Edge detection applied to Cassini images reveals no measurable displacement of Ontario Lacus' margin between 2005 and 2010

Abstract

[1] Ontario Lacus is thus far the largest flat-floored topographic depression of Titan's southern hemisphere interpreted as a permanent or ephemeral lake. From 2005 to 2010, it was imaged several times and at various wavelengths by ISS, VIMS and RADAR instruments onboard Cassini's spacecraft. We analyze the position and uncertainty of Ontario Lacus' margin in all these images using an edge detection method based on image derivation. We find that, given the range of uncertainties in contour locations derived from images, no measurable displacement of Ontario Lacus' margin can be detected between 2005 and 2010 at the actual image spatial resolutions. The discrepancy between this result and previous ones is attributable to differences in (1) the basics behind the methods used, (2) the actual spatial resolutions and contrasts of the available images due to differential atmospheric scattering effects at different wavelengths, and (3) the geomorphological interpretation of contours derived from images acquired at different wavelengths. This lack of measurable displacement in the images suggests that the imaged contour corresponds either (1) to the border of a surface liquid body, provided that potential changes in its extent over five terrestrial years were not sufficiently large to be measured, or (2) to the stationary topographic border between Ontario Lacus' depression and the surrounding alluvial plain. Potential displacements of Ontario Lacus' margin between 2005 and 2010 are thus below the actual resolution of currently available images or have to be sought for within the extent of the topographic depression rather than along its borders.

Modeling of Seasonal Lake Level Fluctuations of Titan's Seas/Lakes

“…The lake may have since then reduced its methane mole fraction (by evaporation) and volume (by seepage and evaporation). This would also reconcile with the geomorphologic evidence of past shorelines around Ontario Lacus indicating a higher lake level some time in the past (Cornet et al, 2012). As already concluded by Dhingra et al (2018), Ontario Lacus may owe its current existence to the huge catchment area surrounding this lake.…”

“…Likewise, Turtle et al () compared ISS images of Ontario Lacus obtained in June 2005 ( L S = 306°) and March 2009 ( L S = 355°) with each other and identified a possible shoreline retreat by 9–11 km. However, it was also pointed out that the apparent shoreline changes observed by ISS may be within measurement error (Cornet et al, ). Furthermore, Arrakis Planitia near the south pole first darkened between July 2004 and June 2005 (Turtle et al, ), but this darkening disappeared by February 2009 (Turtle et al, ).…”

Titan's atmosphere and climate

“…Due to the length of the Cassini mission, it might be possible to detect seasonal changes in lake level by looking for shoreline changes [Mitri et al, 2007]. While some shoreline changes have been reported there are conflicting interpretations of the data and it remains an open question whether Cassini has detected changes in the liquid level of the lakes and seas [Barnes et al, 2009;Turtle et al, 2009;Moriconi et al, 2010;Hayes et al, 2011;Turtle et al, 2011aTurtle et al, , 2011bCornet et al, 2012a;Sotin et al, 2012;Lucas et al, 2014a;Hayes, 2016]. Regardless of whether the lake levels have changed over the past decade, there is evidence for deposits of evaporites on the surface, created by cycles of evaporation and refilling, including around Ontario Lacus [Barnes et al, 2009;Moriconi et al, 2010;Barnes et al, 2011a;Cornet et al, 2012b;MacKenzie et al, 2014].…”

Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT

“…While shoreline recession at Ontario Lacus was reported over the timescale of the Cassini mission Hayes et al, 2011b,a), the shoreline detection algorithms have been disputed (Cornet et al, 2012), leaving the contemporary evaporation rate over lakes uncertain. Cassini has observed albedo variations with both the Imaging Science Subsystem (ISS) and RADAR that depict smaller southern lakes disappearing between adjacent observations (Turtle et al, 2009;Hayes et al, 2011a), which was attributed to either infiltration or evaporation, although the rates could not be quantified.…”