We investigate Titan's low-latitude and midlatitude surface using spectro-imaging near-infrared data from Cassini/Visual and Infrared Mapping Spectrometer. We use a radiative transfer code to first evaluate atmospheric contributions and then extract the haze and the surface albedo values of major geomorphological units identified in Cassini Synthetic Aperture Radar data, which exhibit quite similar spectral response to the Visual and Infrared Mapping Spectrometer data. We have identified three main categories of albedo values and spectral shapes, indicating significant differences in the composition among the various areas. We compare with linear mixtures of three components (water ice, tholin-like, and a dark material) at different grain sizes. Our fits of the data are overall successful, except in some cases at 0.94, 2.03, and 2.79 μm, indicative of the limitations of our simplistic compositional model and the need for additional components to reproduce Titan's complex surface. Our results show a latitudinal dependence of Titan's surface composition, with water ice being the major constituent at latitudes beyond 30°N and 30°S, while Titan's equatorial region appears to be dominated partly by a tholin-like or by a very dark unknown material. The albedo differences and similarities among the various geomorphological units give insights on the geological processes affecting Titan's surface and, by implication, its interior. We discuss our results in terms of origin and evolution theories.

Saturn’s moon Titan has a dense nitrogen-rich atmosphere, with methane as its primary volatile. Titan’s atmosphere experiences an active chemistry that produces a haze of organic aerosols that settle to the surface and a dynamic climate in which hydrocarbons are cycled between clouds, rain and seas. Titan displays particularly energetic meteorology at equinox in equatorial regions, including sporadic and large methane storms. In 2009 and 2010, near Titan’s northern spring equinox, the Cassini spacecraft observed three distinctive and short-lived spectral brightenings close to the equator. Here, we show from analyses of Cassini spectral data, radiative transfer modelling and atmospheric simulations that the brightenings originate in the atmosphere and are consistent with formation from dust storms composed of micrometre-sized solid organic particles mobilized from underlying dune fields. Although the Huygens lander found evidence that dust can be kicked up locally from Titan’s surface, our findings suggest that dust can be suspended in Titan’s atmosphere at much larger spatial scale. Mobilization of dust and injection into the atmosphere would require dry conditions and unusually strong near-surface winds (about five times more than estimated ambient winds). Such strong winds are expected to occur in downbursts during rare equinoctial methane storms—consistent with the timing of the observed brightenings. Our findings imply that Titan—like Earth and Mars—has an active dust cycle, which suggests that Titan’s dune fields are actively evolving by aeolian processes.

In 13 years, infrared observations from the Visual and Infrared Mapping Spectrometer onboard Cassini provided significant hints about the spectral and geological diversity of Titan's surface. The analysis of the infrared (IR) signature of spectral units enables constraining the surface composition, which is crucial for understanding possible interactions between Titan's interior, surface, and atmosphere. Here we investigate a selection of areas in the equatorial regions, imaged by Cassini's instruments, which exhibit an apparent transition from the Visual and Infrared Mapping Spectrometer IR‐bright to the IR‐blue and IR‐brown units (from false‐color composites using red: 1.57/1.27 μm, green: 2.01/1.27 μm, and blue: 1.27/1.08 μm). By applying an updated radiative transfer model, we extract the surface albedo of IR units identified in these regions. Then, we compare them with synthetic mixtures of two expected components on Titan's surface, namely, water ice and laboratory tholins. This allows us to reconnect the derived composition and grain size information to the geomorphology observed from Radio Detection and Ranging instrument (RADAR)/Synthetic Aperture Radar images. We interpret IR‐bright units as hills and plains coated by organic material and incised by fluvial networks. Erosion products are transported downstream to areas where IR‐blue units are seen near the IR‐bright units. These units, enriched in water ice, are most likely outwash plains hosting debris from fluvial erosion. Farther away from the IR‐bright units, the IR‐brown units are dominantly made of organics with varied grain sizes, ranging from dust‐ to sand‐sized particles that form the dune fields. The transition areas therefore exhibit trends in water ice content and grain size supported by geomorphological observations.

In thirteen years, infrared observations from the Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini provided significant hints about the spectral and geological diversity of Titan’s surface. The analysis of the infrared signature of spectral units enables constraining the surface composition, which is essential to understand possible interactions between Titan’s interior, surface and atmosphere. Here, we investigate a selection of areas in Titan's low-latitudes imaged by Cassini's VIMS IR spectrometer, which exhibit an apparent transition from the VIMS IR-bright to the IR-blue and IR-brown spectral units (from false-color composites using red: 1.57/1.27 µm, green: 2.01/1.27 µm, and blue: 1.27/1.08 µm). By applying an updated radiative transfer model [1-3], we extract the surface albedo of IR-units identified in these regions. Then, we compare them with synthetic spectra of mixtures of the two most expected components of Titan’s surface, namely water ice and laboratory tholins. This allows us to reconnect the derived composition and grain size information to the geomorphology observed from Cassini's RADAR/SAR images. Hence, we interpret IR-bright terrains as hills and plains coated by organic material and incised by fluvial networks. The erosion products are transported downstream to areas where IR-blue terrains are seen near the IR-bright terrains. These areas, enriched in water ice, are most likely outwash plains hosting icy and organic debris from fluvial erosion. Farther away from the IR-bright terrains, the IR-brown terrains are dominantly made of organics with varied grain sizes ranging from dust- to sand-sized particles that form the dunes fields. In this work, we show that transition areas exhibit trends in terms of water ice content and grain size supported by geomorphological observations [4]. References: [1] Hirtzig, M. et al. (2013) Icarus, 226. [2] Solomonidou, A. et al. (2014) JGR, 119. [3] Maltagliati, L. et al. (2015) EPSC. [4] Brossier, J. F. et al. (2018) JGR.

We infer surface properties, such as surface albedo and atmospheric contributions in the form of haze content, of the mid-latitude region of Titan. In previous studies [1;2] we reported results on two areas presenting indications for possible changes in surface albedo with time [2]. We also investigate the endogenic or exogenic processes linked to the formation of the various mid-latitude geomorphological units. These could be aeolian, fluvial, sedimentary, cryovolcanic, lacustrine, and more. Furthermore, deposition of organics through the atmosphere seems to be predominantly present [1]. We now focus on constraining the chemical composition of the various geomorphological units [5;6] by investigating the lower atmosphere of Titan from Visual and Infrared Mapping Spectrometer (VIMS) spectro-imaging data by use of a recently updated radiative transfer code in the near-IR range. For the distinction of geomorphological units we use RADAR/SAR data [4]. We study the units of interest identified in [1;3] and [4]: mountains, plains, labyrinths, dune fields, and possible cryovolcanic and/or evaporitic features (the latter two are albedo features, [4;5]). Our findings indicate that many of the regions from the same geomorphological unit show compositional variations depending on location, while units of significant geomorphological differences seem to consist of very similar material mixtures. Preliminary results on the chemical composition of the regions that have shown temporal changes (i.e. Tui Regio and Sotra Patera; [6]) are also presented. The albedo differences and similarities among the various geomorphological terrains set constraints on the possible geological processes that govern Titan's surface. References: [1] Lopes, R.M.C., et al.: Icarus, 270, 162-182, 2016; [2] Solomonidou, A., et al.: Icarus, 270, 85-99, 2016; [3] Lopes, R.M.C., et al.: Icarus, 205, 540-558, 2010; [4] Malaska, M., et al.: Icarus, 270, 130-161, 2016; [4] Barnes, J., et al.: Pl. Scie., 2:1, 2013; [5] Solomonidou, A., et al.: JGR, 119, 1729-1747, 2014; [6] Schmitt, B., et al.: GhoSST database (ghosst.osug.fr).

Charnay, J. F. Kok, R. D. Lorenz, J. Radebaugh, C. Narteau, T . Cornet, O. Bourgeois, A. Lucas, P. Rannou, C. A. Griffith, A. Coustenis, T. Appéré, M. Hirtzig, C. Sotin, J. M. Soderblom, R. H. Brown, J. Bow, G. Vixie, L. Maltagliati, S. Courrech du Pont, R. Jaumann, K. Stephan, K. H. Baines, B. J. Buratti, R. N. Clark, P. D. Nicholson. Laboratoire AIM, Université Paris Diderot – Paris 7/CNRS/CEA-Saclay, DSM/IRFU/SAp, 91191 Gif sur Yvette, France. Laboratoire de Planétologie et Géodynamique (LPGNantes), CNRS-UMR 6112, Université de Nantes, 44322 Nantes, France. University of Idaho, Department of Physics, 875 Perimeter Drive MS0903, Moscow, ID 83844-0903, USA. Virtual Planetary Laboratory, University of Washington, Box 351580, Seattle, Washington 98195, USA. Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA. Department of Geological Sciences, Brigham Young University, Provo, UT 84602, USA. Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris, Cedex 05, France. European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villanueva de la Canada, Spain. Groupe de Spectroscopie Moléculaire et Atmosphérique, UMR CNRS 6089, Université de Reims, U.F.R. Sciences Exactes et Naturelles, Moulin de la Housse B.P. 1039, 51687 Reims Cedex 2, France. Department of Planetary Sciences, University of Arizona, Lunar and Planetary Laboratory, 1629 E. University Blvd., Tucson, AZ 85721, USA. LESIA, Observatoire de Paris, PSLResearch Univ., CNRS, Univ. Pierre et Marie Curie Paris 06, Sorbonne Univ., Univ. Paris-Diderot, Sorbonne Paris-Cité, 5, place Jules Janssen, 92195 Meudon Cedex, France. Institut de Planétologie et d'Astrophysique de Grenoble, Université J. Fourier, CNRS/INSU, Grenoble, France. California Institute of Technology/Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. Laboratoire Matière et Systèmes Complexes, Université Paris Diderot, Paris, France. German Aerospace Centre (DLR), Institute of Planetary Research, 12489 Berlin, Germany. Space Science and Engineering Center, University of Wisconsin, Madison, WI 53706, USA. United States Geological Survey, Denver, Colorado USA. Department of Astronomy, Cornell University, Ithaca, New York USA.

We investigate the surface of Titan using spectro-imaging near-infrared data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS). We apply a radiative transfer code to first determine the contributions of atmospheric haze to the Titan spectrum and then derive the surface albedo (Solomonidou et al. 2014; 2016). We focus here on the geological major units identified in Lopes et al. (2010, 2016), Malaska et al. (2016) and Radebaugh et al. (2016) from Synthetic Aperture Radar (SAR), data including mountains, different types of plains, labyrinths, impact craters, dune fields, and alluvial fans. We find that all regions classified as being the same geomorphological unit in SAR exhibit a coherent spectral response after the VIMS data analysis, thus suggesting a good correlation in the classification between SAR and VIMS. The Huygens landing site appears to be compositionally similar to one type of plains unit (variable plains), suggesting similar plain formation mechanisms. We have sub-categorized the VIMS data into three albedo categories (high, medium, low). By matching the extracted albedos with candidate materials for Titan’s surface (GhoSST database), we find that all regions of interest fall into one out of three main types of major candidate constituents: water ice, tholin-like material, or an unknown, very dark material. This suggests that Titan’s surface is possibly dominated by tholin-like material and a very dark unknown (most likely organic) material, and that most of the surface is covered by atmospheric/organic deposits. Water ice is also present at a number of regions as major constituent at latitudes higher than 30N and 30S. The surface albedo differences and similarities among the various geomorphological units constrain the implications for the geological processes that govern Titan’s surface and interior (e.g. aeolian, fluvial, sedimentary, lacustrine, cryovolcanic, tectonic).References: Lopes et al.: Icarus, 205, 540-558, 2010; Lopes et al.: Icarus, 270, 162-182, 2016; Malaska et al.: Icarus, 270, 130-161, 2016; [4] Solomonidou et al.: JGR, 119, 1729-1747, 2014; [6] Solomonidou et al.: Icarus, 270, 85-99, 2016; [7] Schmitt et al.: GhoSST database (ghosst.osug.fr).

surroundings (16.1 ̊W, 11.3 ̊N), and (3) Menrva crater with Elivagar Flumina (87.2 ̊W, 20.1 ̊N). In this work we used VIMS observations targeting each region After twelve years of exploration, near-infrared imaging data provided from the Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini [1] reveal a variety of surface units that are compositionally and/or structurally distinct [2,3]. The analysis of the infrared signature of these units enables constraining the surface composition of Titan, which is of prime importance for modelling Titan’s interior, surface, and atmosphere, particularly in the search for an endogenic methane source. For this study, we investigate a selection of regions of interest seen in VIMS data by applying methods of correction for the atmospheric contributions.

Since twelve years of exploration, near-infrared imaging data provided from the Visible and Infrared Mapping Spectrometer (VIMS) onboard Cassini reveal a variety of surface units that are compositionally and/or structurally distinct. The analysis of these units enables constraining the surface composition of Titan, which is of prime importance for modelling Titan's interior, surface, and atmosphere, particularly in the search for an endogenic hydrocarbons source. For this study, we investigate a selection of regions of interest seen in VIMS data by comparatively applying methods of correction for the atmospheric contributions.

Since eleven years of observation, near-infrared imaging data from Visual and Infrared Mapping Spectrometer (VIMS) onboard Cassini reveal a variety of surface units that are compositionally and structurally distinct. The analysis of these units enables constraining the surface composition of Titan, which is of prime importance for modeling Titan’s interior, surface, and atmosphere, particularly in the search for an endogenic methane source. For this study, we investigate a selection of units of interest seen in VIMS data by comparatively applying an empirical correction and a radiative transfer code in order to correct for atmospheric contributions and retrieve surface albedo. SAR swaths from the Radar instrument were also used for geomorphological mapping purposes. We focus on the region of Aaru, located in the equatorial belt and centered at 10˚N and 340˚W, where several geological features are have been identified: (1) infrared-brown dunes material; (2) a strongly eroded impact crater named Paxsi; (3) mountainous and infrared-bright terrains; and (4) infrared-blue areas devoid of dunes (similar to those seen in other regions probably enriched in water-ice, such as Chusuk Planitia). By using our radiative transfer model, we estimate the surface albedo of regions of our interest within different infrared units of the Aaru region and compare with spectra of surface candidates, starting with water-ice and tholins. As seen in the similar study over Sinlap crater and its surroundings, this method of analysis allows understanding of the compositional and structural relations between the different spectral units.

We analyze Cassini VIMS data of several areas on Titan's surface looking for variations with time. Three of these locations are near the equator (10-30 degrees S), namely Hotei Regio, Tui Regio and Sotra Patera; in some cases changes in brightness and/or in appearance were reported therein. We also investigate a portion of the undifferentiated plains, areas relatively homogeneous and dark in radar observations, located near 20-25 degrees N. This is a follow-up on a previous paper in which we had inferred surface albedos for some distinct regions of interest (Rols) identified within the Hotei, Tui and Sotra areas through a Principal Component Analysis (PCA) and radiative transfer (RT) modeling (Solomonidou [2014]. J. Geophys. Res. 119, 1729-1747). We apply the same methods here to a larger dataset looking for variations of the surface albedo with time and using the Huygens landing site as the 'ground truth' for calibration purposes. As expected, the undifferentiated plains remain unchanged from January 2010 to June 2012. Our analysis of Hotei Regio data from March 2005 to March 2009 also does not show any significant surface albedo variations within uncertainties. We note however that our RT retrievals are not optimal in this case because of the use of a plane-parallel code and the unfavorable geometry of the associated datasets. Conversely, Tui Regio and Sotra Patera show surface albedo fluctuations with time with pronounced trends for darkening and for brightening respectively. The Tui Regio spectrum exhibits a surface albedo decrease from March 2005 to February 2009, at 0.94, 1.08, 2.03, and 5 mu m wavelengths, while the spectrum shape remains the same over that time. On the contrary, the Sotra Patera area became at least two times brighter within a year (April 2005-February 2006), at 1.58 mu m, 2.03 mu m, and 5 mu m. We also retrieved surface albedo spectra for three reference regions surrounding Hotei, Tui and Sotra and for three additional regions we use as 'test cases' that correspond to dune fields. During the time periods explored here we find that, as expected and contrary to Tui Regio and Sotra Patera, the test cases did not show any significant changes in surface albedo. We therefore suggest that temporal variations of surface albedo exist for some areas on Titan, but that their origin may differ from one region to the other. They could be due to diverse, past and/or ongoing formation processes (endogenic and/or exogenic, possibly cryovolcanic), as discussed here. (C) 2015 Elsevier Inc. All rights reserved.

We use the JPL/Caltech 1-D photochemical model to solve continuity diffusion equation for atmospheric constituent abundances and total number density as a function of radial distance from the planet Venus. Photochemistry of the Venus atmosphere from 58 to 112km is modeled using an updated and expanded chemical scheme (Zhang et al., 2010, Zhang et al., 2012), guided by the results of recent observations and we mainly follow these references in our choice of boundary conditions for 40 species. We model water between 10 and 35ppm at our 58km lower boundary using an SO2 mixing ratio of 25ppm as our nominal reference value. We then vary the SO2 mixing ratio at the lower boundary between 5 and 75ppm holding water mixing ratio of 18ppm at the lower boundary and finding that it can control the water distribution at higher altitudes. SO2 and H2O can regulate each other via formation of H2SO4. In regions of high mixing ratios of SO2 there exists a “runaway effect” such that SO2 gets oxidized to SO3, which quickly soaks up H2O causing a major depletion of water between 70 and 100km. Eddy diffusion sensitivity studies performed characterizing variability due to mixing that show less of an effect than varying the lower boundary mixing ratio value. However, calculations using our nominal eddy diffusion profile multiplied and divided by a factor of four can give an order of magnitude maximum difference in the SO2 mixing ratio and a factor of a few difference in the H2O mixing ratio when compared with the respective nominal mixing ratio for these two species. In addition to explaining some of the observed variability in SO2 and H2O on Venus, our work also sheds light on the observations of dark and bright contrasts at the Venus cloud tops observed in an ultraviolet spectrum. Our calculations produce results in agreement with the SOIR Venus Express results of 1ppm at 70–90km (Bertaux et al., 2007) by using an SO2 mixing ratio of 25ppm SO2 and 18ppm water as our nominal reference values. Timescales for a chemical bifurcation causing a collapse of water concentrations above the cloud tops (>64km) are relatively short and on the order of a less than a few months, decreasing with altitude to less than a few days.

We present a study focused on the mid-latitude and close to the equator surface regions on Titan that present an interest on their spectral behavior and/or morphology. These are regions where spectroscopic anomalies have been reported in the evolution of the brightness and several interpretations have been proposed (cryovolcanic candidates, evaporates, lacustrine, etc [1;2;5]). Also in our work here we have included analysis of some undifferentiated plains (also referred to as ‘blandlands’), which are vast expanses of terrains that appear bland in the radar data [3]. By applying a Radiative transfer code [4;2] we have analyzed these regions to look for evolution with time through their spectral behavior. We use as reference point and calibration tool the surface albedo retrieval of the Huygens Landing site (Titan’s ground truth) and we also check the variability of the surface albedo of these regions against areas that are not expected to change with time (e.g. dune fields), by retrieving their albedo differences at all wavelengths [2]. We report here surface albedo changes with time for some of these regions of interest that imply connection to exogenic and/or endogenic processes.