OPS1

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.

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.

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 (μ_f= 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 μ_f = 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 μ_f = 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.

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.

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.

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.

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.

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.

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.

Satellites are generally believed to form in circumplanetary disks (CPDs): a gas disk containing icy and rocky particles that accumulate to form massive moons over time. The discoveries by the Cassini-Huygens mission have led to a revision of the birth environment of the Saturnian system.

We aim to constrain the formation circumstances of Titan's building blocks by considering the moon's observed characteristics. We use radiation thermo-chemical CPD models and evaluate them on their capacity to reproduce a Titan-like satellite.

To form a moon with Titan's ice-to-rock ratio, we find that the dust-to-gas ratio in the CPD must be in the order of solar nebula values, O(10^-2). The ice availability upon accretion is otherwise incompatible with Titan's moment of inertia. Our models predict a large NH₃ inventory was available upon Titan's formation, ∼10-20wt.% of the total ice. This is consistent with the hypothesis that the observed N₂ in Titan is captured as NH₃ and converted by photolysis and shock heating, and is compatible with the possible presence of a conductive layer at 45±15 km as revealed by the Huygens probe.

How to cite: Immerzeel, C., Cazaux, S., and Oberg, N.: Formation conditions of Titan, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-647, https://doi.org/10.5194/epsc2021-647, 2021.

Titan is the only known moon in the Solar System with a substantial atmosphere of N2 and CH4. However, its origin and evolution are not well understood. Titan’s present amount of atmospheric CH4 was predicted to be destroyed photochemically on very short timescales (~ 10 Myrs, Yung et al. 1984). This suggests that a methane resupply mechanism is necessary. The Huygens probe GCMS measurements of noble gases suggest that Titan’s atmosphere is likely linked to its interior instead of being incorporated during formation (Nieman et al., 2005). Recent theoretical modeling works of Titan’s atmosphere and interior suggest that its atmosphere could have originated partly by outgassing primordial organics in its interior (Neri et al. 2019; Miller et al. 2019). If this theory holds, volatiles like methane could be outgassing from Titan’s interior to sustain its current observed abundances. Insoluble organic matter (IOM) found in carbonaceous chondrites may serve as an analog for the organic material in Titan’s interior and provide experimental constraints on the outgassed component of its atmosphere (Thompson et al. 2021). By heating carbonaceous chondrite samples and measuring the abundances of their released volatiles, specifically methane, we may be able to connect what we see in the lab to species in Titan’s atmosphere today.

We performed outgassing experiments using three primordial CM carbonaceous chondrites: Murchison, Aguas Zarcas, and Jbilet Winselwan. The first two are "fall" meteorite (1969 and 2019), and Jbilet Winselwan is a desert "find'' meteorite (2013). We used two sizes of samples for each CM chondrite for the measurements: a small grain sample with diameters < 20 µm and a normal grain sample with diameters of 20-100µm. Each sample underwent a step heating scheme where they are heated and held at every 100°C from room temperature to 1200° C. The whole heating scheme takes 12 hours. We continuously monitored the partial pressures of 10 outgassed mass peaks using a residual gas analyzer (RGA).

We can estimate how much methane can be outgassed from the insoluble organics in the CM chondrites with the RGA data. We found that chondrite outgassing can resupply methane that can last for ~0.5-2 Gyrs. If organics indeed makes a significant fraction of Titan's interior, outgassing through thermal instability of Titan's interior can potentially resupply Titan's atmospheric methane for a period of time.

How to cite: Yu, X., Thompson, M., Duncan, T., Kim, K., Telus, M., Zhang, X., Joshi, T., and Lederman, D.: Carbonaceous Chondrite Outgassing Experiments: Implications for Methane Replenishment on Titan, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-193, https://doi.org/10.5194/epsc2021-193, 2021.

Benzene (C₆H₆) ice has been observed in the Titan’s stratosphere [1], and benzonitrile (C₆H₅CN) is a possible constituent in the benzene and nitrogen-rich environment of Titan’s atmosphere [2]. The energetic processing of such aromatic molecules can synthesize large and complex aromatic molecules such as the Polycyclic Aromatic Hydrocarbons (PAHs). To-date a number of laboratory experiments have reported the formation of complex organics from the energetic processing of aromatic molecules [3-6]. In particular, Scanning Electron Microscopy (SEM) micrographs of the residues resulting from irradiated benzene ices are found to contain geometrically shaped particles [6]. Therefore, by employing electron microscopes, we can understand the physical nature of the dust leftover from the aromatic molecule irradiation.

In the present investigation, we subjected benzonitrile ice made at 4 K to vacuum ultraviolet (9 eV) radiation at two beamlines, BL03 and BL21A2 of Taiwan Light Source at NSRRC, Taiwan. After irradiation, the ice was warmed to room temperature, which left a brownish residue on the Potassium Bromide (KBr) substrate. The VUV spectrum of the residue is observed to have characteristic aromatic signatures. The residue is then transferred to a quantifoil grid for High-Resolution Transmission Electron Microscope (HR- TEM) imaging. HR-TEM micrographs revealed the presence of graphene in the residue. This result suggests that N-graphene could be present in benzene and nitrogen-rich icy clouds of Titan. The high masses observed by the Cassini plasma spectrometer in Titan’s atmosphere could then be attributed to the presence of N-graphene along with the more common tholins [7].

References

[1] Vinatier S. et al. (2018) Icarus, 310, 89.

[2] Loison J. C. et al. (2019) Icarus 329, 55.

[3] Strazzulla G. et al. (1991) A&A, 241, 310.

[4] Callahan M. P. et al. (2013) Icarus, 226, 1201.

[5] James R. et al. (2019) RSC Adv. 9 (10), 5453.

[6] Rahul K. K. et al. (2020) Spectrochim. Acta A, 231, 117797.

[7] Rahul K. K. et al. (2020) arXiv:2008.10011.

How to cite: Kushwaha, R. K., Mallya, A., Sahu, D., Meka, J. K., Chou, S.-L., Wu, Y.-J., Gupta, D., Das, A., Lo, J.-I., Cheng, B.-M., Rajasekhar, B. N., Bhardwaj, A., Hill, H., Padmanabhan, J., Mason, N. J., and Sivaraman, B.: Graphene in Titan, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-480, https://doi.org/10.5194/epsc2021-480, 2021.

Titan, Saturn’s biggest moon, is an ocean world, covered by organic materials and therefore one of the most promising astrobiological target in the Solar System, likely holding clues on the origin of life on Earth. That is why NASA has selected the Dragonfly mission to send in 2027 a rotorcraft lander to Titan and investigate its prebiotic chemistry and habitability.

Making multiple flights (up to 24 in 2.5 years, starting in 2034), Dragonfly will explore a variety of locations, from the Shangri-La dune field to the rim of the young impact crater Selk (Lorenz et al., 2018), and therefore sample materials and determine surface properties in different geologic settings. The two permittivity probes - called DIEL- on board the Geophysical and Meteorological package (DraGMet) will be especially useful to characterize Dragonfly landing site environment.

DIEL consist of 2 electrodes acting as self-impedance permittivity probes. They will be mounted on each skid of the Dragonfly rotorcraft and operate independently for sake of redundancy and safety. Their objective is to measure the complex ground permittivity at several low frequencies (<10 kHz) which will provide clues on the composition, moisture and porosity of the near-subsurface of Titan as well as on the spatial and temporal variations of such properties. As a reminder, the first permittivity probe on Titan (actually the first ever planetary permittivity probe) successfully determined the complex permittivity of the Huygens landing site in 2005 (Grard et al, 2006; Hamelin et al. 2016).

During this presentation, we will describe the tests that have been performed on prototypes of the DIEL electrode plates in order to estimate their sounding depth and sensitivity to composition variations. Tests were performed e.g., with the electrode lying on reference plastic slabs and natural materials (air, sand, soil, liquid water, etc.) of well-known complex permittivity, at ambient and Titan temperatures. The effect of porosity and of a bad ground-electrode contact was also investigated leading to suggestions to optimize DIEL electrode design, accommodation and performance.

Indeed, the design of the DIEL experiments (size, shape, accommodation of the electrodes, modes of operation ect.) is not frozen yet and we are also conducting modelling simulations with COMSOL Multiphysics © to explore possible better designs and confirm the results obtained in laboratory.

Lastly, in order to relate DIEL measurements to the ground composition, it is crucial to know the electrical properties of materials relevant to Titan’s surface. This is the purpose of the PAP (Permittivité d’Analogues Planétaires) measurement bench that have been developed at LATMOS. This bench includes a cryostat to perform measurements at Titan’s temperature (90 K). It was successfully used to investigate the complex permittivity of analogs of Titan’s organic aerosols called “tholins” (Lethuillier et al., 2018). Future measurements will focus on “eroded” tholins, that is tholins that have been modified during their descent to the surface by processes analogous to those at play in Titan’s atmosphere: UV radiations (Carrasco et al., 2018), interaction with ionosphere’s charged particles (Chatain et al., 2020), deposition of ice in the low stratosphere (Fleury et al., 2019; Dubois et al., 2020), wetting by droplets of liquid methane in the troposphere etc.

References

Beghin et al., Icarus 218 (2012)

Carrasco et al., Nature Astronomy, Nature Publishing Group (2018)

Chatain et al., Icarus 345 (2020)

Dubois et al., Icarus 338 (2016)

Hamelin et al., Icarus 270 (2016)

Grard et al., Planet. Space Sciences 54 (2006)

Fleury et al., Icarus 321 (2019)

Lethuillier et al., Astronomy & Astrophysics 519 (2018)

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

How to cite: Le Gall, A., Chatain, A., Lorenz, R. D., Hamelin, M., Deprez, G., Osiander, R., Berthelier, J.-J., Hassen-Khodja, R., Lebreton, J.-P., Joly-Jehenne, T., Vettier, L., and Carrasco, N.: Measurement of Titan’s surface permittivity with the DraGMet/DIEL sensor on Dragonfly, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-690, https://doi.org/10.5194/epsc2021-690, 2021.

Titan’s atmosphere and surface (a complex system) evolve with season, as Titan follows Saturn in its orbit around the Sun for 30 years with an inclination of about 27°. We performed an analysis of spectra acquired by Cassini/CIRS at high resolution covering the range from 600 to 1500 cm-1 since the beginning and until the last flyby of Titan in 2017 and describe the temperature and chemical composition variations ([1-3]. By applying our radiative transfer code (ARTT) to the high-resolution CIRS spectra we study the stratospheric evolution over almost two Titan seasons [1,2], corresponding to the Cassini mission duration. CIRS nadir and limb spectral together show variations in temperature and chemical composition in the stratosphere during the Cassini mission, before and after the Northern Spring Equinox (NSE) and also during one Titan year.

Since the 2010 equinox we have thus reported on monitoring of Titan’s stratosphere near the poles and in particular on the observed strong temperature decrease and compositional enhancement above Titan’s southern polar latitudes since 2012 and until 2014 of several trace species, such as complex hydrocarbons and nitriles, which were previously observed only at high northern latitudes. This effect followed the transition of Titan’s seasons from northern winter in 2002 to northern summer in 2017, while at that latter time the southern hemisphere was entering winter.

Our data show a continued decrease of the abundances which we first reported to have started in 2015. The 2017 data we have acquired and analyzed here are important because they are the only ones recorded since 2014 close to the south pole in the far-infrared nadir mode at high resolution. A large temperature increase in the southern polar stratosphere (by 10-50 K in the 0.5 mbar-0.05 mbar pressure range) is found and a change in the temperature profile’s shape. The 2017 observations also show a related significant decrease in most of the abundances which must have started sometime between 2014 and 2017 [3]. In our work, we show that the equatorial latitudes remain rather constant throughout the Cassini mission.

We have thus shown that the south pole of Titan is now losing its strong enhancement, while the north pole also slowly continues its decrease in gaseous opacities. It would have been interesting to see when this might happen, but the Cassini mission ended in September 2017. Perhaps future ground-based measurements and the Dragonfly mission can pursue this investigation and monitor Titan’s atmosphere to characterize the seasonal events. Our results set constraints on GCM and photochemical models.

References:

 [1] Coustenis et al., 2016, Icarus 270, 409-420

[2] Coustenis et al., 2018, Astroph. J., Lett., 854, no2

[3] Coustenis et al., 2020. Titan’s neutral atmosphere seasonal variations up to the end of the Cassini mission. Icarus 344, 113413. https://doi.org/10.1016/j.icarus.2019.113413.

How to cite: Coustenis, A., Jennings, D., Achterberg, R., Lavvas, P., Nixon, C., Bampasidis, G., and Flasar, F. M.: Seasonal changes in Titan’s stratosphere during the Cassini mission with CIRS, Europlanet Science Congress 2021, online, 13–24 Sep 2021, EPSC2021-259, https://doi.org/10.5194/epsc2021-259, 2021.