Titan Airglow FUV Limb Spectra From Cassini UVIS Observations JOSEPH AJELLO JPL JACQUES GUSTIN IAN STEWART KRIS LARSEN LARRY ESPOSITO WILLIAM MCCLINTOCK WAYNE PRYOR GREG HOLSCLAW team meeting: june 2008

CASSINI UVIS OBSERVATION Inbound Orbit Tb 13dec04 phase ang.16o at 163,000km 105 km, 200 km resolution EUV/FUV AIRGLOW 2.0 (1.5)x59mrad FOV

The FUV Airglow of Titan Titan FUV UVIS Airglow Limb Spectra from the Surface to Exosphere on 13Dec04.

Regression Spectral Analysis of UVIS FUV Disk Dayglow Spectrum from 13Dec04 Regression Model 4-vectors Rayleigh Scattering by N2 LBH Band system by N2 NI lines PDI by N2 H Lya solar fluorescence by H, …

mixing ratios obtained from fit of UVIS reflected sunlight spectrum of Titan (1500-1880 Å). Liang et al., 2007 tholins 7.7 x 10-7 N2 density is ~ 1 x1016 cm-3 at 300 km. Tholin mixing ratio is then ~ 1 x 10-11

A plot of the geometric albedo of Titan from 1500 to 11000 Å (adapted from Mckay et al., 2001)

The FUV Airglow of Titan Titan FUV UVIS Airglow Limb Spectra from the Surface to Exosphere on 13Dec04.

Key Altitude Limb Profiles for the 13Dec2004 Titan Observations NI 1493 Å LBH SOLAR REFLECTANCE 1600-1900 Å H Ly a

Model Long Wavelength (1500-1900 Å) FUV Reflected Solar Light (step 1 - Vector 1) solar reflection model at Limb (Lommel-Seeliger) law Modeling Parameter is Strong Absorber Mixing Ratio @ at each altitude: fc2H2 fc2H4 fc4H2 Tholins (e.g.C11H11N2)

Spectral Fitting of FUV Airglow In Lower Atmosphere (100 km) Electron impact 18 eV laboratory spectrum. Relative intensities of NI and NII PDI [Bishop and Feldman, 2003]. Rayleigh Scattering HI Lyman-a at 1215 Å. Regression Model

Spectral Fitting of FUV Airglow In Middle Atmosphere (500 km) Electron impact 18 eV laboratory spectrum. Relative intensities of NI and NII PDI [Bishop and Feldman, 2003]. Rayleigh Scattering HI Lyman-a at 1215 Å. Regression Model

Regression Model Fits to Peak (1100 km) FUV Limb Dayglow Spectrum from 13Dec04 Regression Model 4-vectors Rayleigh Scattering by N2 LBH Band system by N2 (+ scattered LBH >1400 Å) NI lines PDI by N2 H Lya solar fluorescence by H, …

Laboratory Spectrum and Model of LBH Band system by 18 eV Electrons

Geometry Distant (13Dec04) UVIS FUV Limb Observation with Lower Atmosphere Scatter Light FOV= 1.5mR (0.09o) Scattered light FOV=.15o (10-3 FOV) Total FOV=0.4o Distance =85,000 to 160,000 km Limb rayheight = 1100 ±560 km Contribution from 500 km to 1100 km spectrum transmitted thru CH4 tslant(1)~1016 cm-2 for CH4 (1200km) at 1300Å

Regression Model Fit to Middle Atmosphere FUV Limb Dayglow Spectrum (Liang et al., 2007) Compare Liang et al., 2007 C4H2 = [4.5e15] C2H4 = [4.0e16] CH4 = [6.0e19] C2H2 = [2.1e17] C2H6 = [7.0e16] HCN = [1.0e17] Regression Model 4-vectors Rayleigh Scattering by N2 LBH Band system by N2 NI lines PDI by N2 H Lya solar fluorescence by H, …

Minimum Ray Height Altitude Profiles of Important Emissions from Cassini UVIS 13Dec04 LBH bands Peak altitude is 1100 km Rayleigh Scattering topside is 500 km H Lya increases with altitude (~doubles) Mystery Line Needs Lower Atmosphere Source CI strong to Ionosphere

Rayleigh Scattering tends to 103 LBH Intensity FUV Observation at 50 km over Full Spectral Range of UVIS (1150-1850 Å) Rayleigh Scattering tends to 103 LBH Intensity

Mixing Ratios Obtained from Fit of UVIS reflected sunlight spectrum at Limb of Titan (study at 1500-1880 Å).

MAJOR RESULTS OF UVIS 13DEC04 LIMB DAYGLOW ANALYSIS ALTITUDE OF FUV DAYGLOW 1100 KM RAYLEIGH SCATTERING TOPSIDE ~500 km ENHANCED LBH INTENSITY @IONOSPHERE (instrumental or real) SECOND SPECTROSCOPIC EVIDENCE FOR THOLINS (LIANG ET AL.2007) FIRST GEOMETRIC ALBEDO OF TITAN (1500-1900 Å) MYSTERY LINE AND CI FEATURES STRONG IN LOWER ATMOSPHERE 2 GRL publications + 1 on Limb (in progress)