The Lakes and Seas of Titan

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The Lakes and Seas of Titan

Rosaly M. C. Lopes, Karl L. Mitchell, Stephen D. Wall, Giuseppe Mitri,

Michael Janssen, Steven Ostro, Randolph L. Kirk, Alexander G. Hayes, Ellen

R. Stofan, Jonathan I. Lunine, et al.

To cite this version:

Rosaly M. C. Lopes, Karl L. Mitchell, Stephen D. Wall, Giuseppe Mitri, Michael Janssen, et al..

The Lakes and Seas of Titan. Eos, Transactions American Geophysical Union, American Geophysical

Union (AGU), 2007, 88, pp.569-570. �10.1029/2007EO510001�. �hal-00405428�

Eos, Vol. 88, No. 51, 18 December 2007

EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION

VOLUME 88 NUMBER 51

18 DECEMBER 2007

PAGES 569-576

The Lakes and Seas of Titan

PAGES 5 6 9 - 5 7 0

T h e study of Titan, Saturn's largest satel­ lite, is a major g o a l of the Cassini-Huygens mission. T h i s j o i n t p r o j e c t b e t w e e n NASA, the E u r o p e a n S p a c e Agency, a n d the Italian S p a c e A g e n c y c o n s i s t s of a Saturn orbiter (Cassini) and a Titan probe (Huygens). Since t h e mission's arrival at Saturn in July 2 0 0 4 , o n e of its most s p e c t a c u l a r d i s c o v e r i e s h a s b e e n the finding of the first extraterrestrial n o n m a g m a t i c s t a n d i n g b o d i e s of liquid: Titan's h y d r o c a r b o n l a k e s a n d s e a s . In July 2 0 0 6 , t h e first s y n t h e t i c a p e r t u r e radar (SAR) i m a g e s o f Titan's north polar region o b t a i n e d by t h e Cassini s p a c e c r a f t s h o w e d dozens of lakes above latitudes of 70° [Stofan

et al, 2007]. Subsequent SAR images obtained

by Cassini have c o v e r e d approximately 6 8 % of Titan's north polar region at latitudes a b o v e 60 d e g r e e s . T h e s e i m a g e s s h o w m o r e than 4 0 0 radar-dark a r e a s that w e interpret a s b e i n g liquid l a k e s (shown in dark blue in Figure 1), including a few that a r e s o large that they rightfully may b e c a l l e d s e a s . W e d i s c u s s h e r e t h e e v i d e n c e for liquids on Titan, the distribution a n d m o r p h o l o g y of lakes, a n d r e c e n t data that i n d i c a t e the p r e s e n c e of lakes in the south polar regions.

R a d a r b a c k s c a t t e r is affected by t h r e e factors: ( 1 ) l o c a l topography (if a surface is a n g l e d toward t h e instrument, it will gener­ ally a p p e a r brighter); ( 2 ) surface r o u g h n e s s , at or n e a r the w a v e l e n g t h of t h e radar instrument ( 2 . 2 - c e n t i m e t e r Ku b a n d ) ; a n d ( 3 ) s u r f a c e d i e l e c t r i c c o n s t a n t (the dielec­ tric c o n s t a n t is t h e relative permittivity of a d i e l e c t r i c , a n d it directly affects h o w m u c h radar is reflected b a c k at the s p a c e c r a f t ) . T h e e x t r e m e l y low radar b a c k s c a t t e r from m o s t of Titan's l a k e s i n d i c a t e s b o t h that t h e s e a r e a s have a low d i e l e c t r i c c o n s t a n t a n d that they a r e very smooth at the 2.2-centi­ meter wavelength. Studies of the lakes' mor­ phology, distribution, a n d a s s o c i a t e d c h a n ­ nels, together with t h e low radar reflectivity a n d c o r r e s p o n d i n g l y high emissivity, all

B Y R . M . C . LOPES, K . L . MITCHELL, S . D . WALL, G . MITRI, M . JANSSEN, S . OSTRO, R . L . KIRK, A . G . HAYES, E . R . STOFAN, J . I. LUNINE, R . D. LORENZ , C.WOOD, J . RADEBAUGH, PPAILLOU, H . ZEBKER, AND FPAGANELLI

s u p p o r t the h y p o t h e s i s that t h e l a k e s cur­ rently c o n t a i n liquid h y d r o c a r b o n s [Stofan

et al, 2 0 0 7 ] , most likely c o n t a i n i n g a mix­

ture of m e t h a n e a n d e t h a n e , a n d p r o b a b l y a l s o dissolved nitrogen [Mitri et al, 2 0 0 7 ] . T h e e x i s t e n c e of liquid h y d r o c a r b o n s on Titan's s u r f a c e had b e e n predicted, b a s e d on t h e r m o d y n a m i c m o d e l s of the stability of liquid h y d r o c a r b o n s at t h e low s u r f a c e

t e m p e r a t u r e s of Titan a n d on t h e o b s e r v a ­ tions of m e t h a n e in t h e a t m o s p h e r e , w h i c h is destroyed by sunlight a n d therefore must b e r e p l e n i s h e d [Lunine et al, 1983]. Obser­ vations by the Huygens p r o b e to Titan, a s well as SAR images at low latitudes, had shown a m p l e e v i d e n c e for p r o c e s s e s involv­ ing liquids, including fluvial networks, r o u n d e d p e b b l e s at t h e Huygens landing site, a n d evaporating liquids. Finding b o d ­ ies of liquids r e m a i n e d elusive, though the Cassini imaging system o b s e r v e d a dark

fea-Fig. 1. This colorized synthetic aperture radar (SAR) image obtained by Cassini is a composite made from seven Titan flybys. The image shows that Titan s north polar region is pitted with giant hydrocar­ bon lakes and seas (shown in dark areas) and that at least one of them (upper right) is larger than Lake Superior, the largest freshwater lake on Earth. Approximately 68% of Titan's north polar region, above 60° latitude, has been mapped by Cassinis radar instrument (used in SAR mode). About 14% of the mapped region is covered by lakes and seas, which are interpreted to be liquid. The 0°-180° longitude line intersects the image from top (180°) to bottom (0°). Latitude circles are in 10° intervals. The image is about 2700 kilometers wide. The image is available at http://photojournal.jpl.nasa .gov/catalog/PIA10008. Original color image appears at the back of this volume.

Fig. 2. Synthetic aperture radar images from the southernmost region of Titan, returned from a 2 October 2007 Cassini flyby show the presence of lakes in the southern hemisphere. Shown here is a portion of the image swath and an inset with details in false color. Titan's south pole is at bottom center. A few small dark patches—interpreted as liquid-hydrocarbon-filled lakes—stand out, at about 70°S. The image swath is 2250 kilometers long and has average spatial resolution of 1.4 kilometers. The inset is 90 x 90 kilometers, centered at 70.5°S and 113.9°W. The image is available at http://photojournal.jpl.nasa.gov/catalog/PIA10018. Original color image appears at the back of this volume.

ture n e a r Titan's s o u t h p o l e , n a m e d Ontario Lacus, that w a s interpreted a s a lake [Turtle

et al., 2 0 0 7 ] .

T h e p r e s e n c e of liquid m e t h a n e a n d e t h a n e at high latitudes a l s o w a s predicted, based on the relative humidity of methane (amount of m e t h a n e relative to the satu­ rated value) in Titan's d e n s e a t m o s p h e r e . E x c e p t at the highest latitudes, the relative humidity of m e t h a n e is u n d e r 100%, a n d it is b e l o w 5 0 % at low latitudes, w h i c h would c a u s e any standing liquids to e v a p o r a t e . M e t h a n e precipitation n e a r the p o l e s is thought to d o m i n a t e t h e "hydrological" c y c l e of Titan [Rannou et al., 2 0 0 6 ] , with m e t h a n e playing a r o l e similar to that of water on Earth. L a k e s c o n t a i n i n g liquid m e t h a n e should b e s t a b l e from the p o l e s down to a latitude d e t e r m i n e d by the abun­ d a n c e of m e t h a n e in the surface-atmosphere system a n d by the p o s s i b l e i n t e r s e c t i o n of surface fluids with putative s u b t e r r a n e a n "methanifers," a n a l o g o u s to terrestrial aqui­ fers [Stofan et al, 2 0 0 7 ] . Mitri et al. [2007] p r o p o s e d that the o b s e r v e d humidity of the a t m o s p h e r e could result from a global liquid c o v e r a g e of as little a s 0 . 2 - 2 % of Titan's area, which is consistent with the radar-dark a r e a s shown in Figure 1.

The Case for Liquids

T h e e v i d e n c e that t h e s e radar-dark lakes a r e liquids c a n b e s u m m a r i z e d a s follows. First, their m o r p h o l o g y a n d relationship with fluvial features give strong e v i d e n c e that the lakes are, o r r e c e n t l y were, filled with liquids. S e c o n d , t h e a n o m a l o u s l y low radar b a c k s c a t t e r (at times, t h e lowest t h e radar c a n s e e ) implies that t h e s e a r e a s a r e e x t r e m e l y s m o o t h at t h e s c a l e of 2.2 centi­ meters and, further, that very little or n o e n e r g y is b a c k s c a t t e r e d from the lake vol­ u m e itself. Third, t h e p r e s e n c e of liquids at the polar regions is c o n s i s t e n t with a t m o ­ s p h e r i c a n d c l i m a t o l o g i c a l m o d e l s . Fourth, the lakes' r a d i o m e t r i c b r i g h t n e s s e s , higher by several d e g r e e s t h a n the surrounding terrain, a r e c o n s i s t e n t with t h e high emis-sivity e x p e c t e d for a s m o o t h surface with the low d i e l e c t r i c c o n s t a n t ( 1 . 7 - 1 . 9 ) of liq­ uid e t h a n e - m e t h a n e solutions. However, it c a n b e argued that t h e liquids h y p o t h e s i s h a s yet to b e tested conclusively.

Since it is currently winter in Titan's northern polar regions, there is not e n o u g h reflected sunlight for o p t i c a l a n d near-infrared instru­ m e n t s on Cassini to o b s e r v e t h e s e regions. A s e a s o n on Titan lasts nearly 7.5 y e a r s , o n e quarter of a Saturn orbit period, w h i c h is 2 9 . 5 Earth y e a r s long. It will b e midsum­ m e r at the north p o l a r regions in 2017, but in only a few more years, there may b e enough sunlight at high northern latitudes for Cassini's near-infrared s p e c t r o m e t e r to o b t a i n o b s e r v a t i o n s that will confirm the e v i d e n c e provided b y SAR i m a g e s .

W h i l e the Cassini mission w a s initially s c h e d u l e d to last o n l y until mid-2008, an additional 2 y e a r s o f mission life a r e cur­

rently p l a n n e d . I n c r e a s e d longevity of t h e mission not only provides the opportunity to o b s e r v e s e a s o n a l c h a n g e s but a l s o per­ mits r e p e a t e d c o v e r a g e of the north p o l a r regions using SAR, a n d c o n s e q u e n t study of p o s s i b l e c h a n g e s in liquid levels. S u c h c h a n g e s would provide the strongest evi­ d e n c e for liquids.

S o m e overlapping c o v e r a g e h a s a l r e a d y b e e n o b t a i n e d , a n d o n e of the lakes s h o w e d significant brightening. However, this may or may not b e due to c h a n g e in t h e level of liquid, a s t h e brightening may b e due to dif­ ferent viewing g e o m e t r i e s b e t w e e n flybys. If the l a k e s a r e not filled with liquid hydro­ c a r b o n s , an alternative possibility to explain t h e l o w - d i e l e c t r i c - c o n s t a n t material is that t h e r e is a layer of low-density solid hydro­ c a r b o n s . W e c o n s i d e r the simplest a n d m o s t likely e x p l a n a t i o n to b e that the p o l a r l a k e s a r e filled with liquid h y d r o c a r b o n s .

Morphology and Sizes

T h e r e is c o n s i d e r a b l e variation in t h e m o r p h o l o g y of lake d e p r e s s i o n s a n d a l s o in their radar b a c k s c a t t e r . S h a p e s r a n g e from c i r c u l a r to c a n y o n - l i k e . S o m e l a k e s a r e within s t e e p s i d e d d e p r e s s i o n s ( o p p o s i t e -l o o k s t e r e o p a r a -l -l a x m e a s u r e m e n t s s h o w s o m e d e p r e s s i o n s to b e s e v e r a l h u n d r e d m e t e r s d e e p ) , w h i l e o t h e r s a p p e a r to have s h a l l o w o r p o o r l y defined m a r g i n s but a r e s u r r o u n d e d by t o p o g r a p h i c a l l y high a r e a s of t h e o r d e r of 1 0 0 0 m e t e r s o v e r d i s t a n c e s of 5 0 - 1 0 0 k i l o m e t e r s . L a k e s in s t e e p - s i d e d d e p r e s s i o n s a p p e a r m o r p h o l o g i c a l l y simi­ lar to v o l c a n i c c r a t e r l a k e s , g l a c i a l l a k e s , or m a s s i v e , k a r s t i c d o l i n e s ( s i n k h o l e s ) . O t h e r l a k e s , i n c l u d i n g t h e largest, have m o r p h o l o g i e s c o n s i s t e n t with f l o o d e d d r a i n e d b a s i n s (e.g., L a k e Powell in Utah a n d A r i z o n a ) . T h e origin of l a k e d e p r e s ­ s i o n s is u n c e r t a i n , but c u r r e n t h y p o t h e s e s

i n c l u d e v o l c a n i c c r a t e r s a n d karst-like d e p r e s s i o n s . S o m e of t h e d e p r e s s i o n s a r e c o m p l e t e l y filled with radar-dark m a t e r i a l , o t h e r s a r e partially filled, a n d s o m e appar­ ently a r e empty.

T h e l a k e s v a r y greatly in s i z e , from t h e limit of SAR r e s o l u t i o n ( w h i c h v a r i e s from 3 0 0 m e t e r s to >1 k i l o m e t e r ) to o v e r 1 0 0 , 0 0 0 s q u a r e k i l o m e t e r s (for c o m p a r i s o n , North A m e r i c a ' s L a k e S u p e r i o r is 8 2 , 0 0 0 s q u a r e kilometers). T h e largest Titan lake o b s e r v e d in its e n t i r e t y c o v e r s a g r e a t e r fraction of T i t a n ( 0 . 1 2 % ) t h a n t h e m o s t e x t e n s i v e ter­ restrial i n l a n d s e a , t h e B l a c k S e a , d o e s of t h e E a r t h ( 0 . 0 8 5 % ) . W e t h e r e f o r e refer to t h e largest of t h e radar-dark f e a t u r e s a s s e a s . T h e International A s t r o n o m i c a l Union h a s r e c e n t l y a p p r o v e d t h e u s e of t h e t e r m "mare" for t h e s e a s of T i t a n . M a r e is a t e r m previously only u s e d for t h e E a r t h ' s M o o n (to d e n o t e large e x p a n s e s of v o l c a n i c d e p o s i t s ) . S m a l l e r l a k e s o n T i t a n a r e desig­ n a t e d "lacus" a n d a r e n a m e d after l a k e s o n E a r t h s i m i l a r in s h a p e . T o date, 12 l a k e s o n T i t a n have b e e n n a m e d ; a figure with l a k e n a m e s c a n b e s e e n o n t h e p l a n e t a r y n o m e n c l a t u r e W e b site at http:// p l a n e t a r y n a m e s . w r . u s g s . g o v / i m a g e s / N P _ l a k e s _ l o w _ n o b o u n d a r i e s . p d f . Global Distribution W h i l e m u c h of t h e n o r t h e r n h e m i s p h e r e h a s b e e n i m a g e d with SAR, o n l y 2 0 % of Titan's total a r e a h a s b e e n i m a g e d s o far. Are l a k e s a n d s e a s present in Titan's south p o l a r regions? O t h e r than t h e o p t i c a l fea­ ture n a m e d Ontario L a c u s , t h e first indica­ tion of t h e p r e s e n c e of l a k e s in t h e south­ ern p o l a r regions c a m e from SAR data a c q u i r e d on 2 O c t o b e r 2007. T h r e e radar-dark lakes, with b a c k s c a t t e r similar to t h o s e in the north polar regions, were detected where the radar swath r e a c h e d its highest

Eos, Vol. 88, No. 51, 18 December 2007

southern latitude, at a b o u t 70° ( F i g u r e 2 ) . O t h e r features in t h e s e data i n c l u d e broad, steep-sided d e p r e s s i o n s adjoined to sinuous d e p r e s s i o n s interpreted to b e empty topo­ g r a p h i c b a s i n s or drained l a k e s fed by c h a n n e l s . Similarities in features b e t w e e n n o r t h e r n a n d southern p o l a r regions imply that t h e c l i m a t i c c o n d i t i o n s might b e simi­ lar, despite the difference in s e a s o n s . S u c h m o r p h o l o g i c a l similarities also make a vol­ c a n i c origin for the majority of lake depres­ s i o n s l e s s likely, u n l e s s o n e c a n argue that v o l c a n i s m c a n b e c o n c e n t r a t e d at the polar regions.

Future Cassini flybys targeting high south­ ern latitudes using radar will s h o w w h e t h e r the distribution of lakes and s e a s near Titan's south p o l e is c o m p a r a b l e to that o b s e r v e d at the north polar regions. If this is the c a s e , t h e reservoir of h y d r o c a r b o n s at Titan may b e m u c h larger than implied by current data. Lorenz et al. [2007] e s t i m a t e d Titan's total inventory of m e t h a n e / e t h a n e lakes, all found in the polar regions, to b e of the order of 3 0 - 3 0 0 , 0 0 0 c u b i c kilometers, c o n t a i n i n g h u n d r e d s of times m o r e liquid hydrocar­ b o n s t h a n the entire k n o w n gas a n d oil r e s e r v e s on Earth.

T h e e x i s t e n c e of lakes and s e a s on another world o p e n s up n e w a v e n u e s of scientific inquiry for limnologists and oceanographers: T h e p h y s i c s of waves, tides, stratification,

a n d s e d i m e n t a t i o n is the s a m e a s on Earth, but the dynamical parameters such as gravity a n d rotation rate, a n d fluid p r o p e r t i e s s u c h a s density a n d viscosity ( s u m m a r i z e d by

Lorenz et al. [ 2 0 0 3 ] ) a r e different. Titan is

therefore a laboratory in w h i c h t h e s e very different c i r c u m s t a n c e s c a n b e u s e d to test our u n d e r s t a n d i n g of Earth's l a c u s t r i n e a n d m a r i n e p r o c e s s e s .

Acknowledgments

Portions of this work w e r e p e r f o r m e d at the J e t Propulsion L a b o r a t o r y (JPL), Califor­ nia Institute of T e c h n o l o g y , u n d e r a c o n t r a c t with NASA. T h e Cassini P r o j e c t is a joint e n d e a v o r of the National A e r o n a u t i c s a n d S p a c e Administration (NASA), the Euro­ p e a n S p a c e A g e n c y (ESA), a n d the Italian S p a c e A g e n c y (ASI). Cassini is m a n a g e d by the J P L , California Institute of T e c h n o l o g y , a l s o u n d e r a c o n t r a c t with NASA.

References

Lorenz, R. D., E. Kraal, E.Asphaug, and R.Thomson (2003),The seas of Titan,£os Trans.AGU, 54(14), 125,131-132.

Lorenz, R. D., et al. (2007),Titan's inventory of organic surface materials, Geophys. Res. Lett., in press. Lunine, J. I., D. J. Stevenson, and Y L.Yung (1983),

Ethane oceans on Titan,Science, 222,1229.

Mitri, G., A. PShowman, J. I. Lunine, and R. D. Lorenz (2007), Hydrocarbon lakes on Titan,Icarus, 756,385. Rannou, P, FMontmessin, FHourdin, and S. Lebonnois

(2006),The latitudinal distribution of clouds on Titan, Science, 311,201-205, doi: 10.1126/ science. 1118424.

Stofan, E.R.,et al. (2007),The lakes of Titan, Nature,

445(4), doi: 10.1038/nature05438.

Turtle, E. P, J. E. Perry A. S. McEwen, R. A. West, D. D. Dawson,C.C. Porco,and S. Fussner (2007),Cassini Imaging Science Subsystem observations of Titan's high latitude lakes, paper presented at Workshop on Ices, Oceans, and Fire: Satellites of the Outer Solar System, Lunar and Planet. Inst., Boulder, Colo., 13-15 Aug.

Author Information

Rosaly M. C. Lopes, Karl L. Mitchell, Stephen D. Wall, Giuseppe Mitri, Michael Janssen, and Steven Ostro,Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena; E-mail: rosaly .m.lopes@jpl.nasa.gov; Randolph L. Kirk, U.S. Geological Survey, Flagstaff, Ariz.; Alexander G. Hayes, JPL; Ellen R. Stofan, Proxemy Research, Gaithersburg, Md.; Jonathan I. Lunine, University of Arizona,Tucson; Ralph D. Lorenz , Johns Hopkins University Applied Physics Laboratory, Laurel, Md.; Charles Wood, Wheeling Jesuit UniversityWVa.; Jani Radebaugh, Brigham Young University, Provo, Utah; Philippe Paillou,Observatoire Aquitain des Sciences de l'Univers, Floirac, France; H. Zebker, Stanford University, Calif.; and Flora Paganelli, European Center for Geodynamics and Seismology, Walferdange, Luxembourg.

NEWS

In Brief

PAGE 5 7 0

NASA m i s s i o n t o m e a s u r e M o o n ' s g r a v i t y NASA h a s s e l e c t e d a new mission

to m e a s u r e the Moon's gravity field in u n p r e c e d e n t e d detail, a c c o r d i n g to the a g e n c y ' s a s s o c i a t e administrator for s c i e n c e , Alan Stern. T h e Gravity R e c o v e r y a n d Inte­ rior L a b o r a t o r y (GRAIL), w h i c h is part of NASA's Discovery Program s e r i e s of s c i e n ­ tist-led, s o l a r system e x p l o r a t i o n missions, is s c h e d u l e d to l a u n c h in 2011 following the a g e n c y ' s 2 0 0 8 l a u n c h of the Lunar R e c o n ­ n a i s s a n c e Orbiter. S c i e n t i s t s plan to u s e gravity field information from GRAIL's two s p a c e c r a f t to X ray the Moon from crust to

c o r e to reveal s u b s u r f a c e s t r u c t u r e s and, indirectly, the Moon's t h e r m a l history. A c a m e r a a b o a r d e a c h s p a c e c r a f t will allow the public to s e e o b s e r v a t i o n s from the sat­ ellites. GRAIL "offers to bring innovative Earth studies t e c h n i q u e s to t h e M o o n a s a p r e c u r s o r to their p o s s i b l e later u s e at Mars a n d o t h e r planets," Stern said. For m o r e information, visit the W e b site: http:// discovery.nasa.gov/.

A t l a n t i c s e a s o n a l h u r r i c a n e f o r e c a s t

Two h u r r i c a n e f o r e c a s t e r s a r e predicting that 2 0 0 8 will b e a n a b o v e - a v e r a g e Atlantic b a s i n tropical c y c l o n e s e a s o n with a n a b o v e - a v e r a g e probability of a major hurri­ c a n e m a k i n g landfall in the United States. During 2 0 0 8 , there c o u l d b e a b o u t s e v e n h u r r i c a n e s (the a n n u a l a v e r a g e is 5.9) a n d

13 n a m e d storms (the average is 9.6), accord­ ing to a 7 D e c e m b e r report by Philip Klotzbach, r e s e a r c h s c i e n t i s t at C o l o r a d o State Univer­ sity in Fort Collins, a n d William Gray, uni­ versity professor e m e r i t u s of a t m o s p h e r i c s c i e n c e s . T h e f o r e c a s t e r s i n d i c a t e that they b e l i e v e the Atlantic b a s i n is in a n a c t i v e h u r r i c a n e c y c l e that is a s s o c i a t e d with a strong t h e r m o h a l i n e c i r c u l a t i o n a n d a n active p h a s e of the Atlantic Multidecadal Oscillation. T h e report n o t e s that, "real-time o p e r a t i o n a l early D e c e m b e r f o r e c a s t s have not s h o w n f o r e c a s t skill over c l i m a t o l o g y during this 16-year p e r i o d [ 1 9 9 2 - 2 0 0 7 ] . T h i s h a s o c c u r r e d despite the fact that t h e skill over the h i n d c a s t p e r i o d . . . s h o w e d appre­ c i a b l e skill." For m o r e information, visit the W e b site: h t t p : / / h u r r i c a n e . a t m o s . c o l o s t a t e . e d u / F o r e c a s t s / 2 0 0 7 / d e c 2 0 0 7 / d e c 2 0 0 7 . p d f .

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Fig. 1. This colorized synthetic aperture radar (SAR) image obtained by Cassini is a composite made from seven Titan flybys. The image shows that Titan s north polar region is pitted with giant hydrocarbon lakes and seas (shown in dark blue) and that at least one of them (upper right) is larger than Lake Superior, the largest freshwater lake on Earth. Approximately 68% of Titan's north polar region, above 60° latitude, has been mapped by Cassini s radar instrument (used in SAR mode). About 14% of the mapped region is covered by lakes and seas, which are interpreted to be liquid. The 0°-180° longitude line intersects the image from top (180°) to bottom (0°). Lati­ tude circles are in 10° intervals. The image is about 2700 kilometers wide. The image is available at http://photojournal.jpl. nasa.gov/catalog/PIA 10008.

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Fig. 2. Synthetic aperture radar images from the southernmost region of Titan, returned from a 2 October 2007 Cassini fly by, show the presence of lakes in the southern hemisphere. Shown here is a portion of the image swath and an inset with details in false color. Titan's south pole is at bottom center. A few small dark patches—interpreted as liquid-hydrocarbon-filled lakes—stand out, at about 70°S, and are colored blue. The image swath is 2250 kilometers long and has aver­ age spatial resolution of 1.4 kilometers. The inset is 90 x 90 kilometers, centered at 70.5°S and 113.9°W. The image is available at http://photojournal.jpl.nasa.gov/catalog/PlA10018.