Influence of methane concentration on the optical indices of Titan’s aerosols analogues

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2012-11, Vol.221 (2), p.670-677
• Main Authors: Mahjoub, A., Carrasco, N., Dahoo, P.-R., Gautier, T., Szopa, C., Cernogora, G.
• Format: Article
• Language: English
• Subjects: Aerosols; Aliphatic compounds; Astronomy; Astrophysics; Constants; Earth and Planetary Astrophysics; Earth, ocean, space; Exact sciences and technology; Methane; Photometry; Physics; Radiative transfer; Saturn satellites; Sciences of the Universe; Signatures; Solar system; Titan; Titan, atmosphere; Wavelengths
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2012.08.015

► Real and imaginary refractive indices of Titan aerosols analogues were determined. ► We study the influence of methane concentration on optical indices. ► High absorption is observed for tholins produced at low methane concentration. ► Mid-IR spectra are reported at different methane concentration. ► Our results are compared to other studies and Titan’s observations. This work deals with the optical constant characterization of Titan aerosol analogues or “tholins” produced with the PAMPRE experimental setup and deposited as thin films onto a silicon substrate. Tholins were produced in different N2–CH4 gaseous mixtures to study the effect of the initial methane concentration on their optical constants. The real (n) and imaginary (k) parts of the complex refractive index were determined using the spectroscopic ellipsometry technique in the 370–1000nm wavelength range. We found that optical constants depend strongly on the methane concentrations of the gas phase in which tholins are produced: imaginary optical index (k) decreases with initial CH4 concentration from 2.3×10−2 down to 2.7×10−3 at 1000nm wavelength, while the real optical index (n) increases from 1.48 up to 1.58 at 1000nm wavelength. The larger absorption in the visible range of tholins produced at lower methane percentage is explained by an increase of the secondary and primary amines signature in the mid-IR absorption. Comparison with results of other tholins and data from Titan observations are presented. We found an agreement between our values obtained with 10% methane concentration, and Imanaka et al. (Imanaka, H., Khare, B.N., Elsila, J.E., Bakes, E.L.O., McKay, C.P., Cruikshank, D.P., Sugita, S., Matsui, T., Zare, R.N. [2004]. Icarus, 168, 344–366) values, in spite of the difference in the analytical method. This confirms a reliability of the optical properties of tholins prepared with various setups but with similar plasma conditions. Our comparison with Titan’s observations also raises a possible inconsistency between the mid-IR aerosol signature by VIMS and CIRS Cassini instruments and the visible Huygens-DISR derived data. The mid-IR VIMS and CIRS signatures are in agreement with an aerosol dominated by an aliphatic carbon content, whereas the important visible absorption derived from the DISR measurement seems to be incompatible with such an important aliphatic content, but more compatible with an amine-rich aerosol.