Aerosol distribution and physical properties in the Titan atmosphere

D. E. Shemansky1, X. Zhang2, M-C. Liang3, and Y. L. Yung2

1 SET/PSSD, California, USA ; 2 CIT California, USA ; 3 Res. Ctr. Env. Change, Taipei, Taiwan

Extinction spectra in Cassini UVIS fuv stellar occultations show a continuum component with a spectral shape similar to the imaginary part of the

refractive index obtained by Khare et al (1984) from deposits (tholin) in a laboratory discharge in an N2-CH4 gas mixture.

• The UVIS fuv experiment also measures emission in scattered solar photons in the 1500 – 1900 A region, interpreted as Rayleigh scattering by tholin.

• The combination of extinction optical depth and measured absolute scattered solar photons in the same upper thermosphere volume is then used to infer a scattering cross section .

Cassini T0 encounter with near 90o solar phase observing geometry

Cassini UVIS fuv emission spectrum from sub-solar 1040 km los at T0Phase ~90o

Tholin vertical distribution extracted in stellar occultations at TB encounter

l(A) 1826 h (km) 1040

FS(9AU)l (ph cm-2 s-1 A-1) 2.7 x 108 I(B)l (R A-1) 1.3

p iso 4.9 x 10-3 mm(Khare) 1.65 + 0.235i

Qs(a, l) (Mie) 4.9 x 10-3 radius a (A) 76

ss(a, l) (Mb) 8900 [tholin]l (cm-2) 6.9 x 1011

[tholin] (cm-3) 3900 [tholin]/[N] 1.2 x 10-6

Calculated tholin physical properties using Mie scattering theory

100 101 102 103 104 105 106 107

[tholin] (cm-3)

200

300

400

500

600

700

800

900

1000

1100

h (k

m)

TB_tholin_lScoTB_tholin_VirT41_tholin_Cma_ing

Derived tholin densities from stellar occultations at TB and T41

Comparison of Cassini UVIS and Voyager 1occultation photometric inversions at ~1650 A

Global distribution

• The high altitude thermosphere tholin content shows dependence on latitude.

• The north and south polar regions have measureable tholin scattering only below 850 km compared to 1040 km at low latitude.

Titan TA encounter near 0o phase Multiple exposures with N/S planet centered UVIS slit

-4000 -3000 -2000 -1000 0 1000 2000 3000 4000RT

0.0

0.5

1.0

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2.0

2.5

3.0

Albe

do X

100

TA_T0_albedo_1827

Albedo at 1827 A X 100

albedoX100 TAalbedoX100 T0

T0 sub solar 90 deg phaseTA zero phase pole to pole

Solar reflection in the Cassini UVIS fuv spectrum at the T0 and TA encounters.T0 is a low latitude 90o phase sub solar los . TA is at 0o phase and scans all latitudes through planet center pole to pole.

Cassini UVIS fuv spectra at selected los impact parameters showing the absence of solar photon scattering at and above 875 km; TA encounter south pole region

Cassini UVIS fuv spectra at selected los impact parameters showing the absence of solar photon scattering at and above 875 km; TA encounter south pole region

Cassini UVIS fuv spectra at selected los impact parameters showing the absence of solar photon scattering at and above 875 km; TA encounter north pole region

Comparison of the south pole region TA spectra with the UVIS simulated fuv solar model spectrum for zero extinction, normalized at 1850 A.

Conclusions• The combined Cassini UVIS occultation and emission measurements of aerosol properties

in the Titan atmosphere allow determination of basic properties. The application of Mie theory with the Khare et al. tholin refractive index measurements are used for the calculation of a mean cross section and particle size (76A radius) at the top of the atmosphere ( 1040 km) at low latitude .

• The derived vertical density profiles from several occultation events show two common deep vertical population depletions in the altitude range 700 – 800 km, and 400 – 500 km. The evidence indicates these features are persistent in location and depth on a ½ century scale based on comparison with Voyager results.

• It is evident from profile scale height that the aerosols must be produced at high altitude and diffuse downward, consistent with ion chemistry production (Coates et al., 2007, 2010) . In addition the source must be a vertically distributed process; regeneration must occur below the depletion layers. The deep layered structure imposes limits on rate modeling. See Kammer et al. EPSC-DPS 2011-347 for relation to lower order hydrocarbons.

• The polar regions show aerosol scattering to be undetectable above 800 km altitude, 200 km below the measured scattering at 1040 km at low latitude.