement and benchmarking. Understanding of impact ejecta is a crucial factor for the current planetary remote sensing and sampling in the future. The following problems will be discussed: (1) why are lunar meteorites more rare than meteorites from Mars; (2) is the K-Pg boundary the result of the Chicxulub impact; (3) what are the possible mechanisms of ejecta separation into two (or more) layers?
The isotope ratio of element vanadium (51V/50V) is interesting for cosmochemical studies because irradiation can theoretically produce large isotope anomalies. It has been suggested as a potential test for the so-called X-wind model. However, due to the low abundance of 50V, no high precision data have thus far been published. In Oxford, we have developed a technique to measure the V isotope ratios at high accuracy and precision. Preliminary data for chondrites and achondrites reveal that all meteorites are significantly offset from silicate samples from Earth. Does this difference reflect solar system heterogeneity? Or is it caused by terrestrial differentiation processes?
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