46th Lunar and Planetary Science Conference (2015) 1538.pdf

TITAN’S MAGIC ISLAND: TRANSIENT FEATURES IN A SEA. J. D. Hofgartner*1, A. G. Hayes1, J. I. Lunine1, H. Zebker2, B. W. Stiles3, C. Sotin3, J. W. Barnes4, E. P. Turtle5, K. H. Baines3, B. H. Brown6, B. J. Buratti3, R. N. Clark7, P. Encrenaz8, R. D. Kirk9, A. Le Gall10, R. M. Lopes3, R. D. Lorenz5, M. J. Malaska3, K. L. Mitchell3, P. D. Nicholson1, P. Paillou11, J. Radebaugh12, S. D. Wall3, C. Wood13, *Presenting author ([email protected]), 1Cornell Uni- versity, Ithaca NY, 2Stanford University, Stanford CA, 3Jet Propulsion Laboratory, Pasadena CA, 4University of Idaho, Moscow ID, 5JHU Applied Physics Lab, Laurel MD, 6University of Arizona, Tucson AZ, 7USGS Denver Federal Center, Denver CO, 8Observatoire de Paris, Paris France, 9USGS Astrogeology Center, Flagstaff AZ, 10LATMOS-UVSQ, Paris France, 11University of Bordeaux, Bordeaux France, 12Brigham Young University, Provo UT, 13 Planetary Science Institute, Tucson AZ.

Introduction: Transient bright features, popularly referred to as Titan’s Magic Island, were observed in Titan’s northern sea, , in the Cassini T92 and T104 observations in July 2013 and August 2014 (Fig. 1) [1]. Multiple images obtained prior to the T92 detection and between the T92 and T104 detections do not include these bright features (Fig. 1). The features are inconsistent with image artifacts and permanent structures and thus are transient. Ephemeral phenome- na most consistent with the observations are waves, floating/suspended solids and bubbles. Additional bright anomalies that are likely also transient features were observed in Ligeia Mare and Kraken Mare during the T104 observation. Titan’s northern hemisphere is transitioning toward the summer season and the timing of the onset of these transients suggests that they are an expression of the changing seasons. The region of the Magic Island will be observed again on January 11th, 2015 (T108). Backscatter Analysis: For a permanent geophysi- cal structure on Titan, the angle of incidence is the dominant parameter that determines its brightness (normalized radar cross-section; NRCS) in Cassini Synthetic Aperture Radar (SAR) images. The angle of incidence and NRCS for a general surface are inverse- ly related; the NRCS is greater for smaller incidence angles. All of the major solid terrain classes on Titan have been shown to have an NRCS-incidence behavior Figure 1: Eight Cassini observations of the region of that is consistent with quasi-specular models [2]. To Titan’s Magic Island. In the T92 and T104 images, test if detection of the transient features is determined transient bright features (circled in red) are observed at simply by the incidence angle we compared the obser- 78O N, 123O E that are not seen in any of the other im- vations to a suite of quasi-specular models (Fig. 2). We ages. find that these models can be ruled out with greater than 88% confidence, indicating that the observations is constrained to be small [3]. Exposure of solid sur- are inconsistent with a permanent structure. face due to liquid-level retreat, as was observed for Dynamic Phenomena: The evolution of the fea- some southern hemisphere during southern tures appears to have included a reversion between the summer [4], remains a plausible explanation. We do T92 and T104 observations and thus is inconsistent not favor this explanation however because of the T93 with the formation of a new permanent structure such non-detection just 16 days after the T92 detection and as an island via cryovolcanism. Variations in absorp- lack of evidence for liquid retreat elsewhere in the sea. tion of the radar energy due to changing sea level is The T92 detection and the T64, T91, T93, and T95 unlikely to explain the transients because the absorp- non-detections all occurred at approximately the same tion of the radar as it propagates through Ligeia Mare true orbital anomaly. Tides due to Titan’s eccentric 46th Lunar and Planetary Science Conference (2015) 1538.pdf

orbit around should result in consistent behavior Future Observations and Modeling: On January at a particular true anomaly and thus are unlikely to 11th, 2015 the region of the transient features will be explain the transients. The remaining transient hy- observed at an incidence angle of approximately 10 potheses include waves, suspended/floating solids and degrees. The seasonal behavior suggests that addition- bubbles. All of these hypotheses have plausible mech- al transients will be discovered during this observation. anisms by which they could commence or become Detailed modeling of the radar backscattering for the enhanced due to the approach of northern summer. remaining transient hypotheses is the next stage of the Local meteorology could also stimulate or enhance analysis and may provide additional insight into the these phenomena but we are unable to constrain its role nature of these enigmatic features. in the appearance of these transients. Acknowledgements: Jason Hofgartner gratefully acknowledges the Cassini RADAR and VIMS teams for the data and opportunity to lead the analysis and the Cassini Project and Natural Sciences and Engineer- ing Research Council of Canada, Post Graduate Schol- arship Program for financial support. References: [1] Hofgartner J. et al., (2014) Nature Geosci. 7, 493. [2] Wye L. (2011) Radar Scattering from Titan and Saturn’s Icy Satellites Using the Cassi- ni Spacecraft. [3] Mastroguiseppe M. et al., (2014) Geophys. Res. Lett. 41, 1432. [4] Hayes A. et al., (2011) Icarus 211, 655.

Figure 2: The normalized radar cross-section of the region of the transient features as a function of inci- dence angle. Quasi-specular models for permanent geophysical structures are inconsistent with the obser- vations indicating that the features are indeed transient.