ISS observations of Titan's northern lakes and evidence for a north-polar surface unit

Titan's hydrocarbon lakes and seas are concentrated at high latitudes (>55°) and preferentially in the northern hemisphere where all of the seas and most of its lakes are found [e.g., 1, 2]; liquid covers ~10% of the surface poleward of 55°N [2-4]. The south polar region has a comparative dearth of lakes, covering less than half a percent of the surface south of 55°S [3-6]. Based on the radar backscatter appearances of lakes, Hayes et al. [2] divided them into three categories: filled lakes, which have very low backscatter cross-sections (often below the noise level) and radiometric signatures consistent with liquid hydrocarbons [1]; empty lakes, which are topographic depressions that have similar morphologies but greater backscatter than the surrounding terrain and are, thus, interpreted to be paleo-lakebeds; and lakes with intermediate radar backscatter, which have been interpreted to be saturated regolith or shallow enough that the incident radar energy can penetrate through the liquid and interact with the lakebed. Morphologic features, including basin shapes ranging from near circular to irregular, with steep walls a few hundred meters deep and slightly raised rims, have suggested that dissolution processes may play a role [7-9]. Intriguingly, recent images acquired by Cassini's Imaging Science Subsystem (ISS) during Titan flybys have revealed that the region encompassing Titan's northern lakes and seas is brighter at 938 nm (the wavelength at which ISS can best observe Titan's surface [4, 10-11]). The boundary of this bright region appears to correlate with the locations of lake features, regardless of whether they are currently filled with liquid hydrocarbons. That this region has a different albedo from the mid-latitude terrain would be consistent with a distinct surface unit in which the lakes have formed. These ISS data also provide important comparisons to observations of Titan's lakes by other Cassini instruments, particularly RADAR and the Visual and Infrared Mapping Spectrometer (VIMS). In general, in the region of numerous small lakes on Titan's leading hemisphere between ~70° and ~85° N the locations and shapes of dark features correlate at both near-infrared and Cassini RADAR (2.2 cm) wavelengths. However, some differences are observed, indicating either differences in the properties of the surface materials or possibly temporal changes. We will present the recent observations and their implications for the nature of Titan's surface at high northern latitudes. [1] Stofan et al. (2007) Nature 445, p. 61. [2] Hayes et al. (2008) GRL 35, L09204. [3] Aharonson et al. (2009) Nature Geosci. 2, p. 851. [4] Turtle et al. (2009) GRL 36, L02204. [5] Hayes et al. (2011) Icarus 211, p. 655. [6] Stofan et al. (2012) DPS 44, Abstract #201.08. [7] Mitchell and Malaska (2011) First International Planetary Caves Workshop, Abstract #1640. [8] Hayes et al. (2012) EGU, #11598. [9] Hayes et al. (2013) LPSC 44, #1719. [10] Porco et al. (2004) Space Sci. Rev. 115, p. 363. [11] Porco et al. (2005) Nature 434, p. 159.