L. A. Stern

United States Geological Survey, Reston, Virginia, United States

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Publications (91)212.9 Total impact

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    ABSTRACT: We show that perchlorate hydrates, which have been detected at high circumpolar martian latitudes, have a dramatic effect upon the rheological behavior of polycrystalline water ice under conditions applicable to the North Polar Layered Deposits (NPLD). We conducted subsolidus creep tests on mixtures of ice and magnesium perchlorate hydrate, Mg(ClO4)2·6H2O (MP6), of 0.02, 0.05, 0.10, and 0.47 volume fraction MP6. We found these mixtures to be increasingly weak with increasing MP6 content. For mixtures with ⩽0.10 volume fraction MP6, we resolved a stress exponent of n ≈ 2 at low stresses transitioning to n ≈ 4 above 10 MPa. Scanning electron microscopy of deformed specimens revealed MP6 to be distributed as an interconnected film between ice grains. These results suggest that grain boundary sliding (GBS) may be enhanced with respect to pure ice. As the enhancement of GBS is expected in polycrystalline aggregates containing a few percent melt or otherwise weak material distributed along grain boundaries, the observed n ≈ 2 is consistent with the mutual accommodation of basal slip and GBS. If ice containing trace concentrations of MP6 is also much weaker than pure ice at low stresses, flow in the NPLD could be significantly enhanced, particularly at the warmer basal temperatures associated with higher martian obliquities.
    Icarus. 08/2013; 225(2).
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    ABSTRACT: Experiments at Brown and MIT show that small amounts of dust significantly affect ice rheology: Differences in lab results may elucidate governing mechanisms.
    03/2013;
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    ABSTRACT: As a consequence of contemporary or longer term (since 15 ka) climate warming, gas hydrates in some settings may presently be dissociating and releasing methane and other gases to the ocean–atmosphere system. A key challenge in assessing the impact of dissociating gas hydrates on global atmospheric methane is the lack of a technique able to distinguish between methane recently released from gas hydrates and methane emitted from leaky thermogenic reservoirs, shallow sediments (some newly thawed), coal beds, and other sources. Carbon and deuterium stable isotopic fractionation during methane formation provides a first-order constraint on the processes (microbial or thermogenic) of methane generation. However, because gas hydrate formation and dissociation do not cause significant isotopic fractionation, a stable isotope-based hydrate-source determination is not possible. Here, we investigate patterns of mass-dependent noble gas fractionation within the gas hydrate lattice to fingerprint methane released from gas hydrates. Starting with synthetic gas hydrate formed under laboratory conditions, we document complex noble gas fractionation patterns in the gases liberated during dissociation and explore the effects of aging and storage (e.g., in liquid nitrogen), as well as sampling and preservation procedures. The laboratory results confirm a unique noble gas fractionation pattern for gas hydrates, one that shows promise in evaluating modern natural gas seeps for a signature associated with gas hydrate dissociation.
    Chemical Geology. 01/2013; 339:242–250.
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    ABSTRACT: Electromagnetic (EM) remote-sensing techniques are demonstrated to be sensitive to gas hydrate concentration and distribution, which complements seismic and other existing resource assessment techniques. Full utilization of EM results requires knowledge of the electrical properties of gas hydrates, mixed phases, and mixing relations. We developed a pressure cell to synthesize gas hydrate while measuring in situ frequency-dependent impedance (Z) and electrical conductivity (σ). Starting samples consisted of either (1) granular ice ( 200 microns grain size) packed to about 25% intergranular porosity, (2) granular ice uniformly mixed with quartz sand (OK#1, 125±50 micron grain size) in various proportions, or (3) ice uniformly mixed with silica glass beads (115±10 micron diameter). The ice was then fully reacted into methane (CH4) hydrate by applying CH4 pore pressure during temperature cycling above and below the ice/water phase boundary. Sediment distribution and grain-scale characterization were determined by post run cryogenic scanning electron microscopy and confirmed no migration of the sediment phase during reaction. Z and σ were measured in samples after hydrate synthesis from -15 to 15 °C, and after dissociation of hydrate into ice from -15 to -2 °C. We determined the σ of unmixed CH4 hydrate to be 10^1.5S/m*exp{(30.6kJ/mol)/RT} and 10^5.0S/m*exp{(45.3kJ/mol)/RT} after dissociation to unmixed ice. For the mixed samples the overall σ measured included significant contribution by surface σ along weathered surfaces of sediments. Samples containing < 30 vol% sand had poorly connected sediment below the percolation threshold, resulting in only a slight increase of up to 0.3 log units in σ and little change in activation energy (Ea) compared to unmixed samples. Conversely, samples containing 47 vol% sand had interconnected sediment above the percolation threshold, with a large increase of 0.7-1.2 log units in σ and a large 80% decrease in Ea. Glass beads had less effect than sand because they have lower surface σ due to less weathering and are less connected due to a lack of angularity. Samples containing 47 vol% glass beads had slight increase of 0.0-0.2 log units in σ and 25% decrease in activation energy (Ea). The addition of sediments had a similar contribution to the total σ of mixtures after dissociation of hydrate to ice. Mixing laws were used to model the relative contributions of CH4 hydrate (or ice) and sediment to total σ. Our results demonstrate that surface σ and connectedness of sediments are important factors in determining σ of natural gas hydrate containing sediment. Support for this work was provided by DOE contract, DE-NT0005668. Partial support was also provided by Interagency Agreement DE-NT0006147 between the USGS Gas Hydrates Project and the U.S. Department of Energy's Methane Hydrates R&D Program. Prepared by LLNL under Contract DE-AC52-158 07NA27344.
    AGU Fall Meeting Abstracts. 12/2011;
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    ABSTRACT: Motivated by a Mars NPLD morphology that suggests a material of weaker rheology than ice, we deformed a polycrystalline sample of solid ice + magnesium perchlorate. The material is profoundly weaker than pure water ice at Mars polar cap temperatures.
    03/2011;
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    ABSTRACT: Neutron powder diffraction data confirm that hydrate samples synthesized with propane crystallize as structure type II hydrate. The structure has been modeled using rigid-body constraints to describe C3H8 molecules located in the eight larger polyhedral cavities of a deuterated host lattice. Data were collected at 12, 40, 100, 130, 160, 190, 220, and 250 K and used to calculate the thermal expansivity from the temperature dependence of the lattice parameters. The data collected allowed for full structural refinement of atomic coordinates and the atomic-displacement parameters.
    Canadian Journal of Physics 02/2011; 81:431-438. · 0.90 Impact Factor
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    ABSTRACT: The polyhedral cage volumes of structure I (sI) (carbon dioxide, methane, trimethylene oxide) and structure II (sII) (methane–ethane, propane, tetrahydrofuran, trimethylene oxide) hydrates are computed from atomic positions determined from neutron powder-diffraction data. The ideal structural formulas for sI and sII are, respectively, S2L6 · 46H2O and S16L'8 · 136H2O, where S denotes a polyhedral cage with 20 vertices, L a 24-cage, and L' a 28-cage. The space-filling polyhedral cages are defined by the oxygen atoms of the hydrogen-bonded network of water molecules. Collectively, the mean cage volume ratio is 1.91 : 1.43 : 1 for the 28-cage : 24-cage : 20-cage, which correspond to equivalent sphere radii of 4.18, 3.79, and 3.37 Å, respectively. At 100 K, mean polyhedral volumes are 303.8, 227.8, and 158.8 Å3 for the 28-cage, 24-cage, and 20-cage, respectively. In general, the 20-cage volume for a sII is larger than that of a sI, although trimethylene oxide is an exception. The temperature dependence of the cage volumes reveals differences between apparently similar cages with similar occupants. In the case of trimethylene oxide hydrate, which forms both sI and sII, the 20-cages common to both structures contract quite differently. From 220 K, the sII 20-cage exhibits a smooth monotonic reduction in size, whereas the sI 20-cage initially expands upon cooling to 160 K, then contracts more rapidly to 10 K, and overall the sI 20-cage is larger than the sII 20-cage. The volumes of the large cages in both structures contract monotonically with decreasing temperature. These differences reflect reoriented motion of the trimethyelene oxide molecule in the 24-cage of sI, consistent with previous spectroscopic and calorimetric studies. For the 20-cages in methane hydrate (sI) and a mixed methane–ethane hydrate (sII), both containing methane as the guest molecule, the temperature dependence of the 20-cage volume in sII is much less than that in sI, but sII is overall larger in volume.
    Canadian Journal of Physics 02/2011; 81:183-189. · 0.90 Impact Factor
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    ABSTRACT: We developed a pressure cell to synthesize and measure sigma of gas hydrateWe determined the sigma of CH4 hydrate to be 5 × 10−5 S/m at 0° CSigma measurements are a factor of ∼4 and Ea is ∼50% lower for CH4 hydrate than ice
    Geophysical Research Letters 01/2011; 38(9). · 3.98 Impact Factor
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    Int. Conf. Gas Hydrates; 01/2011
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    ABSTRACT: Using cryogenic scanning electron microscopy (CSEM), powder X-ray diffraction, and gas chromatography methods, we investigated the physical states, grain characteristics, gas composition, and methane isotopic composition of two gas-hydrate-bearing sections of core recovered from the BPXA–DOE–USGS Mount Elbert Gas Hydrate Stratigraphic Test Well situated on the Alaska North Slope. The well was continuously cored from 606.5 m to 760.1 m depth, and sections investigated here were retrieved from 619.9 m and 661.0 m depth. X-ray analysis and imaging of the sediment phase in both sections shows it consists of a predominantly fine-grained and well-sorted quartz sand with lesser amounts of feldspar, muscovite, and minor clays. Cryogenic SEM shows the gas-hydrate phase forming primarily as a pore-filling material between the sediment grains at approximately 70–75% saturation, and more sporadically as thin veins typically several tens of microns in diameter. Pore throat diameters vary, but commonly range 20–120 microns. Gas chromatography analyses of the hydrate-forming gas show that it is comprised of mainly methane (>99.9%), indicating that the gas hydrate is structure I. Here we report on the distribution and articulation of the gas-hydrate phase within the cores, the grain morphology of the hydrate, the composition of the sediment host, and the composition of the hydrate-forming gas.
    Marine and Petroleum Geology 01/2011; 28(2):394-403. · 2.11 Impact Factor
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    ABSTRACT: Water ice is the principal constituent of the low density moons of the outer solar system, and the flow behavior of ice is of great importance in dynamic processes on icy moons. Our interest here is on the influence that grain growth has on the flow behavior of ice. By understanding grain growth in combination with flow mechanisms it is possible to reconstruct thermal evolutions and tectonic histories of icy moons. Grain growth is expected to influence the evolution of strength of ice by altering the relative contributions to strain rate by grain-size-sensitive (GSS) creep mechanisms, such as diffusion and grain-boundary sliding, and grain-size-insensitive (GSI) creep mechanisms, such as dislocation creep. In particular, we are interested in examining the so-called field boundary hypothesis wherein grain size evolution and GSI and GSS creep mechanisms eventually lead to a dynamic balance involving a distribution of grain sizes and significant operation of both types creep mechanisms. We studied grain size evolution both during static annealing and during deformation. Static anneals ran for up to two weeks at 213
    AGU Fall Meeting Abstracts. 12/2010;
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    ABSTRACT: Ice is one of the most abundant materials in our solar system. It is the principal constituent of most of the moons of the outer solar system. Thus, the flow behavior of ice is of great interest when studying geodynamic processes on icy moons. Grain growth is an elementary process that is assumed to be important in the ice sheet layering of planetary moons, where temperatures 100-273 K exist. We concentrate on the questions to what extent grain growth may influence the evolution of strength of deforming ice and if the grain growth process is independent or dependent of deformation. The answers to these questions will help us to quantitatively test the hypothesis that the progressive evolution of the grain (crystal) size distribution of deforming and recrystallizing ice directly affects its rheological behaviour in terms of composite grain-size-sensitive (GSS) and grain-size-insensitive (GSI) creep, and that this might, after time, result in a steady state balance between mechanisms of GSS and GSI creep. We performed static grain growth experiments at different temperatures and a pressure (P) of 1 atm, and deformation experiments at P = 30-100 MPa starting in the GSS-creep field. The starting material ice I h has a grain size < 2 µm and was generated by a special pressure-release technique described by Stern et al. (1997) resulting in dense ice aggregates. The ice grains of the polycrystalline starting samples were randomly oriented and the material has a porosity of < 0.5%. For the grain growth tests a Hart Scientific temperature bath was filled with d-Limonene as cooling medium. The ice specimens were put into sealed alumina cylinders. For the grain growth tests, temperatures (T) between 213 K and 268 K were chosen. The durations of these tests varied between one day and two weeks. For the deformation experiments, temperatures of > 170 K and strain rates between 10 −8 s −1 and 10 −4 s −1 were chosen. Grain sizes, grain size distributions and grain topologies were measured by cryogenic SEM and image analysis techniques. We found clear evidence of grain growth and a significantly T-dependent variation of grain size distributions. The observations allow us to calibrate values for the grain size exponent n and the activation energy Q as used in conventional grain growth laws. We simulated grain growth of ice based on the microphysical model of Kellermann Slotemaker (2006). This model takes into account full grain size distributions and allows grain boundary migration driven by different acting forces. We will show the importance of these driving forces for grain growth and deformation in polycrystalline ice aggregates.
    S. Geological Survey. 05/2010;
  • Po-Chun Chen, Wuu-Liang Huang, Laura A. Stern
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    ABSTRACT: Polycrystalline methane gas hydrate (MGH) was synthesized using an ice-seeding method to investigate the influence of pressurization and ethanol on the hydrate formation rate and gas yield of the resulting samples. When the reactor is pressurized with CH4 gas without external heating, methane hydrate can be formed from ice grains with yields up to 25% under otherwise static conditions. The rapid temperature rise caused by pressurization partially melts the granular ice, which reacts with methane to form hydrate rinds around the ice grains. The heat generated by the exothermic reaction of methane hydrate formation buffers the sample temperature near the melting point of ice for enough time to allow for continuous hydrate growth at high rates. Surprisingly, faster rates and higher yields of methane hydrate were found in runs with lower initial temperatures, slower rates of pressurization, higher porosity of the granular ice samples, or mixtures with sediments. The addition of ethanol also dramatically enhanced the formation of polycrystalline MGH. This study demonstrates that polycrystalline MGH with varied physical properties suitable for different laboratory tests can be manufactured by controlling synthesis procedures or parameters. Subsequent dissociation experiments using a gas collection apparatus and flowmeter confirmed high methane saturation (CH4·nH2O, with n = 5.82 ± 0.03) in the MGH. Dissociation rates of the various samples synthesized at diverse conditions may be fitted to different rate laws, including zero and first order.
    Energy & Fuels - ENERG FUEL. 03/2010; 24(4).
  • W. B. Durham, A. V. Pathare, L. A. Stern
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    ABSTRACT: We investigate experimentally the effect of dust on the balance of grain-size-sensitive (GSS) vs grain-size-insensitive (GSI) creep mechanisms in ice, and find that small amounts of dust tend to inhibit GSS flow. Application is to Mars and icy moons.
    03/2010;
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    ABSTRACT: We conducted laboratory deformation experiments on sand-rich mixtures of sand + ice under sufficient confinement to inhibit macroscopic dilation. Dry sand packs constrained not to dilate when they are under a shearing load reach an immobile or ``jammed'' state, as load-supporting ``force chains'' of sand particles form after a small amount of strain and cannot be broken without volume expansion. Our research objective here was to find the minimum volume fraction of ice required to overcome the jammed state. The result surprised us: the required volume fraction is not a fixed number, but depends on the packing characteristics of the sand in question. Experiments were carried out in a triaxial gas deformation rig at confining pressures (60 - 200 MPa) always at least twice the level of differential stresses (11 - 50 MPa) in order to suppress dilatancy. Run temperatures were 223 - 243 K. We used two kinds of quartz sand, one well-sorted, with a maximum dry packing density (MDPD) of about 0.68 sand by volume, and the other a mixture of two sizes, having a higher MDPD of 0.75. Ice volume fraction ranged from well below saturation (where unfilled porosity necessarily remained) to slightly greater than the value of porosity at MDPD. We tested these frozen sands in compression under constant applied differential stress (creep). Strain rates were very low at these conditions, and runs took days or weeks to complete. The amount of strain required to reach the jammed state in ice-undersaturated samples was approximately 0.04, and did not show an obvious dependence on ice content. For both sands, the onset of mobility occurred at approximately 5% above the value of pore volume at MDPD. Furthermore, viscosity of mobile frozen sand near the transition point was extremely sensitive to ice fraction, which implies that at geologic strain rates, far slower than we can reach in the lab, the ice fraction at transition may lie closer to that at MDPD. Cryogenic scanning electron microscopy shows that fracturing of sand grains occurs in ice-undersaturated samples, but gradually disappears as saturation is reached. There are no fractured sand grains in deforming mobile frozen sand packs. One application of this work is to the regolith of Mars at mid-latitudes and poleward, where significant ice is expected to be present. Partially relaxed (``softened'') landforms such as craters require the presence of ice, but also suggest strengths far higher than that of ice. The extreme sensitivity of viscosity to ice content near the mobility boundary, and the near coincidence of mobility and saturation at MDPD together suggest a plausible explanation for partial landform softening on Mars that does not require a fortuitous ice content or an unrealistically brief period of saturation; namely, that the water content of the Martian regolith lies at or near saturation. If true, we can estimate the historical water content of the Martian regolith for reasonable soil densities as being between 120 and 240 global meters of water for the upper kilometer of crust. This is somewhat lower than previous estimates.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: The strengths of water ice I and of methane clathrate hydrate at planetary conditions are known individually, but that of ice-clathrate mixtures is largely unconstrained by experiment. Subsurface materials on icy satellites of the outer solar system may well include such mixtures, so an understanding of their mechanical properties is demanded. The question is especially important because methane clathrate and ice I differ hugely in strength, the clathrate being several orders of magnitude more viscous than ice at the same stress and temperature. We present the strengths of various methane clathrate-rich ice + clathrate mixtures (5%, 15%, 25% ice by volume) as measured in triaxial deformation experiments. Our preliminary results show that mixtures with as little as 15% ice exhibit weak, ice-like behavior. A general constitutive law for the strength of two-phase mixtures is elusive, but in mixtures of phases with extreme viscosity contrast substantially more than 25% of the weaker phase is usually required before the strength of the aggregate starts resembling that of the weaker phase. Thus the current results are surprising. Methane clathrate has been suggested as a reservoir of atmospheric methane on Titan, and has been implicated in the origin of geysers on Enceladus, but its presence in massive formations or thick layers has been problematic for tectonic models because of its near rock-like strength. If relatively small amounts of ice can weaken the aggregate, the existence of massive clathrate may become less objectionable, at least on mechanical grounds.
    AGU Fall Meeting Abstracts. 12/2009;
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    ABSTRACT: We present preliminary experimental data indicating a correspondence between the brittle-to-ductile transition of icy sand packs and the maximum packing density of sand in such packs.
    03/2009;
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    ABSTRACT: 1] The physical state of water on Mars has fundamental ramifications for both climatology and astrobiology. The widespread presence of ''softened'' Martian landforms (such as impact craters) can be attributed to viscous creep of subsurface ground ice. We present laboratory experiments designed to determine the minimum amount of ice necessary to mobilize topography within Martian permafrost. Our results show that the jammed-to-mobile transition of icy sand packs neither occurs at fixed ice content nor is dependent on temperature or stress, but instead correlates strongly with the maximum dry packing density of the sand component. Viscosity also changes rapidly near the mobility transition. The results suggest a potentially lower minimum volatile inventory for the impact-pulverized megaregolith of Mars. Furthermore, the long-term preservation of partially relaxed craters implies that the ice content of Martian permafrost has remained close to that at the mobility transition throughout Martian history.
    Geophysical Research Letters 01/2009; 36. · 3.98 Impact Factor
  • W. B. Durham, A. V. Pathare, L. A. Stern
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    ABSTRACT: We present low-temperature experimental runs characterizing the brittle-to-ductile transition in icy materials on Mars, which constrains the global volatile inventory. These runs also reveal the onset of transient creep at very low ice content.
    03/2008;
  • Laura A Stern, Stephen H Kirby
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    ABSTRACT: Using cryogenic SEM, we investigated the physical states of gas-hydrate-bearing samples recovered by drill core from several localities including the SE India margin (NGHP Expedition 01), Cascadia margin (IODP Leg 311), Gulf of Mexico (RV Marion Dufresne 2002), and Mackenzie River Delta (Mallik site, well 5L-38). Core material with a significant fraction of preserved hydrate has only been obtained for cryogenic SEM investigation from relatively few sites worldwide to date, yet certain consistent textural characteristics, as well as some clear differences between sites have been observed. Gas hydrate in cores recovered from Cascadia, Gulf of Mexico, and Mallik often occurs as a dense substrate with typical grain size of 30 to as large as 200 μm. The hydrate often contains a significant fraction of isolated macropores that are typically 5–100 μm in diameter and occupy 10-30 vol. % of the domain. In fine-grained sediment sections of marine samples, gas hydrate commonly forms small pods or lenses with clay platelets oriented sub-parallel around them, or as thin veins 50 to several hundred microns in thickness. In some sections, hydrate grains are delineated by a NaCl-bearing selvage that forms thin rinds along hydrate grain exteriors, presumably produced by salt exclusion during original hydrate formation. Preliminary assessment of India NGHP-01 samples shows some regions consistent with the observations described above, as well as other regions dominated by highly faceted crystals that line the walls or interior of cavities where the hydrate grows unimpeded. Here, we focus on gas hydrate grain morphology and microstructures, pore characteristics and distribution, and the nature of the hydrate/sediment grain contacts of the recovered samples, comparing them to each other and to laboratory-produced gas hydrates grown under known conditions. Other Other
    01/2008;

Publication Stats

1k Citations
212.90 Total Impact Points

Institutions

  • 1993–2013
    • United States Geological Survey
      Reston, Virginia, United States
  • 2010
    • Utrecht University
      Utrecht, Utrecht, Netherlands
  • 2006
    • Kyushu University
      • Faculty of Sciences
      Fukuoka-shi, Fukuoka-ken, Japan
  • 2003
    • Brown University
      Providence, Rhode Island, United States
  • 2001–2003
    • Geological Survey of Alabama
      Tuscaloosa, Alabama, United States
  • 2000
    • Lawrence Livermore National Laboratory
      Livermore, California, United States