L. M. Trafton

University of Texas at Austin, Austin, Texas, United States

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Publications (253)496.52 Total impact

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    ABSTRACT: There is significant scientific interest in simulating the unique atmospheric conditions on the Jovian moon Io that range from cold surface temperatures to hyperthermal interactions which possibly supply the Jovian plasma torus. The Direct Simulation Monte Carlo (DSMC) method is well suited to model the rarefied, predominantly SO2, Ionian atmosphere. High speed collisions between SO2 and the hypervelocity O atoms and ions that compose the plasma torus are a significant mechanism in determining the composition of the atmosphere; therefore, high-fidelity modeling of their interactions is crucial to the accuracy of such simulations. Typically, the Total Collision Energy (TCE) model is used to determine molecular dissociation probabilities and the Variable Hard Sphere (VHS) model is used to determine collision cross sections. However, the parameters for each of these baseline models are based on low-temperature experimental data and thus have unknown reliability for the hyperthermal conditions in the Ionian atmosphere. Recently, Molecular Dynamics/Quasi-Classical Trajectory (MD/QCT) studies have been conducted to generate accurate collision and chemistry models for the SO2–O collision pair in order to replace the baseline models. However, the influence of MD/QCT models on Ionian simulations compared to the previously used models is not well understood. In this work, 1D simulations are conducted using both the MD/QCT-based and baseline models in order to determine the effect of MD/QCT models on Ionian simulations. It is found that atmospheric structure predictions are highly sensitive to the chemistry and collision models. Specifically, the MD/QCT model predicts approximately half the SO2 atmospheric dissociation due to O and O+ bombardment compared to TCE models, and also predicts a temperature rise due to plasma heating further from the Ionian surface than the existing baseline methodologies. These findings indicate that the accurate MD/QCT chemistry and collision models provide a significant improvement over the baseline models for DSMC simulations of the Ionian atmosphere.
    Icarus 09/2014; 239:32–38. · 3.16 Impact Factor
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    ABSTRACT: We model the transport of water to lunar cold traps after a comet impact and discuss how gas dynamic processes constrain where and how much ice is deposited.
    02/2014;
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    ABSTRACT: The sulfur-rich Ionian atmosphere is populated through a number of mechanisms, the most notable of which include sublimation from insolated surface frost deposits, material sputtering due to the impact of energetic ions from the Jovian plasma torus, and plume emission related to volcanic activity. While local flows are collisional at low altitudes on portions of the moon’s dayside, densities rapidly tend toward the free-molecular limit with altitude, necessitating non-continuum (rarefied gas dynamic) modeling and analysis. While recent work has modestly constrained the relative contributions of sputtering, sublimation, and volcanism to Io’s atmosphere, dynamic wind patterns driven by dayside sublimation and nightside condensation remain poorly understood. This work moves toward the explanation of mid-infrared observations that indicate an apparent super-rotating wind in Io’s atmosphere. In the present work, the Direct Simulation Monte Carlo method is employed in the modeling of Io’s rarefied atmosphere; simulations are computed in parallel, on a three-dimensional domain that spans the moon’s entire surface and extends hundreds of kilometers vertically, into the exobase. A wide range of physical phenomena have been incorporated into the atmospheric model, including: [1] the effects of planetary rotation; [2] surface temperature, surface frost inhomogeneity, and thermal inertia; [3] plasma heating and sputtering; [4] gas plumes from superimposed volcanic hot spots; and [5] multi-species chemistry. Furthermore, this work improves upon previous efforts by correcting for non-inertial effects in a moon-fixed reference frame. The influence of such effects on the development of global flow patterns and cyclonic wind is analyzed. The case in which Io transits Jupiter is considered, with the anti-Jovian hemisphere as the dayside. We predict that a circumlunar flow develops that is asymmetric about the subsolar point, and drives atmosphere from the warmer, dayside hemisphere toward the colder nightside. The resultant flow patterns, column densities, species concentrations, and temperatures are discussed in relation to previous simulations of Io in a pre-eclipse configuration. This research is supported via NASA-PATM.
    10/2013;
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    ABSTRACT: The black “butterfly wings” seen at Pele are produced by silicate ash which is to some extent entrained in the gas flow from very low altitudes. These particles are key to understanding the volcanism at Pele. However, the Pele plume is not nearly as dusty as Prometheus, and these are not the only particles in the plume, as the SO2 in the plume will also condense as it cools. It is therefore difficult to estimate a size distribution for the ash particles by observation, and the drag on ash particles from the plume flow is significant enough that ballistic models are also of limited use. Using Direct Simulation Monte Carlo, we can simulate a gas plume at Pele which demonstrates very good agreement with observations. By extending this model down to nearly the surface of the lava lake, ash particles can be included in the simulation by assuming that they are initially entrained in the very dense (for Io) gas immediately above the magma. Particles are seen to fall to the ground to the east and west of the vent, agreeing with the orientation of the “butterfly wings”, and particles with larger diameters fall to the ground closer to the lava lake. We present a model for mapping simulated deposition density to the coloration of the surface and we use it to estimate the size distribution of ash particles in the plume.
    10/2013;
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    ABSTRACT: We analyze ground-based infrared observations of H3+ emission from the upper atmosphere of Uranus using Gemini North/GNIRS, NASA IRTF/SpeX and VLT/CRIRES. These observations were obtained on 15 different nights in late 2011, between day-of-year 251 (5th of September) and 340 (6th of December). The equinox of Uranus occurred in late 2007 and these recent observations quantify the behavior of the planet's upper atmosphere 4 years after equinox, equivalent to 15° of circumsolar rotation. We also present preliminary results from the 2012 observing campaign using the NASA IRTF and Gemini telescopes. The mean temperature of the ionosphere from these measurements is 520 ± 32 K, which is cooler than any of the temperatures determined by the precursor to this study (Melin, H., Stallard, T., Miller, S.,Trafton, L.M., Encrenaz, T., Geballe, T.R. [2011]. Astrophys. J. 729, 134). Thus, the cooling trend that has been observed since the first H3+ observation in 1992 has continued, even as the planet traversed equinox. This suggests that the driver of the elevated thermospheric temperatures cannot be linked to purely seasonal mechanisms, and we consider other sources of variability, such as the changing geometry between the planet, magnetosphere and solar wind.
    09/2013;
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    ABSTRACT: We analyse ground-based infrared observations of H3+ emission from the upper atmosphere of Uranus using Gemini North/GNIRS, NASA IRTF/SpeX and VLT/CRIRES. These observations were obtained on 15 different nights in late 2011, between day-of-year 251 (5th of September) and 340 (6th of December). The equinox of Uranus occurred in late 2007 and these recent observations quantify the behaviour of the planet’s upper atmosphere 4 years after equinox, equivalent to 15° of circumsolar rotation.The mean temperature of the ionosphere from these measurements is 520 ± 32 K, which is cooler than any of the temperatures determined by the precursor to this study (Melin, H., Stallard, T., Miller, S., Trafton, L.M., Encrenaz, T., Geballe, T.R. [2011b]. Astrophys. J. 729, 134). Thus, the cooling trend that has been observed since the first H3+ observation in 1992 has continued, even as the planet traversed equinox. This suggests that the driver of the elevated thermospheric temperatures cannot be linked to purely seasonal mechanisms, and we consider other sources of variability, such as the changing geometry between the planet, magnetosphere and solar wind.
    Icarus 04/2013; 223(2):741–748. · 3.16 Impact Factor
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    ABSTRACT: As a lunar lander approaches a dusty surface, the plume from the descent engine impinges on the ground, entraining loose regolith into a high velocity dust spray. Without the inhibition of a background atmosphere, the entrained regolith can travel many kilometers from the landing site. In this work, we simulate the flow field from the throat of the descent engine nozzle to where the dust grains impact the surface many kilometers away. The near field is either continuum or marginally rarefied and is simulated via a loosely coupled hybrid DSMC - Navier Stokes (DPLR) solver. Regions of two-phase and polydisperse granular flows are solved via DSMC. The far field deposition is obtained by using a staged calculation, where the first stages are in the near field where the flow is quasi-steady and the outer stages are unsteady. A realistic landing trajectory is approximated by a set of discrete hovering altitudes which range from 20m to 3m. The dust and gas motions are fully coupled using an interaction model that conserves mass, momentum, and energy statistically and inelastic collisions between dust particles are also accounted for. Simulations of a 4 engine configuration are also examined, and the erosion rates as well as near field particle fluxes are discussed.
    11/2012;
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    ABSTRACT: Several missions have yielded observations that could indicate the presence of water ice in lunar polar regions. Our work aims to investigate cometary impacts as a mechanism for the delivery of water to permanently shadowed craters (‘cold traps’) at the lunar poles. Of particular interest is the influence of parameters such as impact angle, velocity and location on the long-term retention of cometary water. Our 3D, unsteady simulations use the SOVA hydrocode to model the impact and vaporization of a cometary nucleus composed of pure water ice, 2km in diameter, impacting at 30 km/s. Subsequently, a Direct Simulation Monte Carlo code, designed to handle rarefied planetary flows, is used to simulate the transient water vapor atmosphere that develops. Molecules in this atmosphere collide and migrate across the lunar surface, driven by diurnal variations in surface temperature, and may land in permanently shadowed craters, cold enough to trap water over geological time scales. Here, we discuss the dynamic development of the transient atmosphere and compare initial deposition patterns as gravitationally bound water vapor begins to fall back to the lunar surface, for two different impact angles: 45° and 60° from the horizontal. A greater fraction of water remains gravitationally bound to the Moon in the 60° case, and a less pronounced downrange focusing of the vapor results in a more symmetric initial deposition pattern. On the cold night-side of the Moon, water simply sticks to the surface. However, on the warm day-side, where residence times are much shorter, we observe the development of a relatively dense, low-speed, surface-hugging flow. A particularly interesting depositional feature is the concentration of mass at a point almost antipodal to the point of impact, where a convergence of streamlines results in a shock that channels water to the surface.
    10/2012;
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    ABSTRACT: Io’s Pele plume rises over 300km in altitude and leaves a deposition ring 1200km across on the surface of the moon. Material emerges from an irregularly-shaped vent, and this geometry gives rise to complex 3D flow features. The Direct Simulation Monte Carlo method is used to model the gas flow in the rarefied plume, demonstrating how the geometry of the source region is responsible for the asymmetric structure of the deposition ring and illustrating the importance of very small-scale vent geometry in explaining large observed features of interest. Simulations of small particles in the plume and comparisons to the black “butterfly wings” seen at Pele are used to constrain particle sizes and entrainment mechanisms. Preliminary results for the effects of plasma energy and momentum transfer to the plume will also be presented.
    10/2012;
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    ABSTRACT: Cassini first detected a gas-particle plume over the south pole of Enceladus in 2005. Since then, the plume has been a very active area of research because unlocking its mystery may help answer many lingering questions and open doors to new possibilities, such as the existence of extra-terrestrial life. Here, we present a hybrid model of the Enceladus gas-particle plume. Our model places eight sources on the surface of Enceladus based on the locations and jet orientations determined by Spitale and Porco (2007). We simulate the expansion of water vapor into vacuum, in the presence of dust particles from each source. The expansion is divided into two regions: the dense, collisional region near the source is simulated using the direct simulation Monte Carlo method, and the rarefied, collisionless region farther out is simulated using a free-molecular model. We also incorporate the effects of a sublimation atmosphere, a sputtered atmosphere and the background E-ring. Our model results are matched with the Cassini in-situ data, especially the Ion and Neutral Mass Spectrometer (INMS) water density data collected during the E2, E3, E5 and E7 flybys and the Ultraviolet Imaging Spectrograph (UVIS) stellar occultation observation made in 2005. Furthermore, we explore the time-variability of the plume by adjusting the individual source strengths to obtain a best curve-fit to the water density data in each flyby. We also analyze the effects of grains on the gas through a parametric study. We attempt to constrain the source conditions and gain insight on the nature of the source via our detailed models.
    10/2012;
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    ABSTRACT: The dominant process causing the high thermospheric temperatures observed for the major planets remains an unsolved problem. Uranus is of particular interest for identifying this source of heating because of its extreme obliquity and weak internal heat source, which permit large seasonal extremes driven by radiative and dynamical processes. Sources of thermospheric heating may be investigated indirectly through the energy balance of the offsetting line emission, which radiates the generated heat to space. The cooling rate can be characterized by observing the line emission vs. position over the planet. The primary coolant in Uranus’ thermosphere is emission in the rotational H2 quadrupole lines. We report observations of Uranus’ rotational H2 quadrupole line emission obtained near the 2007 equinox using TEXES at Gemini in late October, 2007. Good data were obtained for the H2 S(1) line, which was scanned longitudinally across Uranus’ disk to make an emission map showing all latitudes. This map shows bimodal emission along Uranus’ central meridian with the brightest peak in the northern (end of winter) hemisphere. Intermittent clouds interfered with the observation of the relatively faint S(2) line, which precluded scanning, thus leaving the observations vulnerable to pointing uncertainties. We combine these data with non-equinox observations of Uranus obtained with TEXES at the IRTF to estimate the positional variation of Uranus’ thermospheric cooling rate, ultimately to help constrain the unknown dominant source of heating.
    10/2012;
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    ABSTRACT: Observations of H+3 emission from the upper atmosphere of Uranus was performed in the latter half of 2011 using Gemini North, NASA Infrared Telescope Facility (IRTF) and the Very Large Telescope (VLT). These determined an average H+3 ionospheric temperature of 520±32 K, which is smaller than any perviously determined value. This long-term cooling, initially observed by Melin et al. (2011, ApJ), was thought to be connected to seasonal mechanisms. However, with Uranus' equinox having occurred in 2007, the planet has already rotated some 15° along its circumsolar orbit. This continued cooling may be due to the changing geometry of the magnetosphere and rotational axis with respect to the solar wind.
    09/2012;
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    ABSTRACT: We report the detection of SO2 emission from Io in Jupiter's shadow, peaking near 26 Rayleigh/Å around 3150 Å, and emission from its associated excitation-dissociation products, SO in the 2550 Å band and S I in the 1800 and 1900 Å multiplets. In addition, an unidentified emission spectrum was discovered between ˜4100 Å and ˜5700 Å, which appears to be a vibronic band. Its spectral lines are listed in neither the GEISA nor HITRAN database. The line spacing and wavelength regime are characteristic of molecular bending modes, which would imply a molecule with three or more atoms; e.g., SO2 or S2O. Alternative candidates for this species are positive or negative ions of SO2 and its daughter species. The wavelength-averaged intensity of this unidentified species is bracketed by intensities imaged through Galileo and Cassini filters when Io was in eclipse. Both the unidentified and SO2 emission are brighter on Io's NE half (in the Jovian system), which is the side closer to Jupiter, but the unidentified emission is more asymmetric, suggesting a connection with Io's wake emission or with volcanic activity. Weakening of the emission intensity between the early eclipse-resolved spectra indicate partial atmospheric collapse due to freezeout of the atmospheric column and the decay of energetic photoelectrons. Specific plume activity is not well constrained through examination of the disk-averaged mid-ultraviolet (MUV) emission spectrum. Simulating the observations using laboratory data published for the electron impact cross sections of SO2 indicates that this emission is consistent with dissociative excitation of SO2 by thermal electrons in the Jovian plasma torus plus a minor non-thermal electron component. Owing to uncertainty in the density and mean energy of non-thermal electrons, the observations are insufficiently constrained to extract the temperature of the upstream electrons. Without any non-thermal electrons, the best fit upstream electron temperature is ˜10 eV; however, prior observations found the Jovian torus thermal electron temperature near Io to be 4-6 eV and thus a non-thermal component is required to reduce the best-fit simulated electron temperature. The upstream temperature of electrons mixed with a non-thermal component that produced agreement between the simulated and observed absolute peak intensities (at 2550 Å and 3150 Å) and their ratio, is Te = 5-6 eV with an accompanying non-thermal component of electrons that is 5% of the thermal density and has a mean electron energy of 35 eV.
    Icarus 08/2012; 220(2):1121-1140. · 3.16 Impact Factor
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    ABSTRACT: Io’s sublimation atmosphere is inextricably linked to the SO2 surface frost temperature distribution which is poorly constrained by observations. We constrain Io’s surface thermal distribution by a parametric study of its thermophysical properties in an attempt to better model the morphology of Io’s sublimation atmosphere. Io’s surface thermal distribution is represented by three thermal units: sulfur dioxide (SO2) frosts/ices, non-frosts (probably sulfur allotropes and/or pyroclastic dusts), and hot spots. The hot spots included in our thermal model are static high temperature surfaces with areas and temperatures based on Keck infrared observations. Elsewhere, over frosts and non-frosts, our thermal model solves the one-dimensional heat conduction equation in depth into Io’s surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The best fit parameters for the SO2 frost and non-frost units are found by using a least-squares method and fitting to observations of the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (HST STIS) mid- to near-UV reflectance spectra and Galileo PPR brightness temperature. The thermophysical parameters are the frost Bond albedo, αF, and thermal inertia, ΓF, as well as the non-frost surface Bond albedo, αNF, and thermal inertia, ΓNF. The best fit parameters are found to be αF ≈ 0.55 ± 0.02 and ΓF ≈ 200 ± 50 J m−2 K−1 s−1/2 for the SO2 frost surface and αNF ≈ 0.49 ± 0.02 and ΓNF ≈ 20 ± 10 J m−2 K−1 s−1/2 for the non-frost surface.These surface thermophysical parameters are then used as boundary conditions in global atmospheric simulations of Io’s sublimation-driven atmosphere using the direct simulation Monte Carlo (DSMC) method. These simulations are unsteady, three-dimensional, parallelized across 360 processors, and include the following physical effects: inhomogeneous surface frosts, plasma heating, and a temperature-dependent residence time on the non-frost surface. The DSMC simulations show that the sub-jovian hemisphere is significantly affected by the daily solar eclipse. The simulated SO2 surface frost temperature is found to drop only ∼5 K during eclipse due to the high thermal inertia of SO2 surface frosts but the SO2 gas column density falls by a factor of 20 compared to the pre-eclipse column due to the exponential dependence of the SO2 vapor pressure on the SO2 surface frost temperature. Supersonic winds exist prior to eclipse but become subsonic during eclipse because the collapse of the atmosphere significantly decreases the day-to-night pressure gradient that drives the winds. Prior to eclipse, the supersonic winds condense on and near the cold nightside and form a highly non-equilibrium oblique shock near the dawn terminator. In eclipse, no shock exists since the gas is subsonic and the shock only reestablishes itself an hour or more after egress from eclipse. Furthermore, the excess gas that condenses on the non-frost surface during eclipse leads to an enhancement of the atmosphere near dawn. The dawn atmospheric enhancement drives winds that oppose those that are driven away from the peak pressure region above the warmest area of the SO2 frost surface. These opposing winds meet and are collisional enough to form stagnation point flow.The simulations are compared to Lyman-α observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-jovian and sub-jovian hemispheres. Lyman-α observations indicate that the anti-jovian hemisphere has higher column densities than the sub-jovian hemisphere and also has a larger latitudinal extent. A composite “average dayside atmosphere” is formed from a collisionless simulation of Io’s atmosphere throughout an entire orbit. This composite “average dayside” atmosphere without the effect of global winds indicates that the sub-jovian hemisphere has lower average column densities than the anti-jovian hemisphere (with the strongest effect at the sub-jovian point) due primarily to the diurnally averaged effect of eclipse. This is in qualitative agreement with the sub-jovian/anti-jovian asymmetry in the Lyman-α observations which were alternatively explained by the bias of volcanic centers on the anti-jovian hemisphere. Lastly, the column densities in the simulated average dayside atmosphere agree with those inferred from Lyman-α observations despite the thermophysical parameters being constrained by mid- to near UV observations which show much higher instantaneous SO2 gas column densities. This may resolve the apparent discrepancy between the lower “average dayside” column densities observed in the Lyman-α and the higher instantaneous column densities observed in the mid- to near UV.
    Icarus 07/2012; 220(1):225–253. · 3.16 Impact Factor
  • Thirteenth ASCE Aerospace Division Conference on Engineering, Science, Construction, and Operations in Challenging Environments, and the 5th NASA/ASCE Workshop On Granular Materials in Space Exploration; 04/2012
  • 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition; 01/2012
  • 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition; 01/2012
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    ABSTRACT: HST/STIS observations of Io obtained in Aug 1999 shortly after umbral ingress into Jupiter's shadow reveal a mid-UV to visual emission spectrum of SO2 excited by impact from Jovian plasma torus electrons (illumination of Io by sunlight refracted by Jupiter's atmosphere is negligible). This spectrum peaks near 3200 Å at 27 Rayleighs/Å. The excitation-dissociation byproducts SO, S I, O I, and potentially S2, are also observed to emit over this range. Two tandem 12-13 min mid-UV exposures obtained with the STIS/MAMA detector beginning 1 min after umbral ingress showed significant weakening of the emission spectrum, which we attribute to partial freezing out of the atmospheric column and the loss of energetic photo-electrons. Similar exposures obtained in the near-UV to visual wavelength range with the STIS/CCD detector beginning 13 min after umbral ingress showed little change in the emission intensity, indicating that most of the freezeout had already occurred. With several minutes between exposures, this time scale is consistent with Io's eclipse light curve taken with the Cassini ISS camera through the clear filter (mid-UV to near-IR), which showed a decline in the disk-averaged intensity in the first 18 min, a relatively flat plateau, then a rise to the pre-eclipse level just prior to egress (Geissler et al. 2004). An unidentified emission source is needed to explain the emission observed longward of the SO2 emission. The low signal level required binning of pixels resulting in only a few spatial resolution elements over Io's disk. Specific plume activity is not well constrained through examination of the disk-averaged MUV emission spectrum. The simulated best fit upstream electron temperature accounting for the peak SO/ SO2 intensity ratios and the absolute intensities is a thermal temperature of 4-5 eV and a non-thermal 30 eV electron density that is 2--5% of the thermal density.
    01/2012;
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    C. K. Sim, S. J. Kim, L. M. Trafton, T. R. Geballe
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    ABSTRACT: We investigated limb brightening phenomenon and east-west asymmetry shown in the 2-μm spectroimages of Titan by using radiative transfer equations. Resultant synthetic spectro-images constructed by an inversion algorithm have been used for studying possible variations of the properties of the haze.
    DPS; 10/2011
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    ABSTRACT: Spectra of Io taken during early umbral eclipse by HST/STIS during the 1999 Io-Galileo Campaign were reanalyzed to improve the S/N of Io's extracted spectrum. This has revealed a spectral emission band extending from 4100 Å to at least 5700 Å that was previously unreported. The integrated intensity appears consistent with Galileo and Cassini images taken through filters that include this wavelength range.
    10/2011;

Publication Stats

2k Citations
496.52 Total Impact Points

Institutions

  • 1973–2014
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 1980
    • National Research Council Canada
      Ottawa, Ontario, Canada