GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L07104, doi:10.1029/2009GL042312, 2010 Click Here for Full Article Specular reflection on Titan: Liquids in Kraken Mare Katrin Stephan,1 Ralf Jaumann,1,2 Robert H. Brown,3 Jason M. Soderblom,3 Laurence A. Soderblom,4 Jason W. Barnes,5 Christophe Sotin,6 Caitlin A. Griffith,3 Randolph L. Kirk,4 Kevin H. Baines,6 Bonnie J. Buratti,6 Roger N. Clark,7 Dyer M. Lytle,3 Robert M. Nelson,6 and Phillip D. Nicholson8 Received 4 January 2010; revised 22 February 2010; accepted 25 February 2010; published 7 April 2010. [1] After more than 50 close flybys of Titan by the Cassini reflections results either because the putative lakes are not spacecraft, it has become evident that features similar in filled with liquids or the lakes have not been observed in morphology to terrestrial lakes and seas exist in Titan’s specular geometry. A further impediment is that the northern polar regions. As Titan progresses into northern spring, polar region, where several extensive features exists, including the much more numerous and larger lakes and seas in the the more than 1000 km wide Kraken Mare [Stofan et al., north‐polar region suggested by Cassini RADAR data, are 2007; Turtle et al., 2009], was not illuminated during Titan’s becoming directly illuminated for the first time since the long winter. Nevertheless, the northern polar region is now arrival of the Cassini spacecraft. This allows the Cassini illuminated for the first time since Cassini arrived at Saturn in optical instruments to search for specular reflections to 2004, thus searches for specular reflections from northern provide further confirmation that liquids are present in bodies of liquid on Titan are now possible, with our first these evident lakes. On July 8, 2009 Cassini VIMS successful observations reported here. detected a specular reflection in the north‐polar region of Titan associated with Kraken Mare, one of Titan’slarge, 2. Observations ’ presumed seas, indicating the lake s surface is smooth and [3] Three of four observations acquired by VIMS during m free of scatterers with respect to the wavelength of 5 m, the 58th flyby of Titan on July 8, 2009 (Table 1) show an where VIMS detected the specular signal, strongly unresolved feature with very high radiance seen only in the suggesting it is liquid. Citation: Stephan, K., et al. (2010), 5‐mm atmospheric window (Figure 1). The feature is located Specular reflection on Titan: Liquids in Kraken Mare, Geophys. near 71°N and 338°W. A quick review of the illumination Res. Lett., 37, L07104, doi:10.1029/2009GL042312. and viewing geometry revealed that the bright spot occurred where the incidence and emission angles were about equal 1. Introduction (∼73°), which would be expected for a specular reflection of sunlight [Shepard et al., 1993]. [2] Observations by the Cassini instruments revealed [4] We projected the VIMS observations accordingly to numerous large‐scale dark surface features in Titan’s polar the geometry of the four VIMS cubes following the method regions [Stofan et al., 2007; Turtle et al., 2009]. Substantial of Jaumann et al. [2006]. In addition, Radar SAR images, evidence provided by the Cassini Radar instrument supports used for comparison to the VIMS data, were processed as the interpretation that these regions are lakes consisting of described by Stephan et al. [2009] following the methods of liquid or solid hydrocarbons [Stofan et al., 2007]. Neverthe- Stiles et al. [2008] that is based on the improved model of less, liquids, mainly in form of ethane, have only been con- Titan’s rotation, including the newly determined pole direc- clusively identified in Ontario Lacus, one of the few lakes in ’ ‐ tion. Based on these data, the location of the specular reflec- Titan s south polar region [Brown et al., 2008]. Detecting tion, with the size of the corresponding VIMS pixels of about specular reflections of sunlight off of the lakes’ surface by 100 km across, can be associated with a Radar‐dark region the optical Cassini instruments, however, which are sensitive (Figure 2a). Although RADAR data do not cover this region in the visible and near‐infrared spectral range, would also completely and future data may reveal it as a separate lake, essentially eliminate anything but a body of liquid [Williams recent data imply that this region is part of the western edge of and Gaidos, 2008]. Until now, however, no specular reflec- Kraken Mare [Stofan et al., 2007; Turtle et al., 2009]. tions have been observed. The lack of observed specular [5] Figures 2b illustrates the predicted footprint of the solar disk derived from ray‐tracing calculations, using 1Institute of Planetary Research, DLR, Berlin, Germany. 2Department of Earth Sciences, Institute of Geosciences, Free position of the Sun, the spacecraft, and Titan (including the University, Berlin, Germany. vector offset determined by the radius of Titan) for each of 3Lunar and Planetary Laboratory, University of Arizona, Tucson, the observations. The size of the solar image was determined Arizona, USA. by projecting the edge of the solar disk onto the surface of 4U.S. Geological Survey, Flagstaff, Arizona, USA. 5Department of Physics, University of Idaho, Moscow, Idaho, USA. Titan in the specular geometry. The size of the solar image 6Jet Propulsion Laboratory, California Institute of Technology, calculated for these observations is sub‐pixel; the semi‐major Pasadena, California, USA. axes of the elliptical footprint of the solar disk are ∼4.4 and 7U.S. Geological Survey, Denver, Colorado, USA. ‐ ‐ 8 1.3 km perpendicular and parallel to the Sun Titan spacecraft Department of Astronomy, Cornell University, Ithaca, New York, (specular) plane, respectively. USA. [6] During our observations, the location of the specular Copyright 2010 by the American Geophysical Union. point moved from east to west (2 h 41 min) as the spacecraft 0094‐8276/10/2009GL042312 flew by Titan. The observed variations in the strength of the L07104 1of5 L07104 Table 1. Observation Parameters of the VIMS Cubes Acquired During the Sequence CLOUDMAP001a Observation S/C‐Titan Center Location of S/C and Location of the Location of the VIMS Cube Time/Integration Distance[km]/Pixel Ground Sub‐Solar Points Brightest Pixel(s) Center of the Brightest Calculated Specular (Cube Name) Time (ms) Resolution (km/pixel) (Lat/Lon) (Sample/Line) Pixel(s) (Lat/Lon) Reflection (Lat/Lon) 1 (1625724038) (05:18:51 − 05:25:31/160) 233370/112.3 34.32128.199−0.371/313.309 5/27 72.98/339.03 71.27/336.22 2 (1625724520) (05:25:53 − 05:55:13/640) 230936/109.25 34.32/128.424−0.371/313.544 4/284/29 70.03/335.1970.82/328.11 71.26/336.49 STEPHAN ET AL.: SPECULAR REFLECTION ON TITAN 3 (1625730355) (07:04:08 − 07:15:18/160) 200 887/95 34.236/129.795−0.370/314.952 7/327/33 71.62/338.9372.34/332.26 71.31/338.22 4 (1625731073) (07:16:06 − 07:59:26/640) 195880/90.4 34.218/130.027−0.370/315.187 6/33 74.13/335.41 71.33/338.52 aFlyby T58. 2of5 and green dots, respectively. tion of the bright spotspot and the the viewing S/C conditions point on indicatedto July by (b) 8, the 2009 the and red corresponding (c) observationsis the geometry loca- restricted at the to bright the∼ VIMS channels atFigure 5 1. 72°N/342°W as seen in VIMS cube #3 (false color) that (a) Bright spot in the northern polar region at m m in comparison L07104 L07104 STEPHAN ET AL.: SPECULAR REFLECTION ON TITAN L07104 footprint (I/F = 2.84) lies almost entirely within the lake, the brightest pixels of VIMS cube #2 have a lower intensity (I/F = 2.1) because a fraction of the solar footprint possibly lies outside the lake. In that case, the observed intensity is lower, arising from a combination of the specular reflection off the lake and from light scattered by the adjacent terrain. In the case of VIMS cube #1, the image footprint is only partially within the lake explaining the weaker (I/F = 0.74), but still recognizable, 5‐mm signal. The predicted position of the solar image for VIMS cube #4 of this observation sequence, whose spectrum shows only a weak signal at 5 mm (I/F = 0.57), is clearly located outside the lake. 3. Direct Versus Diffuse Flux of the Incidence Light Onto Titan’s Surface [7] The question arises as to why the specular reflections are seen only in the 5‐mm window and not in the shorter‐ wavelength atmospheric transmission windows (Figure 3a). The detection of a specular reflection requires that a signifi- cant fraction of the incident sunlight is not scattered out of the beam, nor the light reflected from the surface be scattered during its travel out of the atmosphere. Within the atmo- spheric windows near 1.09, 1.6, 2, and 2.8 mm, less than 30% of the light is transmitted though the atmosphere for i ∼ 73°, and less than 1/300 of this light traverses the atmosphere without being scattered (Figure 3b). Thus at wavelengths <3 mm, only a tiny fraction of the incident light traverses the atmosphere and reflects from the surface fully escaping any scattering event; specular reflection at these wavelengths is suppressed by atmospheric scattering. However, within the atmospheric window centered at 5 mm, the direct flux dom- inates and a significant fraction of the incident light reaching the surface has not been scattered [Griffith et al., 2009]. Thus, we find that light specularly reflected from Titan’s surface is exceptionally evident at 5 mm with a signal that is attenuated Figure 2.