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At least 397 records · Page 22

Grain-Scale Supercharging and Breakdown on Airless Regoliths

Interactions of the solar wind and emitted photoelectrons with airless bodies have been studied extensively. However, the details of how charged particles interact with the regolith at the scale of a single grain have remained largely uncharacterized. Recent efforts have focused upon determining total surface charge under photoemission and solar wind bombardment and the associated electric field and potential. In this work, theory and simulations are used to show that grain-grain charge differences can exceed classical sheath predictions by several orders of magnitude, sometimes reaching dielectric breakdown levels. Temperature-dependent electrical conductivity works against supercharging by allowing current to leak through individual grains; the balance between internal conduction and surface charging controls the maximum possible grain-to-grain electric field. Understanding the finer details of regolith grain charging, conductive equilibrium, and dielectric breakdown will improve future numerical studies of space weathering and dust levitation on airless bodies.

airless↗

In-Flight Calibration of the MMS Fluxgate Magnetometers

We present an overview of the approach to in-flight calibration, which is a coordinated effort between the University of California Los Angeles (UCLA), Space Research Institute, Graz, Austria (IWF) and the NASA Goddard Space Flight Center (GSFC). We present details of the calibration effort at GSFC. During the first dayside season of the Magnetospheric Multiscale (MMS) mission, the in-flight calibration process for the Fluxgate magnetometers (FGM) implemented an algorithm that selected a constant offset (zero-level) for each sensor on each orbit. This method was generally able to reduce the amplitude of residual spin tone to less than 0.2 nT within the region of interest. However, there are times when the offsets do show significant short-term variations. These variations are most prominent in the nighttime season (phase 1X), when eclipses are accompanied by offset changes as large as 1 nT. Eclipses are followed by a recovery period as long as 12 hours where the offsets continue to change as temperatures stabilize. Understanding and compensating for these changes will become critical during Phase 2 of the mission in 2017, when the nightside will become the focus of MMS science. Although there is no direct correlation between offset and temperature, the offsets are seen for the period of any given week to be well-characterized as function of instrument temperature. Using this property, a new calibration method has been developed that has proven effective in compensating for temperature-dependent offsets during phase 1X of the MMS mission and also promises to further refine calibration quality during the dayside season.

Magnetic field reconnection↗

The Chemistry of Pluto and its Satellites

Pluto's bulk composition and the composition of the surface layers hold clues to the origin and evolution of a number of other Solar System bodies of comparable size in the region beyond Neptune. The July 14, 2015 flyby of the Pluto system with the New Horizons spacecraft afforded the opportunity to corroborate and greatly improve discoveries about the planet and its satellites derived Earth-based studies. It also revealed extraordinary details of the surface and atmosphere of Pluto, as well as the geology and composition of Charon and two smaller satellites. With a mean density of 1.86 g/sq cm, the bulk composition of Pluto is about two-thirds anhydrous solar composition rocky material and one-third volatiles (primarily H2O in liquid and solid states) by mass, the surface is a veneer of ices dominated by N2, with smaller amounts of CH4 and CO, as well as limited exposures of H2O ice (considered to be "bedrock"). N2, CH4, and CO occur as solid solutions at temperature-dependent mutual concentrations, each component being soluble in the others. Frozen C2H6 as a minor component has also been identified. Sublimation and recondensation of N2, CH4, and CO over seasonal (248 y) and Milankovich-type megaseasons (approx. 3 My) result in the redistribution of these ices over time and with latitude control. Solid N2 is found in glaciers originating in higher elevations and flowing at the present time into a basin structure larger than the State of Texas, forming a convecting lens of N2 that overturns on a timescale of order 10 My. The varied colors of Pluto's landscape arise from the energetic processing of the surface ices in processes that break the simple molecules and reassemble complex organic structures consisting of groups of aromatic rings connected by aliphatic chains. When synthesized in the laboratory by UV or electron irradiation of a Pluto mix of ice, this material, called tholin, has colors closely similar to Pluto. The Pluto ice tholin analog contains carboxylic acids, urea, ketones, aldehydes, amines, and some nitriles. The largest satellite, Charon has density 1.70 g/sq cm and it is about 3/5 anhydrous solar composition rock, with the remainder in H2O ice. The surface H2O ice is infused in some way with NH3, probably as a hydrate, distributed nonuniformly, but to some degree related to geological structures. Pluto's atmosphere is N2, CH4, with CO, C2-hydrocarbons, HCN, and other molecules in trace but detectable amounts. The atmosphere supports a complex haze structure with about 20 discrete layers, and suspected clouds. The haze is presumed to be made of aggregates of complex hydrocarbons (tholins) produced by photolysis of the atmospheric gases, and with similar composition to the ice tholins made on the planet's surface. Urea and a suite of carboxylic acids are of interest for prebiotic and biological chemistries.

SOLAR SYSTEMS↗

A High-Resolution Measurement of Ball IR Black Paint's Low-Temperature Emissivity

High-emissivity paints are commonly used on thermal control system components. The total hemispheric emissivity values of such paints are typically high (nearly 1) at temperatures above about 100 Kelvin, but they drop off steeply at lower temperatures. A precise knowledge of this temperature-dependence is critical to designing passively-cooled components with low operating temperatures. Notable examples are the coatings on thermal radiators used to cool space-flight instruments to temperatures below 40 Kelvin. Past measurements of low-temperature paint emissivity have been challenging, often requiring large thermal chambers and typically producing data with high uncertainties below about 100 Kelvin. We describe a relatively inexpensive method of performing high-resolution emissivity measurements in a small cryostat. We present the results of such a measurement on Ball InfraRed BlackTM(BIRBTM), a proprietary surface coating produced by Ball Aerospace and Technologies Corp (BATC), which is used in spaceflight applications. We also describe a thermal model used in the error analysis.

Tuttle, Jim↗

Hemispherical Reflectance and Emittance Properties of Carbon Nanotubes Coatings at Far-Infrared Wavelengths

Recent visible wavelength observations of Multiwalled Carbon Nanotubes (MWCNT) coatings have revealed that they represent the blackest materials known in nature with a Total Hemispherical Reflectance (THR) less than .25%. This makes them as exceptionally good absorbers, with the potential to provide order-of-magnitude improvement in stray-light suppression over current black surface treatments when used in an optical system. Here we extend the characterization of this class of materials into the infrared spectral region to further evaluate their potential for use on instrument baffles for stray-light suppression and to manage spacecraft thermal properties to dissipate heat through radiant heat transfer process. These characterizations will include the wavelength-dependent Total Hemispherical Reflectance properties in the mid-IR and far-infrared spectral regions (2-100 micrometers). Determination of the temperature-dependent emittance will be investigated in the temperature range of 20 to 300 K. These results will be compared against other more conventional black coatings such as Acktar Fractal Black or Z-306 coatings among others.

Quijada, Manuel A.↗

Thermal Model Performance for the James Webb Space Telescope OTIS Cryo-Vacuum Test

The James Webb Space Telescope (JWST), set to launch in mid-2020, is currently undergoing a series of system-level environmental tests to verify its workmanship and end-to-end functionality. As part of this series, the Optical Telescope Element and Integrated Science Instrument Module (OTIS) Cryo-Vacuum (CV) test, the most complex cryogenic test executed to date by NASA, has recently been completed at the Johnson Space Center's Chamber A facility. The OTIS CV test was intended as a comprehensive test of the integrated instrument and telescope systems to fully understand its optical, structural, and thermal performance within its intended flight environment. Due to its complexity, extensive pre-test planning was required to ensure payload safety and compliance with all limits and constraints. A system-level pre-test thermal model was constructed which fully captured the behavior of the payload, ground support equipment, and surrounding test chamber. This thermal model simulated both the transient cooldown to and warmup from a 20 K flight-like environment, as well as predicted the payload performance at cryo-stable conditions. The current work is an assessment of thermal model pre-test prediction performance against actual payload response during the OTIS CV test. Overall, the thermal model performed exceedingly well at predicting schedule and payload response. Looking in depth, this work examines both the benefits and shortcomings of assumptions made pre-test to simplify model execution when compared against test data. It explores in detail the role of temperature-dependent emissivities during transition to cryogenic temperatures, as well as the impact that model geometry simplifications have on tracking of critical hardware limits and constraints. This work concludes with a list of recommendations to improve the accuracy of thermal modeling for future large cryogenic tests. The insight gained from the OTIS CV test thermal modeling will benefit planning and execution for upcoming cryogenic missions.

Testing↗

Thermal Model Performance for the James Webb Space Telescope OTIS Cryo-Vacuum Test

The James Webb Space Telescope (JWST), set to launch in early 2019, is currently undergoing a series of system-level environmental tests to verify its workmanship and end-to-end functionality. As part of this series, the Optical Telescope Element and Integrated Science Instrument Module (OTIS) Cryo-Vacuum (CV) test, the most complex cryogenic test executed to date by NASA, has recently been completed at the Johnson Space Center's Chamber A facility. The OTIS CV test was intended as a comprehensive test of the integrated instrument and telescope systems to fully understand its optical, structural, and thermal performance within its intended flight environment. Due to its complexity, extensive pre-test planning was required to ensure payload safety and compliance with all limits and constraints. A system-level pre-test thermal model was constructed which fully captured the behavior of the payload, ground support equipment, and surrounding test chamber. This thermal model simulated both the transient cooldown to and warmup from a 20K flight-like environment, as well as predicted the payload performance at cryo-stable conditions. The current work is a preliminary assessment of thermal model performance against actual payload response during the OTIS CV test. It examines both the benefits and shortcomings of assumptions made pre-test to simplify model execution when compared against test data. It explores in detail the role of temperature-dependent emissivities during transition to cryogenic temperatures, as well as the impact that model geometry simplifications have on tracking of critical hardware limits and constraints. This work concludes with a list of recommendations to improve the accuracy of thermal modeling for future large cryogenic tests. It is hoped that the insight gained from the OTIS CV test thermal modeling will benefit planning and execution for upcoming cryogenic missions.

JWST↗

Influence of Fiber Content, Thermal Cycles and Creep on Electrical Resistivity Measurements of SiCf/SiC Minicomposites

Fiber-reinforced Ceramic Matrix Composites (CMCs) are currently being used in high-temperature applications such as jet engine components. In such extreme environment applications, materials are exposed to many mechanical and thermal loading cycles during the life of the component. This exposure can degrade the component, and thus, components need to be monitored and inspected during service. The electrical resistivity nondestructive evaluation technique has been shown to be very sensitive to damage in CMCs, and although its performance is highly dependent on temperature, this temperature-dependence is still not well understood. Therefore, this current study aims to establish the influence of thermal cycles, creep, CVI-SiC matrix cracks and fiber volume fraction on the electrical resistance of Hi Nicalon and Hi-Nicalon Type S SiC fiber-reinforced minicomposites with BN interphases. Minicomposites with 97% CVI-SiC matrix volume fraction were used to characterize the electrical resistivity of the CVI-SiC matrix. Next, models were constructed to calculate fiber resistivity and onset and evolution of damage during tensile testing at room-temperature. The thermal cycles were achieved by heating as-received and precracked specimens to 1200°C and then cooling them to room temperature, with or without applied stress on fibers.

creep↗

Thermal Model Performance for the James Webb Space Telescope OTIS Cryo-Vacuum Test

The James Webb Space Telescope (JWST), set to launch in early 2019, is currently undergoing a series of system-level environmental tests to verify its workmanship and end-to-end functionality. As part of this series, the Optical Telescope Element and Integrated Science Instrument Module (OTIS) Cryo-Vacuum (CV) test, the most complex cryogenic test executed to date by NASA, has recently been completed at the Johnson Space Center’s Chamber A facility. The OTIS CV test was intended as a comprehensive test of the integrated instrument and telescope systems to fully understand its optical, structural, and thermal performance within its intended flight environment. Due to its complexity, extensive pre-test planning was required to ensure payload safety and compliance with all limits and constraints. A system-level pre-test thermal model was constructed which fully captured the behavior of the payload, ground support equipment, and surrounding test chamber. This thermal model simulated both the transient cooldown to and warmup from a 20K flight-like environment, as well as predicted the payload performance at cryo-stable conditions. The current work is a preliminary assessment of thermal model performance against actual payload response during the OTIS CV test. Overall, the thermal model performed exceedingly well at predicting schedule and payload response. Looking in depth, this work examines both the benefits and shortcomings of assumptions made pre-test to simplify model execution when compared against test data. It explores in detail the role of temperature-dependent emissivities during transition to cryogenic temperatures, as well as the impact that model geometry simplifications have on tracking of critical hardware limits and constraints. This work concludes with a list of recommendations to improve the accuracy of thermal modeling for future large cryogenic tests. It is hoped that the insight gained from the OTIS CV test thermal modeling will benefit planning and execution for upcoming cryogenic missions.

Yang, Kan↗

A Cloud Algorithm Based on the O2-O2 477 nm Absorption Band Featuring an Advanced Spectral Fitting Method and the Use of Surface Geometry-Dependent Lambertian-Equivalent Reflectivity

We discuss a new cloud algorithm that retrieves an effective cloud pressure, also known as cloud optical centroid pressure (OCP), from oxygen dimer (O2-O2) absorption at 477nm after determining an effective cloud fraction (ECF) at 466nm, a wavelength not significantly affected by trace-gas absorption and rotational Raman scattering. The retrieved cloud products are intended for use as inputs to the operational nitrogen dioxide (NO2) retrieval algorithm for the Ozone Monitoring Instrument (OMI) flying on the Aura satellite. The cloud algorithm uses temperature-dependent O2-O2 cross sections and incorporates flexible spectral fitting techniques that account for specifics of the surface reflectivity. The fitting procedure derives O2-O2 slant column densities (SCDs) from radiances after O3, NO2, and H2O absorption features have been removed based on estimates of the amounts of these species from independent OMI algorithms. The cloud algorithm is based on the frequently used mixed Lambertian-equivalent reflectivity (MLER) concept. A geometry-dependent Lambertian-equivalent reflectivity (GLER), which is a proxy of surface bidirectional reflectance, is used for the ground reflectivity in our implementation of the MLER approach. The OCP is derived from a match of the measured O2-O2 SCD to that calculated with the MLER method. Temperature profiles needed for computation of vertical column densities are taken from the Global Modeling Initiative (GMI) model. We investigate the effect of using GLER instead of climatological LER on the retrieved ECF and OCP. For evaluation purposes, the retrieved ECFs and OCPs are compared with those from the operational OMI cloud product, which is also based on the same O2-O2 absorption band. Impacts of the application of the newly developed cloud algorithm to the OMI NO2 retrieval are discussed.

cloud fraction and pressure↗

Simulations of Li+ in Ionic Liquids: Structure, Transport, and Electrochemical Windows

Ionic liquids have been proposed as candidate electrolytes for a number of electrochemical applications. The Li+ solvation structure in these liquids is of central importance to electrolyte properties, like ionic conductivity and electrochemical stability. To this point, we employ simulations at three different size scales to better understand various aspects of the interplay between Li+ solvation structure and dynamics. The smallest systems are Li(Anion)n clusters that are treated with high-accuracy density functional theory (DFT) techniques to provide insight into solvation shell structure through energetics and comparisons to experimental IRRaman spectra. Mid-range sized liquid-phase systems (12-24 ion pairs) are treated with DFT molecular dynamics (MD) to provide temperature-dependent insight into Li+ solvation structure, diffusion, and electrochemical window. The largest systems (144-216 ion pairs) are treated with polarizable MD simulations to evaluate the influence of Li-networks on structure and provide size independent values of transport properties. We perform this procedure on three technologically important ionic liquids and comment on property correlations with solvation structure.

batteries↗

Ab Initio Simulations of Phase Stability and Martensitic Transitions in NiTi

For NiTi based alloys, the shape memory effect is governed by a transition from a low-temperature martensite phase to a high-temperature austenite phase. Despite considerable experimental and computational work, basic questions regarding the stability of the phases and the martensitic phase transition remain unclear even for the simple case of binary, equiatomic NiTi. We perform ab initio molecular dynamics simulations to describe the temperature-dependent behavior of NiTi and resolve several of these outstanding issues. Structural correlation functions and finite temperature phonon spectra are evaluated to determine phase stability. We show that finite temperature, entropic effects stabilize the experimentally observed martensite (B19') and austenite (B2) phases while destabilizing the theoretically predicted (B33) phase. Free energy computations based on ab initio thermodynamic integration confirm these results and permit estimates of the transition temperature between the phases. In addition to the martensitic phase transition, we predict a new transition between the B33 and B19' phases. The role of defects in suppressing phase transformation temperatures is discussed.

Simulations↗

Compensation of Oxygen Transmittance Effects for Proximal Sensing Retrieval of Canopy-Leaving Sun-Induced Chlorophyll Fluorescence

Estimates of Sun-Induced vegetation chlorophyll Fluorescence (SIF) using remote sensing techniques are commonly determined by exploiting solar and/or telluric absorption features. When SIF is retrieved in the strong oxygen (O2) absorption features, atmospheric effects must always be compensated. Whereas correction of atmospheric effects is a standard airborne or satellite data processing step, there is no consensus regarding whether it is required for SIF proximal-sensing measurements nor what is the best strategy to be followed. Thus, by using simulated data, this work provides a comprehensive analysis about how atmospheric effects impact SIF estimations on proximal sensing, regarding: (1) the sensor height above the vegetated canopy; (2) the SIF retrieval technique used, e.g., Fraunhofer Line Discriminator (FLD) family or Spectral Fitting Methods (SFM); and (3) the instrument's spectral resolution. We demonstrate that for proximal-sensing scenarios compensating for atmospheric effects by simply introducing the O2 transmittance function into the FLD or SFM formulations improves SIF estimations. However, these simplistic corrections still lead to inaccurate SIF estimations due to the multiplication of spectrally convolved atmospheric transfer functions with absorption features. Consequently, a more rigorous oxygen compensation strategy is proposed and assessed by following a classic airborne atmospheric correction scheme adapted to proximal sensing. This approach allows compensating for the O2 absorption effects and, at the same time, convolving the high spectral resolution data according to the corresponding Instrumental Spectral Response Function (ISRF) through the use of an atmospheric radiative transfer model. Finally, due to the key role of O2 absorption on the evaluated proximal-sensing SIF retrieval strategies, its dependency on surface pressure (p) and air temperature (T) was also assessed. we combined simulated spectral data with p and T measurements obtained for a one-year period in the Hyytiala Forestry Field Station in Finland. Of importance hereby is that seasonal dynamics in terms of T and p, if not appropriately considered as part of the retrieval strategy, can result in erroneous SIF seasonal trends that mimic those of known dynamics for temperature-dependent physiological responses of vegetation.

"sun–induced chlorophyll fluorescence (SIF); pro↗

Using Defects to Create Ceramics with Giant Permittivity as Ultracapacitors

Dielectrics with a high and weakly- temperature-dependent permittivity are highly desirable for the development of high performance energy storage devices. It is well known that ferroelectrics (FEs) exhibit a high permittivity that is strongly dependent on the temperature due to phase transition. In this project, the FE BaTiO3 (BTO), based ceramics are developed as ultra-capacitors by introducing defects to eliminate the phase transition and enhance the permittivity. The ceramics are fabricated using a unique process: BTO nanoparticles are coated with a nano-layer of SiO2 and, then, sintered in vacuum using SPS process. The ceramics exhibit a giant permittivity and many other unique properties.

Talebinezhad, H.↗

Oil Dispersant Corexit 9500 Is Weakly Estrogenic, but Does Not Skew the Sex Ratio in Alligator Mississippiensis

During the Deepwater Horizon oil spill, vast quantities of the chemical dispersant Corexit 9500 were applied in remediation efforts. In addition to the acute toxicity and obesogenicity of Corexit, it is important to further evaluate Corexit with long-term sublethal endocrine endpoints. The American alligator (Alligator mississippiensis) exhibits temperature-dependent sex determination, in which egg incubation temperatures during a thermosensitive period (TSP) in embryonic development determine sex of the embryos. Estrogen signals also play critical roles in this process. For example, a single exposure to exogenous estrogen during the TSP overrides the effects of temperature and leads to skewed sex ratios. At a dose of 100 ppm, Corexit significantly induced transcriptional activity of both alligator nuclear estrogen receptors 1 and 2 in vitro using the reporter gene assays. To investigate the estrogenic effects on gonadal development in ovo, alligator eggs were exposed to Corexit at environmentally relevant concentrations (0.25, 2.5, and 25 ppm) prior to the TSP. Significant delays in the hatching and growth were observed in 0.25 and 25 ppm Corexit exposed groups. At one week of age, no significant effects on sex ratios or testicular mRNA abundance were observed as a result of exposure to Corexit at the tested exposure levels, suggesting Corexit does not have strong estrogenic activity in ovo. Further investigations are required using individual components of Corexit to better understand their estrogenic effects on the development and reproductive health of alligators and other coastal aquatic wildlife.

Nicole Mcnabb↗

LongRunMIP - Motivation and Design for a Large Collection of Millennial-Length AO-GCM Simulations

LongRunMIP is the first collection of millennial-length simulations of complex coupled climate models and enables investigations of how these models equilibrate in response to radiative perturbations.We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1000+ year) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time-and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1000 years. LongRunMIP is a MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields, and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of summer 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submission of more simulations in the future.

Maria Rugenstein↗

Disentangling the Drivers of the Summertime Ozone‐Temperature Relationship Over the United States

Summertime surface-level ozone (O3 ) is known to vary with temperature, but the relative roles of different processes responsible for causing the O3 -temperature relationship are not well quantified. In this study we use simulations of NASA's Global Modeling Initiative (GMI) chemical transport model (CTM) to isolate and assess the relative impact of atmospheric transport, chemistry, and emissions on O3 variability, events, and its correlation with temperature. Using observations from CASTNet in the contiguous United States, we show that the GMI CTM reproduces the spatiotemporal variability of O3 and the O3 -temperature relationship during the summer. We primarily focus on the total change in O3 due to a change in temperature (d O3 /d T). In regions with strong positive correlations between temperature and O3 such as the Northeast, Great Lakes, and Great Plains, temperature's association with transport primarily drives d O3 /d T with smaller contributions from temperature-dependent chemistry and anthropogenic emissions. There are regions, however, with near-zero correlation between temperature and O3 , and our findings suggest that transport is still an important driver of O3 variability in these regions, albeit not correlated with temperature. Transport is not directly dependent on temperature but rather is linked through an indirect association, and it is therefore important to understand the exact mechanisms that link transport to O3 and how these mechanisms will change in a warming world.

ozone, temperature, CTM↗

Thermoplastic Matrix Composite Design for Cryotanks Using Multiscale Modeling and Bayesian Optimization

Designing lightweight, robust cryogenic storage tanks is critical for future launch vehicles, in-space propellant storage, and hydrogen powered aircraft. This work presents a multiscale modeling and Bayesian optimization framework for the design of thermoplastic matrix composite cryotanks. Molecular dynamics simulations are first used to determine temperature-dependent constituent properties for candidate thermoplastic matrices, which are homogenized to the lamina scale using NASA’s Multiscale Analysis Tool (NASMAT). These lamina properties, in combination with laminate family generation rules, are evaluated in HyperX structural optimization software to identify stacking sequences that meet all cryogenic load requirements. A Bayesian optimization framework is applied, with HyperX in the loop (via the HyperX API) to efficiently search across material and laminate design variables, yielding an optimized cryotank configuration with significant reductions in design cycle time compared to exhaustive search approaches.

thermoplastics↗