Mass Loss Processes in Titan's Upper Atmosphere

Johnson, R. E.; Tucker, O. J.; Michael, M.; Sittler, E. C.; Smith, H. T.; Young, D. T.; Waite, J. H.

doi:10.1007/978-1-4020-9215-2_15

Cited by 53 publications

(46 citation statements)

References 75 publications

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“…Thus, when ignoring chemistry, one must compensate by imposing hydrodynamic outflow to match INMS data. By contrast, by including direct photolytic, neutral-neutral, and ion-neutral methane chemical losses, model A is able to match INMS data while simulating escape rates that are consistent with pre-Cassini estimates [see Johnson et al, 2009].…”

Section: Discussionmentioning

confidence: 99%

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

et al. 2014

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In this study, we develop a best fit description of Titan's upper atmosphere between 500 km and 1500 km, using a one‐dimensional (1‐D) version of the three‐dimensional (3‐D) Titan Global Ionosphere‐Thermosphere Model. For this modeling, we use constraints from several lower atmospheric Cassini‐Huygens investigations and validate our simulation results against in situ Cassini Ion‐Neutral Mass Spectrometer (INMS) measurements of N2, CH4, H2, 40Ar, HCN, and the major stable isotopic ratios of 14N/15N in N2. We focus our investigation on aspects of Titan's upper atmosphere that determine the amount of atmospheric escape required to match the INMS measurements: the amount of turbulence, the inclusion of chemistry, and the effects of including a self‐consistent thermal balance. We systematically examine both hydrodynamic escape scenarios for methane and scenarios with significantly reduced atmospheric escape. Our results show that the optimum configuration of Titan's upper atmosphere is one with a methane homopause near 1000 km and atmospheric escape rates of 1.41–1.47 ×1011 CH4 m−2 s−1 and 1.08 ×1014 H2 m−2 s−1 (scaled relative to the surface). We also demonstrate that simulations consistent with hydrodynamic escape of methane systematically produce inferior fits to the multiple validation points presented here.

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Section: Discussionmentioning

confidence: 99%

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

et al. 2014

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“…For Event 1 we estimated an ion flux $ 7 Â 10 6 ions/cm 2 /s so this gives a mean ion outflow $ 8 Â 10 6 ions/cm 2 /s, which is close to their lower estimate. This estimate is important since it represents a continual loss of ionospheric plasma to Saturn's magnetosphere and must be continuously replaced by the ionospheric sources, which in the case of T9 are expected to be dominated by solar EUV (see Cravens et al, 2005 andJohnson et al, 2009). …”

Section: Eventmentioning

confidence: 99%

Saturn's magnetospheric interaction with Titan as defined by Cassini encounters T9 and T18: New results

Sittler

Hartle

Johnson

et al. 2010

Planetary and Space Science

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“…[3][4][5] However, the large amount of data on Titan's atmosphere from the Cassini spacecraft 6 led to the use of models for thermal escape that disagreed considerably even for a single component, spherically-symmetric atmosphere. [6][7][8] This disagreement persisted due to a lack of understanding on how the escape changes in character from evaporation on a molecule by molecule basis to an organized macroscopic flow, often referred to as hydrodynamic escape. 2 At a large distance from a body, each of these gives way to almost free molecular flow (FMF) and, therefore, the basic assumptions of the continuum models of hydrodynamic escape 9,10 become inconsistent with flow conditions.…”

Section: Introductionmentioning

confidence: 97%

Kinetic simulations of thermal escape from a single component atmosphere

et al. 2011

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The one-dimensional steady-state expansion of a monatomic gas from a spherical source in a gravity field is studied by the direct simulation Monte Carlo method. Collisions between molecules are described by the hard sphere model, the distribution of gas molecules leaving the source surface is assumed to be Maxwellian, and no heat is directly deposited in the simulation region. The flow structure and the escape rate (number flux of molecules escaping the atmosphere) are analyzed for the source Jeans parameter λ0 (ratio of the gravitational energy to thermal energy of the molecules) and Knudsen number Kn0 (ratio of the mean free path to the source radius) ranging from 0 to 15 and from 0.0001 to ∞, respectively. In the collisionless regime, flows are analyzed for λ0=0-100 and analytical equations are obtained for asymptotic values of gas parameters that are found to be non-monotonic functions of λ0. For collisional flows, simulations predict the transition in the nature of atmospheric loss from escape on a molecule-by-molecules basis, often referred to as Jeans escape, to an organized outflow, often referred to as hydrodynamic escape. It is found that the structure of the flow and the escape rate exhibit drastic changes when λ0 varies over a narrow transition range 2-3. The lower limit of this range approximately corresponds to a critical Jeans parameter equal to 2.06, which is the upper limit for isentropic, supersonic outflow of a monatomic gas from a body in a gravity field. Subcritical, λ0≤2, flows are qualitatively similar to free outgassing in the absence of gravity, resulting in hypersonic terminal Mach numbers and escape rates that are independent of λ0 in the limit of small Knudsen numbers. Supercritical, λ0≥3, flows are controlled by thermal conduction and demonstrate qualitatively different trends. The ratio of the actual escape rate to the Jeans escape rate at the source surface is found to be a non-monotonic function of Kn0 spanning the range from ∼0.01 to ∼2. At λ0≥6, the ratio of the actual escape rate to the Jeans escape rate at the exobase is found to be ∼1.4–1.7. This is unlike the predictions of the slow hydrodynamic escape model, which is based on Parker’s model for the solar wind and intended for the description of the atmospheric loss at λ0>∼10. At λ0<6, the actual escape rate can be well approximated by a modified Jeans escape rate, which accounts for non-zero gas velocity.

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Mass Loss Processes in Titan's Upper Atmosphere

Cited by 53 publications

References 75 publications

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

Developing a self‐consistent description of Titan's upper atmosphere without hydrodynamic escape

Saturn's magnetospheric interaction with Titan as defined by Cassini encounters T9 and T18: New results

Kinetic simulations of thermal escape from a single component atmosphere

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