Wind, Waves, and Magic Islands at Titan’s Largest Sea: Kraken Mare

Introduction: For the majority of the Cassini Mission, Titan’s hydrocarbon lakes and seas were observed to be quiescent and remarkably calm, with vertical deviations of less than a millimeter[1,2,3,4,5]. This lack of observable surface roughness has been attributed to a seasonal effect in which polar winds were too weak to create waves or other dynamic features[6]. As northern spring approached, the previous series of nondetections began to give way to indications of potential activity. Specifically, apparent sunglints offset from the geometric specular points were observed in several lakes and seas[7] and a transient bright radar signature, known as Titan’s Magic Island, was observed to both appear and disappear over the surface of Ligeia Mare[8,9]. Herein, we report the observation of a new Magic Island (Fig. 1), in addition to an isolated patch of transient surface roughness (Fig. 2/3), observed within Titan’s largest sea Kraken Mare. The data were acquired during flyby T104 (08/21/2014) and included nearly coincident observations by the radar and Visual and Infrared Mapping Spectrometer VIMS). Kraken Magic Island: The primary objective of the T104 flyby was a radar altimetry observation over Kraken Mare in order to look for surface activity and measure the depth of the sea. While shallow (~30 m) bottom detections were observed over Moray Sinus, the majority of Kraken was too deep to detect the seafloor[10,11]. During the observation, the radar was cycled between the nadir beam and two adjacent beams with incidence angles of 3–5°. These off-nadir beams were processed into a Synthetic Aperture Radar (SAR) strip that revealed transient bright features similar to the Magic Island observed at Ligeia Mare (Fig. 1)[8,9]. These features have been dubbed the Kraken Magic Island and are located in an area that was previous observed to be quiescent sea. The Kraken Magic Island has an average backscatter (so) of 3.6±1.7 dB at 5.3° incidence. The nearby physical island has an average so of -1.2±2.9 dB at 5.1° and the surrounding sea has an average 5° incidence so of -15.4±2.9 dB, which is well below the noise-equivalent so of -11.5 dB and consistent with no detectable signal. In a fortuitous circumstance, VIMS spectra of Kraken Mare were collected ~2 hours after the radar swath during a ride-along observation. Furthermore, the geometric specular point (location of the specular reflection of the Sun from a flat surface on Titan as viewed by Cassini) was located within the sea, and only ~0.5° (as viewed by Cassini) from the Kraken Magic Island. At the position of the Kraken Magic Island, an anomalous 5-μm bright signal was detected that is