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: firstname.lastname@example.org) or Patricia Craig (phone: 281-486-2144; e-mail: email@example.com). A map of the Clear Lake area is available here. This schedule is subject to revision.
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Around the Noachian/Hesperian transition on Mars, and perhaps as late as the middle of the Hesperian, an epoch of Earthlike climate conditions prevailed. This epoch appears to have begun and ended abruptly and lasted on the order of 1,000-100,000 years, as shown by relatively straight fluvial channels, low drainage densities, and deltas that experienced little or no flow while lake levels were declining. During this time, many previously enclosed basins overflowed, lower reaches of valley networks were incised, a number of deltas and alluvial fans formed, and fluvial channels record Earthlike discharge and runoff production rates. This climate may have varied regionally. The prevailing conditions during the Noachian were considerably drier on average but may have included episodes of warmer climate, perhaps encouraged by impacts. Conditions after this late stage of fluvial activity have been extremely dry, with very low weathering and denudation rates on rocky surfaces.
After decades of research on how the Earth's lithosphere deforms, there is still no consensus on the magnitude or distribution of its strength. The debate becomes even more challenging given that laboratory experiments suggest that the rheology of the lower crust and upper mantle are stress-dependent, implying significant temporal and spatial variations in lithospheric strength, but little evidence of this behavior has yet been found from in situ measurements. Here, I use an earthquake as a large rock deformation experiment in which sudden stress changes induce viscous flow in warm, deep regions of the of the lower crust and upper mantle that lead to observable surface deformations. By using GPS observations of postseismic deformation following the 2002, M7.9 Denali, Alaska earthquake as a constraint on a model of viscoelastic relaxation, I find the rheology of the upper mantle to be consistent with a laboratory-derived, stress-dependent, power-law for hot, wet olivine.
Numerical modeling has become an important approach for understanding active crustal deformation and earthquakes both in interplate and intraplate regions; however, the progress in developing sophisticated and integrated numerical models largely lags behind the progress of geodetic, geological and geophysical data acquisition. In my thesis study, I have developed a 3D parallel visco-elasto-plastic finite element model that is capable of simulating simultaneously elastic deformation, viscous relaxation, and frictional failure in the crust and lithospheric mantle. The finite element model has been successfully applied to several research projects, which include (1) stress and strain energy evolution difference between intraplate and interplate fault zones, (2) stress evolution following the 1811-1812 large earthquakes in the New Madrid Seismic Zone, (3) geometrical impact of the San Andreas Fault (SAF) on stress and seismicity in California, and (4) interaction between the San Andreas and San Jacinto faults (SJF) in southern California. The main findings in these researches are: Intraplate seismic zones (e.g., the New Madrid fault zone) tend to remain in a Coulomb stress shadow for thousands of years following large earthquakes. On the other hand, a significant amount of the stress relieved from large intraplate earthquakes, and the associated strain energy, may migrate to and be trapped within the ambient crust, mainly near the tips of the fault zones (e.g., southern Illinois and eastern Arkansas). These are some of the fundamental differences from interplate seismic zones, where the evolution of stress and strain energy are dominated by tectonic loading. Within California plate boundary zone, the observed along-strike variation of slip rate, stress, and seismicity along the SAF, and the initiation and growth of the SJF may reflect the geometrical impact of the SAF. Once the SJF has initiated, it causes decrease of fault slip rate on the southernmost SAF, consistent with observations. The initiation of the SJF also causes less strain energy release rate in regions to the west of the SJF, and diffuses a belt of concentrated strain energy along the East California Shear Zone.
This talk will discuss the motivation, guiding strategy, and geophysical techniques being employed to search for water on Mars - with a particular emphasis on the activities of the Mars radar group at LPI. Among the topics that will be addressed are the size and nature of the Martian inventory of water, strategies for assessing its distribution and state, current and future spacecraft investigations (including a preview of some of the early MARSIS orbital radar sounding results), and the importance of supporting laboratory and field work of Mars analog environments on Earth. The presentation will conclude with an informal travelog of the recent LPI-led multi-team geophysical investigation of the West Egyptian Desert, describing some of the highs and lows of what happens when an international group of hi-tech western investigators encounters the friendly people, natural beauty, and logistical and bureaucratic realities associated with working in Egypt.
The recent Australian film, The Dish, highlighted the role played by the Parkes Radio Telescope in tracking and communicating with the Apollo 11 mission. However, the events depicted in this film represent only a single snapshot of the role played by Parkes in the exploration of the Solar System by NASA.
As the fledgling Deep Space Network was being established in the early 1960's, one of the world's major radio telescope facilities was being built at Parkes, in western New South Wales, Australia. This 64-metre diameter dish, designed and operated by the Commonwealth Scientific and Industrial Research Organisation (CSIRO), was well-suited for deep space tracking work: its design was in fact, the inspiration for the 64-metre dishes of the Deep Space Network. From Mariner 2 in 1962 to Huygens at Titan in 2005, the Parkes Radio Telescope has been contracted by NASA on many occasions to support interplanetary spacecraft. The highlight of the NASA support was its critical role in several of the Apollo lunar landing missions, especially Apollo's 11 and 13. This talk will outline the role played by Parkes in these historic missions and its relationship with the stations in the Deep Space Network.
Despite highly successful missions to the surfaces of the Moon and Mars, the complimentary knowledge gained from petrological, chemical and chronological studies of meteorites from these and perhaps other planetary bodies is of unique value. Beyond the 41 currently known lunar meteorites and 37 known Martian meteorites, intriguing but inconclusive arguments can be made for derivation of angrites (10 known specimens) from Mercury. Compelling evidence for decompressive tectonics on the angrite parent body is preserved in the form of anorthite+olivine coronas and clinopyroxene+spinel symplectites in angrite Northwest Africa 2999.
The hemispheric dichotomy on Mars is one of the most prominent topographic structures on the planet, with a typical elevation difference of 3 to 4 kilometers between the southern highlands and the northern lowlands. The dichotomy formed very early in martian history, and previous work has suggested both mantle convection and large impacts as the cause for the dichotomy. Topographically, the dichotomy boundary has two distinct segments. In Arabia Terra, the elevation change occurs over 2000 to 3000 kilometers of gently sloping terrain. In eastern Mars, the dichotomy boundary is more scarp-like, with the transition from highland to lowland occurring over just a few hundred kilometers. My recent work has shown that the gravity anomalies along the dichotomy boundary also show distinct differences between Arabia Terra and eastern Mars. The correlated nature of the variations in the topography and the gravity anomalies suggest that at least two different processes contributed to the formation and early modification of the hemispheric dichotomy. One possibility is that the dichotomy was initially formed by convective processes and that large impacts, such as the Utopia basin, later modified the dichotomy boundary on a regional scale.
Equilibrium thermal states have been computed for Europa's mantle and ice shell in isolation, due to the lack of a reliable parameterization of heat flow through an ice layer heated from within and below. This limitation has been overcome with a new parameterization and coupled models of thermal equilibrium in Europa are calculated. There are two disjoint sets of equilibria determined by the thermal state of the silicate interior: Moon-like and Io-like. Moon-like cases have a cold, non-dissipative interior and the thermal state of the ice shell is determined by the balance between tidal heating and convection in the shell itself. Such states have total heat flows around 1 TW, and the thickness of the shell is determined by the rheology (grain size) of the shell. Io-like states are dominated by the heat coming from the partially-molten silicate interior and the ice shell is essentially heated from below. The heat flow in these cases is about 10 TW, requiring a very low viscosity (small grain size) in the convecting shell or a very thin conducting shell. The pattern of heating in the Io-like case is dominated by dissipation in the mantle which peaks at the poles, while the pattern in the Moon-like case is determined by dissipation in the shell which has peaks near the equator and minima at the poles. The ocean may also smooth the heating pattern from the mantle (though this depends on unknown ocean properties). The thickness of the shell, the pattern of heating, the response of the shell to eccentricity variations, and the lateral thickness variations in the shell all depend on the mode of equilibrium and may provide observational clues to the state of Europa's silicate interior.
Precambrian spherule layers are a largely untapped source of information about impacts and early Earth history. They form where ejecta fall and get buried in low energy environments, thereby outlasting the craters they came from. All spherule layers occur in stratigraphic context, so they provide good baselines to assess environmental change (or lack thereof) before and after major impacts. Although they have been replaced wholesale, many spherules retain textures that offer clues to their original composition, which is i largely inherited from target rocks. Precambrian spherules appear to be more basaltic on average than Phanerozoic ones, perhaps reflecting a secular increase in the area of granitic continental crust. As the only surviving record of terrestrial impacts prior to ca. 2 Ga, spherule layers may shed light on the flux and composition of early Earth impactors, but only if more can be identified. Ultimately, they might serve as time planes with a precision on the order of days for correlating stratigraphic successions that are billions of years old on different continents.
Hydrothermal systems may have been a "cradle" for early biosphere development on Earth. The deepest and shortest branches of the Domains Bacteria and Archaea are all populated by hyperthermophiles that grow at temperatures >80° C. The most deeply rooted organisms are chemolithotrophs that utilize hydrogen and sulfur in their metabolism, properties regarded to be likely features of the last common ancestor of life on Earth. This view from molecular biology is consistent with a variety of independent geological evidence that suggests the presence of widespread hydrothermal conditions on the early Earth. Supporting observations include results of model-based predictions of higher crustal temperatures on the early Earth, due to heightened impact rate and size, internal heat from the decay of the radioactive elements and convective transfers of deep internal heat to the shallow crust through widespread magmatic and geothermal processes. Empirical observations include eruptions of komatiitic lavas during the early Archean, indicating average crustal temperatures much higher than today and oxygen isotope fractionations measured in well-preserved siliceous sediments (cherts) that suggest the early Earth was an open hydrothermal system with surface (climatic) temperatures in the range, 55-85° C. In so far as the Earth's early history represents a general pattern of planetary evolution, hydrothermal systems may have provided major habitable environments on Mars and also some of the outer Solar System moons (e.g. the icy Galilean moons, Enceladus, etc.), throughout the history of our Solar System. This leads to the larger question: Given the general physical and chemical features of hydrothermal systems and their integral role in planetary evolution, could thermophily represent a common pathway for early biosphere emergence elsewhere in the Cosmos?
I will summarize several areas of my research that deal with the role of water on ancient Mars. This will include an update of ongoing global valley network analysis and then a focus on the Meridiani region where the Opportunity Rover landed. In the former case I will argue for precipitation and a clement climate and in the latter I will argue the opposite. This will illustrate some of the current conundrums of ancient Mars and our lack of constraints in the spatial and temporal domains.
Some occurrences of igneous rock (hotspots) and some topographic or bathymetric elevations (Swells or highspots) at the Earth's surface have been suggested to be linked to mantle plumes (here defined as buoyant objects in the convecting mantle). With colleagues I have begun to test this idea by concentrating on two subsets of the population of possibly plume related features:
- Using paleomagnetic rotation we have restored 25 of the Earth's 28 Large Igneous Provinces (LIPS) of the past 200 Million years to their locations at the time of eruption. Radials dropped from 24 of those restored sites intersect the Core-Mantle Boundary (CMB) at the edges of one or other of two Large Low Velocity Provinces (LLVPs) defined by seismic tomography of the deep mantle. We conclude that long-lived structure at the CMB has controlled the development of deep mantle plumes responsible for the eruption of LIPS.
- Basins and swells (d=ca.1000 km) dominate the topography of the African continent. Continents on other plates are very different. Because the African plate is at rest w.r.t. Earth's spin axis the basins and swells have long been attributed to interaction with a shallow mantle convection pattern not developed under moving plates. But no such pattern has been seen in seismic studies. Using surface wave records from a major instrumental deployment in southern Africa Aibing Li has now discerned a low velocity zone at ca.160-260 km in which shallow mantle convection involving plumes of the postulated shallow population may be present. High resolution dating shows that the present basin and swell topography of Africa began to develop when the Ethiopian LIP was erupted at ca.31 Ma. That eruption pinned the slowly rotating African plate setting the stage for new shallow mantle convection to develop. Deep and shallow mantle plumes are thus not fully independent.
- Mars and Venus, which are "one plate planets" (Solomon & Head 1982), show intriguing resemblances both to Africa and to LIPS.
Significant and perhaps complete melting of the young terrestrial planets is expected from radiogenic heating, heat of accretion, and the potential energy release of core formation. The rate of subsequent magma ocean solidification is tempered by volatiles that are degassed to the atmosphere, where they decrease atmospheric transmission, reduce heat flux from the planet's surface, and slow solidification. I will present results from a new model that integrates silicate mantle solidification with atmospheric formation, linking heat flux, convective velocity, and cooling rates with resulting mantle and atmospheric compositions. Magma ocean processes may be responsible for initial atmospheric mass and composition, initial mantle heterogeneity and compositional stability, and formation of both an early crust and planetary magnetic field.
Over the last two decades, deep earth geophysicists have simulated planetary core processes via two main approaches: laboratory experiments and numerical modeling. Laboratory experiments typically use water and liquid metals to replicate various aspects of core flows, including rotating convection, rotating magnetoconvection, precession and dynamo generation. Over this time period, major increases in computational power have allowed numerical simulations of core processes to be carried out in parameter regimes comparable to those of laboratory experiments. Each approach has its advantages. For example, laboratory experiments can contain fine scale flow structures that are not resolved computationally. Numerical models are able to generate dynamo action at moderate parameter values by using simulated physical properties not found in laboratory fluids. Future advances in our understanding of planetary core processes will depend on successfully combining these two complementary approaches. Towards this goal, I will focus in this seminar on integrating results from recent experimental and numerical studies of planetary core dynamics. I will present experimentally-generated scaling laws for convective heat transfer and compare these with scaling laws from numerical simulations.
Reactions of aqueous fluids with rocks shortly after formation of the solar system affected the oxidation states, mineralogy, organic speciation, ice composition, and surface/atmospheric chemistry of asteroids, icy satellites of giant plants, and possibly Kuiper belt objects. Competitive oxidation and hydration by water affected both inorganic and organic compounds in rocks. High water/rock ratios, elevated temperatures and low pressures favored oxidation. Low temperatures supported hydration. In some icy satellites (Europa, Ganymede) high water content and hydrothermal processes during differentiation may have caused profound oxidation leading to carbonates and sulfates. H2 was produced in all oxidation reactions and separated into the gas phase. Escape of H2 into space promoted oxidation. Low porosity and permeability, filling of pore space with secondary minerals, and sealing of outer zones with ice restricted H2 escape and caused incomplete reduction of minerals formed earlier by oxidation. Interaction of water with organic compounds and carbon grains caused disproportionation of carbon leading to O-bearing organic compounds and CO2, as well as to hydrogenated organic compounds and methane, consistent with observations (e.g., Titan, Triton). Although prolonged heating of bodies caused dehydration and some reduction, water-rock reactions led to net oxidation of primary rocky components, consistent with the elevated oxidation states of metamorphosed chondrites and Io. In most bodies, net oxidation of organic compounds can be attributed to preservation of CO2 and O-bearing organics in solution, ices and carbonates, and escape of H2 formed through oxidation reactions.
A fundamental process of planetary differentiation is the segregation of metal-sulfide and silicate phases, leading eventually to the formation of a metallic core. Asteroidal meteorites provide a glimpse of this process frozen in time from the early solar system. While chondrites represent starting materials, iron meteorites provide an end product where metal has been completely concentrated. A complimentary end product is seen in metal-poor achondrites that have undergone significant igneous processing, such as angrites, HED's and the majority of aubrites. Metal-rich achondrites such as acapulcoite/lodranites, winonaites, ureilites, and metal-rich aubrites may represent intermediate stages in the metal segregation process. Among these, acapulcoite/lodranites and ureilites are examples of primary metal-bearing mantle restites where metallic partial melting was captured in progress. Chemical analysis of siderophile elements in these meteorites provides a means to observe this record.
Mantle-derived orogenic ultramafic bodies, which are often exposed in the high and ultra-high pressure cores of collisional mountain belts, are critical for understanding lithosphere evolution because they often preserve mantle structures and lithologic associations that are unlikely to be preserved in mantle xenoliths. Although orogenic ultramafic rocks can contain a record of the physical and chemical evolution of lithospheric mantle, their emplacement into the crust is an important but often poorly understood process. In this presentation, I will focus on the ~3 billion year evolution of Mg-Cr peridotite bodies from the Western Gneiss Region (WGR), Norway as well as focus on the mechanisms for the incorporation of these mantle lithologies into the crust.
The WGR contains several extremely fresh pod-shaped ultramafic bodies enclosed by paragneiss and orthogneiss; many of the bodies are mostly composed of dunite and harzburgite and several of these contain garnet-bearing peridotite and pyroxenite. This talk focuses on a small outcrop of garnet peridotite and associated garnet pyroxenite at Sandvik on the island Gursk°y that contains four of the numerous mineral assemblages that have been identified in WGR peridotites: 1) ol + grt + opx + cpx: (porphyroclasts) 1000 °C, 50 kbar; 2) ol + grt + spl + am ± opx: (kelyphite around Stage 1 garnet) 700-750 °C, 12-18 kbar; 3) ol + spl + opx + am: (matrix for Stage 1 porphyroclasts) ~700 °C, ~10 kbar; and 4) ol + chl + opx + am: (the predominant, Caledonian assemblage in WGR peridotites) ~600 °C, ~5 kbar. Combined Sm-Nd, Lu-Hf, and Rb-Sr age and isotope data of the stage 1 assemblage indicates that these rocks originated from depleted Archean lithospheric mantle that was chemically and physically modified in Middle Proterozoic time. Sm-Nd dating of the stage 2 kelyphite assemblage yields an age of 600 Ma which is roughly 200 Ma older than the 400 Ma Caledonian orogeny to which it has been previously attributed. The age and P-T data of the kelyphite indicates that the emplacement of these peridotite bodies into the crust is unrelated to Caledonian orogeny and may instead be related to an earlier anorogenic process.
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