T. V. Johnson

University of Chicago, Chicago, IL, United States

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Publications (358)1492.46 Total impact

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    ABSTRACT: The CO2 frost on Enceladus' surface is suggested to come from venting pockets of subsurface gas. This presentation traces the life cycle of such a gas pocket.
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    ABSTRACT: Titan may have an upper crust rich in methane clathrates which would have formed early in Titan’s history [1-3]. The abundance of atmospheric methane, which has a limited lifetime, and the presence of 40Ar require replenishment over time. Volcanic processes may release these gases from Titan’s interior, although, so far, no conclusive evidence of an ongoing volcanic event has been observed: no “smoking gun” has been seen. Still, some process has recently supplied a considerable amount of methane to Titan’s atmosphere. We have investigated the emplacement of “cryolavas” of varying composition to quantify thermal exchange and lava solidification processes to model thermal wave penetration into a methane-rich substrate (see [4]), and to determine event detectability. Clathrate destabilisation releases methane and other trapped gases, such as argon. A 10-m-thick cryolava covering 100 km2 raises 3 x 108 m3 of substrate methane clathrates to destabilization temperature in ~108 s. With a density of 920 kg/m3, and ≈13% of the mass being methane, 4 x 1010 kg of methane is released. This is an impressive amount, but it would take 5 million similar events to yield the current mass of atmospheric methane. However, meeting Titan’s current global methane replenishment rate is feasible through the thermal interaction between cryolavas and methane clathrate deposits, but only (1) after the flow has solidified; (2) if cracks form, connecting surface to substrate; and (3) the cracks form while the temperature of the clathrates is greater than the destabilisation temperature. The relatively small scale of this activity may be hard to detect. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Choukroun, M. and Sotin, C. (2012) GRL, 39, L04201. [2] Tobie, G. et al. (2006) Nature, 440, 61-64. [3] Lunine, J. et al. (2009) Origin and Evolution of Titan, in Titan From Cassini-Huygens, ed. R. Brown et al., 35-59, Springer. [4] Davies, A. G. et al. (2013) LPSC 44 abstract 1681.
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    ABSTRACT: Traces of CO2 ice have been found on Enceladus’ surface (Brown et al., 2006 p. 1427 [1]). We suggest that the CO2 came from subsurface pockets of gas. Such pockets are a natural consequence of the hydrothermal circulation of Enceladus’ gas-rich ocean water [2]. Ocean water rises through fissures in the icy crust and brings heat and chemicals to the surface. Near the surface the water flows horizontally below the relatively thin cap ice before returning to the ocean. The horizontal flow allows some of the CO2 bubbles in the seawater to raise and collect as gas pockets in irregularities along the bottom of the cap ice. Subsequent fissuring (e.g., as suggested by Hurford et al. [3]) at irregular intervals allows the gas to escape to the surface where it can condense as CO2 ice and is seen by Cassini VIMS. This work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. 2006. [2] Matson D. L. et al., Icarus 221, 53-62, 2012. (also see Matson et al. LPS 44 Abstract 1371, 2013). [3] Hurford T. A. et al., Nature 447, 292-294, 2007.
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    ABSTRACT: We explore the effects of reported differences in C/O values for exoplanet host stars on the composition of planetesimals formed beyond the snow line in these systems. Since the value of C/O in a planet forming nebula has a strong effect on amount of oxygen available for water ice in an oxidizing nebula, exoplanet systems for host stars with C/O greater than the solar value may have planetesimals with very little or no water ice. Thus one the key chemical ingredients for habitability may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the 'habitable zone'.
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    ABSTRACT: "We found traces of free CO2 …." (Brown et al., 2006 p. 1427 [1]). The profound question for which Brown et al. did not have an answer is how did pure CO2 ice come to be on the surface. We suggest that the CO2 came from gas pockets trapped below the ice. We show that such pockets are a natural consequence of the hydrothermal circulation of Enceladus' gasrich ocean water.
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    ABSTRACT: We identify four new hot spots on Io. An infrared ratio technique applied to Galileo NIMS data is shown to be sensitive to faint thermal sources.
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    ABSTRACT: "We found traces of free CO_2 ice…" [Brown et al., 1976]. How did pure CO_2 ice come to be on the surface? We offer an explanation.
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    ABSTRACT: We predict the abundances of volatiles in the envelope of CoRoT-2b and derive the mass of heavy elements to match the abundances derived from observations.
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    ABSTRACT: We calculate planetesimal compositions for exoplanet systems with different C/O ratios, where 0.55 of C is in the form of organic CHON grains.
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    ABSTRACT: The thermal destabilisation of methane clathrates by cryolava flows and intrusions is sufficient to resupply Titan’s current atmospheric methane.
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    ABSTRACT: Motivated by recent spectroscopic observations suggesting that atmospheres of some extrasolar giant-planets are carbon-rich, i.e. carbon/oxygen ratio (C/O) ≥ 1, we find that the whole set of compositional data for Jupiter is consistent with the hypothesis that it be a carbon-rich giant planet. We show that the formation of Jupiter in the cold outer part of an oxygen-depleted disk (C/O ˜1) reproduces the measured Jovian elemental abundances at least as well as the hitherto canonical model of Jupiter formed in a disk of solar composition (C/O = 0.54). The resulting O abundance in Jupiter's envelope is then moderately enriched by a factor of ˜2 × solar (instead of ˜7 × solar) and is found to be consistent with values predicted by thermochemical models of the atmosphere.
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    ABSTRACT: Brown et al. (2006) identified CO2 ice within Enceladus’ South Polar Terrain using Cassini VIMS data and suggested that it resulted from an active replenishment process. Until now this process has been a mystery. Although there is a relatively small amount of CO2 in the water vapor erupted by the plumes, the spectra of the resulting deposits are expected to be dominated by water frost. We point out that CO2 frost deposits are a possible product of the water circulation model proposed by Matson et al. (2012). In this model, buoyant CO2-bubble-rich water rises up from the ocean and into fissures in the icy crust. When a neutral buoyancy level is reached, the water flows horizontally along the fissures under a relatively thin ice cap. Heat lost from the water beneath the ice supplies heat for the thermal anomalies identified on the surface. Even as the water is flowing horizontally, it continues to lose CO2 because bubbles continue to rise. Recesses and other irregularities on the bottom of the surface ice allow the bubble-gas to collect in pockets. When these are fissured by recurring tidal stresses, the CO2 gas can escape and condense nearby on surfaces that are cold enough. References: Brown et al. (2006) Science, 311, 5766; Matson et al. (2012) Icarus, in press, doi 10.1016/j.icarus.2012.05.031. This work has been conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract to NASA.
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    ABSTRACT: Motivated by recent spectroscopic observations suggesting that atmospheres of some extrasolar giantplanets are carbon-rich, i.e. carbon/oxygen ratio (C/O) ≥ 1, we find that the whole set of compositional data for Jupiter is consistent with the hypothesis that it be a carbon-rich giant planet. The Jovian oxygen abundance to be measured by NASA's Juno mission en route to Jupiter will provide a direct and strict test of our predictions.
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    ABSTRACT: Saturn’s satellite Phoebe is the best-characterized representative of large outer Solar System planetesimals, thanks to the close flyby by the Cassini spacecraft in June 2004. We explore the information contained in Phoebe’s physical properties, density and shape, which are significantly different from those of other icy objects in its size range. Phoebe’s higher density has been interpreted as evidence that it was captured, probably from the proto-Kuiper-Belt. First, we demonstrate that Phoebe’s shape is globally relaxed and consistent with a spheroid in hydrostatic equilibrium with its rotation period. This distinguishes the satellite from ‘rubble-piles’ that are thought to result from the disruption of larger proto-satellites. We numerically model the geophysical evolution of Phoebe, accounting for the feedback between porosity and thermal state. We compare thermal evolution models for different assumptions on the formation of Phoebe, in particular the state of its water, amorphous or crystalline. We track the evolution of porosity and thermal conductivity as well as the destabilization of amorphous ice or clathrate hydrates. While rubble-piles may never reach temperatures suitable for porous ice to creep and relax, we argue that Phoebe’s shape could have relaxed due to heat from the decay of 26Al, provided that this object formed less than 3 Myr after the production of the calcium–aluminum inclusions. This is consistent with the idea that Phoebe could be an exemplar of planetesimals that formed in the transneptunian region and later accreted onto outer planet satellites, either during the satellite’s formation stage, or still later, during the late heavy bombardment.
    Icarus 05/2012; 219(1):86–109. DOI:10.1016/j.icarus.2012.02.002 · 2.84 Impact Factor
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    ABSTRACT: We have used recent surveys of the stellar abundances of solid forming elements in a sample of exoplanet host stars discussed by Bond et al. (Astrophys. J. 715, 1050-1070, 2010) to calculate the expected composition of silicate and ice planetesimals formed beyond the snow line in these systems. The refractory silicate and metal composition is derived following Johnson and Lunine (Nature 435, 69-71, 2005) and Wong et al. (in Oxygen in the Solar System Vol. Reviews in Mineralogy and Geochemistry Vol. 68 (ed G. J. MacPherson) Ch. 10, 241-246, 2008). The nebula gas C and O composition was set based on amount of O tied up in refractories and the volatile condensation sequence for ices in the 5-10 AU region of the stellar systems calculated following Mousis et al. (Astrophys. J. 727, 7pp, 2011). The resultant condensate compositions show that planetesimal compositions in exoplanet systems may differ significantly from solar system planet forming materials. The C/O abundance of the exoplanet host star has the strongest effect on planetesimal composition, strongly affecting the relative proportions of refractory materials and volatile ices, particularly water ice and C-bearing ices. For stars with sub-solar C/O values H2O and silicate plus metal dominate the condensate composition with CO2 as the next most abundant species at < ~0.10 mass fraction. Minor species (CH3OH, H2S, NH3, CH4, PH3), with mass fractions of 10-4to 10-2, are present in approximately the relative proportions as for the solar nebula. As stellar C/O increases, H2O decreases and beyond the solar value ([C/O] =0, C/O = 0.55), rapidly disappears as the C/O = ~0.8 is approached, with CO2 and CH3OH ices becoming more important. Planetesimals in these systems will have refractory, silicate plus metal rich compositions compared with solar system conditions. If the midplane temperatures in the circumstellar nebula are
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    ABSTRACT: The relationships between water, carbon dioxide, and heat in Enceladus are used to obtain the temperatures of water in the plume-formation chambers and in the subsurface ocean.
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    ABSTRACT: We have calculated the ≈5-μm radiant flux for every volcanic hot spot in every one of the 190 Galileo Near-Infrared Mapping Spectrometer (NIMS) tube observations of Io obtained between 28 June 1996 and 16 October 2001 in order to determine the variability of thermal emission from Io's volcanoes at local, regional and global scales, and to identify individual eruption episodes where thermal emission waxes and wanes. The resulting NIMS Io Thermal Emission Database (NITED) allows the comparison of activity at individual volcanoes and different regions of Io. The database contains over 1000 measurements of radiant flux at approximately 5 μm, corrected for emission angle, range to target and incident sunlight (where necessary). We examine the data for Loki Patera, Io's most powerful volcano. For data acquired in local darkness we use two-temperature fits to nighttime spectra and prior knowledge of emitting area to determine total radiated thermal emission. For other data we use the constancy of the integrated thermal emission spectrum to determine total thermal emission from measurements of radiant flux at 5 μm. As seen by NIMS, total thermal emission from Loki Patera varies between 7600 GW and 17000 GW. We revise upwards previous estimates of thermal emission from NIMS data. NIMS 3.5-μm radiant fluxes (both measured and estimated) are consistent with measurements from ground-based telescopes. This work highlights the value of NITED as a research tool.
    Geophysical Research Letters 01/2012; 39(1):1201-. DOI:10.1029/2011GL049999 · 4.46 Impact Factor
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    ABSTRACT: In order to determine the magnitude of thermal emission from Io's volcanoes and variability with time at local, regional and global scales, we have calculated the 4.7 or 5 μm radiant flux for every hot spot in every Galileo Near Infrared Mapping Spectrometer (NIMS) observation obtained during the Galileo mission between June 1996 and October 2001. The resulting database contains over 1000 measurements of radiant flux, corrected for emission angle, range to target, and, where necessary, incident sunlight. Io's volcanoes produce the most voluminous and most powerful eruptions in the Solar System [1] and NIMS was the ideal instrument for measuring thermal emission from these volcanoes (see [1, 2]). NIMS covered the infrared from 0.7 to 5.2 μm, so measurement of hot spot thermal emission at ~5 μm was possible even in daytime observations. As part of a campaign to quantify magnitude and variability of volcanic thermal emission [1, 3-5] we examined the entire NIMS dataset (196 observations). The resulting NIMS Io Thermal Emission Database (NITED) allows the charting of 5-μm thermal emission at individual volcanoes, identifying individual eruption episodes, and enabling the comparison of activity at different hot spots [e.g., 6] and different regions of Io. Some ionian hot spots were detected only once or twice by NIMS (e.g., Ah Peku Patera, seen during I32), but most were detected many times (e.g., Culann, Tupan and Zamama, [6]). For example, the database contains over 40 observations of Loki Patera (some at high emission angle, and two partial observations). There are 55 observations of Pele. The 27 nighttime observations of Pele show a remarkably steady 5-μm radiant flux of 35 ± 12 GW/μm. There are 34 observations of Pillan, which erupted violently in 1997. Although in many observations low spatial resolution makes it difficult to separate hot spot pairs such as Susanoo Patera and Mulungu Patera; Tawhaki Patera and Hi'iaka Patera; and Janus Patera and Kanehekili, comparison with Galileo SSI camera data sometimes allows the active partner to be identified. The inability to resolve individual hot spots does not affect calculations of regional or hemispherical thermal emission and variability. Having quantified 5-μm radiant flux using NIMS data, we are extending analysis to determine total thermal emission [6] and incorporating other data to complete the database of thermal emission and variability [e.g., 3-5]. This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and is supported by the NASA OPR Program. Copyright 2011 Caltech.
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    ABSTRACT: Detections of Io's hot spots and identification of volcanic features have been catalogued by various workers [e.g., 1-4]. However, to understand the role played by volcanism in global heat transport, thermal emission from Io's volcanoes has to be quantified, locally, regionally and globally. Only then can robust estimates be made of volcanic advection, which may reveal internal heating patterns controlled by the evolving tidal resonance between Io, Europa and Ganymede. We have completed an analysis of all suitable spacecraft data and, using additional ground-based data, have quantified the thermal emission from all of Io's volcanoes during the Galileo epoch down to the limit of detection [5-7]. Galileo identified many dark features on Io that did not exhibit obvious anomalous thermal emission, yet their low albedo suggested that these features were at least warm (cool, high albedo sulphurous deposits had not formed on them). We used dark areas identified from the recently-published Io Global Map [3] and a knowledge of the detection limit of the Galileo NIMS instrument to quantify the thermal emission from these areas. In all, our analysis includes 272 individual thermal sources yielding ~60 TW. Our "snapshot" of global volcanic activity shows that Io's paterae yield ~80% of this amount, with a preponderance of thermal emission emanating from the northern hemisphere. This is strongly biased by Loki Patera and, to a lesser extent, by recent outburst locations. Of the remaining identified hot spot thermal emission, ~15% comes from active or recent lava flow fields, and the remaining 5% comes from massive outburst eruptions (some in paterae) and very small hot spots. The energy accounted for makes up ~60% of Io's total thermal emission of ~100 TW [8]. It is possible that a multitude of very small hot spots beneath instrument detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds may be responsible for the unaccounted heat flow. This work was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA, and is supported by the NASA OPR and PG&G Programs. Copyright 2011 Caltech.

Publication Stats

8k Citations
1,492.46 Total Impact Points


  • 2012
    • University of Chicago
      • Enrico Fermi Institute
      Chicago, IL, United States
  • 1975–2011
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, California, United States
  • 2007
    • University of Michigan
      • Department of Atmospheric, Oceanic and Space Sciences
      Ann Arbor, Michigan, United States
  • 1981–2007
    • NASA
      Вашингтон, West Virginia, United States
  • 1981–2005
    • Cornell University
      • Department of Astronomy
      Ithaca, NY, United States
  • 1998
    • United States Geological Survey
      • Astrogeology Science Center
      Reston, Virginia, United States
  • 1997
    • Honolulu University
      Honolulu, Hawaii, United States
    • University of Hawaiʻi at Mānoa
      • Institute of Geophysics and Planetology
      Honolulu, HI, United States
  • 1990
    • University of Toronto
      Toronto, Ontario, Canada
  • 1988
    • Pasadena City College
      Pasadena, Texas, United States
  • 1970–1973
    • Massachusetts Institute of Technology
      • Department of Earth Atmospheric and Planetary Sciences
      Cambridge, Massachusetts, United States