Potentially active regions on Titan: New processing of Cassini/VIMS data

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) obtained data of Titan's surface from flybys performed during the last seven years. In the 0.8-5.2 µm range, these spectro-imaging data showed that the surface consists of a multivariable geological terrain hosting complex geological processes. The data from the seven narrow methane spectral "windows" centered at 0.93, 1.08, 1.27, 1.59, 2.03, 2.8 and 5 µm provide some information on the lower atmospheric context and the surface parameters that we want to determine. Atmospheric scattering and absorption need to be clearly evaluated before we can extract the surface properties. We apply here a statistical method [1, 2] and a radiative transfer method [3, 1] on three potentially "active" regions on Titan, i.e. regions possibly subject to change over time (in brightness and/or in color etc) [4]: Tui Regio (20°S, 130°W) [5], a 1,500-km long flow-like figure, Hotei Regio (26°S, 78°W) [6], a 700-km wide volcanic-like terrain, and Sotra Facula (15°S, 42°W) [7], a 235-km in diameter area. With our method of Principal Component Analysis (PCA) we have managed to isolate specific regions of distinct and diverse chemical composition. We have tested this method on the previously studied Sinlap crater [8], delimitating compositional heterogeneous areas compatible with the published conclusions by Le Mouélic et al. (2008). Our follow-up method focuses on retrieving the surface albedo of the three areas and of the surrounding terrains with different spectral response by applying a radiative transfer (RT) code. We have used as input most of the Cassini HASI and DISR measurements, as well as new methane absorption coefficients [9], which are important to evaluate the atmospheric contribution and to allow us to better constrain the real surface alterations, by comparing the spectra of these regions. By superposing these results onto the PCA maps, we can correlate composition and morphology. As a test case, we used our RT code to verify the varying brightness of Hotei Regio reported by other investigators based on models lacking proper simulation of the atmospheric absorption [10]. Even though we have used exactly the same dataset, we did not detect any significant surface albedo variations over time; this led us to revise the definition of "active" regions: even if these regions have not visually changed over the course of the Cassini mission, the determination of the chemical composition and the correlation with the morphological structures [11] observed in these areas do not rule out that past and/or ongoing cryovolcanic processes are still a possible interpretation. [1] Solomonidou, A. et al. (2011). Potentially active regions on Titan: New processing of Cassini/VIMS data. In preparation. [2] Stephan, K. et al. (2008). Reduction of instrument-dependent noise in hyperspectral image data using the principal component analysis: Applications to Galileo NIMS data. Planetary and Space Science 56, 406-419. [3] Hirtzig, M. et al. (2011). Applications of a new methane linelist to Cassini/VIMS spectra of Titan in the 1.28-5.2 µm range . In preparation. [4] Wall, s. D. et al. (2009). Cassini RADAR images at Hotei Arcus and western Xanadu, Titan: Evidence for geologically recent cryovolcanic activity. Journal of Geophysical Research 36, L04203, [5] Barnes, J.W. et al. (2006). Cassini observations of flow-like features in western Tui Regio, Titan. Geophysical Research Letters 33, L16204. [6] Soderblom, L.A. et al. (2009). The geology of Hotei Regio, Titan: Correlation of Cassini VIMS and RADAR. Icarus 204, 610-618. [7] Lopes, R.M.C. et al. (2010). Distribution and interplay of geologic processes on Titan from Cassini radar data. Icarus 205, 540-558. [8] Le Mouélic et al. (2008). Mapping and interpretation of Sinlap crater on Titan using Cassini VIMS and RADAR data. Journal of Geophysical Research 113, E04003. [9] Campargue, A. et al. (2011). An empirical line list for methane at 80 K and 296 K in the 1.26-1.71 µm region for planetary investigations. Application to Titan. Icarus. Submitted. [10] Nelson, R. et al (2009). Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity. Icarus 199, 429-441. [11] Solomonidou, A. et al. (2011). Possible morphotectonic features on Titan and their origin. Planetary and Space Science. Submitted

Titan as an Exobiotical Environment

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Organic chemistry on planetary satellites around the gas giants and implications for habitability

International audienceThe icy satellites of the outer solar system present a variety of chemical compositions where initial complements of ices, minerals, and elements provided during formation have been subjected to various internal and external processes. Additional material has been gained via cometary and meteoritic infall, transfer of material between satellites and magnetospheric interactions. Exchanges also occur between the surface and the interior.Organic compounds have been detected on several of our Solar System's satellites around the gas giant planets, each with unique characteristics as to organic chemistry. Jupiter's Europa, Ganymede and Callisto show evidence of undersurface layers of liquid water that offer potentially interesting environments for organic synthesis. Spectra obtained by Galileo's NIMS show absorption bands indicative of C-H and C≡N organic compounds. Potential organics on the large Galilean moons include CO2, carbonic acid and different kinds of carbonates, hydrocarbons and nitriles.Saturn's Titan is a chemical factory, where the mother molecules N2 (at about 95%), CH4 (at 1.5-5%) and H2 (at 0.1%) produce a host of hydrocarbons and nitriles. The Cassini-Huygens mission has shown Titan to be indeed very rich in organic molecules, which are formed in the upper atmosphere and then deposited on the surface. Some of these species are of prebiotic importance, such as C6H6, HC3N and HCN. Titan's surface displays unique geomorphological features while it probably overlies an internal liquid water ocean. The organic deposits, if in contact with the underground liquid water could undergo an aqueous chemistry that could replicate aspects of life's origins. Enceladus, a smaller moon of Saturn, ejects large amounts of water and organics in the space from plumes located in its southern pole. The implied requirement for liquid water reservoirs under its surface, significantly broadens the diversity of solar system environments which could be habitable worlds.References: [1] Coustenis, A., Tokano, T., Burger, M. H., et al., 2010. Space Sci. Rev. 153, 155-184. [2] Coustenis and Encrenaz, 2013, CUP, ISBN: 9781107026179

Definition and archiving of ground-based observations in support of space missions

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Organic chemistry in planetary satellites of gas giants and implications for habitability

International audienceWe look at the icy moons in our outer solar system in which we find organics and the possibility for habitabile conditions therein

Formation and Evolution of Titan's Atmosphere

International audienceThe origin and evolution of Titan's enigmatic atmosphere is reviewed. Starting with the present-day volatile inventory, the question of what was the original composition on Titan and how a satellite of similar size to other Galilean moons managed to acquire and hold on to the required material is discussed. In particular the possible sources and sinks of the main mother molecules (nitrogen, methane and oxygen) are investigated in view of the most recent models and laboratory experiments. The answers expected to be provided by the instruments aboard the Cassini-Huygens mission to some of the most prominent current questions regarding Titan's atmosphere are defined

Catalogue of IR and Raman spectra of gas CH4 and other molecules' coefficients, organics, minerals and ices

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Titan's atmospheric composition: from Voyager to Cassini and beyond

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Laboratory and theoretical work applied to planetary atmospheres

International audienceWe look at applications of recent work in theoretical and experimental spectroscopy for the analysis of IR data concerning giant planets, Titan and possibly exoplanets

Titan's Atmosphere From Recent Space And Earth Observations : Last Call Before Cassini/Huygens

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