Domenico Casarano

Active shoreline of Ontario Lacus, Titan: A morphological study of the lake and its surroundings

Of more than 400 filled lakes now identified on Titan, the first and largest reported in the southern latitudes is Ontario Lacus, which is dark in both infrared and microwave. Here we describe recent observations including synthetic aperture radar (SAR) images by Cassinis radar instrument (λ = 2 cm) and show morphological evidence for active material transport and erosion. Ontario Lacus lies in a shallow depression, with greater relief on the southwestern shore and a gently sloping, possibly wave-generated beach to the northeast. The lake has a closed internal drainage system fed by Earth-like rivers, deltas and alluvial fans. Evidence for active shoreline processes, including the wave-modified lakefront and deltaic deposition, indicates that Ontario is a dynamic feature undergoing typical terrestrial forms of littoral modification.

Cassini Radar Data: Estimation of Titan's Lake Features by Means of a Bayesian Inversion Algorithm

The analysis derived from the Cassini SAR imagery reflects the complex Titans surface morphology with a wide range of backscattering coefficients and peculiar features such as periodic structures and lakelike features, which were observed on July 22, 2006, when polar areas were first imaged, and are considered good candidates to be filled with liquid hydrocarbons. In this paper, the modeling description of lakes is addressed by means of a double-layer model which considers an upper liquid-hydrocarbon layer and a lower layer compatible with the radar response of the neighboring areas. This model is introduced into a Bayesian framework for the purpose of inferring the likely ranges of some parameters and, in particular, of the optical thickness of the hypothesized liquid-hydrocarbon layer and of the wind speed. The main idea is to use the information contained in the parameter probability density function, which describes how probability is distributed among the different values of parameters according to the various scenarios considered. The analysis carried out on lakes and surrounding areas on flybys T16 and T19 determines optical thickness values from 0.2 to 6. For T25 flyby, the inferred values of optical thickness indicate that a limit value of optical thickness may be 9. Considering that, beyond these values, the signal from the bottom layer is completely attenuated, information on the wind speed on the upper layer can be inferred. The found mean values of wind speed are around 0.2-0.3 m/s according to different hypotheses on the upper layer dielectric constant.

Cassini Radio Detection and Ranging (RADAR): Earth and Venus observations

The Cassini Radio Detection and Ranging (RADAR) was operated in scatterometric and radiometric modes during the Venus 1 and Earth swingbys to verify its functionality. At Venus, only the thermal emission from the thick absorbing atmosphere was detected. At Earth both the radar echo and the microwave emission from the surface were detected and reveal ocean surface disturbances, the rough, high, and cold Andes mountains, and surface features including a small reservoir in Brazil. Instrument performance appears to be excellent.

Use of a C-Band Ground-Based Scatterometer to Monitor Surface Roughness and Soil Moisture Changes

Eight experiments on remote sensing of soil moisture and surface roughness were carried out over bare fields with a microwave C-band scatterometer from 1998 until 2001. This device is able to provide backscattering coefficients in the range of +10 dB and −40 dB for incidence angles between 10° and 60°. The objective is to assess the conditions (of roughness and incidence angle) in which it is possible to separate the effects of roughness and soil moisture, thus allowing a reliable estimation of soil moisture from backscattering coefficients. In particular, starting with measurements carried out at different incidence angles, we experimented with an approach to normalize the backscattering coefficients to a reference angle with the principal aim of comparing data sets acquired in different conditions and putting in evidence the radar response to soil moisture variations. A sensitivity analysis, performed over the whole acquired data set, confirmed that the dependence of backscattering coefficients on the incidence angle is influenced more by surface roughness than by soil moisture, as indicated also by theoretical models and other similar data sets. A significant result is that a simple model for comparing data acquired with different incidence angles works better for rough surfaces: only in this case an acceptable correlation between backscattering coefficients and soil moisture is retained. In order to better understand this behavior, the experiments carried out in 2001 were designed to acquire radar measurements on a test site where soil moisture was controlled with artificial irrigation and constantly monitored during the dry-down phase. This allowed a direct estimation of the relationship between radar responses and soil moisture, a quantitative evaluation of the sensitivity of our device and a test for the model developed using the previous acquisitions.

Retrieval of soil moisture profile by combined C-band scatterometer data and a surface hydrological model

The objective of this work is to develop a method to use radar scatterometer data and a hydrological model in order to retrieve soil behaviour at a level greater than C-band microwave penetration depth. For microwave measurements a C-band FM-CW scatterometer has been employed in two campaigns; the device is able to provide backscattering coefficients in the range of+10 dB and -40 dB for incidence angles between 10° and 60°. Subsequently, microwave scatterometer data have been analysed to estimate their sensitivity to the soil moisture patterns of topsoil comparing them with ground truth measurements. For the validation of these radar data, a coupled heat and moisture balance model has been run to predict the hydrological behaviour of the same topsoil starting from point ground truth measurements. In a second run, soil moisture values derived from scatterometer data should have been used for the initialisation of the model. First attempts have been carried out to propagate the surface physical parameters to unreachable soil layers, such as vertical soil moisture profiles.

Synergy of Cassini SAR and altimeter acquisitions for the retrieval of dune field characteristics on Titan

This work focuses on the retrieval of Titan’s dune field characteristics addressing different radar modes. The main purpose of the proposed work is to exploit a possible synergy between SAR and altimeter acquisitions modes to provide information about dune field. Cassini has performed 86 Titan flybys in which several observations of dune fields have been collected in altimetry mode. There are several cases in which SAR and altimeter have been acquired over same areas covered by dune fields, such as during T28 (SAR) and T30 (altimeter) flybys. Altimetry together with SAR data have been used to derive the rms slopes of dunes (large scale) over Fensal area, this information has been employed to calculate SAR incidence angle with respect to dunes. We extracted backscattering coefficients of bright and dark areas detected in the analyzed SAR image in order to evaluate the angular response of scattering. Through the Geometric Optics model we retrieve roughness values (small scale rms slope) for both dune bright and dark areas.

Dune Height Estimation on Titan Exploiting Pairs of Synthetic Aperture Radar Images With Different Observation Angles

Widespread longitudinal dunes have been identified on Titan thanks to the 2.2-cm wavelength Cassini Synthetic Aperture Radar (SAR) instrument. Understanding the properties of these surface features, such as material composition and dune height, is very important for giving new clues about the Titan geology and climate. One of the major difficulties in the estimation of dune heights using SAR occurs when the material composition of the dunes is heterogeneous. In this paper, we propose a novel method for dune height estimation, which takes into account material heterogeneity, and in particular, the case in which the interdune exhibits different dielectric properties with respect to the remaining part of the dune. Paired data acquisitions with orthogonal observations are considered for separating the dielectric from the geometric effect on the backscattering coefficients in order to retrieve the slope and thus the height of the dunes. The results for a test area located in the Fensal region indicate that the slopes of the dune faces are generally lower than 5° and the heights range between 40 and 110 m.

Local Scale Seismic Landslide Susceptibility Assessment Based on Historic Earthquake Records Combined with Accelerometer Monitoring and Ambient Noise Data

Uncertainty in the quantification of earthquake loading poses one of the major difficulties in local scale seismic landslide susceptibility assessments. This problem can be exacerbated for slope settings that are likely to produce considerable amplifications of seismic shaking. We address this issue by examining the case of a historic landslide triggered by the 1627 Apulian (southern Italy) earthquake (epicentral intensity X on the MCS scale) in the peri-urban area of Caramanico (central Italy), distant ~120 km from the epicenter. The failure caused a large downslope displacement and destroyed several buildings. The slope seismic response is assessed using data from long-term accelerometer monitoring of the hillslope and from recent ambient noise measurements. This provided evidence of significant directional amplifications, e.g., by a factor of approximately 4 and 20, respectively in terms of peak horizontal acceleration and Arias Intensity during the 2009 Mw 6.3 L’Aquila earthquake that occurred 60 km NW of Caramanico. Then taking into account the site amplification, permanent displacements are calculated by applying a rigorous Newmark approach. This study shows that historical information on landslides triggered at apparently anomalously large distances from an earthquake epicentre can help to identify hillslopes influenced by site effects and that reconnaissance-type measurements of ambient noise can be useful to reveal directional amplifications. The importance of accurate assessments of other relevant input parameters (e.g., material properties, slip surface geometries, groundwater conditions) used in seismic slope modeling is also recognized.

HR Satellite Imaging and Borehole Monitoring of Landslides in Daunia, Italy

We examine the instability of slopes in a catchment traversed by a 11 km long mid-slope road, characterized by the widespread presence of clay materials with poor geotechnical properties. High Resolution (HR) multispectral satellite imagery is used to provide focus on the landslides affecting the road and on their close association with poorly drained sites (wet areas, concentration of roadway runoff). The origin of many wet areas mapped from the satellite imagery is linked to the seasonally persistent high groundwater levels revealed through the piezometer monitoring. The remotely sensed data integrated with the in situ controls and subsurface monitoring show that the slopes traversed by the road are poorly drained and marginally stable. Thus even relatively moderate rainfall events (e.g. several tens of mm of rain in few days) can lead to landslide re-activations, which typically occur in the second half of fall and winter periods characterized by higher groundwater levels.

Combined use of Cassini Radar active and passive measurements to characterize Titan morphology

This paper focuses on the Titan surface parameters retrieval with emphasis on a combination of passive and active microwave measurements from Cassini spacecraft on the areas characterized by large liquid surfaces and neighboring land areas. The methodology consists of a combination of direct modeling and inversion algorithms. First, these surfaces have been described by means of a double layer model which considers an upper liquid hydrocarbons layer and a lower layer compatible with the radar response of the neighboring areas. This model is introduced into a Bayesian framework for the purpose of inferring the likely ranges of some parameters, in particular the optical thickness of the hypothesized liquid hydrocarbons layer and the wind speed. Second, the optical thickness information is used as an input to a forward radiative transfer model calculation to obtain simulated brightness temperatures. Comparison of the observed and computed brightness temperatures allows addressing the consistency of the observations from the two instruments.