NoteShoreline retreat at Titan’s Ontario Lacus and Arrakis Planitia from Cassini Imaging Science Subsystem observations
Introduction
Observations of Titan’s south-polar regions acquired by Cassini’s Imaging Science System (ISS) over several years (2004–2009) have revealed differences indicative of changes in the distribution of liquids on the surface. In images acquired of Ontario Lacus in March 2009, the shoreline appears to have retreated from its location in June 2005 by 9–11 km in the southwest (Fig. 1). A distant observation taken in February 2009 suggests that the large dark area that appeared between July 2004 and June 2005 (Turtle et al., 2009) may have subsequently faded (Fig. 2). To date, we have not detected differences in repeated ISS observations of the lakes and seas at high northern latitudes. The recent changes are interpreted to be the result of the evaporation and infiltration of surface liquids, essential components of Titan’s methane cycle, and they provide insight into and allow us to put constraints on the amount of precipitation from a storm observed in October 2004 (Fig. 2; Schaller et al., 2006). The observations also demonstrate that the bed of Ontario Lacus is not dark at near-infrared wavelengths, an important piece of information regarding the poorly understood nature of Titan’s surface materials.
Section snippets
Ontario Lacus
ISS observed Ontario Lacus (∼70°S, ∼180°W) in July 2004 (designated T0), in June 2005 (Rev009), and in March 2009 (T51). Fig. 1 shows images from the last two of these; observational parameters are listed in Table 1. The 2009 observation was acquired during a targeted Titan flyby and, thus, at much closer range, so the exposure times were shortened significantly (by more than a factor of 4) to prevent smearing. Moreover, the wide-angle camera (WAC) was used and the images were summed over 4 × 4
Discussion
The observations reveal retreat of the shoreline of Ontario Lacus of up to 9–11 km along its southwestern margin, while the southern and southeastern margins appear unchanged at the resolution of these observations (Fig. 1). The variability of the changes along the shoreline is consistent with variability in the nature of the shore along the lake’s perimeter documented by RADAR (Hayes et al., 2010b, Wall et al., 2010): the eastern shore is quite steep; the western shore much more gradual.
Conclusions
The changes observed in ISS observations of Titan’s south polar lakes suggest that liquid deposited during late summer storms is being lost to evaporation and infiltration into the subsurface as southern fall progresses. Using loss rates derived from repeat south-polar observations by Cassini RADAR (Hayes et al., 2010a), we derived precipitation rates for the October 2004 storm of at least a few to several centimeters, consistent with model predictions. The ISS observations also demonstrate
Acknowledgments
We are deeply grateful to all who have worked to create and operate the Cassini–Huygens mission, thereby making possible the acquisition and analysis of its wealth of data. We also thank J. Soderblom and an anonymous reviewer for their careful assessments and suggestions for improving the manuscript. This work was supported by the Cassini–Huygens mission, which is a joint endeavor of NASA, ESA, and ASI and is managed by JPL/Caltech under a contract with NASA.
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