SEGMENTATION OF LAKES FROM THE LOCAL BACKGROUND ON THE SURFACE OF TITAN USING CASSINI SAR IMAGES Emmanuel Bratsolis University of Athens, Department of Physics, 15784 Athens, Greece ebrats@phys.uoa.gr 1. INTRODUCTION The lakes of Titan, the largest satellite of Saturn, are bodies of liquid hydrocarbon that have been detected by the Cassini space probe, and had been suspected long before by the Voyager 1 and 2. The large ones are known as seas and the small ones as lakes. Synthetic Aperture Radar (SAR) images of Titan’s surface reveal quasi-circular to complex shape lakes which vary from <10 to more than 100,000 km². Lake-like features are separated into 3 classes; dark lakes, granular lakes, and bright lakes. Dark lakes are interpreted as liquid filled while bright lakes are interpreted as empty basins. Granular lakes are inferred as transitional between dark and bright lake features. The morphology of lakes on Titan span the range of observed morphologies on Earth [1]-[3]. Cassini carries a multimode Ku-band (13.78 GHz, Ȝ =2.17 cm) radar instrument designed to probe the surface of Titan and that of other targets in the Saturn system in four operating modes: imaging, altimetry, scatterometry, and radiometry. The Synthetic Aperture Radar (SAR) mode is used at altitudes under ~4000 km, resulting in spatial resolution ranging from ~350 m to >1 km. Images are acquired either left or right of nadir using 2–7 looks. A swath 120–450 km wide is created from 5 antenna beams. SAR coverage is dependent on spacecraft range and orbital geometry. Radar backscatter variations in SAR images can be interpreted in terms of variations of surface slope, near-surface roughness, or near- surface dielectric properties. The images obtained using SAR revealed that Titan has very complex surface [4]-[5]. The evidence that SAR-dark lakes are liquids is summarized because their morphology and relationship with fluvial features give strong evidence that the lakes are, or recently were, filled with liquids. The anomalously low radar backscatter (at times, the lowest the radar can see) implies that these areas are extremely smooth at the scale of 2.17 centimeters and, further, that very little or no energy is