Europlanet Science Congress 2021
Virtual meeting
13 – 24 September 2021
Europlanet Science Congress 2021
Virtual meeting
13 September – 24 September 2021

OPS1

"Planet" Titan

Saturn's moon Titan, despite its satellite status, has nothing to envy to planets: it has planetary dimensions, a substantial and dynamic atmosphere, a carbon cycle, a variety of geological features (dunes, lakes, rivers, mountains and more), seasons, and a hidden ocean. It even now has its own mission: Dragonfly, selected by NASA in the frame of the New Frontiers program.
In this session, scientific presentations are solicited to cover all aspects of current research on Titan: from its interior to its upper atmosphere, using data collected from the Cassini-Huygens mission (2004-2017) and/or from ground-based telescopes (e.g., ALMA) and/or based on modelling and experimental efforts to support the interpretation of past and future observations of this unique world.

Convener: Anezina Solomonidou | Co-conveners: Sam Birch, Alice Le Gall, Shannon M. MacKenzie, Marco Mastrogiuseppe

Session assets

Discussion on Slack

Oral and Poster presentations and abstracts

Chairpersons: Anezina Solomonidou, Marco Mastrogiuseppe
Oral presentation
EPSC2021-137
Anezina Solomonidou, Athena Coustenis, Alice Le Gall, Rosaly Lopes, Michael Malaska, Bernard Schmidtt, Kenneth Lawrence, Charles Elachi, Ashley Schoenfeld, Christophe Sotin, Stephen Wall, Yannis Markonis, and Christos Matsoukas

The investigation of Titan’s surface chemical composition is of great importance for the understanding of the atmosphere-surface-interior system of the moon. The Cassini cameras and especially the Visual and infrared Mapping Spectrometer has provided a sequence of spectra showing the diversity of Titan’s surface spectrum from flybys performed during the 13 years of Cassini’s operation. In the 0.8-5.2 μm range, this spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral “windows” centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 μm provide some information on the lower atmospheric context and the surface parameters. Nevertheless, atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. In various studies (Solomonidou et al., 2014; 2016; 2018; 2019; 2020a, 2020b; Lopes et al., 2016; Malaska et al., 2016; 2020), we used radiative transfer modeling in order to evaluate the atmospheric scattering and absorption and securely extract the surface albedo of multiple Titan areas including the major geomorphological units. We also investigated the morphological and microwave characteristics of these features using Cassini RADAR data in their SAR and radiometry mode. Here, we present a global map for Titan’s surface showing the chemical composition constraints for the various units. The results show that Titan’s surface composition, at the depths detected by VIMS, has significant latitudinal dependence, with its equator being dominated by organic materials from the atmosphere and a very dark unknown material, while higher latitudes contain more water ice. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan’s surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.

References: [1] Solomonidou, A., et al. (2014), J. Geophys. Res. Planets, 119, 1729; [2] Solomonidou, A., et al. (2016), Icarus, 270, 85; [3] Solomonidou, A., et al. (2018), J. Geophys. Res. Planets, 123, 489; [4] Solomonidou, A., et al. (2020a), Icarus, 344, 113338; [5] Solomonidou, A., et al. (2020b), A&A 641, A16; [6] Lopes, R., et al. (2016) Icarus, 270, 162; [7] Malaska, M., et al. (2016), Icarus 270, 130; [8] Malaska, M., et al. (2020), Icarus, 344, 113764.

Acknowledgements: This work was conducted at the California Institute of Technology (Caltech) under contract with NASA. Y.M. and A.S. (partly) was  supported by the Czech Science Foundation (grant no. 20-27624Y). ©2021 California Institute of Technology. Government sponsorship acknowledged.

How to cite: Solomonidou, A., Coustenis, A., Le Gall, A., Lopes, R., Malaska, M., Schmidtt, B., Lawrence, K., Elachi, C., Schoenfeld, A., Sotin, C., Wall, S., Markonis, Y., and Matsoukas, C.: Compositional mapping of Titan’s surface using Cassini VIMS and RADAR data, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-137, https://doi.org/10.5194/epsc2021-137, 2021.

EPSC2021-155
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ECP
Martin Kihoulou, Ondřej Čadek, Klára Kalousová, Gaël Choblet, and Gabriel Tobie

INTRODUCTION

The thermo-mechanical evolution of Titan's ice shell is primarily controlled by the mode of the heat transfer in the ice shell and the amount of heat coming from the ocean. Numerical models of thermal convection in Titan's ocean have suggested that the heat flux from the core can be strongly affected by the Coriolis effect, leading to a latitudinal gradient in the heat flux at the base of the ice shell [1,2,3]. These heat flux variations are of the order of mW/m2 and may induce phase changes at the ice/ocean interface, affecting the thickness of the ice shell. Until now, the effect of lateral variations in the heat flux has been studied under the assumption that the ice shell is highly viscous and the heat transfer in the ice occurs by conduction. In such a case, the heat flux from the ocean is negatively correlated with the ice shell thickness [3]. A higher-than-average heat flux in polar regions then causes melting of ice and thinning of the ice shell, possibly explaining the fact that the poles on Titan are about 300 m lower than the equator [4].

The issue of whether Titan's ice shell is convecting or not has not yet been resolved. The mode of heat transfer depends on the ice shell thickness and the viscosity of ice, which in turn depends on the grain size and the temperature of the ocean. Since these parameters are not accurately known, the convection model cannot be excluded from analysis. The response of the convecting ice shell to heat flux variations from the ocean is likely to be different from that described for the conduction model [5]. When heat is transferred by convection, the ice shell is on average warmer and less viscous than in the conductive case. The loss/gain of ice mass at the ice/ocean interface caused by melting/freezing is compensated by flow of low-viscosity ice and, therefore, the shape of the ice/ocean interface remains unchanged. The shape of the surface topography is difficult to assess a priori because it may depend on the vigor of convection.

 

METHOD

In this study, we investigate the effect of spatial variations in the heat flux from the ocean on the behavior of a convecting ice layer. The models discussed below have been obtained by simultaneously solving the momentum equation, the transport equation for temperature and the mass conservation equation for an incompressible fluid. The temperature is fixed to 90 K on the upper boundary and to 265 K on the bottom (ice/ocean) boundary. The computational domain is 100 km thick and has an aspect ratio of 16:1. We assume that the deformation of ice is controlled by diffusion creep and the viscosity of ice only depends on temperature and grain size.

The surface is treated as a material boundary while the ice/ocean interface is open and its shape can change due to phase changes. The heat flux variations from the ocean enter the problem through the energy conservation law which relates the jump in the heat flux across the boundary with the relative motion of the ice/ocean interface [3,6,7]. Note that the heat flux across the phase boundary is generally not continuous (as assumed, for example, by [8]), but depends on the velocity of ice flow, which cannot be neglected if the viscosity of ice is low (< 1016 Pa.s). The lateral variation of the heat flux imposed at the bottom boundary of the domain is scaled to correspond to 50 % of the average heat flux (Fig. 1a).

 

RESULTS

In Figs. 1 and 2, we compare the results obtained for a grain size of 2 mm and 3 mm. Although these values are close to each other, the models represent two end-member cases discussed above. While in the former case (2 mm), the heat transfer is controlled by convection, in the latter case (3 mm), convection does not develop and the heat is transferred by conduction.

 

               

 

Inspection of Figs. 1 and 2 shows that the convection model gives the opposite sign of the surface topography than the conduction model (Fig. 1b) and predicts a significant (8 K) increase in ice temperature above the positive heat flux anomaly (Fig. 2a). The conduction and convection models also give different orientations of tectonic stresses - extension in the case of convection and compression in the case of conduction (Fig. 2b). Both models predict only small amplitudes of the geoid (Fig. 2c), suggesting that the equilibrium state can be approximated by either Airy or Pratt isostatic model, depending on the mode of heat transfer.

 

CONCLUSIONS

Variations in the heat flux from the ocean can significantly influence Titan’s topography and the distribution of stress near the surface. The topography is negatively correlated with the heat flux when the heat in the ice shell is transferred by conduction, whereas a positive correlation and more complex topography response is found for the convective heat transfer.  In contrast to the topography, the heat flux variations have only a small effect on the geoid. A careful analysis of the geoid and topography data collected by Cassini could thus provides an insight into the processes occurring in Titan's interior and helps answer the question of which heat transfer mode dominates in Titan's ice shell.

 

ACKNOWLEDGEMENTS

This research was supported by the Czech Science Foundation through project No. 19-10809S. M.K. acknowledges the support from the Charles University project SVV-2020-260581. K.K. was supported by the Charles University Research program No. UNCE/SCI/023. G.C. and G.T. acknowledge the support from the ANR COLOSSe project. 

 

REFERENCES

[1] Soderlund, 2019, Geophys. Res. Lett. 46, 8700–8710.

[2] Amit et al., 2020, Icarus 338, 113509.

[3] Kvorka et al., 2018, Icarus 310, 149–164.

[4] Corlies et al., 2017, Geophys. Res. Lett. 44, 11754–11761.

[5] Nimmo and Bills, 2010, Icarus 201, 896–904.

[6] Čadek et al., 2019a, Icarus 319, 476–484.

[7] Čadek et al., 2019b, Geophys. Res. Lett. 46, 14299–14306.

[8] Ojakangas and Stevenson, 1989, Icarus 81, 242–270.

 

 

 

How to cite: Kihoulou, M., Čadek, O., Kalousová, K., Choblet, G., and Tobie, G.: Effect of ocean heat flux on Titan's topography and tectonic stresses, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-155, https://doi.org/10.5194/epsc2021-155, 2021.

EPSC2021-450
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ECP
Liliane Burkhard, Bridget Smith-Konter, Sarah Fagents, Marissa Cameron, Geoff Collins, and Robert Pappalardo

Titan's geology is complex, with a wide range of surface morphology, including fluvial, aeolian, and cryovolcanic and tectonic activity. Surface observations have not yet revealed large-scale and distinct characteristics of strike-slip faulting (i.e., en echelon structures, fault duplexes, and clear offset features) that have been observed on other ocean worlds such as Europa, Ganymede, and Enceladus [1,2]. However, the SW Xanadu region shows offsets in fluvial networks that seem to have been caused by strike-slip faulting [3]optimal shear failure conditions may be present within Titan's shallow subsurface, where the existence of a porous ice cover filled with liquid hydrocarbons can create an environment for areas of frictional instability [4], shear heating [5], and possibly cryovolcanism [6,7]. In this work, we examine Titan’s ability to host shear deformation at identified rectilinear fluvial features that are inferred to be strike-slip faults [3] through a sensitivity analysis guided by Coulomb failure laws [8] and tidal stress mechanisms [9]. Our modeling technique includes considerations for how the presence of near-surface liquid hydrocarbons and the crustal porosity of the ice significantly reduce the resistance to shear failure of strike-slip faults subjected to diurnal tidal stresses through a pore pressure parameter.

For this study, we examine failure conditions at proposed example strike-slip faults [3] in SW Xanadu (Figure 1): (A) 9ºS 138ºW. Using the SatStress [9,10] tidal stress model for Titan-appropriate rheology [6], we compute the diurnal tidal stress tensor and resolve shear and normal stresses onto shallow fault planes (100 m depth) with azimuthal orientations consistent with mapped observations [3], as well as for the full range of orientations. We explore candidate coefficients of friction (μf= 0.3-0.5) [11] and hydrostatic pore fluid pressure ratios for Titan (l = 0.67-0.9) [4]. Figure 2 illustrates where the magnitudes of frictional stress and absolute value of shear stress overlap and the Coulomb criterion is met using the mapped azimuth of the suggested fault structure, permitting a finite slip-window. At a shallow fault depth of 100 m and for the inferred oriented faults, our model suggests that shear failure is possible under diurnal tidal stresses subjected to the studied parameters. For an assumed coefficient of friction of μf = 0.4, shear failure can only be achievable with intermediate to high pore fluid pressure ratios (λ > 0.75). At location (A), the resulting left-lateral slip direction from the model agrees with the inferred left-lateral shear direction of the proposed mapped strike-slip fault [3]. However, right- and left-lateral slip windows are achieved for a range of possible azimuths at this location with μf = 0.4 and a pore fluid pressure ratio of l = 0.75 (Figure 2, top). While a variety of factors can control this observed fluvial drainage system on Titan, the general azimuthal orientation of these features in this region suggests that there is a subsurface tectonic fabric trending roughly east-west guiding the flow of these rectilinear fluvial systems. Overall, our findings show that on Titan, the crustal porosity of ice and the inclusion of near-surface liquids will minimize the resistance to shear failure of faults subjected to diurnal tidal stresses. The optimal combination of tensile and shear tractions can allow for finite slip windows where the Coulomb criteria is met.

References:  [1] Cameron, M.E. et al. (2018) Icarus, 315, 92-114.  [2] Martin, E.S. (2016) Geophys. Res. Let., 43, 2456–2464.  [3] Matteoni, P. et al. (2020) JGR: Planets, 125(12).  [4] Liu, Z. et al. (2016) Icarus, 270, 2-13.  [5] Nimmo, F. et al. (2007) Nature, 447, 289.  [6] Sohl, F. et al. (2014) JGR: Planets, 119, 1013-1036.  [7] Malaska, M.J. et al. (2016) Icarus, 270, 130-161.   [8] Byerlee, J. (1978) Pageoph, Vol. 116.  [9] Wahr, J. et al. (2009) Icarus, 200, 188-206. [10] Patthoff, A. et al. (2016) AGU, Abstract #P51B-2147.  [11] Schulson, E.M. (2016) Int. Material Reviews, 60:8, 451-478. 

How to cite: Burkhard, L., Smith-Konter, B., Fagents, S., Cameron, M., Collins, G., and Pappalardo, R.: Investigating inferred strike-slip features on Titan: Modeling possible shear failure due to tidal stresses and pore fluid interactions, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-450, https://doi.org/10.5194/epsc2021-450, 2021.

EPSC2021-472
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ECP
Audrey Chatain, Alice Le Gall, Michel Hamelin, Jean-Jacques Berthelier, Ralph D. Lorenz, Rafik Hassen-Khodja, Jean-Pierre Lebreton, and Grégoire Déprez

 

Context

Titan, the largest moon of Saturn, is the place in the Solar System owning the most Earth-like landscapes. Titan’s dense atmosphere and cold temperatures enable a complex methane hydrological cycle that have shaped the surface, very similarly to the water cycle on Earth. Titan has another peculiar feature: a wealth of organic grains is created by photochemistry in its atmosphere and progressively deposited at its surface. Such atmospheric production of organics likely occurred on Earth before the apparition of life; that is the reason why a better understanding of the formation processes, chemical composition and physical properties of these grains is of great interest.

The Dragonfly mission has recently been selected by NASA to explore Titan’s surface with a rotorcraft circa 2034 (Lorenz et al., 2018). Dragonfly will explore a region of organic sand dunes with monthly flights of a few kilometres each aiming to an impact crater named Selk. In addition to chemical analyses, Dragonfly is equipped with several sensors intended to characterize its environment. Among them, as part of the Dragonfly Geophysical and Meteorological (DraGMet) package, the EFIELD instrument will record the AC electric field at low frequencies (~5-100 Hz).

EFIELD consists in two spherical electrodes accommodated at different locations on the rotorcraft. The main scientific objective of EFIELD is to measure Schumann Resonances on Titan. Such resonances may have been detected by the Huygens probe in 2005 (unless it was an artefact of probe motion; Lorenz and Le Gall, 2020) and would be an indication of the existence of an underground global salty ocean (Beghin et al., 2012). Another scientific objective of EFIELD is the detection and characterization of charged grains. This work is dedicated to this secondary objective.

The exploration area of Dragonfly is covered by sand grains, most likely organic in nature, maybe mixed with ice. Surface winds can sometimes put them in saltation or suspension. In the process, these organic grains are likely to get charged by friction (triboelectric effect; Méndez-Harper et al., 2017), and would then induce a perturbation on the electric field detectable by the EFIELD antennas (see Figure 1).

 

Numerical simulation

To estimate the significance of this perturbation and test the possibility to measure it, we have developed a numerical model that simulates the trajectory of charged particles in the probe environment, subjected to turbulent wind flows, gravity and electrostatic forces. Results show that charged particles will induce a strong measurable signal on the EFIELD spectra (see Figure 2). Particles > 100 µm should be detectable with a 1 mV resolution limit (see Figure 3).

          

 

Experimental simulation

To test our numerical model, we built two prototypes of EFIELD antennas, adapted from previous models of Huygens/PWA AC and DC antennas (Fulchignoni et al, 2002). As analogues of sand organic grains, we used small polystyrene balls charged by friction in a rotating cylinder, itself coated with small polystyrene balls as done in (Méndez-Harper et al., 2017). Charged balls are dropped one by one to fall close to the antennas. The experiment is performed in a nearly-closed Faraday cage to avoid any electromagnetic disturbances. Examples of particle detections are given in Figure 4.

 

Retrieval of sand grains properties

Finally, we investigated how we could exploit these signals to derive information on the grains (number, charge, velocity). A simple approximation of particles with a linear trajectory and constant speed give a simple equation to fit the data, from which we can infer the charge/velocity ratio of the particle, as well as the minimum distance to the probe/particle velocity ratio. We first validated the approximation with simulated data in Titan’s surface conditions. Figures 4 and 5 give results obtained with experiments done with 3-mm polystyrene balls. The fit results have a dispersion due to the variations in the particle charges inherent to our charging method. Nevertheless, the mean values are similar to a numerical simulation performed in the laboratory conditions. Further developments are on-going to independently measure particle charges.

      

 

Conclusion

The EFIELD sensor on board Dragonfly will be able to investigate charged organic grains blown by winds at the surface of Titan. The signal created by the passage of a particle in particular gives access to its charge/velocity ratio. It will be possible to estimate the particle velocity from other instruments (wind sensor, cameras…), and therefore estimate the particle charge. The measurement of particle charges with the EFIELD antenna will be a valuable asset to understand the interaction between the particles and Titan surface environment (winds, saltation, friction).

 

References

Beghin et al., Icarus 218 (2012)

Fulchignoni et al, Space Science Reviews 104 (2002)

Lorenz et al., Johns Hopkins APL Technical Digest 34 (2018)

Lorenz and Le Gall, Icarus 351 (2020)

Méndez-Harper et al., Nature Geoscience 10 (2017)

How to cite: Chatain, A., Le Gall, A., Hamelin, M., Berthelier, J.-J., Lorenz, R. D., Hassen-Khodja, R., Lebreton, J.-P., and Déprez, G.: Detection of sand grains blown by Titan surface winds with the DraGMet/EFIELD sensor on Dragonfly, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-472, https://doi.org/10.5194/epsc2021-472, 2021.

EPSC2021-245
Benjamin Charnay, Gabriel Tobie, Sébastien Lebonnois, and Ralph Lorenz

Just as Saturn’s massive gravity causes tides both in Titan’s interior as well as its surface seas, it causes a tide in the atmosphere (Lorenz 1992; Tokano & Neubauer 2002). Previous modelling work with a 3D Global Climate Model found that gravitational tides should produce a surface pressure variation of ∼1.5 hPa through the orbit of Titan and tidal winds in the troposphere (Tokano & Neubauer 2002). Here, we revisit gravitational atmospheric tides on Titan with analytical calculations and with a 3D Global Climate Model (the IPSL-Titan GCM). We show that the surface pressure field quickly adjust to the tidal potential, strongly decreasing the amplitude of tidal winds. We analyze the impact of the deformation of Titan’s interior and crust on the amplitude of the tidal pressure variations. Finally, we discuss how measurements of pressure variations by Dragonfly could help to constrain Titan’s interior and crust.

How to cite: Charnay, B., Tobie, G., Lebonnois, S., and Lorenz, R.: Gravitational atmospheric tides as a probe of Titan’s interior: application to Dragonfly, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-245, https://doi.org/10.5194/epsc2021-245, 2021.

EPSC2021-268
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ECP
Demian Marchione, Luca Mancini, Pengxiao Liang, Gianmarco Vanuzzo, Piergiorgio Casavecchia, Dimitrios Skouteris, Marzio Rosi, and Nadia Balucani

Several studies have reported model vertical profiles and/or abundance measurements for the main nitrile species observed in Titan’s atmosphere including: hydrogen cyanide (HCN), cyanoacetylene (HC3N), and vinyl cyanide (H2CCHCN) [1-4]. One of the main mechanisms leading to the formation of these nitriles (molecules bearing the CN group) is the reaction of CN radicals with hydrocarbons (molecules with general formula CxHy) involving H elimination (e.g. CN + C2H4 = H2CCHCN + H) or H abstraction (e.g. CN + CH4 = HCN + CH3) [5-9]. In this regard, it is worthwhile to highlight the recent spectroscopic detection of H2CCHCN, obtained using archival data from the Atacama Large Millimeter/submillimeter Array (ALMA), because this species is the best candidate molecule to form stable cell membranes/vesicle structures of potential astrobiological importance in Titan’s hydrocarbon-rich lakes and seas [10-12]. 

In the upper atmosphere of Saturn’s moon, photolysis of N2 and HCN leads to a continuous supply of reactive neutral species including N(4S, 2D) atoms and CN radicals [2, 13-16] that can further react with hydrocarbons and nitriles leading to two possible outcomes: 1) these reactions can directly contribute to the growth of molecular complexity, leading to a variety of N-rich organic species which, in turn, can act as (some of the) building blocks of life as we know it; 2) break the molecules into smaller but more stable fragments.  

A detailed knowledge of the relevant potential energy surface for each reactive systems on the basis of quantum-mechanical calculations and laboratory experiments is of pivotal importance in identifying the nature of the primary products and estimating their branching ratios, and hence providing the modellers with solid ground to describe and predict the rich, complex chemical evolution of Titan’s planetary atmosphere. 

Within this framework, we will report on two elementary chemical processes: 1) N(2D) + H2CCHCN and 2) CN(X2Σ+) + H2CCHCN with emphasis on the N addition or CN addition (followed by H elimination) reactive channel by means of the crossed molecular beams (CMB) technique coupled with mass spectrometric detection and time-of-flight analysis, and we will report on the primary products from the analysis of our measurements. Additional insight on the micromechanism at play and the interpretation of the scattering results is given by new electronic structure calculations of stationary points and product energetics in the potential energy surface of the investigated systems. RRKM statistical calculations for the CN(X2Σ+) + H2CCHCN system allows us to derive the product branching ratios and rate constants under the conditions of the present experiments and of the atmosphere of Titan. Preliminary results and astrophysical implications will be presented. 

ACKNOWLEDGMENTS:  

This research was supported by the Italian Space Agency (ASI, DC-VUM-2017-034, Grant n° 2019-3 U.O Life in Space) and the Marie Skłodowska-Curie project "Astro-Chemical Origins” (ACO), grant agreement No 811312. 

References:

[1]   P.P. Lavvasa, A. Coustenis, I.M. Vardavas, Planet. Space Sci56, 67 (2008) 

[2] K. Willacy, M. Allen, and Y. Yung, Astrophys. J.829:79 (2016) 

[3] A. E. Thelen, C.A. Nixon, N.J. Chanover, M.A. Cordiner, E.M. Molter, N.A. Teanby, P.G.J. Irwin, J. Serigano, S.B. Charnley, Icarus319, 417 (2019)  

[4] V. Vuitton, R.V. Yelle, S.J. Klippenstein, S.M. Hörst, P. Lavvas, Icarus324, 120 (2019) 

[5] L. C. L. Huang, Y. T. Lee, and R. I. Kaiser, J. Chem. Phys. 110, 7119 (1999) 

[6] L. C. L. Huang, O. Asvany, A. H. H. Chang, N. Balucani, S. H. Lin, Y. T. Lee, R. I. Kaiser, and Y. Osamura, J. Chem. Phys. 113, 8656 (2000) 

[7] S. C. S. Ely, S. B.Morales, J.-C. Guillemin, S. J.Klippenstein, and I. R. Sims, J. Phys. Chem. A117, 12155 (2013) 

[8] F. Leonori, R. Petrucci, X. Wang, P. Casavecchia, N. Balucani, Chem. Phys. Lett.553, 1 (2012) 

[9] N. Balucani, D. Skouteris, F. Leonori, R. Petrucci, M. Hamberg, W. D. Geppert, and P. Casavecchia, J. Phys. Chem. A116, 10467 (2012) 

[10] M. Y. Palmer, M. A. Cordiner, C. A.  Nixon, S. B. Charnley, N. A. Teanby, Z. Kisiel. P. G. J. Irwin, M. J. Mumma, Sci. Adv.3, e1700022 (2017) 

[11] J. C.-Y. Lai, M. A. Cordiner, C. A. Nixon, R. K. Achterberg, E. M. Molter, N. A. Teanby, M. Y. Palmer, S. B. Charnley, J. E. Lindberg, Z. Kisiel, Astron. J.154:206 (2017) 

[12] J. Stevenson, J. Lunine, P. Clancy, Sci. Adv., 1, e1400067 (2015) 

[13] L. M. Lara, E. Lellouch and V. Shematovich, Astron. Astrophys.341, 312 (1999) 

[14] A. Marten, T. Hidayat, Y. Biraud and R. Moreno, Icarus158, 532 (2002)  

[15] E. H. Wilson and S. K. Atreya, Planet. Space Sci.51, 1017 (2003).  

[16] N. Balucani, Chem. Soc. Rev. 41, 5473 (2012) 

 

How to cite: Marchione, D., Mancini, L., Liang, P., Vanuzzo, G., Casavecchia, P., Skouteris, D., Rosi, M., and Balucani, N.: Reactivity of N(2D) atoms and of the cyano radical (CN) with vinyl cyanide (H2CCHCN) in Titan’s atmosphere: a combined crossed beam and theoretical study, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-268, https://doi.org/10.5194/epsc2021-268, 2021.

EPSC2021-778
Anne Wellbrock, Andrew Coates, Geraint Jones, Richard Haythornthwaite, Oleg Shebanits, Erik Vigren, Panayotis Lavvas, and Véronique Vuitton

The discovery of heavy organic anions by in situ measurements using Cassini’s CAPS Electron Spectrometer (ELS) in Titan’s ionosphere was an unexpected result of the Cassini mission (Coates et al, 2007, Waite et al, 2007); a complete reconsideration of chemical processes in this enigmatic atmosphere was necessary as a result. These negative ions can be associated with complex hydrocarbon and nitrile processes which are linked to haze formation at lower altitudes. Cassini’s CAPS ELS observed negative ions during Titan encounters at altitudes below 1400 km. The ions can reach masses over 13,000 amu/q (Coates et al., 2009), while recurring peaks in the mass spectra can be used to identify different mass groups as reported by Coates et al. (2007) and Wellbrock et al. (2013, 2019). Studying density and mass trends of these groups helps to identify controlling factors of the production and destruction mechanisms, and ultimately to improve our understanding of how organic macromolecules can be produced by naturally occurring abiotic processes. In this study we examine the effects different solar zenith angle conditions might have on both the light and heavy negative ion mass groups, and consider the role of processes such as photodetachment and dissociative electron attachment. We also compare the negative ion data with RPWS electron measurements and discuss the possible implications associated with the above processes.

How to cite: Wellbrock, A., Coates, A., Jones, G., Haythornthwaite, R., Shebanits, O., Vigren, E., Lavvas, P., and Vuitton, V.: Cassini CAPS-ELS Observations of Negative Ions in Titan's Ionosphere: Solar Zenith Angle - Density Trends, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-778, https://doi.org/10.5194/epsc2021-778, 2021.

EPSC2021-566
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ECP
David Dubois, Laura Iraci, Erika Barth, Sandrine Vinatier, Farid Salama, and Ella Sciamma-O'Brien

1. Introduction

Following the northern spring equinox in August 2009, Titan’s global atmospheric circulation reversed within the next two years. This event increased the mixing ratios of benzene (C6H6) and other species at the South pole. Simultaneously, a strong cooling with temperatures dropping below 120 K favored the condensation of organic hydrocarbon molecules at unusually high altitudes (>250 km). The Cassini Composite Infrared Spectrometer (CIRS) detected for the first time an IR spectral signature consistent with the presence of C6H6 ice in the South polar region at these high altitudes [1]. Previous laboratory data used for the interpretation of CIRS data [2], however, was insufficient to allow models to reproduce the formation of this high-altitude cloud system.

2. Methods

We have combined laboratory [3], modeling [4] and observational [1] efforts to investigate the chemical and microphysical processes leading to the formation of the cloud system that formed at unusually high altitude (> 250 km) over Titan’s South pole after the northern spring equinox. We present here a study focused on the formation of C6H6 ice clouds at 87ºS. We have experimentally measured, for the first time, the equilibrium vapor pressure of pure C6H6at low temperatures (134-158 K) representative of Titan’s atmosphere using the NASA Ames Atmospheric Chemistry Laboratory [3,5] (ACL). The ACL (Figure 1) has enabled us to monitor the condensation of C6H6 in the IR and determine the nucleation supersaturation and equilibrium vapor pressure of pure C6H6 at Titan-relevant temperatures (134–158 K).

Figure 1. Schematic diagram (not to scale) of the ACL experimental apparatus used for benzene ice condensation and growth studies (adapted from [3]), including the capability for cyclohexane calibration measurements. Pressure is measured with a capacitance manometer (P1) and an ion gauge (P2). The inset shows a top view of the sample holder with the positions of the two K-type thermocouple gauges (red dots) used for temperature measurements. The ice sample forms on either or both sides of the silicon substrate (grey), which is in the path of the IR beam. Infrared transmission spectra are collected with an external DTGS detector (adapted from Dubois et al. 2021).

 

We have used our benzene experimental vapor pressure to re-analyze CIRS observational data from May 2013 covering the 68°S to 87°S latitudes and derive benzene volume mixing ratios. The experimental data along with the temperature profiles and C6H6 mixing ratios derived from CIRS observational data at 87ºS were then used as input parameters in the coupled microphysics radiative transfer Community Aerosol and Radiation Model for Atmospheres (CARMA) to simulate C6H6 ice cloud particle size distribution, gas volume mixing ratios, gas relative humidity and cloud altitudes. CARMA simulates the microphysical evolution of aerosol particles in a column of atmosphere. Benzene cloud particle formation and growth is controlled by the vapor pressure of C6H6. All particles are transported vertically through sedimentation, eddy diffusion, and a vertical wind simulating Titan’s Hadley cell. We simulated the southern polar atmosphere in CARMA by initializing the model with a temperature/pressure profile from CIRS data at 87ºS.

 

3. Results & Conclusions 

Figure 2 shows the first experimental measurements of benzene vapor pressure at Titan-relevant temperatures compared to extrapolations from higher temperatures [2,6-8]. Our vapor pressures can be fitted with two slopes (T < 146 K and T > 146 K) that we have parameterized. Because the temperature regime in Titan’s atmosphere at the South pole where the benzene cloud formed is less than 146 K, we used our low temperature experimental parameterization in the CIRS re-analysis and with CARMA. The CIRS re-analysis have resulted in benzene condensation occurring at lower altitudes in the stratosphere than previously thought [5]. The CARMA simulations at 87ºS have helped us constrain C6H6ice particle size distribution, gas volume mixing ratios, gas relative humidity and cloud altitudes down to lower altitudes that are not accessible with CIRS observations. The simulations predict greater C6H6 gas mixing ratios below the condensation level than with previous vapor pressure extrapolations, resulting in more C6H6 being available per CCN to condense at stratospheric levels (< 250 km) and hence a growth in size distribution, in particular between 125 km and 50 km. At 87ºS, as observed with the CIRS data re-analysis, the CARMA model predicts benzene condensation occurring deeper in the stratosphere. From the re-analysis of Cassini CIRS observations at latitudes spanning from 68ºS to 87ºS, we also inferred that the vortex polar boundary in 2013 resided between 78ºS and 83ºS. From 83ºS to 87ºS, the cloud top would be located between 246-256 km, and from 68ºS to 78ºS it would be located between 90 and 110 km.

Figure 2. Experimental vapor pressure measurements of C6H6 ice (black) with associated temperature and pressure uncertainties, along with two new vapor pressure parameterizations fitted to the experimental measurements following the colder (134-146 K – brown) and warmer (146-158 K – black) temperature ranges. For comparison, the extrapolations from higher temperature measurements [2, 6-8] (represented as dashed lines) from J74 (blue), H76 (green), F&S09 (red) and Růžička et al. (2014, cyan) parameterizations are also plotted on this figure. Pressures are given in both Torr (experimental measurement unit) and mbar (for comparison to observational data). The orange shaded area represents the temperature range relevant to Titan’s stratosphere in the South pole in May 2013, where benzene ice clouds were detected, i.e., < 145 K (reproduced from Dubois et al. 2021).

 

Acknowledgements

Funding for this project was provided through NASA SMD CDAP.

 

References                 

[1] Vinatier, S. et al., Icarus, 310, 89–104, 2018.[2] Fray & Schmitt, Planet. Space Sci., 57, 2053–2080, 2009.

[3] Iraci, L. T. et al. Icarus, 210, 985–991.

[4] Barth, E. L. Planet. Space Sci., 137, 20–31, 2017.

[5] Dubois et al., PSJ, 2021 (in press).

[6] Ha, H. et al., J. Chem. Soc., 72, 1051–1057, 1976.

[7] Jackowski, A. W., J. Chem. Therm., 6, 49–52, 1974.

[8] Růžička, K. et al., J. Chem. Therm., 68, 40–47, 2014.

How to cite: Dubois, D., Iraci, L., Barth, E., Vinatier, S., Salama, F., and Sciamma-O'Brien, E.: Multidisciplinary investigation of benzene (C6H6) condensation in Titan’s South polar cloud system, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-566, https://doi.org/10.5194/epsc2021-566, 2021.