Modeling the Stability of Ontario Lacus on Titan

Introduction: Since the arrival of the CassiniHuygens mission to the Saturn system, numerous liquid-filled lakes have been identified to date. The area of the lakes vary between <10 km to over 100,000 km [1]. The largest of them in the south polar region is Ontario Lacus, that was first identified as a radar dark feature having low reflectivity and smooth, rounded boundary, resembling shorelines on Earth. During the T38 flyby, Brown et al. 2008 [2] identified absorption features characteristic of ethane in VIMS spectra in Ontario Lacus, probably present in liquid solution with methane, nitrogen and other low molecular-weight hydrocarbon species. However, the chemical composition of the lakes is still under debate and is a challenge to determine precisely, partially because of the strong atmospheric absorption of CH4. Cordier et al. 2009 [3] proposed that the lakes are indeed composed of CH4 to a large extent (∼5-10%), following C2H6. The first detailed geometric analysis of Ontario Lacus’ shore reveals two distinct annuli around the lakebed itself, indicating shoreline variations over time [4]. Moriconi et al. 2010 [5] characterized Ontario Lacus and its adjacent regions with three distinct regions based on VIMS data in the 5 μm spectral window: the lake itself, which appears to be dark as a result of nearly full extinction of solar radiation, a dark-gray ”ramp”, most probably adequate with exposed lakebed sediments, and a bright, outer irregular ridge. It was also proposed that the weak C2H6 absorption feature detected by Brown et al. 2008 [2] at shorter wavelengths is rather present in the region adjacent to the lake [5]. The supposed lakebed sediments may have been exposed by seasonal evaporation of methane and is indicative of lake level change over time [4]. Furthermore, the asymmetric morphology of Ontario Lacus found by Wall et al. 2009 [6] and the raised beach site on the north-eastern side also indicate former lake shorelines from times when the lake level was probably higher. Recent results of Wye et al. 2010 [7] estimate the maximum depth of Ontario Lacus to be 9 m, while depths over the rest of the lake are proposed to be less than 5 m. Figure 1: Radar image of Ontario Lacus, located at 72◦ S, 183◦ W. Credit: NASA/JPL.

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