LPI Seminar Series


The LPI Seminar Series brings prominent scientists to the LPI to present on a broad array of scientific disciplines that advance our understanding of the solar system. The seminar series, which began in September 1969, has brought many notable contributors from numerous research and academic institutions to the LPI. Seminars are typically held on Thursdays from 3:00-4:00 p.m. US/Central, but dates and times are subject to change. All seminars will be held virtually until further notice.

Sign up for LPI Seminars to receive email notifications of upcoming seminars and details on how to join the virtual seminar. For more information, please contact Patrick McGovern ([email protected]) and Sam Crossley ([email protected]).

See also the Rice University Department of Physics and Astronomy Colloquia and the Department of Earth Science Colloquia pages for other space science talks in the Houston area.

View Recordings  

January 2004

Friday, January 30, 2004 - Lecture Hall, 3:30 PM
Munir Humayun, University of Chicago
Niobium in Meteoritic Troilite: Implications for the Earth's Core

Niobium (Nb) is a nominally lithophile element, until recently believed to be distributed only in the silicate portions of planetary bodies. Evidence for a depletion of Nb relative to Ta (Tantalum), its geochemical twin, has led to proposals that it may have been selectively incorporated into the Earth's core. High pressure experiments have provided supporting evidence for a weakly siderophile behavior of Nb relative to other high field strength elements (Ta, Zr and Hf), with the implication that Nb is siderophile only at high pressures. I will present new data obtained by laser ablation microanalysis to show that Nb is selectively distributed in troilite from Canyon Diablo and other IAB irons, but that it is absent from Fe-Ni metal. These results show that Nb behaves like a chalcophile element (S-loving) and that composition (not pressure) is the controlling parameter in Nb partitioning. This finding has important implications for the dating of the Canyon Diablo troilite by the Nb-92/Zr-92 chronometer, and possibly for dating the time of formation of the Earth's core.

February 2004

Wednesday, February 4, 2004 - Lecture Hall, 3:30 PM
Catherine Hier Majumder, University of Minnesota
Advances in Visualization and Numerical Simulation: Examples from Finite Prandtl Convection, Wavelets Analysis, and Dark Streaks on Mars

This talk focuses on the use of visualization and numerical techniques to address problems related to planetary science. I will discuss finite Prandtl number convection, inverse methods and data assimilation, wavelet analysis of convection and geoid data, and numerical simulations of dark streaks on Mars. The finite Prandtl number convection studies examine how convection behavior changes as Prandtl numbers are increased to as high as 2 x 104, on the order of Prandtl numbers expected in very hot magmas or mushy ice diapirs. I found that there are significant differences in the convection style between finite Prandtl number convection and the infinite Prandtl number approximation even for Prandtl numbers on the order of 10r. This indicates that the infinite Prandtl convection approximation might not accurately model behavior in fluids with large, but finite Prandtl numbers. The section on inverse methods and data assimilation discusses the technique of four dimensional variational data assimilation (4D-VAR) developed by meteorologists to integrate observations into forecasts. This technique allows us to study the predictability and dependence on initial conditions of finite Prandtl simulations. It promises to be useful in a wide range of geological and geophysical fields, including mantle convection, hydrogeology, and sedimentology. Examples will be shown of the use of wavelet analysis to help image and scrutinize at small-scales both convection simulation and geoid data. I will also show examples of simulations investigating whether liquid water plays a role in the formation of active dark streaks on Mars.

Friday, February 13, 2004 - Lecture Hall, 3:30 PM
Anne Peslier, University of Houston
Water in the Earth's Mantle - The Perspective from Peridotite "Anhydrous" Minerals

Small amounts of water in rock-forming minerals can potentially have important influence on the chemical and physical properties of their host rocks. The melting point, ionic diffusion properties, phase relations, redox conditions, transmission of seismic waves, deformation, and conductivity may be affected to varying degrees by the water contents in minerals. The focus here will be on the role of water in the Earth's upper mantle. More exactly, we will look at the amount of water, or more exactly hydrogen, that so-called "nominally anhydrous" minerals such as olivine and pyroxenes can incorporate in their structure. Knowledge of the amount of water in these main phases of the mantle is key to constraining many essential parameters for understanding mantle geodynamics, and has potential bearing upon the entire cycle of hydrogen in the Earth. In particular, our Fourier transform infrared (FTIR) data are here used to determine the olivine and pyroxene water contents from sub-arc mantle-wedge peridotite xenoliths and to compare them with those of spinel peridotite xenoliths farther inboard from the continental margin, taken to represent the ambiant mantle unaffected by subduction. The calculated water contents of the whole-rock peridotites range from 34 to 195 ppm H2O. The olivine and pyroxene water contents correlate positively, though roughly, with indices of melting (e.g. bulk-rock aluminum content), and correlate negatively with Fe3+/(Fe total) ratios in spinel and oxygen fugacities calculated from them. These correlations indicate that although H behaves as an incompatible element in these minerals during partial melting, the main control of H incorporation in their structure is the oxidation state of the peridotite. The anhydrous phases of the mantle wedge, a typically oxidized environment, thus appear to incorporate very little water from fluids or melts from the slab that may transfer through it to feed volatile-rich highly explosive volcanoes lying above.

Wednesday, February 25, 2004 - Lecture Hall, 3:30 PM
Nicolle Zellner, Lawrence Livermore National Laboratory
Impacts on Earth - Flux and Delivery of Biomolecules

Understanding the impact flux in the Earth-Moon system and the delivery of organic molecules has important implications for the origin and evolution of life on Earth. Geochemical analyses of lunar impact glasses and chemical analyses of shocked organic liquids can aide us in this understanding.<br><br>Lunar impact glasses possess the composition of the original fused target materials at the sites of impact and allow for geochemical exploration of the Moon, including places not visited by the astronauts. Additionally, they can be used to constrain the impact history in the Earth-Moon system. A subset of these glasses has been dated by the 40Ar/39Ar method in order to determine the impact flux, and results will be presented.<br><br>Recent detections of biomolecules in space and their presence in meteorites lead us to believe that they can form easily and survive extreme impact events. While shock experiments have already been carried out to understand the effects of pressure and temperature on the chemistry of amino acids during deliver to Earth by comets, no experiments regarding the impact delivery and survivability of sugars have been done. I propose to carry out a study to understand how glycolaldehyde and dihydroxyacetone, two of the simplest sugars, react under the extreme pressures and temperatures of simulated terrestrial impact events. Ideas will be presented.

April 2004

Monday, April 5, 2004 - Lecture Hall, 3:30 PM
Molly McCanta, Brown University
Pyroxene as a Recorder of Oxygen Fugacity: Implications for the Martian Interior

The oxidation state of volcanic materials on a planet reflects the degree of oxidation of the magma source region and, possibly, the additional effects of magma interaction with the near surface environment. The intrinsic oxygen fugacity (fO2) of a magma is a direct measure of its oxidation state. It is vital to understand the fO2 of a magma as it influences the crystallization sequence of the magma, as well as the composition of the resulting minerals. To this extent, we have developed and applied two Ca-pyroxene oxybarometers to the Martian meteorites in order to gain insight into magmatic processes on Mars. These oxybarometers rely on the partitioning of the multivalent elements Eu and Fe.

Friday, April 16, 2004 - Lecture Hall, 3:30 PM
Harry Becker, LPI
New Estimates of Highly Siderophile Element Abundances in the Earth's Primitive Mantle: a Poor Match With Chondrites and a Link to Lunar Impact Breccias

The relatively high abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh. Pd and Au in the Earth's mantle suggest that mantle and core are not in equilibrium regarding these elements, which should have been extracted nearly quantitatively into the core. Hypotheses proposed to explain the HSE excess and near-chondritic ratios of these elements in the mantle include late accretion of a small mass fraction of chondritic material after core formation (Late veneer hypothesis), decrease of metal-silicate partition coefficients at high pressures and metal-silicate equilibration at the bottom of a magma ocean, incomplete metal-silicate separation during core formation, or contamination of the mantle with outer core material. Each of these models has its problems. More precise estimates of the HSE composition of the primitive mantle may provide further clues. Recent analytical progress now permits concentration measurements of HSE in peridotites at a 2-7% level of precision. New concentration data for HSE in terrestrial mantle peridotites will be presented along with new estimates for HSE abundances in the primitive mantle. The results show that the abundance pattern of the HSE in the primitive mantle is unlike that of any known chondrite class. The only material that provides a good match for the HSE pattern of the Earth's mantle so far are the lunar impact melt breccias from Serenitatis. If confirmed, this would represent a strong argument for a common origin of the HSE on the lunar surface and in the Earth's mantle. More detailed studies of the HSE distribution in lunar impact breccias and their compositional variability are necessary.

Friday, April 23, 2004 - Lecture Hall, 3:30 PM
Tracy Gregg, University at Buffalo
Loki: A Round 'Mid-Ocean Ridge' on Io

Ionian paterae are a class of volcanic feature that are characterized by irregular craters with steep walls, flat floors, and arcuate margins that may or may not exhibit nesting. Loki (310\260W, 12\260N) is Io's largest patera at ~200 km in diameter, and may account for 25% of Io's total heat flow when erupting. Earth-based infrared data, as well as information collected using the Galileo Near-Infrared Mapping Spectrometer (NIMS) and the Photopolarimeter Radiometer (PPR) have been used to interpret Loki's eruption style. Debate continues over whether Loki's occasional (periodic or not) temperature increases are due to an overturning lava lake within the patera, or to an eruption of surface flows on the patera floor. Interpretation of model results and comparisons with active terrestrial lava lakes suggest that Loki behaves quite differently from active lava lakes on Earth, and that the best possible terrestrial analog--volcanologically and magmatically, although not tectonically--is a fast-spreading mid-ocean ridge.

May 2004

Wednesday, May 19, 2004 - Lecture Hall, 3:30 PM
Ben Bussey, Applied Physics Laboratory, Johns Hopkins University
The Lunar Poles: An Ideal Location for a Future Outpost?

The Moon's poles have been identified as a target for extensive investigations by the Lunar Reconnaissance Orbiter, to be launched in 2008. This is because the poles possibly represent an ideal location for a long duration stay on the lunar surface. The prospect of constant solar energy exists, possibly negating the requirement for a nuclear power source. Additionally the illumination conditions near the poles permit operations in a relatively benign thermal environment. Both Clementine and Lunar Prospector have identified resources in the permanently shadowed areas in the polar regions.<br><br>This seminar will review the search for water ice in permanently shadowed craters, and show the results of my research into the illumination conditions at the lunar poles. I will then discuss the overall implications for the ideal location of a possible future outpost.

Friday, May 21, 2004 - Lecture Hall, 3:30 PM
Tom Stepinski, Lunar and Planetary Institute
Martian Valley Networks - New Approaches to Old Problems

Valley networks are common geomorphic features on Mars that are visually reminiscent of terrestrial river systems. This resemblance gave rise to an early suggestion that valley networks are remnants of ancient river systems. However, even so the overall morphology of valley networks continues to suggest their origin by runoff, many detailed features point to their origin by groundwater sapping. Despite many studies the process of valley networks formation has not been positively identified. The topic is of high interest, because of its consequences for our understanding of history of water on early Mars. In this talk I will present a new approach to study Martian valley networks that can be succinctly described as "computational geomorphology." The networks and their underlying drainage basins are identified and acquired using a computer algorithm. Subsequently, they are analyzed and classified using integral-geometry and neural maps methods. This modern analysis reveals a systematic difference between Martian valley networks and terrestrial river systems. I will discuss valleys formation scenarios that are compatible with observed morphologies.

June 2004

Friday, June 4, 2004 - Lecture Hall, 3:30 PM
Gareth Collins, Lunar and Planetary Laboratory, University of Arizona
The Effect of Target Rheology on Impact Crater Morphology: Insight from Nume rical Simulations of the Silverpit and Chesapeake Bay Impacts

The geologic history of the Earth is punctuated by large meteor impacts. The local, regional and global environmental consequences of these collisions have played an influential role in the evolution of the Earth and life as it exists today. Assessing the magnitude of impact-related environmental catastrophes requires accurate knowledge of the energy released during the impact, which depends upon the size, composition and velocity of the impactor. In many cases, simple empirically-derived scaling relations may be used to establish reasonable estimates of the impact energy from measurements of the final crater. However, in situations where strong rheologic variations exist between layers within the target these simple relations do not hold. Two prime examples of the effect of target strength variations on crater morphology are the Chesapeake Bay structure, Virginia, USA, and the putative Silverpit impact structure, North Sea, UK. I will discuss recent numerical modeling of these two craters that provides insight into how target rheology can influence final crater morphology, and establishes more reliable estimates for the energy released during these impact events.

Friday, June 11, 2004 - Lecture Hall, 3:30 PM
Walter Kiefer, Lunar and Planetary Institute
The Devana Chasma Rift System, Venus: Morphology, Evolution, and Comparison with Earth

Devana Chasma is the type example of an extensional rift system on Venus and is similar in some ways to continental rift systems on Earth. Recent gravity and topography analyses have improved our knowledge of the structure and evolution of Devana Chasma. Continental rifts on Earth typically have a half-graben morphology, with a single dominant boundary fault in each rift basin. In contrast, Devana Chasma has a mixture of half-graben and full graben, in which there are prominent boundary faults on both sides of the basin. This morphological difference may be due to differences in the lithospheric structure on the two planets. There is a 600 km offset in the trend of Devana Chasma near 8 North latitude. This offset zone is marked by a a change in fault orientation, by decreases in fault density, tectonic extension, and rift flank height, and by the absence of mantle density anomalies. Devana Chasma probably formed as two distinct, mantle plume-driven rifts. One rift propagated southward from Beta Regio and the other propagated northward from Phoebe Regio. The offset zone in the rift structure is the zone where the two rift tips interacted.

Friday, June 18, 2004 - Lecture Hall, 3:30 PM
Richard Schultz, University of Nevada, Reno
Growth of Faults and Deformation Bands in Layered Sequences

Faults that are initially unrestricted by mechanical or lithologic stratigraphy follow a displacement-length (D-L) scaling law with steep (n = 1) slope. Vertical restriction leads to nonproportional growth and reduced slope until a critical threshold is reached for the fault or precursory deformation band to break out of the layer and cut across the stratigraphic sequence. Mechanical models of fault growth assist in the interpretation of these relationships and provide predictive capability in both terrestrial and planetary settings. Recent field and theoretical advances reveal how cataclastic deformation bands in porous sandstones form spatially organized arrays and networks prior to faulting, with implications for damage-zone development, fault localization, and fluid-seal ruptures within the network.

Monday, June 21, 2004 - Lecture Hall, 3:30 PM
Duncan Young, University of Minnesota-Duluth
Strain Localization and Tectonic Interactions Within Venus' Lowlands: Constraints From Mapping

Strain within the lowland regions of Venus, as observed by Magellan radar, is typically low in magnitude and regional in extent. Examples include wrinkle ridges and fracture suites. However, exceptions exist in the form of deformation belts and coronae, which combine higher strains, spatially restricted expression and evidence of volcanic activity. The interactions between regional and localized expressions of tectonism are one of the few available constrains on lithospheric evolution and structure. By combined geological mapping and analysis of altimetry data, we examine these constraints in Rusalka Planitia and other lowland regions. Local stratigraphic and geometric considerations indicate that vertical, out-of-plane processes are responsible for much of the structure found within deformation belts, as opposed to the inplane forces that are likely to drive regional deformation. Thus, deformation belts may be analogous to coronae, representing a direct manifestation of small scale mantle convection. Such a result may have implications for the evolution of Venus' general convective regime. An alternative interpretation is that major horizontal heterogeneities exist in the Venusian lithosphere.

Tuesday, June 22, 2004 - Lecture Hall, 3:30 PM
Lisa Danielson, Arizona State University
Explorations and New Approaches to Accretion and Core Formation Processes of the Earth

The processes of accretion and core segregation are fundamental to understanding the evolution of our solar system and, indeed, set the stage for a planet's subsequent geologic evolution. Today, a remnant trace element signature in the Earth's mantle from these processes, the excess siderophile element problem, fuels ongoing investigations. Two hypotheses have emerged: (1) Earth accreted heterogeneously from compositionally diverse material derived from varying annuli within the solar system. Core formation was an episodic process involving periods of metal-silicate equilibrium following impact of differentiated chondritic material. Mantle trace elements reflect only the very last episode of accretion, a late chondritic veneer. (2) Earth accreted from a narrow band of material around 1 AU. The mantle trace element signature was derived from metal-silicate equilibrium at high pressures and temperatures in the deep mantle. This study investigates (2) by conducting element partitioning and phase equilibria experiments at pressures, temperatures, and a composition relevant to the early Earth. Partitioning of Au between metal-sulfide liquid and silicate liquid [D(Au)met/sil] was chosen because almost no partitioning data exist for the highly siderophile elements at the investigated conditions and new microanalytical techniques allow more thorough characterization of run products. Results of these experiments show D(Au)met/sil decreases with increasing pressure and temperature, and also as a response to sulfur content of the metallic fraction. One interpretation of these results allows for magma ocean equilibration conditions at lower pressures but higher temperatures than in previous models. This possibility is highlighted by the recent addition of the H-chondrite to the P-T phase diagram data suite, where the best fit of P-T equilibration conditions lies above the Allende and Richardton liquidus.

Friday, June 25, 2004 - Lecture Hall, 3:30 PM
Rebecca Ghent, CEPS/Smithsonian
Dark-haloed Craters on the Lunar Nearside: Implications for Regolith Properties

We use recently acquired radar observations at 70-cm wavelength to examine the distribution of ejecta around lunar impact craters. Numerous craters with haloes characterized by low 70-cm radar return occur throughout the maria and highlands. These haloes extend 1-2 crater diameters from the rim, outboard of a zone of radar-bright rough ejecta that varies in width between craters; in some instances the dark halo reaches the rim of the crater. The new 70-cm radar observations allow detailed analysis of the backscatter variations associated with these dark haloes. Clementine multispectral data are used to infer mineralogical and maturity properties. Taken together, these data permit characterization of the surface and subsurface regolith properties in the vicinity of these craters.

July 2004

Friday, July 9, 2004 - Lecture Hall, 3:30 PM
Catherine Corrigan, Smithsonian Instituteion National Museum of Natural History
Multiple Generations of Carbonate in ALH 84001: Implications for Environmental Conditions on Ancient Mars

The carbonate mineralogy of several complex carbonate-rich regions in ALH 84001 has been examined. These regions contain familiar forms of carbonate as well as textural forms previously unreported. "Slab" carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite-rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite-siderite layers on the exterior surfaces of the carbonate. "Post-slab" magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts. The new carbonate textures, including post-slab magnesites and slab carbonates, were analyzed for oxygen isotopes and trace elements. Isotopic analyses of carbonates extend the previous range of chemical compositions by ~28 mol% Ca and the range of isotopic compositions by ~+9*. Isotopic analyses also suggest that post-slab magnesites are of a later generation. ALH 84001 carbonates exhibit similar trace element patterns, and are LREE-depleted relative to HREEs, indicating that they formed from a consistent initial fluid (possibly in trace elemental equilibrium with orthopyroxene), precipitating quickly from high-temperatures. Variations in individual trace elements confirm the presence of multiple generations of carbonate and suggest localized mineral interaction. ToF SIMS analyses also confirm the presence of multiple generations of carbonate. Martian meteorites exhibit little aqueous alteration, even taking into account the multiple generations of carbonate seen here. This supports suggestions that the martian hydrosphere and lithosphere interacted with less frequency than is sometimes suggested.

Friday, July 16, 2004 - Lecture Hall, 3:30 PM
Chris Okubo, University of Nevada, Reno
The Strain Energy Density Criteria for Predicting Fault-related Damage Zone Geometry in Porous Rock, with Martian Implications

Understanding the tendency for the nucleation and growth of secondary fractures induced by slip along an adjacent fault is a key to evaluating the fluid reservoir potential of fault-related folds on Earth and Mars (e.g. within wrinkle ridges or across normal faults). I will present results of a study in which the tendencies for both pre-peak and post-peak fracture growth within granular solids are independently quantified by strain energy density-based criteria. Specifically, critical volumetric and distortional strain energy densities are used to separately describe the tendencies of deformation band nucleation and propagation around a slipped fault. Predictions of these fault-related damage zone locations and densities provide insight into attendant changes in the porosity and permeability of the host rock, which has a potential for compartmentalizing fluids within a large-scale reservoir.<br><br>Results of laboratory testing and numerical modeling of strain energy density predict distributions and densities of deformation bands that are consistent with field observations of damage zones within the Laramide-aged Uncompahgre uplift in western Colorado, as well as in other field sites in southern Utah. Deformation band nucleation is a precursor to the formation of through-going faults within porous materials (e.g. sandstone, volcanic tuff). Erosional exposures of fault-related deformation band damage zones form high-relief ridges that are on the order of meters, to in excess of hundreds of meters, in length. These deformation band damage zones are visible in aerial photographs of Earth, and morphologically similar ridges are present on Mars within the interior layered deposits, along-strike of thrust fault-related folds (wrinkle ridges) apparent along the tops of the surrounding plateaus. Interpretations of deformation band damage zones within the interior layered deposits implies that these deposits are granular in nature and are have the potential for compartmentalization and directed flow of any near-surface volatiles. Future work will use the 4-D finite element code ABAQUS to track cumulative mode I and mode II/III strain localization within pre-and syn-fault damage zones during fault-related fold growth on Earth and beyond.

Friday, July 23, 2004 - Lecture Hall, 3:30 PM
Antoine Pommerol, Ecole Normale Superieure, Lyon, France
Toward a Dielectric Map of the Martian Surface

Low frequency sounding radar will be used in the on going decade to probe the Martian subsurface. The MARSIS instrument, on-board the Mars Express mission will be deployed in couple of months and will sound the subsurface to detect the presence of hypothetical liquid water at depth ranging from few hundreds meters to few kilometers. The 2005 Mars Reconnaissance Orbiter will use a similar instrument named SHARAD to perform shallow sounding of the Martian permafrost in the first hundreds of meters only. The performances of these instruments, in terms of penetration depth, vertical resolution and ability to unambiguously detect interfaces depend mainly on two parameters: the surface and subsurface geometry (roughness and stratigraphy) and the geoelectric and geomagnetic properties of the surface and subsurface materials. While the first parameter has been partly revealed from the MOLA data, the geoelectric properties remain quite unknown and unrelated to a geographical description of the planet surface.<br><br>I will present a frequency dependent model (1 MHz to 3 GHz) of the dielectric map of the Martian surface based on a homogenous description of the surface dust composition. I will discuss two implications of this map. In a first step, I will present the effects of the variations of the surface dielectric constant on the performances of the radar instruments and hence water detectability. Second, I will discuss the future use of this map in terms of refining our state of knowledge on the surface properties as density and mineralogy.

August 2004

Friday, August 13, 2004 - Lecture Hall, 2:00 PM
Daniel Nunes, Washington University, St. Louis
Viscous Relaxation, State of Compensation, and the Evolution of Crustal Plateaus on Venus

Crustal plateaus correspond to one of the main types of physiographic features and the oldest surface on Venus; a detailed understanding of these highlands will likely yield important clues to the evolution of the planet itself. Still, there is no consensus on what mechanisms led to their formation and controlled their evolution. While both mantle upwellings and downwellings have been proposed as the origin of crustal plateaus, viscous relaxation has been suggested frequently as a mechanism controlling such plateaus after their emplacement. This study assesses the hypothesis that crustal plateaus evolved from high-standing plateaus to low-standing rimmed highlands and arcuate tessera patches in the plains through viscous relaxation. It uses a combination of finite element and analytical methods to model the viscous flow in two end member cases: fully compensated (Airy) and uncompensated topography. The models also incorporate a range of thermal states consistent with different formation scenarios - hot upwelling and cold downwelling - as well as different surface temperatures caused by possible variations in greenhouse efficiency. Results indicate that the initial state of topographic compensation is crucial in determining the rate and mode of relaxation. Compensated topography relaxes more slowly and retains its plateau aspect or becomes more domical, like Ovda Regio perhaps. In contrast, uncompensated topography subsides more quickly and produces elevated rims, as in Alpha Regio. It is likely, however, that crustal plateaus did not originate as either end member, fully supported or not. The most recent modeling in this study incorporates a sub-crustal buoyancy source and addresses the evolution of plateau topography resulting from the interaction between relaxation timescales and the loss of buoyancy. In these cases, rim formation is modulated by buoyancy lifetime and is precluded in cases where buoyancy persists over timescales longer than those characteristic of lower crustal flow.

Friday, August 20, 2004 - Lecture Hall, 3:30 PM
Carrine Blank, Washington University, St. Louis
Using Phylogenomic Dating Techniques to Reconstruct the Microbial Composition of the Early Earth

Phylogenomic dating involves three steps, one constructing well-resolved phylogenetic trees using large datasets obtained from whole genome sequences, two mapping the evolution of traits (metabolic, ecological, morphological) onto the tree, and three correlating changes in the appearance of certain traits with changes (in microfossils and/or biomarkers) that occurred on the early Earth. Using this method, we can better understand how certain traits arose in the two prokaryotic domains of life (the Archaea and Bacteria) and how early life co-evolved with the Archean and Paleoproterozoic Earth. My talk will focus on three major findings: the establishment of age constraints on the origin of the Cyanobacteria and mesophilic sulfate reducers, the establishment of age constraints on multiple lineages of archaea (none of which left distinctive traces in the geologic record), as well as tracing the early evolution within the Cyanobacterial group itself.

September 2004

Friday, September 3, 2004 - Lecture Hall, 3:30 PM
Pierre Hudon, JSC
Ureilite Formation Models

Ureilites are ultramafic achondrites composed primarily of olivine and pyroxene, and containing up to 66 mg/g elemental carbon. Recent petrologic studies have supported carbon-silicate redox ("smelting") reactions as the major petrogenetic process responsible for the positive correlation between modal pigeonite and mg# and the negative Fe/Mn-mg# trend seen in mineral and bulk compositions of ureilites. In this model, ureilites with the largest mg# are the most reduced, experienced the highest temperatures, and formed at the lowest pressures, nearer the ureilite parent asteroid surface. Ureilites with the highest mg# also have the most negative \30417O. To further investigate possible correlations, we did carbon content and isotopic composition, and petrologic studies of a suite of ureilites to test the smelting model. Results do not support smelting as the major petrogenetic process responsible for the genesis of ureilites; alternate models are thus presented and the case of a few anomalous ureilites is discussed.

October 2004

Friday, October 29, 2004 - Berkner Room, 3:30 PM
David R. Criswell, University of Houston
<a href="/lpi/criswell041029.pdf">Industrial-Scale Lunar Sciences</a>

November 2004

Friday, November 5, 2004 - Lecture Hall, 4:00 PM
Allan Treiman, Lunar and Planetary Institute
The AMASE II Expedition to Spitsbergen, Norway: The Sverrefjell Volcano and its Carbonate Minerals as Mars Analogs

The 'Arctic Mars Analog Svalbard Expeditions,' AMASE, are sponsored by the University of Oslo and the Carnegie Institution to allow studies in the Svalbard Archipelago, Norway, as analogs for Mars. Svalbard, in the high arctic, presents many opportunities for testing equipment and procedures in cold and difficult conditions. Geologically, many sites on Svalbard are analogs (known and potential) for features on Mars and in the Martian meteorites, including: carbonate and clay alterations of igneous rocks, an evaporite sequence with concretions, endolithic biota, and sediments rich in organics. After a photo tour AMASE II (2004), I will talk briefly about iron-magnesium carbonate deposits at the Sverrefjell volcano, Spitsbergen Island.

Tuesday, November 9, 2004 - Lecture Hall, 3:30 PM
Robert Herrick, University of Alaska, Fairbanks
Houston to Fairbanks, a Bicycling Odyssey

In April I left the homeland of Texas to take a job in Alaska. To make the move a little more interesting, I decided to ride up by bicycle. Starting in April and finishing in June, I rode 4400 miles and crossed the continental divide seven times. This will be a "how I spent my summer vacation" slide show of the journey.

December 2004

Friday, December 3, 2004 - Lecture Hall, 3:30 PM
Marc Norman, Lunar and Planetary Institute
Targeting the Impactors: Ages and Compositions of Lunar Impact Breccias

Lunar impact melt breccias provide valuable clues to the composition of the deep lunar crust, the chronology of impact events in the inner Solar System, and the types of impactors colliding with the Moon. Most lunar impact breccias in our collection probably were deposited within or around large complex craters or multi-ring basins, although linking individual breccias to specific impact events has been frustratingly difficult. Pervasive shock and brecciation produced by 4.5 billion years of exposure to impact events has obscured many of the primary petrological and geochemical characteristics of the lunar crust. Criteria that distinguish lunar impact breccias from primary igneous rocks include enrichments in highly siderophile elements derived from the meteoritic impactor, textural evidence for mechanical mixing, and bulk compositions that cannot plausibly be produced by melting of the lunar interior. The predominance of 3.7-4.0 Ga crystallization ages in lunar impact melt breccias probably records a short-lived spike in the flux of large of bodies to the Moon, and presumably the Earth, although other explanations are possible. Additional work linking the ages of lunar impact breccias, the nature of the impacting bodies, and petrologic and geochemical compositions of crustal target rocks is needed to better understand the impact history of the Moon and early Earth, and the possible effects on biology and crustal evolution.

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