L. M. Trafton

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

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Publications (271)600.47 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The Enceladus plume is composed of primarily water vapor and tiny ice grains. Various observations suggest that it most likely originates from vents along the Tiger Stripes. Consequently, understanding the expansion process of the two-phase flow from the vents into vacuum is crucial. Our goal is to investigate the important physical processes and the interaction between the gas and the grains associated with the expansion process. To do so, we compute the expansion flow from the vents out to higher altitudes using the direct simulation Monte Carlo (DSMC) method with two-way coupling between the gas and the grains. The expansion flow passes through multiple regimes, from continuum (very collisional) near the vents to free-molecular (collisionless) at higher altitudes. This transition occurs at a few kilometers high for meter-sized vents and is higher up for larger vents. During expansion, the Mach number increases rapidly in the first few vent diameters mainly due to a drop in gas temperature rather than an increase in gas speed. Collisions at the vent strongly affect the molecular speeds in the far-field (as the flow becomes free-molecular). If the flow is sufficiently collisional at the vent, the molecular speeds approach the ultimate speed of adiabatic expansion, which depends on the source conditions (∼1005 m/s for the triple-point of water). The flow is highly supersaturated as it emerges from the vents, thus condensation grain growth is very likely. We find that condensation growth via heterogeneous nucleation above the vents is proportional to vent size and that fairly large vents several tens to hundreds of meters in size are required to produce the micron-sized grains detected by CDA (radii ⩾1.6 μm) if the grains start with a negligible size at the vents. We also examine how the effects of grains on the gas vary with grain size, grain/gas (ice/vapor) mass ratio and gas–grain velocity difference at the vent. The effects of grains increase with mass ratio and velocity difference at the vent due to greater exchange of momentum and energy. Moreover, smaller grains have a stronger effect for the same mass ratio. However, the effects of grains are minimal for plausible mass ratios ⩽1.0. Our studies show that nanometer-sized grains decouple from the gas at altitudes of 10–100 vent diameters and spread more with the gas while micron-sized grains decouple at <10 vent diameters and remain in collimated beams. Even with large velocity differences at the vent, the micron-sized grains spread by ⩽12°. Consequently, the inferred spreading angles ⩾30° (Ingersoll, A.P., Ewald, S.P. [2011]. Icarus 216, 492–506; Postberg, F. et al. [2011]. Nature 474, 620–622) cannot be caused by velocity difference at the vent alone. Furthermore, nanometer-sized grains are accelerated close to gas speeds while micron-sized grains tend to retain the initial speeds they had at the vent. We determine that the largest grain size that can be accelerated from rest to escape speeds (>240 m/s) is proportional to vent size and that small vents (<1 m) are enough to accelerate large micron-sized grains to escape speeds.
    Icarus 06/2015; 253. DOI:10.1016/j.icarus.2015.02.020 · 3.04 Impact Factor
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    ABSTRACT: Io’s giant Pele plume rises high above the moon’s surface and produces a complex deposition pattern. We use the direct simulation Monte Carlo (DSMC) method to model the flow of SO2 gas and silicate ash from the surface of the lava lake, into the umbrella-shaped canopy of the plume, and eventually onto the surface where the flow leaves black “butterfly wings” surrounded by a large red ring. We show how the geometry of the lava lake, from which the gas is emitted, is responsible for significant asymmetry in the plume and for the shape of the red deposition ring by way of complicated gas-dynamic interactions between parts of the gas flow arising from different areas in the lava lake. We develop a model for gas flow in the immediate vicinity of the lava lake and use it to show that the behavior of ash particles of less than about 2 μm in diameter in the plume is insensitive to the details of how they are introduced into the flow because they are coupled to the gas at low altitudes.
    Icarus 04/2015; 257. DOI:10.1016/j.icarus.2015.03.019 · 3.04 Impact Factor
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    ABSTRACT: The LCROSS (Lunar Crater Observation and Sensing Satellite) impacted the Cabeus crater near the lunar South Pole on 9 October 2009 and created an impact plume that was observed by the LCROSS Shepherding Spacecraft. Here we analyze data from the ultraviolet–visible spectrometer and visible context camera aboard the spacecraft. We use these data to constrain a numerical model to understand the physical evolution of the resultant plume. The UV–visible light curve peaks in brightness 18 s after impact and then decreases in radiance but never returns to the pre-impact radiance value for the ∼4 min of observation by the Shepherding Spacecraft. The blue:red spectral ratio increases in the first 10 s, decreases over the following 50 s, remains constant for approximately 150 s, and then begins to increase again ∼180 s after impact. Constraining the modeling results with spacecraft observations, we conclude that lofted dust grains remained suspended above the lunar surface for the entire 250 s of observation after impact. The impact plume was composed of both a high angle spike and low angle plume component. Numerical modeling is used to evaluate the relative effects of various plume parameters to further constrain the plume properties when compared with the observational data. Dust particle sizes lofted above the lunar surface were micron to sub-micron in size. Water ice particles were also contained within the ejecta cloud and simultaneously photo-dissociated and sublimated after reaching sunlight.
    Icarus 03/2015; 254. DOI:10.1016/j.icarus.2015.02.026 · 3.04 Impact Factor
  • Aaron B. Morris · David B. Goldstein · Philip L. Varghese · Laurence M. Trafton ·
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    ABSTRACT: When a lander approaches the lunar surface, the plume from the descent engine impinges on the ground and entrains loose regolith into a high-velocity spray. This problem is simulated with a hybrid continuum-kinetic solver, where the internal nozzle flow and near-field plume are simulated with a continuum solver (data parallel line relaxation) and the regions of two-phase flow are simulated via the direct simulation Monte Carlo method. Fully coupled two-phase dust and gas motions and an inelastic grain-grain collision model have been implemented for a polydisperse distribution of particles sizes. This work examines the sensitivity of the solutions to these models for the case of steady axisymmetric plume impingement at different hovering altitudes. Of the physical phenomena modeled in this work, the dust sprays were found to be most sensitive to grain-grain collisions. The coefficient of restitution for colliding grains was parametrically studied and has a significant impact on how grain-grain collisions scatter the dust sprays. Copyright © 2014 by the American Institute of Aeronautics and Astronautics, Inc.
    Journal of Spacecraft and Rockets 01/2015; 52(2):1-13. DOI:10.2514/1.A33058 · 0.53 Impact Factor
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    Laurence M. Trafton ·
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    ABSTRACT: Compositional analyses of Pluto's surface ice in the literature typically include large areas on the body where CH4 and other volatiles are segregated in the pure form from the solid solution N2:CH4 in which CH4 is diluted. However, the existence of continent-size areas of pure CH4 are in conflict with both of the alternative models that successfully explain the enhancement of CH4 in Pluto's atmosphere, the Detailed Balancing thermal equilibrium model and the Hot Methane Patch model. Pluto's spectrum includes an apparently unshifted CH4 component while Triton's does not, and 93% of the concentration range of the binary phase diagram at 38 K shows that these species exist as a mixture of two saturated solid solution phases. Recognizing this, we propose that both of these saturated phases are present on Pluto and the CH4-rich phase of the mixture, CH4:N2, is the source of the relatively unshifted CH4 spectrum attributed to pure CH4. We also propose that CH4 is less abundant in Triton's ice to the point where either the ice is not saturated or the saturated CH4:N2 phase has not been detected. In this scenario, the partial vapor pressures do not change when the relative proportions of these saturated phases are varied in the mixture. Thus, the partial vapor pressures are independent of N2-CH4 concentrations if both saturated phases are present. Accordingly, the longitudinal and seasonal variations of CH4 and N2 features in Pluto's spectrum would be attributed to spatial variations in the relative proportions of these species. This may occur during volatile transport in the sublimation wind through extensive influences. The lower, unsaturated, values of the mole fraction of CH4 in the ice reported by Owen et al. (Owen et al. [1993]. Science 261, 745-748) and Cruikshank et al. (Cruikshank, D.P., Rush, T.L., Owen, T.C., Quirico, E., de Bergh, C. [1998]. The surface compositions of Triton, Pluto, and Charon. In: Solar System Ices. Astrophysics and Space Science Library Series, vol. 227. Kluwer Academic Publishers, Dordrecht), and by Doute et al. (Doute, S., Schmitt, B., Quirico, E., Owen, T.C., Cruikshank, D.P., de Bergh, C., Geballe, T.R., Roush, T.L. [1999]. Icarus 142, 421-444) based on a compositional analysis of Pluto's surface, were not obtained using optical constants for components consistent with the constraints of the phase diagram.
    Icarus 12/2014; 246. DOI:10.1016/j.icarus.2014.05.022 · 3.04 Impact Factor
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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. DOI:10.1016/j.icarus.2014.05.041 · 3.04 Impact Factor
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    ABSTRACT: In recent decades, several missions have detected signs of water and other volatiles in cold, permanently shadowed craters near the lunar poles. Observations suggest that some of these volatiles could have been delivered by comet impacts and therefore, understanding the impact delivery mechanism becomes key to explaining the origin and distribution of lunar water. During impact, the constituent ices of a comet nucleus vaporize; a significant part of this vapor remains gravitationally bound to the Moon, transforming the tenuous, collisionless lunar exosphere into a collisionally thick, transient atmosphere. Here, we use numerical simulations to investigate the physical processes governing volatile transport in the transient atmosphere generated after a comet impact, with a focus on how these processes influence the accumulation of water in polar cold traps. It is observed that the transient atmosphere maintains a certain characteristic structure for at least several Earth days after impact, during which time volatile transport occurs primarily through low-altitude winds that sweep over the lunar day-side. Meanwhile, reconvergence of vapor antipodal to the point of impact results in preferential redistribution of water in the vicinity of the antipode. Due to the quantity of vapor that remains gravitationally bound, the atmosphere is sufficiently dense that lower layers are shielded from photodestruction, prolonging the lifetime of water molecules and allowing greater amounts of water to reach cold traps. Short-term ice deposition patterns are markedly non-uniform and the variations that arise in simulated volatile abundance between different cold traps could potentially explain variations that have been observed through remote sensing.
    Icarus 02/2014; 255. DOI:10.1016/j.icarus.2014.10.017 · 3.04 Impact Factor
  • William McDoniel · D. B. Goldstein · P. L. Varghese · L. M. Trafton ·
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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.
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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.
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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.
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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. DOI:10.1016/j.icarus.2013.01.012 · 3.04 Impact Factor
  • A. B. Morris · D. B. Goldstein · P. L. Varghese · L. M. Trafton ·
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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; 1501(1):1220-1227. DOI:10.1063/1.4769681
  • William McDoniel · D. Goldstein · P. Varghese · L. Trafton ·
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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.
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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.
  • Laurence M. Trafton · G. S. Orton · T. K. Greathouse · J. H. Lacy · T. Encrenaz ·
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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.
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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.
  • H. Melin · T. S. Stallard · S. Miller · T. R. Geballe · L. M. Trafton · J. O'Donogue ·
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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.
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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. DOI:10.1016/j.icarus.2012.06.025 · 3.04 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.
    Icarus 07/2012; 220(1):225–253. DOI:10.1016/j.icarus.2012.05.001 · 3.04 Impact Factor
  • A. B. Morris · D. B. Goldstein · P. L. Varghese · L. M. Trafton ·

    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

Publication Stats

3k Citations
600.47 Total Impact Points


  • 1971-2015
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
  • 1991-2012
    • Concordia University Texas
      Austin, Texas, United States
  • 1997
    • University of Michigan
      • Department of Atmospheric, Oceanic and Space Sciences
      Ann Arbor, Michigan, United States
  • 1996
    • Boulder County
      Boulder, Colorado, United States
  • 1993
    • University of Oxford
      • Department of Earth Sciences
      Oxford, England, United Kingdom