ANOMALOUS EMISSION IN THE 3.28 ANOMALOUS EMISSION IN THE 3.28 µµm IN THE TITAN UPPER ATMOSPHEREm IN THE TITAN UPPER ATMOSPHERE

B.M. Dinelli1, A. Adriani2, M. Lopez-Puertas3, M. L. Moriconi1, Maya Garcia-Comas3, Angioletta Coradini2, Emiliano D’Aversa2, Gianrico Filacchione4 and Federico Tosi2

1ISAC-CNR, Bologna and Roma, Italy, 2IFSI-INAF, Roma, Italy, 3IAA, CSIC, Granada, Spain, 4IASF-INAF, Roma, Italy

31 August 2007 16:0331 August 2007 16:03sub solar point: lat/lon -10.63/179.85W degsub solar point: lat/lon -10.63/179.85W degsub spacecraft point: lat/lon -2.60/154.12W deg sub spacecraft point: lat/lon -2.60/154.12W deg SOLAR_PHASE_ANGLE = 26.0000 (deg)SOLAR_PHASE_ANGLE = 26.0000 (deg)CASSINI_TARGET_DISTANCE = 236000 (km)CASSINI_TARGET_DISTANCE = 236000 (km)

SOA simulation for the V1567269485_1V1567269485_1 observation. The sub solar (red triangle), the sub reflection (orange triangle) and the sub spacecraft (green triangle) positions are reported.

4e-08 1.16e-06

Case study: S33-CIRSSTARE001-V1567269485_1.QUBCase study: S33-CIRSSTARE001-V1567269485_1.QUB

The frame at 3.33 micron (CH4 Q branch), masked for the surface contributions. In the two images the color scale (shown on the left) indicates the measured radiation intensity in W/(m2 nm sr) (false colors). The color scale range has been chosen from the noise level up to the highest radiance in the analyzed wavelength range. Superimposed in blue on the left image are the latitudes for nadir and limb viewings and, on the right image, the tangent height levels plotted every 500 km. In these images the geographical poles appear inverted. The yellow arrow indicates the latitude path of the observations shown in this poster. They represent a typical example of the spectral behavior of the observations taken of Titan’s atmosphere.

Upper panels: Comparison of simulated (blue line) and measured (green line) spectra in the CH4 spectral region.

Lower panels: Difference between observed and simulated spectra (orange line), compared with the noise level of the measurements (red line).

The comparison shows that there is a spectral feature centered at 3280 nm that cannot be reproduced by just CH4 non-LTE emission

Comparison between observed and calculated spectraComparison between observed and calculated spectra

Observed spectra, LTE simulations, non-LTE simulationsObserved spectra, LTE simulations, non-LTE simulations

Spectral observations collected along the vertical to the surface at 60°SW latitude (shown as an example). On the plot the Solar Zenith Angle (SZA in deg) and the corresponding tangent altitude (TH) in km are reported using the same color of the spectrum. The CH4 emissions are clearly visible in the spectra.

CH4

R

P

Q

R

Q

P

Non-LTE simulations for the same wavelength range and tangent altitudes of the observations. Vibrational temperatures for CH4 energy levels have been taken from the non-LTE model described below. CH4 contributions in the four most important bands (v3 fundamental, and v4+v3, v2+v3 and v2+v4+v3 hot bands have been included.

Local Thermal Equilibrium (LTE) simulations for the same wavelength range and tangent altitudes of the observations. Only CH4 line parameters have been used. Note that the shape of the CH4 spectral signature is very different than in the observations. The radiance scale is about 4 order of magnitudes lower than the one used to plot the observations.

Simulated LTE integrated intensities for the P, Q and R branches of CH4 as a function of tangent altitude.

Simulated non-LTE integrated intensities for the P, Q and R branches of CH4 as a function of tangent altitude

Measured integrated intensities for the P, Q and R branches of CH4 as function of tangent altitude.

The comparison between the observed integrated intensity for the three branches of methane with LTE simulations shows huge differences in the relative behaviour of all branches.The comparison between the observed integrated intensity for the three branches of methane with Non-LTE simulations shows that P and Q branches are in agreement with observations while the R branch has a different behaviour

Band profiles simulationBand profiles simulation

ABSTRACT: Earliest Cassini VIMS limb observations at Titan taken in October 26th, 2004 show a strong methane non-LTE limb emission at high atmospheric altitudes. During that pass at Titan, VIMS vertical resolution was about 110 km and the analyzed spectral interval corresponds to the methane emission band centered around 3.33 micron. A detailed analysis of the radiances versus altitudes shows an anomalous emission above around 900 km at wavelengths close to the methane R branch around 3.28 micron. The nature of such emission is under investigation. Different spectral databases and codes have been used for calculating the expected CH4 non-LTE limb emissions. The “anomalous” emission could not be reproduced using all the known CH4 bands. Its spectral position hints at a molecule containing C-H or C-N bonds. Different molecules and ions have been tested unsuccessfully. We propose that, given its spectral features and modeled abundances in the upper atmosphere, benzene (C6H6), the phenyl radical (C6H5) or aromatic species are likely candidate for such emission. We choose the observation on Oct. 26th, 2004 for its very good signal to noise ratio and because of the favorable illumination (low phase angle) of the atmosphere as seen from the Cassini spacecraft. Other observations also exhibit the same feature.

CONCLUSIONSThere are differences between the measured and expected relative behavior of the integrated intensities in the 3 branches of the methane emission. The biggest difference is due to the intensity of the R branch at high altitudes. If this difference was due to rotational NLTE effects, the relative intensity of the 3 branches should not be affected. The possibility that one particular vibration of methane could be affected by anomalous NLTE effects has been explored, but no measured hot bands could explain the observed behavior.

REFERENCESREFERENCES

Friderichsen et al., J. Am. Chem. Soc., 2001

Vuitton et al, JGR, 113, 2008

Coustenis et al., Icarus, 189, 2007

Funke et al., International Radiation Symposium, Korea, 2004

Stiller et al., JQSRT,72, 249, 2000

Yelle, ESA Spec. Publ. 1177, 243–256.

Wavelength (m)

C4H6 (1,3-butadiene)

Integrated intensities & R-branch anomalies over 500 kmIntegrated intensities & R-branch anomalies over 500 km

Left panel: Spectrally integrated intensity profiles for the selected path and a 70° solar zenith angle. The anomalous behavior of the CH4 R-branch integrated intensity (cyan) has been evaluated by subtracting to the observed R-branch the calculated R-branch contribution, estimated by P-branch/1.3.

Right panel: R-branches-anomaly profiles for different illumination angles

Experimental Infrared Absorption Frequencies (), Polarizations, and Absolute Intensities for C6H5 (© JCR, 2001)

Candidate Candidate moleculesmoleculesC6H5 (Phenyl radical)

C6H6, C6H5

In the upper figure modeled neutral vertical profiles and comparison with observations of C6H6 and aromatic species, are reported. The observed C6H6 mole fraction in the stratosphere was retrieved by CIRS at 15 °S [Coustenis et al., 2007]. Data points represent averages over 12 Titan passes of the C6Hx density measured by INMS. Error bars include statistic uncertainties but not calibration uncertainties (© Icarus, 2008)

C6H5

=> ~ 3240 nm

Before performing the 3.3-micron radiance simulations, we have calculated the vibrational temperatures of 13 CH4 vibrational levels from the ground to 1500 km. For that, we have used:

• The GRANADA non-LTE model (Funke et al., 2004), including:

•Radiative processes: absorption of solar radiation, spontaneous emission, radiation exchange among atmospheric layers

•Collisional processes: CH4-N2 and CH4-CH4 VT and VV collisions

•Radiative transfer calculations with the KOPRA model (Stiller et al., 2000)

• The HITRAN spectroscopic database

•The vibrational temperatures have been calculated for a kinetic temperature profile composed by the HASI measurements and the model profile from Yelle (Yelle, 1997)

CH4 vibrational temperatures (Tv) for the v3, v4+v3, and v2+v3, levels responsible for the CH4 3.3 µm emission, for SZA=60º. The v3 Tv is completely dominated by absorption of solar radiation above 800 km. Below, collisions start to be important and completely determine the behavior below 400 km. The v2 and v4 levels vibrational temperatures are also shown. The solid black line is the kinetic temperature.

CH4 v3 vibrational temperatures for SZAs from 40º to 150º (in 10º step up to 80 and 5º up to 140º). As the SZA increases, the Tv decreases. It is noticeable the large variability of Tv with SZA. The discontinuities at SZA>120º show the limit where Titan’s atmosphere is illuminated. Complete night in the altitude range shown occurs for SZA>145º whereas complete day applies to SZA<115º.

The Non-LTE vibrational temperaturesThe Non-LTE vibrational temperatures