The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

Koskinen, Tommi; Yelle, R. V.; Snowden, D.; Lavvas, P.; Sandel, B. R.; Capalbo, Fernando J.; Bénilan, Y.; West, Robert A.

doi:10.1016/j.icarus.2011.09.022

Cited by 136 publications

(174 citation statements)

References 56 publications

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“…An infrared limb spectrum taken by the Voyager I spacecraft produced the first vertical abundance profile of the gas, with a vertical resolution of ∼200 km (Coustenis et al 1991). Hidayat et al (1997) and Marten et al (2002) CN and HC 15 N using the CIRS instrument (Vinatier et al 2007), as well as extensive mapping of the vertical and horizontal distributions of HCN (Teanby et al 2007;Vinatier et al 2010;Koskinen et al 2011).…”

Section: Introductionmentioning

confidence: 99%

Alma Observations of HCN and Its Isotopologues on Titan

Molter

Nixon

Cordiner

et al. 2016

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General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. , and HCN/H 13 C 15 N=5800±270 (1σ errors). The carbon and nitrogen ratios are consistent with and improve on the precision of previous results, confirming a factor of ∼2.3 elevation in 14 N/ 15 N in HCN compared to N 2 and a lack of fractionation in 12 C/ 13 C from the protosolar value. This is the first published measurement of D/H in a nitrile species on Titan, and we find evidence for a factor of ∼2 deuterium enrichment in hydrogen cyanide compared to methane. The isotopic ratios we derive may be used as constraints for future models to better understand the fractionation processes occurring in Titan's atmosphere.

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Section: Introductionmentioning

confidence: 99%

Alma Observations of HCN and Its Isotopologues on Titan

Molter

Nixon

Cordiner

et al. 2016

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“…Cassini observations revealed the presence of aerosols in multiple regions of the atmosphere, from the troposphere (7), stratosphere (8), and mesosphere (9,10) up to the thermosphere where the detection of large mass positive and particularly negative ions has been suggested to be the signature of aerosol formation (11)(12)(13)(14). During a recent flyby (T70), the Cassini spacecraft penetrated to deeper regions of Titan's thermosphere than usual, reaching altitudes close to 880 km that had not previously been sampled with in situ measurements.…”

mentioning

confidence: 99%

Aerosol growth in Titan’s ionosphere

Lavvas

Yelle

Koskinen

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.planetary sciences | heterogeneous chemistry | particles charging | plasma P hotochemical aerosols are observed in many atmospheres of our solar system (1-3), and their presence is also detected in exoplanet atmospheres (4). Apart from their direct influence on the atmospheric properties through their interaction with the radiation field, aerosols have further astrobiological implications; their presence in the early Earth's atmosphere could have protected the surface and any life evolving there from UV radiation (5), and laboratory studies of Titan aerosol analogs have identified an in vitro formation of amino acids during aerosol production (6). However, the general mechanisms involved in the production and growth of photochemical aerosols from atmospheric gases have remained elusive. Titan, as the most extreme example of an aerosol-dominated atmosphere, provides a unique opportunity to investigate these mechanisms.Cassini observations revealed the presence of aerosols in multiple regions of the atmosphere, from the troposphere (7), stratosphere (8), and mesosphere (9, 10) up to the thermosphere where the detection of large mass positive and particularly negative ions has been suggested to be the signature of aerosol formation (11)(12)(13)(14). During a recent flyby (T70), the Cassini spacecraft penetrated to deeper regions of Titan's thermosphere than usual, reaching altitudes close to 880 km that had not previously been sampled with in situ measurements. Measurements from the Langmuir probe (LP) during this unique flyby reveal a high abundance of negative ions at closest approach, comparable to the positive ion density, and a corresponding decrease in the electron density (15). This conclusion is supported by the analysis of multiple Cassini observations, which reveals that the observed electron density is smaller than the density anticipated from photochemical equilibrium, implying that the photochemical models are missing a significant electron loss mechanism (16, 17). Thus, current observations demonstrate a significant decrease of the electron density relative to the positive ion density in the lower ionosphere with a concurrent increase in the density of the negative ions.Aerosols, or dust particles in general, are known to interact with free electr...

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“…It is stressed, however, that systematic errors in the utilized neutral number densities cannot explain the magnitude difference between modeled and observed electron fluxes. First, the utilized densities (or calibration factor) match those derived from measurements by the Cassini Ultraviolet Spectrograph (Koskinen et al 2011), as well as those inferred from the Cassini Attitude and Articulation Control Subsystem and the Huygens Atmosphere Structure Instrument (Strobel 2010). Second, by systematically changing the utilized densities by ±50% removes the agreement with observed profile shapes, regarding both the model output of suprathermal electron fluxes and thermal electron number densities (see Vigren et al 2013).…”

Section: Comparisons Of Modeled and Observed Electron Intensitiesmentioning

confidence: 82%

“…(2012) used a calibration factor of 2.9 (see also Koskinen et al 2011). While using as default the correction factor 2.9, we present in Section 3 also P e,EI,M /P e,EI,O ratios obtained from the use of a calibration factor of 2.1 (slightly reduced from the value of 2.2 suggested by Teolis et al 2015).…”

Section: Calculations Of Electron-impact Electron Production Ratesmentioning

confidence: 99%

Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model–observation Comparisons

Vigren

Galand

Wellbrock

et al. 2016

ApJ

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The dayside ionosphere of the Saturnian satellite Titan is generated mainly from photoionization of N 2 and CH 4 . We compare model-derived suprathermal electron intensities with spectra measured by the Cassini Plasma Spectrometer/Electron Spectrometer (CAPS/ELS) in Titanʼs sunlit ionosphere (altitudes of 970-1250 km) focusing on the T40, T41, T42, and T48 Titan flybys by the Cassini spacecraft. The model accounts only for photoelectrons and associated secondary electrons, with a main input being the impinging solar EUV spectra as measured by the Thermosphere Ionosphere Mesosphere Energy and Dynamics/Solar EUV Experiment and extrapolated to Saturn. Associated electron-impact electron production rates have been derived from ambient number densities of N 2 and CH 4 (measured by the Ion Neutral Mass Spectrometer/Closed Source Neutral mode) and related energy-dependent electron-impact ionization cross sections. When integrating up to electron energies of 60 eV, covering the bulk of the photoelectrons, the model-based values exceed the observationally based values typically by factors of ∼3 ± 1. This finding is possibly related to current difficulties in accurately reproducing the observed electron number densities in Titanʼs dayside ionosphere. We compare the utilized dayside CAPS/ELS spectra with ones measured in Titanʼs nightside ionosphere during the T55-T59 flybys. The investigated nightside locations were associated with higher fluxes of high-energy (>100 eV) electrons than the dayside locations. As expected, for similar neutral number densities, electrons with energies <60 eV give a higher relative contribution to the total electron-impact ionization rates on the dayside (due to the contribution from photoelectrons) than on the nightside.

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The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations

Cited by 136 publications

References 56 publications

Alma Observations of HCN and Its Isotopologues on Titan

Alma Observations of HCN and Its Isotopologues on Titan

Aerosol growth in Titan’s ionosphere

Suprathermal Electrons in Titan’s Sunlit Ionosphere: Model–observation Comparisons

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