JOSEPH AJELLO
JPL
JACQUES GUSTIN
IAN STEWART
KRIS LARSEN
LARRY ESPOSITO
WILLIAM
MCCLINTOCK
WAYNE PRYOR
GREG HOLSCLAW

EUV/FUV AIRGLOW
2.0 (1.5)x59mrad FOV
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Inbound Orbit Tb 13dec04 phase ang.16
o at
163,000km
10 5 km, 200 km resolution

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
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Regression Model 4-vectors
Rayleigh Scattering by N
2
LBH Band system by N
2
NI lines PDI by N
2
H Ly a solar fluorescence by H, …

mixing ratios obtained from fit of UVIS reflected sunlight spectrum of Titan (1500-1880 Å).
5.4 x 10 -8
2.4 x 10 -6
3.6 x 10 -5 6.9 x 10 -6
1.1 x 10 -8
Liang et al., 2007 tholins
7.7 x 10 -7
N
2 density is ~ 1 x10 16 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
LBH
NI 1493 Å
SOLAR
REFLECTANCE
16001900 Å
H Ly a

Model Long Wavelength (1500-1900 Å) FUV
Reflected Solar Light (step 1 - Vector 1)
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 solar reflection model at
Limb
(Lommel-
Seeliger) law
Modeling
Parameter is
Strong Absorber
Mixing Ratio
@ at each altitude: f f f c2H2 c2H4 c4H2
Tholins
(e.g.C
11
H
11
N
2
)

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 Lymana at 1215 Å.
• Regression Model

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
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Regression Model 4vectors
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Rayleigh Scattering by N
2
LBH Band system by N
2 scattered LBH >1400 Å)
(+
NI lines PDI by N
2
H Ly a 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
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FOV= 1.5mR (0.09
o )
Scattered light
FOV=.15
o ( 10 -3 FOV)
Total FOV=0.4
o
Distance =85,000 to
160,000 km
Limb rayheight =
1100 ± 560 km
Contribution from
500 km to 1100 km spectrum transmitted thru
CH
4 t slant
CH
4
(1)~
10 16 cm -2 for
(1200km) at 1300Å

Regression Model Fit to Middle Atmosphere FUV
Limb Dayglow Spectrum (Liang et al., 2007)
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Compare Liang et al., 2007
C
4
H
2
C
2
H
4
CH
4
C
2
C
2
H
2
H
6
= [4.5e15
]
= [4.0e16]
= [6.0e19]
= [2.1e17]
= [7.0e16]
HCN = [1.0e17
]
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Regression Model 4-vectors
Rayleigh Scattering by N
2
LBH Band system by N
2
NI lines PDI by N
2
H Ly a solar fluorescence by H, …

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

FUV Observation at 50 km over Full Spectral
Range of UVIS (1150-1850 Å)
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Rayleigh Scattering tends to 10 3 LBH Intensity

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

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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)