Cassini VIMS and Altimeter Joint Study of Titan Surface

The joint NASA-ESA-ASI Cassini- Huygens mission reached the saturnian system on July 1st 2004. It started the observations of Saturn's environment including its atmosphere, rings, and satellites (Phoebe, Iapetus and Titan). Titan, one of the primary scientific interests of the mission, is veiled by an ubiquitous thick haze [1]. Its surface is unreachable to ultraviolet and visible wavelengths, but can be seen in some infrared atmospheric windows and for greater wavelengths, in the case of an unclouded low atmosphere [2,3]. Onboard the Cassini spacecraft, the VIMS (Visual and Infrared Mapping Spectrometer) instrument has already proved to be able to successfully pierce the veil of the hazy moon and image its surface in the infrared wavelengths, taking hyperspectral images in the range 0.4 to 5.2 ?m. Since July 2004, VIMS acquired image cubes with spatial resolution ranging from a few tens of kilometers down to less than one kilometer per pixel, demonstrating its capability for mapping more than 70% of Titan's surface and studying its composition and geology [4,5,6,7,8,9,10,11]. Also in the Cassini orbiter payload is the Ku-band RADAR experiment that can operate in altimeter mode. Exclusively dedicated to Titan's observations, this second active mode has been designed primarily to retrieve Titan's surface elevation and study its topography. We present here the comparative analysis of the altimeter track recorded during the first Titan flyby (26 October 2004, tagged TA) and VIMS images over the same regions acquired during the 13th flyby (30 April 2006). In particular, we present here the first nontopographic analysis of Cassini altimeter data along with a tentative correlation with VIMS observations

Cassini VIMS and Altimeter Joint Study of Titan Surface

The joint NASA-ESA-ASI Cassini- Huygens mission reached the saturnian system on July 1st 2004. It started the observations of Saturn's environment including its atmosphere, rings, and satellites (Phoebe, Iapetus and Titan). Titan, one of the primary scientific interests of the mission, is veiled by an ubiquitous thick haze [1]. Its surface is unreachable to ultraviolet and visible wavelengths, but can be seen in some infrared atmospheric windows and for greater wavelengths, in the case of an unclouded low atmosphere [2,3]. Onboard the Cassini spacecraft, the VIMS (Visual and Infrared Mapping Spectrometer) instrument has already proved to be able to successfully pierce the veil of the hazy moon and image its surface in the infrared wavelengths, taking hyperspectral images in the range 0.4 to 5.2 ?m. Since July 2004, VIMS acquired image cubes with spatial resolution ranging from a few tens of kilometers down to less than one kilometer per pixel, demonstrating its capability for mapping more than 70% of Titan's surface and studying its composition and geology [4,5,6,7,8,9,10,11]. Also in the Cassini orbiter payload is the Ku-band RADAR experiment that can operate in altimeter mode. Exclusively dedicated to Titan's observations, this second active mode has been designed primarily to retrieve Titan's surface elevation and study its topography. We present here the comparative analysis of the altimeter track recorded during the first Titan flyby (26 October 2004, tagged TA) and VIMS images over the same regions acquired during the 13th flyby (30 April 2006). In particular, we present here the first nontopographic analysis of Cassini altimeter data along with a tentative correlation with VIMS observations

Monitoring of water and carbon fluxes using a land data assimilation system: a case study for southwestern France

International audienceA Land Data Assimilation System (LDAS) able to ingest surface soil moisture (SSM) and Leaf Area Index (LAI) observations is tested at local scale to increase prediction accuracy for water and carbon fluxes. The ISBAA-gs Land Surface Model (LSM) is used together with LAI and the soil water content observations of a grassland at the SMOSREX experimental site in southwestern France for a seven-year period (2001-2007). Three configurations corresponding to contrasted model errors are considered: (1) best case (BC) simulation with locally observed atmospheric variables and model parameters, and locally observed SSM and LAI used in the assimilation, (2) same as (1) but with the precipitation forcing set to zero, (3) real case (RC)simulation with atmospheric variables and model parameters derived from regional atmospheric analyses and from climatological soil and vegetation properties, respectively. In configuration (3) two SSM time series are considered: the observed SSM using Thetaprobes, and SSM derived from the LEWIS L-band radiometer located 15m above the ground. Performance of the LDAS is examined in the three configurations described above with either one variable (either SSM or LAI) or two variables (both SSM and LAI) assimilated. The joint assimilation of SSM and LAI has a positive impact on the carbon, water, and heat fluxes. It represents a greater impact than assimilating one variable (either LAI or SSM). Moreover, the LDAS is able to counterbalance large errors in the precipitation forcing given as input to the model

Modeling SAR Backscattering of Bright Flows and Dark Spots on Titan

We present first analysis of radar-bright and radar-dark features on Titan obtained from the Cassini Radar Ta flyby, based on the use of two-layer SAR backscattering model

Effect of multiple scattering on the phase signature of wet subsurface structures: applications to polarimetric L- and C-band SAR

We propose a two-layer integral equation model (IEM) model including multiple-scattering terms to reproduce the phase signature of buried wet structures that we observed on L-band synthetic aperture radar (SAR) images. We have good agreement between the extended (single+multiple scattering) IEM model and previous results obtained using a single-scattering IEM model combined with finite-difference time-domain simulations. We show that the multiple scattering not only significantly influences the copolarized phase difference but can also be related to the soil moisture content. In order to assess the validity of our extended model, we performed radar measurements on a natural outdoor site and showed that they could be fairly well fitted to the extended model. A parametric analysis presents the dependence of the copolarized phase difference on roughness parameters (rms height and correlation length) and radar parameters (frequency and incidence angle). Our study also shows that the phase signature should allow detection of buried wet structures down to a larger depth for C-band (3.8 m) than for L-band (2.6 m). This signature could then be used to map subsurface moisture in arid regions using polarimetric SAR systems

Models of SAR Backscattering for Bright Flows and Dark Spots on Titan

The SAR imaging mode of the Cassini RADAR instrument enables us to map the surface of Titan through its thick atmosphere. The first Cassini close flyby Ta, acquired on 26 October 2004, revealed a complex surface, with areas of low relief and dome-like volcanic constructs, flows and sinuous channels. In particular, fan-like features with strong radar backscattering were observed. Such structures, extending from tens of kilometers to more than 200 km in length, could be the result of cryovolcanism. Several radar-dark spots, up to 30 km across, were also observed: they may correspond to smooth hydrocarbon deposits. We present here a first modeling of these radar-bright and radar-dark features, based on classical radar backscattering models. We considered two main materials which could constitute the surface of Titan, tholins and water-ammonia ice, and modeled both single and two- layer cases, taking into account volume and sub-surface scattering. Our results show that SAR-bright regions could better be explained by the effect of a thin layer of water-ammonia ice covering a tholin substratum. Radar-dark spots can be modeled two-ways: a rough tholin surface or a smooth one with some volume scattering. We show that multi-incidence SAR data could help discriminate between the various scenarios proposed

Cassini VIMS and RADAR Altimeter Joint Study of Titan Surface

Correlations between Cassini/Altimeter data and VIMS underlying images of Titan's surface suggest the presence of very local enrichments in water ice linked with smooth depressions, maybe hinting an ancient channel connected to a large basin

Cassini VIMS and Altimeter joint study of Titan surface

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Comparison between VIMS and radar data on Sinlap crater on Titan

Only few impact craters have been observed so far on Titan by the CASSINI imaging instruments. A 80 km diameter crater named Sinlap has been observed by the ISS, radar and VIMS instruments, with a resolution up to 14 km/pixel for VIMS. Observations at infrared and radar wavelengths provide complementary information on the composition, topography and surface roughness of the surface materials. Despite scattering by haze particles and strong absorption of light by methane contained in the atmosphere, there are several infrared windows that allow the observations of the surface of Titan with VIMS, enabling first order analysis of surface heterogeneities thanks to the use of band ratios. Several distinct units appear in these band ratios, with a possible enrichment in water ice around the main bright ejecta blanket. We report here on the cross comparison between VIMS band ratios and the radar T3 observations, in order to investigate the compositional and physical characteristics of the materials excavated by the impact