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Titan airglow spectra from the Cassini Ultraviolet Imaging
Spectrograph: FUV disk analysis
Joseph M. Ajello,1 Jacques Gustin,2 Ian Stewart,2 Kristopher Larsen,2 Larry Esposito,2
Wayne Pryor,3 William McClintock,2 Michael H. Stevens,4 Charles P. Malone,1
and Dariusz Dziczek5
Received 30 October 2007; revised 18 January 2008; accepted 6 February 2008; published 21 March 2008.
[1] We present a spectral analysis of the far ultraviolet
(FUV: 1150 – 1900 Å) disk airglow observations of Titan’s
atmosphere by the Cassini Ultraviolet Imaging Spectrograph
(UVIS). The FUV spectrum consists of emissions from the
Lyman-Birge-Hopfield (LBH) band system of N2 excited by
photoelectrons (a 1Pg ! X 1S+g ), N I multiplets from solar
photodissociative excitation of N2, resonantly scattered
solar H Ly-a and sunlight reflected by N 2 in the
mesosphere-stratosphere and modified by aerosols (e.g.,
tholins) and hydrocarbon absorption. Below 1450 Å, the
strongest emissions arise from H Ly-a with an intensity of
208 Rayleighs (R), LBH bands with an intensity of 43 R,
and the N I multiplets with a combined intensity of 16 R.
Above 1450 Å, most of the UVIS signal is due to reflected
sunlight. Mixing ratios of tholins, C2H2, C2H4 and C4H2
have been derived from the reflected sunlight using a
Rayleigh scattering model. The derived mixing ratios are in
good agreement with Voyager infrared observations and
with FUV photochemical models, assuming solar energy
deposition above 1450 Å occurs near 250 km (Wilson and
Atreya, 2004). We also present the first geometric albedo
measurement of Titan from 1500 – 1900 Å. Citation: Ajello,
J. M., J. Gustin, I. Stewart, K. Larsen, L. Esposito, W. Pryor,
W. McClintock, M. H. Stevens, C. P. Malone, and D. Dziczek
(2008), Titan airglow spectra from the Cassini Ultraviolet
Imaging Spectrograph: FUV disk analysis, Geophys. Res. Lett.,
35, L06102, doi:10.1029/2007GL032315.
1. Introduction
[2] We have recently provided an analysis of the extreme
ultraviolet (EUV) spectrum (900– 1150 Å) from the Ultraviolet Imaging Spectrograph (UVIS) on Cassini, hereafter
referred to as Paper 1 [Ajello et al., 2007]. In the current
paper we analyze the far ultraviolet (FUV) spectrum (1150 –
1900 Å) of the disk of Titan. The FUV spectral region is a
probe of the stratosphere-mesosphere-thermosphere region
from 200 to 1600 km [Wilson and Atreya, 2004]. For the
1
Jet Propulsion Laboratory, California Institute of Technology,
Pasadena, California, USA.
2
Laboratory for Atmospheric and Space Physics, University of
Colorado, Boulder, Colorado, USA.
3
Science Department, Central Arizona College, Coolidge, Arizona,
USA.
4
Space Science Division, Naval Research Laboratory, Washington,
D. C., USA.
5
Nicolaus Copernicus University, Torun, Poland.
Copyright 2008 by the American Geophysical Union.
0094-8276/08/2007GL032315$05.00
first time, UVIS provides a spectrum of the FUV that
bridges the gap between the EUV in Paper 1 and the middle
ultraviolet (MUV) observations of the Titan albedo from
International Ultraviolet Explorer (IUE) [McGrath et al.,
1998]. Indeed, previous Titan FUV observations by Voyager
1 (VI) were poorly defined because of lack of instrumental
sensitivity and resolution [Broadfoot et al., 1981] and did
not allow an accurate analysis of the emission in this
wavelength range.
[3] McKay et al. [2001] and Cortin et al. [1991] have
shown that in the UV (down to 2000 Å), the astronomically
observed UV geometric albedo is determined by extinction
properties of absorbing haze and intense Rayleigh scattering. The organic haze is most likely formed in the 200–
500 km altitude region, initiated with the thermosphere
photolysis of CH4 into CH radicals and photoelectron
excitation of N2 into predissociating states yielding atomic
N that diffuses to the stratosphere, where the dissociation
products recombine as nitriles and hydrocarbons and eventually coagulate into tholin particles [McKay et al., 2001].
The attendant collisions result in a likely bimodal distribution with the small aerosol particles (average radius that is
smaller than 200 Å) acting as an efficient UV absorber
[Cortin et al., 1991]. Using the UV cross sections for
extinction by aerosols computed by Khare et al. [1984]
and assuming a mean particle radius of 125 Å, Liang et al.
[2007] have analyzed a Cassini UVIS stellar occultation
light curve over the range of 330– 970 km minimum ray
heights and derived an altitude distribution of tholins.
[4] The current state of the photochemistry of Titan’s
atmosphere has been reviewed by Wilson and Atreya
[2004]. Using a two-stream model algorithm to model the
scattering contribution to the solar flux, they found that the
solar flux between 1450 and 2000 Å reached altitudes
between 200 and 350 km, where it is absorbed by molecules
and aerosols [see Wilson and Atreya, 2004, Figure 3]. The
airglow and reflected sunlight are produced at two different
altitudes. The longer wavelength airglow emissions of the
FUV (1450 – 1900 Å) are dominated by Rayleigh scattering
produced in the stratosphere near 250 km compared to the
shorter wavelength airglow emissions (1150 – 1450 Å) that
occur higher in the thermosphere at 1100 km (Paper 1).
This is the wide altitude range that we are probing.
[ 5 ] The optically forbidden Lyman-Birge-Hopfield
(LBH) band system of N2 (a 1Pg X S+g ) was observed
by VI in the FUV and was the brightest N2 UV emission
from the disk of Titan [Strobel et al., 1991]. The LBH
intensities measured in 1980 by VI at solar maximum and in
2004 by UVIS at low solar activity track time changes in the
solar XUV flux (0 to 450 Å). The VI observations did not
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resolve the LBH emissions from the Rayleigh scattered
background.
[6] The goal of this paper is to provide a spectral analysis
of the FUV Titan airglow disk spectrum using a regression
analysis on the three main N2 sources of the FUV airglow
spectrum: (1) the LBH band system excited by photoelectrons; (2) the atomic N I lines produced primarily by
photodissociative ionization (PDI) of N2 (Paper 1); and
(3) solar UV emission scattered by the N2 atmosphere
embedded with absorbing hydrocarbons and tholin absorbers.
The FUV limb spectra analysis is the subject of a forthcoming paper. The limb spectra are complicated with a
critical dependence on minimum ray height, dominated by
solar reflectance at low altitudes and LBH airglow emissions in the ionosphere, near 1100 km.
[7] We have discussed the geometry, performance and
calibration procedures of the EUV channel in Paper 1. (See
Ajello et al. [2007, Figure 1] for viewing geometry.) In
brief, Cassini made its second close pass by Titan on
13 December 2004 (13Dec04) (11:36 UT DOY-348). The
performance of the FUV channel is different than described
in the instrument paper [Esposito et al., 2004] in two
respects. The data pipeline includes a flat-field correction
[Steffl et al., 2004] and takes into account the instrumental
sensitivity variation with time. In flight calibrations using
bright O-B stars (September 1999 – November 2007) have
revealed that the FUV detector absolute sensitivity (l >
1600 Å) is slowly increasing with time. The uncertainty for
the 13Dec04 FUV intensities is a root-sum-square error of
16% based on Poisson statistics (<5%) and calibration
uncertainty (15%). The UVIS FUV line spread function
was based on the near-Earth interplanetary Ly-a line.
2. FUV Solar Reflectivity From Rayleigh
Scattering
[8] We begin the problem by modeling the long wavelength portion of the FUV disk spectrum of Titan from 1500
to 1900 Å. This is the first step to provide one of the N2
input vectors to fit the entire FUV by regression analysis. To
model the reflected sunlight from Titan, we use the LommelSeeliger (LS) law that includes multiple scattering and
assumes a homogeneous atmosphere (refer to equation (47)
of Wallach and Hapke [1985], applied to disc viewing
geometry, i.e. pixels at the limb are excluded). We use the
single scattering albedo in a gaseous mixture as defined by
Gladsone and Yung [1983].
[9] The calibrated Titan FUV airglow spectrum that we
analyzed is the sum of all the spatial pixels that intersect
Titan’s disk (sum of all 1723 readouts). Since the incidence,
emission and phase angles are generally very different for
each pixel along the slit, we applied the reflectivity model to
each individual pixel, computed an average spectrum, and
weighted each spectrum by each pixel’s integrated intensity
above 1500 Å (see Figure 1). The reflectivity calculated
from the model above is then multiplied by a solar spectrum
provided by the TIMED Solar EUV Experiment (SEE)
[Woods et al., 2005], measured the same day as the
observation. A least-square regression between the observed
data and model is then applied, yielding the best-parameters,
which are the mixing ratios of the absorbing components.
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[10] The main hydrocarbons in Titan’s atmosphere that
have a significant absorption cross-section in the 1500–
1900 Å window are: C2H2, C2H4 and C4H2. With these
hydrocarbons, we obtain a best fit shown in green in
Figure 1, compared to the UVIS FUV disk spectrum in
black. In this spectral window, the absorption cross-sections
of these hydrocarbons decrease very steeply with increasing
wavelength, which means that the more we add hydrocarbons, the larger is the discrepancy in the range 1650–
1850 Å. To improve the fit, we add an organic aerosol
known as tholin, produced on Titan by photochemical
reactions [McKay et al., 2001]. The mixing ratios of these
four species are included in the single-scattering albedo
calculation. We use tholins produced from N2/CH4 discharges, which have the required absorption cross-section
dependence in this wavelength range [Liang et al., 2007].
The best-fit model with N2/CH4 tholins is greatly improved compared to the hydrocarbon-only fit, as seen in
red in Figure 1. We find the mixing ratios of hydrocarbons
relative to N2 to be: 2.4 106 for C2H2, 5.7 108 for
C2H4, 1.2 108 for C4H2 and 2.4 1011 for 125 Å
radius particle tholins. The quality of the fit shows these
four species are sufficient to explain the observations. One
UVIS reflection spectrum alone can not determine particle
size; a similar fit is achieved with 25 Å radius particles by
adjusting the tholin mixing ratio upward by a factor of 25.
The exact optical properties for the type of tholin(s) present
are unknown and LS results are a first order model of the
FUV reflection spectrum.
[11] The mixing ratio values we obtain characterize the
atmosphere where the extinction optical depth is unity. The
best-fit values are in excellent agreement with the hydrocarbon mixing ratios modeled by Wilson and Atreya [2004,
Figure 10] and with the tholin mixing ratio modeled by
Liang et al. [2007, Figure 2], by assuming that the bulk of
sunlight is scattered near 250 km. These results are also
consistent with Figure 3 of Wilson and Atreya [2004] which
exhibits that the maximum solar scattering occurs between
200 and 350 km, and with a limb profile of the integrated
1500– 1900 Å band pass of the UVIS spectral intensities
versus altitude (not shown) which exhibit an emission
peaking around 250 km, that is 5 times more intense than
the emission at 800 km. The mixing ratios also compare
well with the VI infrared values [Hanel et al., 1981;
Coustenis et al., 1991]. The best-fit model is used for the
next step in the analysis.
3. Spectral Analysis
[12] The UVIS disk airglow spectrum over the spectral
range of 1150– 1750 Å is shown in black in Figure 2. It is
observed that there are about 20 well-defined emission
features from the LBH bands of N2, atomic multiplets of
N and H Ly-a from 1150– 1570 Å along with about a halfdozen solar chromospheric lines from 1570 – 1750 Å
[Cohen, 1981]. Clearly identified in the airglow spectrum
are the strong atomic lines of N I at 1200 Å, 1243 Å, and
1493 Å. The positions of weaker N I features are identified
by tick marks. The strongest part of the LBH spectrum can
be found between 1300– 1500 Å [Ajello and Shemansky,
1985]. Beyond 1500 Å, the spectrum is dominated by solar
reflectance from Rayleigh scattering.
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Figure 1. Fit to the UVIS dayglow FUV disk spectrum
from 1500– 1900 Å with two models. The differences
between the model in green (hydrocarbons only) and the
model in red (hydrocarbons + tholins) show the importance
of tholins to accurately model this portion of the FUV
spectrum. The mixing ratio of C2H2 (the main absorbing
hydrocarbon in this spectral region) is 10 times higher when
tholins are not included.
[13] The regression fit (orange) in Figure 2 consists of
four independent spectra: (1) an optically thin 18 eV direct
electron impact laboratory fluorescence spectrum (the N2
gas, rotationally cooled [Ajello et al., 1998], simulating the
Titan airglow LBH spectrum) (red). The spectral fittings of
both the FUV and EUV (paper 1) observations of UVIS at
5.6 Å FWHM are insensitive to variations in N2 rotational
temperatures of laboratory spectra between 150– 300 K;
(2) a PDI spectrum of N2 to produce the N I FUV emissions,
based on the relative intensities given by Bishop and
Feldman [2003] (purple); (3) the H I Ly-a emission line
excited mainly by solar fluorescence at 1215.6 Å (green);
and (4) reflected sunlight, calculated for the entire FUV
using the mixing ratios obtained in Section 2 (blue). The
best fit to the data from the regression in Figure 2 (orange)
determines the contributions from the four components.
[14] The medium resolution laboratory spectrum of electron-excited N2 (1.0 Å FWHM, prior to convolution with
the Cassini line spread function) shows the presence of
about 100 LBH spectral features over the FUV spectral
range of UVIS. The strongest LBH features are the (2,0) +
(5,2) and (3,0) + (6,2) bands at 1383 Å (5.5%) and 1356 Å
(5.9%) [Ajello and Shemansky, 1985]. Based on the laboratory spectrum, the disk intensity of the LBH band system
(1250 – 2600 Å) from Titan is 43 ± 7 R. The regression fit
shows significant Rayleigh scattering background to the
LBH system down to a wavelength of 1350 Å with an
intensity of 458 R from 1350 to 1750 Å. Below 1350 Å the
LBH, PDI and H Ly-a vectors include absorption by a slant
column of CH4 of 3 1016 cm2. The model also places an
upper limit of the slant column of C2H2 of 3 1014 cm2,
which is consistent with the fact that the density of C2H2 is a
factor of 100 below that of CH4 near 900 km [Shemansky et
al., 2005].
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[15] The N I features can be identified from the work of
Ajello and Shemansky [1985]. The strongest N I features in
the FUV spectrum are 1200 Å at 6.8 R, 1243 Å at 0.6 R,
1493 Å at 3.0 R and 1743 Å at 1.6 R. The N I lines yield
16 R and the LBH N2 bands (1250 – 2600 Å) furnish 43 R.
The H Ly-a feature dominates the intensity of the FUV at
208 R. The sources of H Ly-a, given by Strobel and
Shemansky [1982], are due to resonance fluorescence in
Titan’s atmosphere and background emissions from the
local interstellar medium and particle excitation.
[16] Tentative identification of atomic carbon airglow
features are made from deficits in the regression fit compared to the UVIS FUV spectrum. These same C I features
also occur in the solar spectrum [Prekke et al., 1991] and
contribute to the reflection spectrum from Rayleigh scattering. The 31.4° ecliptic longitude difference between Cassini
(Saturn) and SEE (Earth) contributes 2% uncertainty to
the solar spectrum estimate at Saturn and to the tentative
identification. The C I features are all identified in the figure
caption and are located, also, by tick marks. The C I features
are most likely due to electron-excited CH4 spectrum [Pang
et al., 1987] with contributions from resonantly scattered
sunlight and photoelectron excitation of atomic C. The
strongest feature is 1657 Å followed by 1561 Å. Solar
XUV photodissociative excitation produces a similar FUV
spectrum of C I multiplets. One ‘‘mystery feature’’ at 1597
Å has not been positively identified. There are several
candidate emission features at this wavelength. We have
discounted the following because of lack of other supporting emissions: (1) CO A 1PX 1S (0,1) fourth positive
Figure 2. Regression model fit to the FUV dayglow disk
spectrum from 13Dec04 in black. The regression model
(orange) includes four independent spectra: (1) LBH
spectrum in red (see text); (2) PDI spectrum from solar
photo-dissociative ionization of N2 in purple; (3) the solarscattered component described in Figure 1 in blue; and (4) the
H Ly-a feature at 1216 Å in green. The C I airglow features
(tentative identification) and ‘‘mystery feature’’ at 1597 Å are
not included in the regression but are identified by tick marks.
The C I multiplets are found at 1464 Å, 1561 Å and 1657 Å.
Solar lines are found in the UVIS data from C IV at 1548 Å,
C I at 1561 Å and 1656 Å, Fe II at 1686 Å, 1697 Å, 1702 Å,
1712 Å, and 1721 Å.
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scattering spectral region. The major difference between the
model and observed albedo curves is the presence of the C I
multiplets from the upper atmosphere of Titan at 1561 and
1657 Å in the observed spectrum. The long wavelength
value at 1850 Å joins smoothly on to the low wavelength
IUE geometric albedo value of 0.028 at 2225 Å for the Faint
Object Spectrograph (FOS) grating G270H. McGrath et al.
[1998] also give a value of geometric albedo at 1800 Å of
0.02 for FOS grating G190H in agreement with our value.
Further into the visible spectral region McKay et al. [2001]
have noted that the haze extinction coefficient switches
from mostly absorbing to mostly scattering. The general
trend of the reddening of Titan was the original reason
prompting tholin studies by Khare et al. [1984].
Figure 3. A plot of the geometric albedo of Titan from
1500 to 11000 Å, adapted from McGrath et al. [1998] for
the FOS grating G270H (2200– 3300 Å) (red) for the MUV
and McKay et al. [2001] (green) for the visible and near IR.
The plot includes measured FUV values from 1500 – 1900 Å
for the geometric albedo. Inset: The FUV model geometric
albedo of Titan of the solar reflection (only) spectrum
(blue) is compared to the observed UVIS data (black).
1 +
band at 1597 Å [Beegle et al., 1999]; (2) the N++
2 (D Su 1 +
X Sg ) (0,0) and (1,1) transitions from 1587 – 1594 Å
[Lilensten et al., 2005]; and (3) N+(3s 1Po3s 3P) between
1595 – 1598 Å (NIST http://physics.nist.gov/PhysRefData/
ASD/lines_form.html).
[17] The nightglow spectrum (not shown) indicates a
strong H Ly-a feature of about 80 R intensity and some
weak N lines (1200 Å, 1243 Å and 1493 Å) each with an
intensity <0.5 R. The LBH band system is not observed in
the 13Dec04 nightglow spectrum.
4. Summary
[18] We have modeled the FUV disk airglow spectrum of
Titan from the upper atmosphere, probing altitudes between
200 km (reflected sunlight) and 1100 km (airglow). Our
study of the UVIS FUV dayglow spectrum demonstrates
that tholins are a key component of Titan’s atmosphere. The
geometric albedo is a useful way for studying the nature of
the haze [McKay et al., 2001]. In Figure 3 we show for the
first time the FUV geometric albedo of Titan between
1500 – 1900 Å. Here, the observed albedo is the UVIS
FUV disk-averaged surface brightness divided by the solar
flux (4pIl/pFl) corrected for Titan distance as measured by
TIMED SEE [Woods et al., 2005]. Our albedo results are
compared to complementary results for the MUV from the
IUE for phase angles of 1° to 5° [McGrath et al., 1998] and
for the visible/near IR [McKay et al., 2001].
[19] The modeled albedo (blue, in Figure 3 inset, representing solar reflection from the lower atmosphere) and the
UVIS observations of the FUV albedo (black, Figure 3
inset) indicate a value of 0.017 at about 1850 Å with a
general trend to decline with decreasing wavelength. Superimposed on this trend are the molecular band features of the
principal organic hydrocarbon absorber C2H2 in the Rayleigh
[20] Acknowledgments. The research was carried out at the Jet
Propulsion Laboratory (JPL), California Institute of Technology. The work
was supported by the NASA Planetary Atmospheres Program and by the
Cassini Project. M.H.S. was supported by the Office of Naval Research.
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