Complex molecules in Titan s upper atmosphere

Transcription

1 Complex molecules in Titan s upper atmosphere Panayotis Lavvas GSMA/CNRS Roger V Yelle LPL, University of Arizona 52 nd ESLAB Meeting, ESTEC, 14 th May 2018

2 INTRODUCTION From ATOMS to MOLECULES to MACROMOLECULES Interstellar medium Biological systems Protoplanetary Disks Combustion Global Climate Combustion Research Laboratory Nasa Earth Observatory

3 INTRODUCTION From ATOMS to MOLECULES to MACROMOLECULES Interstellar medium Biological systems Protoplanetary Disks Combustion Global Climate Combustion Research Laboratory Nasa Earth Observatory

4 INTRODUCTION to PHOTOCHEMICAL AEROSOLS in PLANETARY ATMOSPHERES Earth s O 2 Level Lyons et al Trainer et al Titan Cassini-Huygens

5 Voyager 1 (1981) Visible wavelengths RADIUS: 2575 km COMPOSITION: N 2 (95%), CH 4 (5%) GRAVITY: 1.35 m/s 2 TEMPERATURE: 94 K SURFACE PRESSURE: 1.5 atm ORBIT: 10 AU Cassini (2005) Composite EARTH 6371 km N 2 (78%), O 2 (21%), Ar(1%) 9.81 m/s K 1 atm 1 AU

6 THERMAL STRUCTURE Titan Earth

7 Cassini-Huygens Cassini Instruments* amu Vuitton et al INMS CAPS C 2 H 2 Altitude (km) 800 UVIS ISS 200 VIMS DISR CIRS UV Vis nir TIR *not exhaustive Wavelength Altitude (km) Latitude 78.5 N 68.5 N 54 N 46.5 N 39 N 29.5 N 3.5 N 20.5 S 54 S Mole Fraction INMS (Yelle et al. 2006, Waite et al. 2007, Vuitton et al. 2008, ) UVIS (Koskinen et al, 2011) CIRS (Coustenis et al., 2007, Vinatier et al., 2011, Teanby et al., 2007, )

8 CASSINI BREAKTHROUGH INMS CAPS Vuitton et al. 2007, 2008 Coates et al. 2007, Crary et al Muñoz et al A large number of new species identified for the first time in Titan s upper atmosphere with masses up to ~1000 Da. Aerosol formation takes place in the upper atmosphere!

9 Photochemistry (first steps) CH 4 + hv > Galand et al Altitude (km) Fe + Icy Meteoroid Rocky Si + CH 3 + H CH 2 + H 2 CH e - CH H + e -... T5 H + E > 30 kev Typical H + E > 30 kev O + E > 2 kev T5 e 91 SZA Production rate (cm 3 s 1 ) N 2 + hv > N + N( 2 D) N + N + + e - N e - Cassini observations show that solar input has the dominant role in the upper atmosphere GCR... Lavvas et al Models Strobel et al Yung et al Toublanc et al Lara et al Lebonnois et al Wilson & Atreya 2004 Vuitton et al. 2007/8 Horst et al Lavvas et al. 2008a,b Krasnopolsky 2009/12 Yelle et al Mandt et al Vuitton et al Loison et al Vuitton et al s s of reactions Immense efforts from experimental & theoretical investigations

10 re 36: INMS observed during T40 (symbols) and modeled (lines) mass spectrum at 1100 Ion Neutral Chemistry Positive Ions! Radiative association A + B! AB + h Positive ions (Dissociative) photoionization AB + h! AB + (A + + B)+e (Dissociative) electron ionization AB + e S! AB + (A + + B)+2e Bimolecular reactions A + + BC! AB + + C Termolecular association A + + B + M! AB + + M Radiative association A + + B! AB + + h Dissociative recombination AB + + e T! A + B Radiative recombination A + + e T! A + h Negative ions Vuitton et al. 2018, Icarus Charge transfer from species with low proton (hydrocarbons) affinity to species with higher proton affinity (nitrogen bearing species).

11 Negative Ions Ion Neutral Chemistry! Radiative recombination A + e T! A + h are the signatures of negative ions rather than electrons because Negative ions ions have thermal velocities that are small Ion-pair formation AB + h! A + B + compared to the spacecraft velocity whereas electrons are more isotropic due to Dissociative attachment their supra-thermal velocities AB + and e S! thus A are + Bdetected from any Radiative attachment actuator position. Negative A + ions e T were! A detected + h for approximately Bimolecular 73.5 reactions minutes around closest A approach, + BC! corresponding AB + C to an upper Photodetachment altitude of 1250 km. A + h! A + e Ion recombination The spacecraft ram velocity A + BC separates +! AB the+ masses C of the cold Associative detachment ions in detected energy per A charge. + B! The AB peak + e energy per charge, once corrected for the spacecraft potential, can then be converted to mass per charge knowing the spacecraft speed. For example, a Mass group 6kms Mass 1 range flyby (u) speed Negative givesions m amu/q ¼ 5.32E ev/q (Coates et al., a) It is assumed H,CH that 2,CH ions 3 are,csingly 2 H,CN charged,,o,oh which is 2 probably reasonable C 4 for H,C the 3 N lighter ions (o100 amu). However, 3 this assumption may C 6 be H questionable,c 5 N for high m/q, giving an even higher m in the aerosol range in those cases (Coates et al., 2007a; Waite et al., 2007) a As seen on the spectrogram in Fig. 2, two C 2 H - bnegative ion peaks centered at 2274 and 4478 amu/q are clearly identified. A third peak at amu/q CH 2 may be present as well. Keeping in mind the finite energy width of the detector (DE/E ¼ 16.7%), the peak width CH is consistent with the presence of a single ion species per peak, CN though the presence of multiple - species cannot be rejected. Altitude (km) Fig. 1. Energy time spectrogram retrieved by ELS and the corresponding actuator/ram angle during the T40 encounter. The vertical spikes systematically occur at a 01 angle between the actuator and ram and can consequently be attributed as negative ions O - OH - 3. Chemistry H Various negative ions have been 1000 suggested as potential candidates for the low mass peaks observed by ELS (Coates Altitude (km) 900 b C 4 H - Vuitton 1400 et al C 6 H Fig. 2. Negative ions measured in each CAPS-ELS energy bin at an altitude of C 4 H - c Group km during the T40 encounter. The attribution of the peaks is based on the C 6 H - ionospheric chemistry 1400 model. C 3 N - C 5 N Altitude (km) C 3 N Density (cm -3 ) ions likely present 1200in Titan s ionosphere based on their electron Altitude (km) for negative ions 1100 include radiative and dissociative electron photodetachment, 1000 ion ion recombination and ion neutral Group 2 Group 3 et al., 2007a; Waite et al., 2007). In this section, we discuss the affinity and gas phase acidity (Table 1). Production mechanisms attachment and ion-pair formation, while loss processes are associative detachment. Chemical reactions such as proton Total C Density (cm -3 ) Density (cm -3 ) Density (cm -3 )

12 Ion Neutral Chemistry Neutrals Formation of benzene from ion-neutral chemistry Vuitton et al C 6 H 6 Altitude (km) C 6 H 5 Latitude 78.5 N 68.5 N 54 N Mole Fraction

13 CASSINI BREAKTHROUGH INMS CAPS Vuitton et al. 2007, 2008 Coates et al. 2007, Crary et al Muñoz et al A large number of new species identified for the first time in Titan s upper atmosphere with masses up to ~1000 Da. Aerosol formation takes place in the upper atmosphere!

14 Photochemical products Positive ions MOLECULAR GROWTH electrons Macromolecules from ion chemistry

15 MOLECULAR GROWTH Macromolecules Photochemical products

16 MOLECULAR GROWTH Macromolecules electrons attach on macromolecules

17 MOLECULAR GROWTH Macromolecules Charged macromolecules attract positive ions

18 MOLECULAR GROWTH Macromolecules Recombination leads to mass transfer to macromolecules

19 MOLECULAR GROWTH Macromolecules Recombination leads to mass transfer to macromolecules Charge balance based on collisions with ion/electrons & photoelectric effect Positive Negative Neutral Total

20 MOLECULAR GROWTH Macromolecules Recombination leads to mass transfer to macromolecules Lavvas et al Negative Ions Positive Ions

21 MOLECULAR GROWTH Aerosol Mass flux from DISR observations : ~3x10-14 g cm -2 s -1 (Lavvas et al. 2010) Ion chemistry drives a rapid formation of aerosol embryos but is not sufficient to explain the total mass flux of aerosols observed.

22 HETEROGENEOUS CHEMISTRY assuming 1% efficiency Heterogeneous reactions with neutral components can provide a further addition to the aerosol mass flux as well as affect the gaseous abundances.

23 HETEROGENEOUS CHEMISTRY assuming 1% efficiency Lavvas et al Sekine et al Heterogeneous reactions with neutral components can provide a further addition to the aerosol mass flux as well as affect the gaseous abundances.

24 HETEROGENEOUS CHEMISTRY Particle ineption Coagulation Surface growth Particle rounding due to surface growth Lavvas et al Heterogeneous reactions affect the particle shape and the transition from spheres to aggregates Need new input from laboratory studies

25 Hot N 2 P. Lavvas et al. / Icarus 260 (2 P. Lavvas et al. / Icarus 260 (2015) Lavvas et al Fig. 24. Overview for the vibrational distribution and density of the ground state levels. Left: (blue lines and numbers). The black crosses represent the anticipated Boltzmann distributions the sensitivity of the 800 and 1000 km distributions on the vibrational energy exchange rate w relative to the nominal case used (k CH4 = cm 3 s 1 for the blue lines). Right: Density profil with the total N 2 (blue) and CH 4 (red) density profiles. The thin gray lines represent the same interpretation of the references to color in this figure legend, the reader is referred to the we Chemical implications unknown Fig. 33. Limb radiances for the CY(0, 0), CY(0, 1), and CY(0, 2) transitions and the altitude profile of CY(0, 1)/CY(0, 2). Each panel presents the calculated emissions in spherical geometry, assuming different N 2 state vibrational distributions: solid lines correspond to a Boltzmann distribution at 150 K, while the broken lines correspond to the ground

26 Conclusions Titan s atmosphere is the most complex organic laboratory in our Solar system Ion-Neutral chemistry has a fundamental role in the formation of complex molecules and eventually to the birth of the photochemical aerosols in Titan s atmosphere Neutral chemistry generates the most abundant photochemical products and drives that growth of the photochemical aerosols through heterogeneous reactions Titan, through Cassini-Huygens, has provided valuable lessons on complex chemistry and photochemical aerosol formation and evolution that help us understand other environments (Pluto, Giant Planets, Exoplanets)