Showing papers by "J. Hunter Waite published in 2010"

Titan from Cassini-Huygens

Abstract: Preface.- Chapter 1 Overview.- Chapter 2 Earth-based Perspective and pre-Cassini-Huygens Knowledge of Titan.- Chapter 3 The Origin and Evolution of Titan.- Chapter 4 Titan's Interior structure.- Chapter 5 Geology and Surface Processes on Titan.- Chapter 6 Composition of Titan's Surface.- Chapter 7 Volatile Origin and Cycles: Nitrogen and Methane.- Chapter 8 High-altitude Production of Titan's Aerosols.- Chapter 9 Titan's Astrobiology.- Chapter 10 Atmospheric Composition and Structure.- Chapter 11 Composition and Structure of the Ionosphere and Thermosphere.- Chapter 12 Aerosols in Titan's Atmosphere.- Chapter 13 Atmospheric Dynamics and Meteorology.- Chapter 14 Seasonal Change on Titan.- Chapter 15 Mass loss processes in Titan's Upper Atmosphere.- Chapter 16 Energy Deposition Processes in Titan's Upper Atmosphere and Its Induced Magnetosphere.- Chapter 17 Titan in the Cassini-Huygens Extended Mission.- Chapter 18 Titan Beyond Cassini-Huygens.- Chapter 19 Mapping Products of Titan's Surface.- Appendix.

Negative ions at Titan and Enceladus: recent results

Abstract: The detection of heavy negative ions (up to 13 800 amu) in Titan's ionosphere is one of the tantalizing new results from the Cassini mission. These heavy ions indicate for the first time the existence of heavy hydrocarbon and nitrile molecules in this primitive Earth-like atmosphere. These ions were suggested to be precursors of aerosols in Titan's atmosphere and may precipitate to the surface as tholins. We present the evidence for and the analysis of these heavy negative ions at Titan. In addition we examine the variation of the maximum mass of the Titan negative ions with altitude and latitude for the relevant encounters so far, and we discuss the implications for the negative ion formation process. We present data from a recent set of encounters where the latitude was varied between encounters, with other parameters fixed. Models are beginning to explain the low mass negative ions, but the formation process for the higher mass ions is still not understood. It is possible that the structures may be chains, rings or even fullerenes. Negative ions, mainly water clusters in this case, were seen during Cassini's recent close flybys of Enceladus. We present mass spectra from the Enceladus plume, showing water clusters and additional species. As at Titan, the negative ions indicate chemical complexities which were unknown before the Cassini encounters, and are indicative of a complex balance between neutrals and positively and negatively charged ions.

Simulating the one‐dimensional structure of Titan's upper atmosphere: 1. Formulation of the Titan Global Ionosphere‐Thermosphere Model and benchmark simulations

Abstract: [1] We employ a newly developed Navier-Stokes model, the Titan Global Ionosphere-Thermosphere Model (T-GITM) to address the one dimensional (1-D) coupled composition, dynamics, and energetics of Titan's upper atmosphere. Our main goals are to delineate the details of this new theoretical tool and to present benchmark calibration simulations compared against the Ion-Neutral Mass Spectrometer (INMS) neutral density measurements. First, we outline the key physical routines contained in T-GITM and their computational formulation. Then, we compare a series of model simulations against recent 1-D work by Cui et al. (2008), Strobel (2008, 2009), and Yelle et al. (2008) in order to provide a fiducial for calibrating this new model. In paper 2 and a future paper, we explore the uncertainties in our knowledge of Titan's atmosphere between ∼500 km and 1000 km in order to determine how the present measurements constrain our theoretical understanding of atmospheric structures and processes.

Cassini INMS observations of neutral molecules in Saturn's E‐ring

Abstract: [1] In 2008, the Cassini ion neutral mass spectrometer (INMS) investigation made in situ measurements of neutral species near Saturn's equatorial plane within 0.5 Saturn radii (RS) of the orbit of Enceladus. After removing the large background and modeling to interpret instrumental effects, the data provide rough constraints on the neutral distribution and composition. These data show an azimuthal asymmetry in the neutral densities and provide measurements used to compare to simulations of neutral H2O emitted from Enceladus. Far from Enceladus, the neutral water densities, at a few times 103 molecules/cm3, are near the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, the INMS detects particles within 5000 km of Enceladus, with the density increasing to approximately 105 molecules/cm3 at the equatorial plane. The observations also show CO2 in the form of its dissociated product, CO. On the basis of the spatial distribution of CO2 counts, the scale height of the neutral cloud above and below the equatorial plane is less than 7000 km. Far from Enceladus, the concentration of CO2 with respect to H2O increases, a consequence of the predicted decline in H2O density. Relatively high counts at 2 amu are infrequently observed. These measurements indicate H2 released during ice-grain impacts and provide a constraint on the frequency and distribution of small ice grains.

Simulating the one-dimensional structure of Titan's upper atmosphere: 2. Alternative scenarios for methane escape

Abstract: [1] In Bell et al. (2010) (paper 1), we provide a series of benchmark simulations that validate a newly developed Titan Global Ionosphere-Thermosphere Model (T-GITM) and calibrate its estimates of topside escape rates with recent work by Cui et al. (2008), Strobel (2009), and Yelle et al. (2008). Presently, large uncertainties exist in our knowledge of the density and thermal structure of Titan's upper atmosphere between the altitudes of 500 km and 1000 km. In this manuscript, we explore a spectrum of possible model configurations of Titan's upper atmosphere that are consistent with observations made by the Cassini Ion-Neutral Mass Spectrometer (INMS), Composite Infrared Spectrometer, Cassini Plasma Spectrometer, Magnetospheric Imaging Instrument, and by the Huygens Gas Chromatograph Mass Spectrometer and Atmospheric Science Instrument. In particular, we explore the ramifications of multiplying the INMS densities of Magee et al. (2009) by a factor of 3.0, which significantly alters the overall density, thermal, and dynamical structures simulated by T-GITM between 500 km and 1500 km. Our results indicate that an entire range of topside CH4 escape fluxes can equivalently reproduce the INMS measurements, ranging from ∼108 − 1.86 × 1013 molecules m−2 s−1 (referred to the surface). The lowest topside methane escape rates are achieved by scaling the INMS densities by a factor of 3.0 and either (1) increasing the methane homopause altitude to ∼1000 km or (2) including a physicochemical loss referred to as aerosol trapping. Additionally, when scaling the INMS densities by a factor of 3.0, we find that only Jeans escape velocities are required to reproduce the H2 measurements of INMS.

Design and performance evaluation of a two-stage resistively-heated thermal modulator for GC × GC

Abstract: The design and performance evaluation of a two-stage resistively-heated thermal modulator that does not use any cryogenic consumables is described. A commercially available piece of stainless-steel wall coated capillary column is used as the modulator. Cooling of the modulator is provided by a two-stage refrigeration unit with an external heat exchanger and a closed-loop recirculating air system. The modulator is resistively heated by sending a current pulse through each stage of the modulator tube. Studies evaluating the modulator trapping efficiency, cooling efficiency, desorption efficiency, and resultant peak shape are presented and discussed. Performance evaluation of this modulator is compared to similar single-stage modulators, both qualitatively and quantitatively, and has exceeded the previous data reported for the single stage versions. GC × GC chromatograms of a C6-C20 n-alkane mixture, a petroleum sample, and a 38-component mixture of aliphatic, aromatic, and halogenated volatile organic compounds are provided. This two-stage modulator proves to be a viable alternative to commercially available modulators which require large amounts of cryogenic fluids.