The Exploration of Titan with an Orbiter and a Lake Probe

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The exploration of Titan with an orbiter and a lake probe

Giuseppe Mitri a,n, Athena Coustenis b, Gilbert Fanchini c, Alex G. Hayes d, Luciano Iess e, Krishan Khurana f, Jean-Pierre Lebreton g, Rosaly M. Lopes h, Ralph D. Lorenz i, Rachele Meriggiola e, Maria Luisa Moriconi j, Roberto Orosei k, Christophe Sotin h, Ellen Stofan l, Gabriel Tobie a,m, Tetsuya Tokano n, Federico Tosi o a Université de Nantes, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, France b Laboratoire d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC University Paris 06, University Paris-Diderot, Meudon, France c Smart Structures Solutions S.r.l., Rome, Italy d Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, United States e Dipartimento di Ingegneria Meccanica e Aerospaziale, Università La Sapienza, 00184 Rome, Italy f Institute of Geophysics and Planetary Physics, Department of Earth and Space Sciences, Los Angeles, CA, United States g LPC2E-CNRS & LESIA-Obs., Paris, France h Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States i Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, United States j Istituto di Scienze dell‘Atmosfera e del Clima (ISAC), Consiglio Nazionale delle Ricerche (CNR), Rome, Italy k Istituto di Radioastronomia (IRA), Istituto Nazionale di Astrofisica (INAF), Bologna, Italy l Proxemy Research, Rectortown, VA, United States m CNRS, LPGNantes, UMR 6112, 2 rue de la Houssinière, F-44322 Nantes, France n Institut für Geophysik und Meteorologie, Universität zu Köln, Cologne, Germany o Istituto di Astrofisica e Planetologia Spaziali (IAPS), Istituto Nazionale di Astrofisica (INAF), via del Fosso del Cavaliere 100, Rome, Italy article info abstract

Article history: Fundamental questions involving the origin, evolution, and history of both Titan and the broader Received 17 January 2014 Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present Received in revised form the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. 18 July 2014 Observations from an orbit-platform can improve our understanding of Titan's geological processes, Accepted 21 July 2014 surface composition and atmospheric properties. Further, combined measurements of the gravity field, rotational dynamics and electromagnetic field can expand our understanding of the interior and Keyword: evolution of Titan. An in situ exploration of Titan's lakes provides an unprecedented opportunity to Titan understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and habitability potential, and study meteorological and marine processes in an exotic environment. We briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a lake probe. & 2014 Elsevier Ltd. All rights reserved.

1. Introduction similar to water on Earth. The Cassini–Huygens mission to the Saturn system, which is still ongoing after 10 years of operations, Titan, Saturn's largest satellite, is a unique object in the Solar accomplished a first in-depth exploration of Titan (Lebreton et al., System, having a substantial nitrogen atmosphere, a surface with a 2009; Coustenis et al., 2009a). The data revealed hydrocarbon complex interplay of geological processes (Lopes et al., 2010) and lakes and seas located mostly in Titan's polar regions (Stofan et al., an outer ice shell overlying a subsurface ocean (Iess et al., 2007; Hayes et al., 2008), which, combined with extensive 2012; Mitri et al., 2014). The climate on this icy satellite boasts a equatorial dune deposits (Lorenz et al., 2006), form a major multi-phase alcanological cycle where methane plays a role reservoir of organics in the Solar System. The presence of standing bodies of liquid, dissected fluvial channels (Lopes et al., 2010), tectonic features (Radebaugh et al., 2007; Mitri et al., 2010), vast n Corresponding author. dune fields (Lorenz et al., 2006) and putative cryovolcanic flows E-mail address: [email protected] (G. Mitri). http://dx.doi.org/10.1016/j.pss.2014.07.009 0032-0633/& 2014 Elsevier Ltd. All rights reserved.

(Lopes et al., 2007; 2013) shows striking analogies with terrestrial understand the hydrocarbon cycle, investigate a natural laboratory geological activity. Titan is the only other place in the Solar for prebiotic chemistry and the limits of life, and study meteor- System, with the Earth, to have exposed lakes and seas, albeit ological and marine processes in an exotic environment. In Section with different materials. 4 we present the scientific investigations, measurements and The initial discoveries of the Cassini–Huygens mission inspired example approaches. Finally, in Section 5 we review the mission an extension of the mission in 2008, first as the Cassini Equinox concept legacy and in Section 6 we present a mission scenario for Mission and then as the Cassini Solstice Mission to 2017, as well as the future exploration of Titan with an orbiter and a lake probe. multiple proposals to return to the Saturnian system for further exploration of this enigmatic satellite. Two mission proposals, Titan and Enceladus Mission TandEM (Coustenis et al., 2009a) 2. Scientific objectives and investigations for a future and Titan Explorer (Lorenz, 2009) for ESA and NASA respectively, exploration of Titan merged to form the Titan Saturn System Mission TSSM (Coustenis et al., 2010; ESA, 2009) which sought further exploration of Titan In spite of all its advances, the Cassini–Huygens mission, due to with both a dedicated orbiter and in situ probes (balloon and lake its limited instrumentation and timeline, among other things, probe) and a large instrument suite. Ultimately ESA and NASA cannot address some fundamental questions regarding Titan such decided to prioritize the Europa Jupiter System Mission (EJSM), as (1) What are the complex organic molecules that form the dune which would later become ESA's first Large (L) L1 mission of its material, and how have they been formed? (2) What is the Cosmic Vision Program (2015–2025), the JUpiter ICy moon composition of the lakes and seas? (3) What are the processes Explorer (JUICE) with a planned launch date of 2022 (Grasset that form and maintain the atmosphere? and (4) Is Titan endo- et al., 2012). The demise of TSSM inspired mission proposals genically active? In fact, these central themes can be investigated smaller in scale and cost including the Titan Mare Explorer TiME by a future mission using an orbiter and a lake probe. (Stofan et al., 2010, 2013), the Titan Aerial Explorer TAE (Lunine To answer these questions among others, the scientific objec- et al., 2011) and the Journey to Enceladus and Titan JET (Sotin tives for a future exploration of Titan include (Table 1) et al., 2011), using either a probe or an orbiter but not both, and a (i) Determine how Titan was formed and evolved, and its internal small instrument suite. Building on this heritage, the exploration structure (Goal 1); (ii) Determine the nature of the geological of Titan with an orbiter and a lake probe was presented as a White activity on Titan (Goal 2); (iii) Characterize the atmosphere of Paper in response to the 2013 European Space Agency (ESA)'s Call Titan and determine how the alcanonological cycle works (Goal 3); for Science Themes for the Large-class (L) L2 and L3 missions and (iv) Determine the marine processes and chemistry of Titan's (Mitri et al., 2013). This paper is based upon the fore-mentioned lakes and seas (Goal 4). White Paper. The Online Supplementary material for this paper includes the list of the authors and the supporters of the White 2.1. Geophysics Paper from the scientific community. Future exploration of Titan with a dedicated orbiter and an The combination of measurements of the gravity field, rota- in situ lake probe would address central themes regarding the tional dynamics and electromagnetic field will contribute to the nature and evolution of this icy satellite and its organic-rich achievement of the scientific objective Goal 1 (Table 1). Static and environment. While the term lake probe is used in this paper dynamic gravity field measurements (Iess et al., 2010, 2012), and and in previous literature, it should be noted that the ideal target shape models (Zebker et al., 2009; Mitri et al., 2014) from the of any future exploration of the standing liquid bodies on Titan Cassini–Huygens mission have greatly expanded our knowledge of would mostly likely be one of the large seas found in the north the geophysical processes and structure of Titan's interior and as a polar region rather than a lake. Bodies of standing liquid on Titan consequence its formation and evolution have also become better with large dimensions are classified as seas (mare) rather than understood. Data from the Cassini–Huygens mission suggest that lakes (lacus). Prior to an orbiter phase around Titan and an in situ either Titan has no induced magnetic field or else it is not yet exploration of its lakes, mission scenarios could also involve detectable, while an intrinsic magnetic field is found to be absent investigations of the planet Saturn and its other satellites includ- (Wei et al., 2010). While there is still not yet a unique internal ing the geologically active icy moon Enceladus. A companion structural model of Titan, constraints provided by gravity, topo- White Paper proposed the exploration of Enceladus with flybys graphy and rotation data suggest that Titan's interior has some and Titan with an orbiter and a balloon probe (Tobie et al., 2014, degree of differentiation and possesses a water-ice outer shell in this issue). While the exploration of the Saturn system and its separated from its deep interior by a dense subsurface water ocean moons, Enceladus and Titan, was ultimately not selected for the L2 (Mitri et al., 2014). Thermal models suggest the presence of a high- and L3 missions, the scientific case for such exploration received pressure polymorph ice layer between the deep interior and the strong support from the scientific community (Mitri et al., 2013; subsurface ocean (Tobie et al., 2005; Mitri and Showman, 2008; Tobie et al., 2014, in this issue). Mitri et al., 2010). Section 2 summarizes the scientific objectives for Titan's Radio-tracking data registered during multi-flybys of Titan has exploration and how future scientific investigations of Titan yielded gravity field measurements with a spherical harmonic with a dedicated orbiter and lake probe can address some of these expansion to degree-3 (Iess et al., 2010, 2012). The quadrupole objectives. In Section 3 we discuss how in situ exploration (J2 and C22) moments of the gravity field were used to constrain of Titan's lakes can provide an unprecedented opportunity to the normalized axial moment of inertia which value was inferred

Table 1 Scientiﬁc objectives for a future exploration of Titan.

Goal 1 Determine how Titan was formed and evolved, and its internal structure Goal 2 Determine the nature of the geological activity of Titan Goal 3 Characterize the atmosphere of Titan and determine how the alcanological cycle works Goal 4 Determine the marine processes and chemistry of Titan's lakes and seas

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 3 using the Radau–Darwin approximation as MoIffi0:341470:0005 model and gravity anomalies of Titan's interior and define the (Iess et al., 2010). The high moment of inertia suggests that Titan's level of hydrostatic compensation of the shape and the outer ice deep interior is formed of relatively low-density material shell. A refinement of the shape of Titan at 1 m vertical (2600 kg mÀ3) that is consistent with a deep interior differen- resolution will allow the characterization of the tidal deformation tiated and composed of silicate hydrates such as antigorite of the outer layer and the determination of the Love number h2. (Castillo-Rogez and Lunine, 2010; Fortes et al., 2007) or only Radar altimetry is a suitable technique to determine the elevation partially differentiated and composed of a mixture of ice and of Titan's surface as demonstrated during the Cassini mission. rocks (Iess et al., 2010). The state of the internal structure of Titan Interferometric Synthetic Aperture Radar (InSAR) is also a possible is still debated, and future investigations are therefore necessary technique that could be used to generate digital elevation models to determine its internal structure. The interior structure of Titan (DEMs) of the surface and to produce maps of surface deformation can be inferred from a combined measurement of the gravity field, in combination with the imaging of the surface. rotational dynamics and electromagnetic field. The measurement of the imaginary part of the tidal Love

Future gravity measurements from a spacecraft (S/C) in a polar number k2 using gravity field measurements from the radio orbit will provide a significant improvement to the gravity field science experiment can be used to estimate the tidal quality factor determination with respect to Cassini's gravity measurements. We Q ¼ k2 =Imðk2Þ. The estimation of Q would be used to constrain consider a nominal final orbit of the S/C around Titan as a 1500 km the present internal tidal dissipation of Titan providing informa- altitude circular near-polar orbit. Using such an orbit, the static tion on the internal heat production as well as on the tidal gravity field of Titan can be resolved to degree-10 and possibly evolution of Titan. The estimation of Q would also provide higher orders. Also the gravity anomalies can be determined with information on the viscosity of the ice shell and indirectly on the a spatial resolution lower than 260 km allowing a better char- thermal structure as well as on the thermal state (conductive or acterization of the density contrasts within the ice shell as well as convective) of the ice shell. within the deep interior. The determination of rotational dynamics is crucial to under- Analysis based on Titan's shape suggests that Titan's outer ice stand the internal structure and geophysical processes on Titan. shell is thicker at the equator and thinner at the poles (Nimmo and Evolution models of the orbital dynamics use the obliquity along Bills, 2010; Hemingway et al., 2013; Mitri et al., 2014). Estimates with the eccentricity as a constraint. When the quadrupole gravity for the thickness of the ice shell vary from thin (50 km) to thick field is known, the obliquity may also provide the moment of (200 km) (e.g., Tobie et al., 2005, 2006; Mitri et al., 2008), and inertia (Bills and Nimmo, 2008), a fundamental quantity to determining which model is correct will require observations constrain the internal structure of Titan. The spin rate and the beyond the capability of Cassini mission. The thickness of Titan's physical librations in longitude can provide essential information ice shell has major implications for the thermal state of the crust. on the degree of internal differentiation and the ice shell thick- In fact, geological activities and processes on Titan could be related ness. Currently the only information about the rotational dynamics to the onset of thermal convection in the outer ice shell (Tobie of Titan is derived from Cassini mission data (e.g., Stiles et al., et al., 2005, 2006; Mitri and Showman, 2008). Analysis of Titan's 2008; Meriggiola et al., 2011) and astronomical observations topography suggests that the ice shell is currently in a conductive (Seidelmann et al., 2006). state (Nimmo and Bills, 2010; Hemingway et al., 2013; Mitri et al., During Cassini observations the pole location was found to be 2014); however, during Titan's evolution it is possible that the ice not fully compatible with the occupancy of a Cassini state shell may have transitioned between a convective to a conductive (Meriggiola et al., 2011). The estimated obliquity (0.31) is not in state (Mitri et al., 2008, 2010). agreement with the predicted value (0.151) inferred from the polar, The radio science experiment during the Cassini mission dimensionless moment of inertia (0.34) derived from gravity field allowed the measurement of the tidal Love number, k2. The tidal measurements (Iess et al., 2010). The interpretation of the dis- Love number k2 characterizes the tidal deformation of the interior. agreement between the measured and predicted values is con- The relatively high measured tidal Love number k2 (0.58970.150) troversial and it seems to point to a differentiated internal of Titan indicates that its outer ice shell is decoupled from the structure characterized by complex rotational dynamics. For this deep interior by a global subsurface ocean (Iess et al., 2012). The reason the pole location in terms of obliquity and the occupancy of tidal Love number k2 (Iess et al., 2012) in conjunction with a Cassini state should be investigated by a dedicated experiment. altimetric measurements of the tidal deformation of the outer Physical librations in longitude are due to the eccentricity. Titan is ice shell (Love number h2 measurement) could be used to in a 1:1 resonance and it is orbiting around Saturn at the same rate determine the thickness of the ice shell (see similar experiment of its spin motion (Meriggiola et al., 2011). Since Titan's orbit is proposed for Europa by Wahr et al. (2006)) during a future eccentric, the Saturn-satellite line and the long axis of the satellite mission. Using a nominal polar circular orbit around Titan with a are not aligned and they differ by a libration angle. The libration

1500 km altitude, the measurement of the tidal Love number k2 amplitude is inversely proportional to the moment of inertia of the σ Â À 4 fi can be obtained with an expected uncertainty k2 5 10 body, and it is possible, if the quadrupole gravity eld is known, to improving the accuracy of the determination of k2 achieved during infer the thickness (h) of the outer icy shell from the libration the Cassini mission by two orders of magnitude. In addition, radar amplitude. The determination of the libration amplitude can sounder observations with a penetration depth up to 10 km with provide crucial information on the dimension as well as on the a vertical resolution of 30 m from an orbital platform could liquid/solid state of the shell and the deep interior. The rotational directly determine the relict brittle–ductile transition of the ice dynamics of Titan can be inferred from tracking surface landmarks shell revealing its thermal state. along time using, for example, Synthetic Aperture Radar (SAR) Surface heights derived during the Cassini mission from the images with a spatial resolution of 30 to 50 m, and/or from Cassini RADAR altimeter (Elachi et al., 2004) and SAR-Topography gravity field measurements (see Cicalò and Milani (2012)). technique (Stiles et al., 2009) are sparsely distributed and cover Observations of Cassini's magnetic field on Titan show that it only 1% of Titan's surface. Using the available altimetric data, currently does not have an intrinsic field generated by a dynamo Zebker et al. (2009) and Mitri et al. (2014) determined the shape of (upper limit is 2 nT surface field at the equator). Even though the Titan up to the seventh order of the spherical harmonic expansion. Cassini mission has not been successful thus far in inferring an A quasi-global coverage of altimetric data with a 10 m vertical electromagnetic induction response from a subsurface salty ocean resolution for a future mission is necessary to refine the shape as measured during the Galileo mission for Europa, Ganymede and

Callisto, future, orbiting spacecraft can provide better information Sections 2.2.1 and 2.2.2 discuss how observations from the on the interior (subsurface ocean) from this technique. The orbital-platform will improve our understanding of the geological discovery of periodic changes in the quadrupole gravity field of processes and surface composition, respectively. Titan at 4% of the static value indicates the presence of an internal ocean (Iess et al., 2012). Further independent confirmation of Titan's subsurface ocean is necessary. 2.2.1. Geology Detection of electromagnetic induction from the interior of Our current knowledge of Titan's geology comes from Cassini Titan induced by time-varying external magnetic fields could observations, which show a geologically complex surface strik- provide such a confirmation as has been successfully demon- ingly similar to Earth's. Titan has been called the Earth of the outer strated in the discoveries of subsurface oceans of Europa, Callisto Solar System. Titan has a dense atmosphere (1.5 bar) and the and Ganymede (Khurana et al., 1998; Zimmer et al., 2000; Kivelson present-day surface–atmospheric interactions make eolian, fluvial, et al., 2002) and the magma ocean of Io (Khurana et al., 2011). The pluvial and lacustrine processes important on a scale previously basic principle of electromagnetic induction relies on the fact that seen only on Earth, despite vast differences in material properties subsurface conductors generate eddy currents on their surfaces in and ambient conditions. These atmospheric and surface processes response to time varying electric fields imposed by the changing interact to create similar geologic features to Earth's: lakes and primary magnetic field. The secondary field generated by the eddy seas filled with liquid, rivers caused by rainfall, and vast fields of currents is dipolar in nature if the conductor has a spherical shape dunes caused by wind (e.g. Lorenz et al., 2006; Stofan et al., 2007; and can be easily characterized by a spacecraft bearing a magneto- Hayes et al., 2008; Radebaugh et al., 2008; Barnes et al., 2008; Burr meter. Modeling of the secondary field provides information on et al., 2013). The scarcity of impact craters indicates that Titan's the thickness and conductivity of the subsurface conductor. For the surface is relatively young, with estimates ranging from 200 Ma to Galilean satellites, the time varying driving primary field is 1Ga(Lorenz et al., 2007; Wood et al., 2010). Mountains indicate provided by the rotating tilted magnetic field of Jupiter and its that tectonic activity has taken place (Radebaugh et al., 2007; Mitri magnetosphere and has an amplitude ranging from 40 nT near et al., 2010) although it may not necessarily be endogenic in nature Callisto to 800 nT at Io. However, Saturn's internal magnetic field is (Moore and Pappalardo, 2011). Several putative cryovolcanic completely axisymmetric and therefore does not impose a time- features have been identified (e.g. Lopes et al., 2007, 2013; varying field on its satellites. However a cyclical magnetic field of Soderblom et al., 2009) and the existence of some ongoing unknown origin has recently been detected; this field is supported volcanic activity has been proposed (Nelson et al., 2009a, by current systems in Saturn's magnetosphere (Andrews et al., 2009b), though the cryovolcanism interpretation has been dis- 2010). This magnetic field has an amplitude of 2–4 nT near Titan puted (Moore and Pappalardo, 2011). The variety of geologic and has a period close to 10.8 h which is an ideal sounding processes on Titan and their relationship to the methane cycle frequency. Assuming that the ice crust has a thickness of 100 km make it particularly significant in Solar System studies. or less, a highly conducting ocean would then create a secondary The Cassini RADAR swaths have revealed a wide variety of induced field with a magnitude of 0.6 to 1.3 nT just above Titan's geologic features, examples of which are shown in Fig. 1. However, atmosphere (assuming an RÀ3 falloff for the dipolar response) interpretations of Titan's surface properties and geologic history which would be detectable by a magnetometer. Another inducing have been hindered by observational challenges primarily related field with an amplitude exceeding 10 nT is imposed by the to its dense obfuscating atmosphere. Currently, the highest resolu- periodic exits of Titan from Saturn's magnetosphere. The several tion observations of Titan's surface come from SAR images with a nT induction response to this signal would not be sinusoidal but pixel-scale of 300 m. At this resolution, Cassini has imaged would be easily characterized by an orbiting spacecraft. Unfortu- 43% of Titan's surface. Including lower resolution HiSAR data nately, the plasma interaction of Titan with the magnetosphere of (few km pixel scale), obtained at further distances from Titan Saturn creates magnetic perturbations with an amplitude of 10 nT than standard SAR, Cassini's surface coverage increases to 60%. or more (Wei et al., 2010), therefore, it has so far not been possible A follow-up mission to Titan should image the surface at a pixel to confirm the presence of an electromagnetic induction signal scale of 30 m or better with an almost complete surface coverage from Titan. This situation can be remedied if continuous observa- using Synthetic Aperture Radar (SAR) technique. As has been tions from an orbiter were available. The spatial forms and shown in both the terrestrial and Martian communities, resolution frequency contents of magnetic fields generated by plasma inter- increases of an order of magnitude or better results in funda- action and by electromagnetic induction are distinct and can be mental advances in the interpretability of observed morphologies. easily separated by harmonic analysis if continuous observations Altimetric data with a vertical resolution of 10 m is necessary for from an orbiter were available. Such an experiment will be the geological interpretation of surface features. Both radar alti- performed by the JUpiter ICy moons Explorer – JUICE spacecraft metry and Interferometric Synthetic Aperture Radar (InSAR) are mission at Ganymede where researchers hope to disentangle the suitable techniques to determine the elevation of Titan's surface. inducing field from its ocean with an amplitude of 60 nT from Radar sounder measurements with a penetration depth up to few the much stronger permanent internal magnetic field with a km and a vertical resolution of 10–30 m are capable of character- magnitude of 1500 nT. izing the subsurface structures and determining their correlation with surface features allowing the determination of the internal 2.2. Surface processes and composition processes that form the surface geological features. In addition, radar sounder measurements will determine the thickness and A combination of measurements and observations from surface structure of surface organic-material deposits, which can be used imaging, altimetric data, spectral data and radar sounding will to ascertain the global volume of organic material on Titan. enable the achievement of the scientific objective Goal 2 (Table 1). The major geologic unit types and their possible genesis have The Cassini mission has revealed for the first time the complexity been recently reviewed by Lopes et al. (2010).Below,webriefly of the surface processes and composition of Titan. However, it is describe each of the major geomorphic units that have been still not understood if Titan is endogenically active and if cryo- identified and, where appropriate, state how higher resolution data volcanic activity is present. Other unsolved questions include could provide fundamental advancements in their interpretability. determining the role of the lakes, channel networks and dune Hummocky and mountainous terrain: This unit consists of fields in the alcanological cycle (e.g. Aharonson et al., 2012). numerous patches of radar-bright, textured terrains, some of

Fig. 1. Examples of geological features on Titan. Top row from left: Sinlap crater with its well-defined ejecta blanket. Middle: Crateriform structure (suspected impact crater) on Xanadu. Right: Ontarius Lacus near Titan's south pole. Middle row, left: Putative cryovolcanic flow Winia Fluctus (lobate, radar bright), note the dunes overlaying part of flow. The flow overlays radar-dark undifferentiated plains. Right: Dunes abutting against hummocky and mountainous terrain. Bottom left: mountains (radar bright) in the equatorial region. Right: Elivagar Flumina, interpreted as a large fluvial deposit, showing braided radar-bright channels. Deposits overlay radar-dark undifferentiated plains. which have been interpreted to be of tectonic origin (Radebaugh Titan that may have been caused by impacts. Radar-bright com- et al., 2007; Mitri et al., 2010). Among these features are long plete or incomplete circles are seen all over the surface. Wood mountain chains, ridge-like features, elevated blocks that stand et al. (2010) identified 44 possible impact craters with a wide generally isolated, and bright terrains of a hilly or hummocky variety of morphologies and, since then, others have been recog- appearance. This terrain is mostly found as isolated patches or nized and mapped. Neish et al. (2013) found Titan's craters are long mountain chains, small in areal extent. The exception is shallower than those on Ganymede, probably due to eolian Xanadu, a large area about 4500 km across. The hummocky and infilling, with possible contributions from viscous relaxation and mountainous terrain unit, including Xanadu, is thought to be the fluvial modification. Radar sounding can be used to determine the oldest geologic unit exposed on Titan's surface (Lopes et al., 2010). structure of the craters. Detailed morpho-dynamic analysis of Deciphering the origin of this feature class awaits high-resolution Titan's craters, however, requires high resolution stereo models imagery that can resolve the complex lineations and brightness that allow quantitative comparison with numerical simulations. variations that make up the hummocky appearance; altimetric Dunes: Eolian transportation and deposition is a major and data can be used to determine the elevation of this terrain. widespread process on Titan. Features resembling linear dunes on Impact craters: Few well-preserved impact craters have been Earth have been identified by SAR, covering regions hundreds, identified on Titan. However, there are numerous structures on sometimes thousands, of kilometers in extent (Lorenz et al., 2006).

Dunes are dark to RADAR and Imaging Science Subsystem (ISS) thought to be cryovolcanic in origin (Sotin et al., 2005; Barnes and correlate well with dark surface units identified by the Visual et al., 2006; Nelson et al., 2009a, 2009b). As more data were and Infrared Mapping Spectrometer (VIMS). The dunes are acquired, particularly overlapping SAR coverage from which stereo thought to be made of organic particles (Soderblom et al., 2007), could be derived, several putative cryovolcanic units were re- which may have originated as photochemical debris rained out assessed and thought to be of a different origin (Lopes et al., 2010) from the atmosphere, deposited and perhaps later eroded by leading Moore and Pappalardo (2011) to argue that there may be fluvial processes into sand-sized particles suitable for dune for- no cryovolcanic units on Titan at all. However, radar stereogram- mation (Atreya et al., 2006; Soderblom et al., 2007). Titan's sand metry has revealed a tall mountain (Doom Mons) next to a deep dunes represent a detailed geomorphic record of both current and elongated pit (depth 41 km), adjacent to several flow-like fea- past wind regimes that can reveal global changes in climate and tures. This is considered the strongest evidence for a cryovolcanic surface evolution (e.g. Radebaugh et al., 2008). Studies of Titan's region on Titan (Lopes et al., 2013). Higher resolution images and dune morphologies, how the morphologies spatially vary and topography are needed in order to further scrutinize potential how the dunes interact with local topography provide a cryovolcanic features. Radar sounder measurements can distin- basis for interpreting wind direction, sediment supply, and atmo- guish cryovolcanic features such as calderas from other features sphere conditions such as boundary layer depth. Thus dunes that have similar morphology (e.g. impact craters) imaging the represent a morphologic unit where even slight increases in image subsurface structures. resolution can reveal patterns that have significant interpretive Undifferentiated plains: There are vast expanses of plains, implications. mostly at mid-latitudes, that appear relatively homogeneous and Channels: Many radar-bright and radar-dark channels are radar-dark, and are classified as plains because they are really seen in SAR images, widely distributed in latitude and longitude. extensive, relatively featureless, and generally appear to be of low It is likely that many more channels exist on Titan, but are too relief. The units may be sedimentary in origin, resulting from small for the Cassini spacecraft SAR data to resolve. Channels are fluvial or lacustrine deposition, or they may be cryovolcanic, but seen connecting some lakes at high latitudes and cutting the their relatively featureless nature and gradational boundaries hummocky, mountainous terrain at lower latitudes, possibly prevent interpretation using SAR data alone. Radar sounder indicating orographic rainfall. Channels, lakes and dunes are measurements will determine the layering of the undifferentiated thought to be the youngest geologic units on Titan (Lopes et al., plains to determine if they are sedimentary or cryovolcanic in 2010). The Labyrinthic terrains unit, identified by Malaska et al. origin. (2010) is a probable end-member of the different types of channel Mottled plains: The mottled plains are irregularly shaped and valley network morphologies identified on Titan. There are terrains having dominantly intermediate backscatter that appear few areas of this type of terrain exposed, mostly at high latitudes. to have relatively small topographic variations. Mottled terrains Higher resolution images of Titan's channel networks can allow may be erosional and a mix of several other units, for example, pattern analysis, such as branching angle distribution, that will some may be undifferentiated plains covered over in small areas address the strength of the bedrock and origin (e.g., overland flow by isolated dunes. vs. subsurface drainage) of the network (e.g., Burr et al., 2013). Lakes: Hydrocarbon lakes are found preferentially in the polar regions at the current season on Titan (Stofan et al., 2007). While 2.2.2. Surface composition Section 3 discusses the benefits of in situ lake exploration, there is Observations of Titan's surface are severely hindered by the also much to learn from studying the morphology and distribution presence of an optically thick, scattering and absorbing atmo- of lacustrine features on Titan. Titan's lakes and seas are found sphere, allowing direct observation of the surface within only a both connected to and independent of regional and local drainage few spectral windows in the near-infrared. Based on the 1.29/ networks. They have shoreline transitions varying from sharp to 1.08 μm, 2.03/1.27 μm and 1.59/1.27 μm band ratios measured by diffuse and include a variety of plan-form shapes ranging from Cassini's VIMS, three main spectral units can be distinguished on near-circular to highly-irregular. Exposed surface areas span from the overall surface of Titan: whitish material mainly distributed in the limits of detection (1km2) to more than 105 km2 (Hayes the topographically high areas; bluish material adjacent to the et al., 2008). Along with the surrounding landscape topography, bright-to-dark boundaries; and brownish material that correlates these lacustrine and features express a range of morphologic with RADAR-dark dune fields (Barnes et al., 2007; Soderblom et al., characteristics that constrain basin formation and evolution. For 2007; Jaumann et al., 2008). Even though the spectral units are instance, the largest lakes, or seas, in the north polar region, distinct, their actual composition remains elusive. Bright materials consisting of Ligeia, Kraken and Punga Mare, have shorelines that may consist of precipitated aerosol dust composed of methane- include shallow bays with well-developed drowned river valleys derived organics (Soderblom et al., 2007) superimposed on water- (Stofan et al., 2007). Such drowned valleys indicate that once well- ice bedrock. The bluish component might contain some water ice drained upland landscapes have become swamped by rising fluid as its defining feature (Soderblom et al., 2007), even though levels where sedimentation is not keeping pace with a rising base organics cannot be ruled out. Finally, the brownish unit reveals level. These features also imply that liquid levels were previously lower relative water-ice content than Titan's average that is lower and stable long enough to allow the currently drowned compatible with an enhancement in complex organic material networks to be incised into the terrain. In contrast the largest and/or nitriles clumping together after raining onto the surface southern lake, Ontario Lacus, expresses depositional morphologies (Barnes et al., 2008). along its western shoreline that include lobate structures inter- A number of chemical species, including water ice, have been preted as abandoned deltas (Wall et al., 2010). Obtaining higher proposed to exist on the surface of Titan including carbon dioxide, resolution images and topography data of Titan's lakes will allow ammonia, ammonia hydrate, cyanoacetylene, benzene, hydrogen quantitative morphology analysis that will lead to new hypotheses cyanide, acetylene, acetonitrile, and liquid alkanes. But only a few for landscape formation and evolution. absorptions have been unambiguously detected in Titan's methane Candidate cryovolcanic terrain: The Cassini's SAR swaths also windows from remote sensing observations carried out during revealed several lobate, flow-like features and constructs inter- Cassini fly-bys. Evidence from VIMS infrared spectra suggests CO2 preted as resulting from cryovolcanism (Lopes et al., 2007; Wall frost, which is probably condensed from the atmosphere (McCord et al., 2009) and VIMS data also identified some features that were et al., 2008). Liquid ethane has been identified in Ontario Lacus

(Brown et al., 2008) and, although the exact blend of hydrocarbons The Cassini mission has revealed the presence of large organic in polar lakes and seas is unknown, liquid methane is almost molecules with high molecular masses, above 100 amu, in the certainly present. Finally, the correlation between the 5-μm-bright thermosphere and ionosphere. This is an indication of the photo- materials and RADAR-empty lakes suggests the presence of chemical production of Titan's aerosols starting in the upper sedimentary or organic-rich evaporitic deposits in dry polar atmosphere. While the identities of these molecules are still lakebeds (Barnes et al., 2011). unknown, their presence suggests a complex atmosphere that Spectral mapping of Titan's diverse surface at both high spatial could hold the precursors for biological molecules such as those and high spectral resolution could close a major gap in knowl- found on Earth. edge after the Cassini mission. Cassini's VIMS instrument is The thermal structure of Titan's atmosphere is similar to the expected to cover only a small percent of Titan's surface at Earth's, with a troposphere, a stratosphere, a mesosphere and a kilometer-or-better resolution, even after the extended mission. thermosphere; however these regions develop on a larger scale Thus a prime element in the orbiter payload is a spectral mapper. height due to Titan's lower gravity and lower temperatures. In essence this instrument addresses two main goals: high- A detached layer of haze, the result of photochemical reactions resolution near-IR imaging and spectroscopy of the surface. Other in the upper atmosphere, is visible at altitudes greater than spectral end-members will likely appear as a dedicated orbiter 520 km (Porco et al., 2005). The Voyager spacecraft showed mission improves the spatial resolution and coverage by orders of the vertical structure of the haze to consist of a main layer, located magnitude. An important augmentation beyond Cassini is that at an altitude below 200 km, separated from a detached layer of the spectral mapper should cover the wavelength range beyond haze at an altitude of 500 km by a gap of approximately 300 km; 5 μm(ideallyupto6μm). This spectral region (in a methane these two layers merge near the north pole (Porco et al., 2005). window, where the atmosphere is quite transparent) is highly The extended haze layer has been found to have reduced its diagnostic of several organic compounds expected on Titan's altitude from 500 km to 380 km between 2007 and 2010 (West surface. Cassini VIMS data has already shown some tentative et al., 2011). Cassini's images also confirmed the existence of bright signatures around 5 μm (possibly associated with aromatic and/ clouds in Titan's atmosphere, which were already inferred from or polycyclic compounds like phenanthrene, indole, etc.), how- ground-based observations (Brown et al., 2002). Titan's clouds are ever robust identification is impossible due to inadequate spec- probably composed of methane, ethane, or other simple organics. tral resolution. Cassini VIMS has shown that Titan's surface can be Their presence seems scattered and variable, interspersing the observed and mapped at 5 μm(Sotin et al., 2012) despite the low overall haze. signal-to-noise ratio of the VIMS instrument at this wavelength, A striking feature of Titan's atmospheric dynamics is the the small amount of solar flux and the small surface albedo. With strong stratospheric super-rotation in either hemisphere the current technologies, detectors in this wavelength domain (Achterberg et al., 2011). The global profile of zonal winds could provide high resolution mapping as well as diagnostic between 100 and 500 km altitude can be reconstructed from signatures of several organic compounds likely to be present on the temperature data retrieved from Cassini Composite Infrared Titan's surface either in the dune fields or on the shores of lakes Spectrometer (CIRS), except very close to the equator (Achterberg (Stofan et al., 2007; Barnes et al., 2011). Finally, correlations et al., 2011). On the other hand, the wind profile below 100 km between high-resolution infrared hyperspectral images and sur- altitude is known only at the entry site of the Huygens Probe. face images (e.g. SAR data) will enable the identification of terrain Huygens' Doppler Wind Experiment (Bird et al., 2005) found that units at various spatial scales, which are homogeneous both in the superrotation gradually weakens below 150 km altitude, but terms of surface composition (abundance of non-water-ice com- it also detected an unexpected layer of extremely weak winds ponents compared to water ice) and surface properties (e.g., near 80 km altitude and a complex wind pattern near the surface. roughness and porosity) (Tosi et al., 2010). This demonstrates that in situ measurements can detect wind patterns especially in the troposphere that cannot be detected 2.3. Atmosphere from remote measurements in other parts of the atmosphere. The wind speed and direction in selected areas of the troposphere can An instrument suite, enabling both remote and in situ measure- be constrained by cloud tracking (Turtle et al., 2011a)butonly ments and observations of Titan's atmosphere including, for where clouds exist. The observed orientation and appearance of example, lidar and infrared spectrometers on an orbiter platform dunes have been used to constrain the surface wind, but this led and mass spectrometers, physical property (temperature, pressure, to ambiguous interpretations on the prevailing surface wind humidity, wind speed) sensors and an imaging camera onboard a direction (Lorenz and Radebaugh, 2009; Rubin and Hesp, 2009; lake-probe could be used to achieve scientific objective Goal 3 Tokano, 2010). The wind direction near the surface and its (Table 1). Tobie et al. (2014, in this issue) discuss in detail the possible seasonal variation are crucial measurements for the scientific case for the in situ examination of Titan's atmosphere angular momentum budget of the atmosphere as well as using a balloon probe. length-of-day variation of the surface (Tokano and Neubauer, Unlike the other moons in the Solar System, Titan has a dense 2005). It is unknown from atmospheric observations whether nitrogen-dominated (98.4%) atmosphere, presenting unique atmo- there is an Earth-like large angular momentum exchange spheric dynamics, atmospheric chemistry and alcanonological between the atmosphere and surface, which is equilibrated on cycle. Besides nitrogen, the atmosphere consists of 1.5% methane, an annual average, or there is almost no angular momentum as well as trace amounts of hydrocarbons such as ethane, acet- exchange. Geodetic observations should ideally be accompanied ylene, and diacetylene, benzene and nitriles, such as hydrogen with wind observations to establish a link between possible cyanide (HCN), cyanogen (C2N2) and cyanoacetylene (HC3N). atmospheric forcing and response of the surface. Near-surface Somewhat more complex molecules such as propane, butane, wind measurements by a lake lander will be meaningful in this polyacetylenes, and complex polymeric materials might follow regard since the wind at high latitudes is more indicative of from these simpler units based on experiments in the laboratory. seasonal wind fluctuations than equatorial winds and there are In addition, several oxygen compounds, including water vapor, CO no apparently active dunes at high latitudes that could be used to and CO2 exist in the atmosphere, probably of exogenic origin. The guess the wind direction. Cassini–Huygens mission detected and measured many of the Another major characteristic of Titan's meteorology is the constituents in Titan's atmosphere (Coustenis and Hirtzig, 2009). methane weather, which most impressively manifests itself in

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i 8 G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ the visible clouds (Roe, 2012). During the southern summer most near the surface (Tokano and Neubauer, 2002; Mitchell et al., 2011; convective clouds were seen at high southern latitudes but the Lebonnois et al., 2012). Some of the waves may become visible as main cloud activity moved northward around the recent equinox clouds but most other waves may not. The most reliable data to (Schaller et al., 2009; Turtle et al., 2011a). More persistent clouds verify these waves are in situ time series measurements of surface at higher altitudes were seen near the north pole (Rodriguez et al., wind and atmospheric pressure over several days. The various 2009). For many years, the equatorial region was devoid of visible waves have different frequencies and amplitudes and may thus be clouds and thus was expected to be dry. However, the vertical recognized in the power spectra of the time series. profile of temperature and pressure (Fulchignoni et al., 2005) and methane mole fraction (Niemann et al., 2005) measured by Huygens found that methane condensation and weak precipita- 3. In situ exploration of a Titan sea tion in the form of drizzles occur even near the equator where no visible cloud was present (Tokano et al., 2006). On the other hand, The Cassini spacecraft has unveiled a world that is both strange occurrence of convective clouds strongly depends on the global and familiar to our own, with vast equatorial dune fields (e.g., circulation pattern, static stability and methane humidity in the Lorenz et al., 2006), well-organized channel networks that drain lower troposphere (Mitchell et al., 2011; Schneider et al., 2012). from mountains into large basins (e.g., Burr et al., 2013) and, While the vertical temperature profile down to the surface can be perhaps most astonishingly, lakes and seas (e.g., Stofan et al., 2007) sounded by radio occultation (Schinder et al., 2012), the methane filled with liquid hydrocarbons (Brown et al., 2008). Titan is the humidity is difficult to measure remotely. In situ measurement of only extraterrestrial body currently known to support standing the near-surface methane humidity in the polar region would bodies of liquid on its surface and is one of only three places in the therefore provide valuable insight into the mechanism behind the Solar System, along with Earth and Mars, that we know to posses seasonality of Titan's methane weather. There is also evidence of or have possessed an active hydrologic system (e.g., Atreya et al., substantial methane rainfall on Titan, but the evidence is indirect 2006). Just as Earth's history is tied to its oceans, Titan's origin and in that it came from temporary surface darkening and temporal evolution are chronicled within the nature and evolution of its correlation with a cloud system (Turtle et al., 2011b). The pre- lakes and seas and their interactions with the atmosphere and cipitation amount is unknown despite the huge role it plays for surface. While Cassini has provided a wealth of information Titan's geomorphology. Only direct measurements of rainfall on regarding the distribution of liquid deposits on Titan (e.g., Hayes the surface would unambiguously show in which seasons it does et al., 2008; Turtle et al., 2009; Aharonson et al., 2009), it has or does not rain in the polar region, how much it rains and the provided only the most basic information regarding their compo- chemical composition of the rains. sition (e.g., Brown et al., 2008) and role in Titan's volatile cycles The meridional circulation pattern in the troposphere pre- (e.g., Lunine and Atreya, 2008). In order to address these funda- dicted by general circulation models (GCMs) differs from model mental questions, which link the lakes to Titan's origin and to model, particularly at high latitudes. To some extent the global evolution, we must visit them in situ. In situ exploration of Titan's statistics of clouds constrain the meridional circulation pattern lakes and seas presents an unprecedented opportunity to under- (Rodriguez et al., 2009). However, clouds do not develop in the stand its hydrocarbon cycles through the abundance and varia- upwelling branch of the circulation if the near-surface air is too bility of the liquid's compositional components (methane, ethane, dry and not every condensation causes visible clouds. Therefore, a propane, etc.), glimpse into the history of the moon's evolution as more certain quantity to verify the meridional circulation at high chronicled by noble gas abundance (argon, krypton, xenon) and latitudes is the surface pressure, which can be compared to the isotopic composition (e.g., 12C vs. 13C), investigate a natural in situ surface pressure data at the Huygens landing site laboratory for prebiotic chemistry and the limits of life through (Fulchignoni et al., 2005) after a sea-level correction. A surface an examination of trace organics, and study meteorological and pressure higher and lower than the Huygens pressure would marine processes in an exotic environment that is drastically indicate downwelling and upwelling, respectively, in the season distinct from terrestrial experience (scientific objectives 1, 2, 3, of observation. and 4, Table 1). Theoretical studies predict some significant changes when the Titan's lakes are found mostly poleward of 551 latitude and north polar lake area approaches summer during the next few encompass 1.2% of the surface that has been observed (50%) by years. Seasonal change in the global circulation pattern may cause Cassini's instruments (Hayes et al., 2008, 2011). Lacustrine features substantial precipitation in the north polar region (Schneider et al., are predominantly found in the north, where 11% of the observed 2012), warming of the sea surface may cause a sea breeze/land area poleward of 551N(60%) is covered by radar-dark lakes. breeze depending on the sea composition (Tokano, 2009), fresh- Poleward of 551S, only 0.5% of the observed area (60%) has been ening of the near-surface wind may cause previously undetected interpreted as potentially liquid-filled. This dichotomy has been waves on the seas (Hayes et al., 2013) and evaporation of a large attributed to Saturn's current orbital configuration, in which amount of methane from the seas might even cause tropical Titan's southern summer solstice occurs nearly coincident with cyclones (Tokano, 2013). These meteorological features cause perihelion and results in a 25% higher peak flux than what is substantial surface pressure variations and thus can be detected encountered during northern summer (Aharonson et al., 2009). in situ by continuous surface pressure measurements and optical Over many seasonal cycles, this flux asymmetry can lead to net monitoring of the environment. The unknown exact chemical transport of volatiles (methane/ethane) from the south to the composition in the sea, as well as in the air over sea, is important north. As the orbital parameters shift, however, the net flux of for many questions related to the polar region. Evaporation of northward-bound volatiles is expect to slow and eventually methane from a sea itself is difficult to observe in situ unless reverse, predicting a liquid distribution opposite of today's in accompanied with fog, but it can be quantified from the methane 35 kyr (i.e., more liquid in the south than the north). If this vapor pressure and methane partial pressure. The mole fractions hypothesis is correct, the distribution of liquid deposits on Titan is of other species in the sea and air over sea would indicate whether expected to move between the poles with a period of 50 kyr in a the lake and atmosphere are in short-term or long-term chemical process analogous to Croll–Milankovich cycles on Earth. In situ equilibrium, if any. measurement and comparison between the relative abundance of GCMs predict gravitational tides and various waves in the volatiles that are mobile over these timescales (e.g., methane, troposphere, which affect the wind, pressure and temperature ethane) vs. those that are involatile (e.g., propane, benzene), can be

Fig. 2. Distribution of lacustrine and fluvial morphologies in Titan's polar regions (see also Hayes et al. (2008) and Aharonson et al. (2012)). Blue polygons depict modern lakes and seas filled with liquid hydrocarbons. The red polygons show the locations of radar-bright basins interpreted as paleolakes. Cyan polygons depict the distribution of albedo-dark features that are above the radar noise floor and express lacustrine-like morphologies. These features are believed to represent anything from mud flats to shallow liquid deposits. Fluvial valleys are represented by blue lines. Dashed lines were drawn on ISS images while the remaining polygons were created using Synthetic Aperture Radar (SAR) images generated from Cassini RADAR observations. SAR incidence angle and swath coverage are shown. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

provided a wide range of evidence that the largest of Titan's lakes are indeed liquid-filled basins (e.g., Brown et al., 2008), the returned data have only been able to provide lower limits on their depth that are highly dependent on the unknown dielectric properties of the both the liquid and the lakebed (e.g., Paillou et al., 2008; Hayes et al., 2010, 2011). In the absence of a substantial subsurface reservoir, these maria represent the dominant liquid hydrocarbon inventory on Titan. Prior to Cassini, a global ocean of liquid hydrocarbon was predicted to cover the surface and act as both a source of atmospheric methane and sink for photolysis products such as ethane and higher order hydrocarbon/nitriles (Lunine et al., 1983). The absence of this global reservoir questions the long-term stability of Titan's atmosphere, which would be depleted in 107 years in the absence of a crustal source or methane outgassing from the interior (e.g., Tobie et al., 2006). Determining the bathymetry of one of Titan's Mare, which can be reliably accomplished in situ, is essential to understanding the total volume of liquid available for interaction with the atmosphere. Remote sensing radar techniques should also be used to determine Fig. 3. Cassini SAR mosaic images of the north polar region showing Kraken, Ligeia the lakes' bathymetry. The inventory of methane in Titan's maria, – and Punga Maria. Black yellow color map was applied to the single band data. (For which requires knowledge of both depth and composition, will interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) provide a lower limit on the length of time that the lakes can sustain methane in Titan's atmosphere (Mitri et al., 2007) and help to quantify the required rate of methane resupply from the interior used to test this hypothesis and understand volatile transport on and/or crust. Similarly, the absolute abundance of methane photo- thousand year timescales. Volatile transport over shorter time- lysis products (e.g., ethane, propane) will determine a lower limit for scales (diurnal, tidal, and seasonal) can be investigated via in situ the length of time that methane has been abundant enough to drive measurements of the methane evaporation rate and associated photolysis in the upper atmosphere and deposit its products onto the meteorological conditions (e.g., wind speed, temperature, humid- surface and, ultimately, into the lakes. Furthermore, a comparison ity). These measurements can be used to ground-truth methane amongst the relative abundance of photolysis products could reveal transport predictions from global climate models (e.g., Mitchell, fractionation processes such as crustal sequestration, as would 2008; Tokano, 2009; Schneider et al., 2012). happen if one constituent (e.g., propane) were preferentially intro- In the north, 87% of the area of observed lacustrine deposits is duced into a repository such as crustal clathrate hydrate. contained within the three largest seas, Kraken Mare (496,380 km2), Similar to the Earth's oceans, Titan's seas record a history of Ligeia Mare (133,160 km2), and Punga Mare (49,400 km2)(Figs. 2 and their parent body's origin and evolution. Specifically, the noble gas 3). Bodies of standing liquid on Titan with large dimensions are and isotopic composition of the sea can provide information classified as seas (mare) rather than lakes (lacus). The remainder of regarding the origin of Titan's atmosphere, reveal the extent of north polar lakes approximately follow a lognormal distribution with communication with the interior, potentially constrain the condi- ameandiameterof10 km (Hayes et al., 2008). While Cassini has tions in the Saturn system during formation, and refine estimates

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i 10 G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ of the methane outgassing history. The Huygens probe detected a Finally, lacustrine settings on Titan represent exotic environ- small argon abundance in Titan's atmosphere, but was unable to ments that are being sculpted by many of the same meteorological detect the corresponding abundance of xenon and krypton and oceanographic processes that affect lake and ocean settings on (Niemann et al., 2005). The non-detection of xenon and krypton Earth. In situ exploration of such an environment provides an supports the idea that Titan's methane was generated by serpen- opportunity to study these processes under vastly different envir- tinization of primordial carbon monoxide and carbon dioxide onmental conditions (scientific objectives 2 and 4, Table 1). While delivered by volatile depleted planetesimals originating from the Huygens probe provided atmospheric data over equatorial within Saturn's subnebula (e.g., Atreya et al., 2006). A competing latitudes around winter solstice, models of Titan's climate predict model of methane origination, however, suggests that methane is that polar weather could be quite different from the equatorial primordial and was sourced from planetesimals originating from environment investigated by Huygens. In situ measurements of the outer solar nebula (Mousis et al., 2009). This competing model such an environment would provide invaluable ground-truth for was originally suggested as an attempt to explain the measured global climate models (GCM) of Titan's surface/atmosphere inter- À ratio of deuterium/hydrogen (D/H) (1:32 Â 10 4), which Mousis actions. Ultimately, this could lead to a better understanding of et al. (2009) suggest would require a D/H ratio in Titan's water ice how to accurately predict the evolution of our own climate system that does match that of Enceladus. In order to support a primordial and even how to model extreme climates on extrasolar planets, as methane source, xenon and krypton would both have to be Titan may represent a common climate system amongst solid sequestered from the atmosphere. While xenon is soluble in liquid bodies in Earth-like orbits around M-dwarf stars. hydrocarbon (solubility of 10À3 at 95 K) and could potentially be sequestered into liquid reservoirs, argon and krypton cannot. Therefore, the absence of measureable atmospheric krypton 4. Scientific investigations, measurements and approaches requires either sequestration into non-liquid surface deposits, such as clathrates (Mousis et al., 2011), or depletion in the noble All of the previously discussed objectives of the proposed gas concentration of the planetesimals that originally delivered exploration of Titan and the broader Saturnian system (Table 1) Titan's atmosphere (Owen and Niemann, 2009). The low, albeit can be achieved by complementary investigations of a dedicated measureable, argon-to-nitrogen ratio has been used to suggest orbiter and a lake probe. The in situ exploration of one of Titan's that Titan's atmospheric nitrogen was sourced from ammonia, as seas can contribute to the achievement of all of these scientific ammonia is significantly less volatile than both molecular nitrogen objectives. Table 2 summarizes the scientific investigations, mea- and argon (Niemann et al., 2005), which have similar volatility. surements and examples of approaches considered for an explora- The presence of xenon in Titan's seas would indicate that the tion of Titan from an orbiter and lake probe and ties them to the absence of atmospheric krypton is a result of sequestration into scientific objectives presented in Table 1. crustal sources (Mousis et al., 2011) as opposed to the result of carbon delivery via depleted planetesimals (Owen and Niemann, 2009). In addition to noble gas concentration, isotopic ratios can 5. Mission concept legacy also be used to decipher the history of Titan's atmosphere. For 13 12 example, the C/ C ratio of CH4 was used by Mandt et al. (2009) Since the Cassini spacecraft and Huygens probe arrived at Titan to conclude that methane last outgassed from the interior 107 in 2004, there have been many proposed mission concepts using a years ago. However, this calculation assumes that the exposed variety of vehicles, including orbiters, landers, balloons and of methane reservoir has an isotopic composition that is in equili- course, nautical probes, including floating, propulsion and sub- brium with the atmosphere. If the carbon isotope ratio of hydro- mersible types, to expand both orbital and in situ exploration of carbons in Titan's lakes/seas were found to be different than in the Titan inspired by the initial discoveries of the Cassini Huygens atmosphere, it would imply alteration of the isotopic composition mission. In spring of 2013, ESA put out a request for White Papers by evaporation and condensation and indicate a different time- to the scientific community to propose Science Themes for its scale for the history of methane outgassing. large (L)-scale L2 and L3 missions. The exploration of Titan with an Titan's lakes and seas collect organic material both directly, orbiter and a lake probe was presented as a White Paper in through atmospheric precipitation of photolysis products, and response to this request, which is presented in an extended form indirectly, through eolian/fluvial transport of surface materials in this paper (see Supplementary material). A companion White (e.g., river systems flowing into the Mare). As a result, the lakes Paper was submitted, which presented the scientific case for the and seas represent the most complete record of Titan's organic exploration of Titan with an orbiter and a balloon probe (Tobie complexity and present a natural laboratory for studying prebiotic et al., 2014, in this issue). The concept for a mission capable of organic chemistry. Titan's environment is similar to conditions on implementing the science described in previous sections is in part Earth four billion year ago and presents an opportunity to study based on the Titan and Enceladus Mission TandEM (Coustenis active systems involving several key compounds of prebiotic et al., 2009a) and the Titan Explorer (Leary et al., 2007; Lockwood chemistry (Raulin, 2008). For instance, the complex aerosols pro- et al., 2008) mission proposals for ESA and NASA respectively and duced in Titan's atmosphere via photolysis, which are presumably which were merged into the joint space agency Titan Saturn transported into the lake via pluvial, fluvial, and eolian processes, System Mission Study TSSM (Coustenis et al., 2010; ESA, 2009) are similar to the initial molecules implicated in the origin of life on for consideration for a Large (L) class or Flagship type mission. The Earth. Other organics can act as tracers for specific environmental TSSM mission proposal included an orbiter with two types of conditions. For example, the presence of oxygen-bearing organic probe vehicles to enable in situ exploration of Titan: a balloon and species would suggest that redox reactions have taken place and a lake lander, with NASA responsible for the orbiter and ESA that the organic deposits have, at some point, been in at least responsible for the probe vehicles. The target of the proposed lake transient contact with liquid water. In general, simple abiotic probe was the Kraken Mare. In addition, TSSM included a tour of organic systems exhibit monotonically decreasing abundance with the Saturnian system with flybys at Enceladus. The proposed increasing chain complexity. Distinctive deviations from such a mission would have had a launch date between 2023 and 2025, distribution can mark the signs of prebiotic chemistry and poten- however ultimately the mission was not chosen for further studies tially represent the initial markers on a pathway toward self- by the involved space agencies. This initiated several proposals for replication and a minimally living system. smaller type missions in scale and cost for both lake, Titan Mare

Table 2 Science Investigations, measurements and example of approaches.

Science investigations Measurements Example of approaches

Internal structure Determine the internal structure Gravity field S/C radio-tracking to measure static gravity field (Goal 1) Obliquity, libration, occupancy of a Cassini state Imaging (e.g. SAR) (S/C) with spatial resolution 30 to 50 m S/C radio-tracking to measure static gravity field Gravity anomalies in the ice shell and High order gravity field components S/C radio-tracking to measure static gravity field deep interior (Goal 1) Topography Altimetry/stereo imaging/interferometric topography (S/C) with vertical resolution of 10 m Present tidal quality factor Q (Goal 1) Time variation of the degree-2 gravity field S/C radio-tracking to measure time dependent gravity field Hydrostatic compensation (Goal 1) Gravity field S/C radio-tracking to measure static gravity field Topography Altimetry/stereo imaging/interferometric topography (S/C) with vertical resolution 10 m

Subsurface ocean Extent of the subsurface ocean Time variation of the degree-2 gravity ﬁeld (k2) S/C radio-tracking to measure static gravity ﬁeld (Goal 1) Detection of electromagnetic induction from a Magnetometer measurements (S/C) subsurface ocean

Tidal deformation (h2) Altimetry and/or interferometric topography (S/C) with 1 m vertical resolution Libration and obliquity Imaging (e.g. SAR) (S/C) with 30–50 m resolution S/C radio-tracking to measure the static gravity ﬁeld

Ice shell and geology Thickness of the ice shell (Goal 1) Time variation of the degree-2 gravity ﬁeld (k2) S/C radio-tracking to measure time dependent and composition gravity ﬁeld

Tidal deformation (h2) Altimetry and/or interferometric topography (S/C) with 1 m vertical resolution Libration SAR imaging (S/C) S/C tracking to measure the static gravity ﬁeld Characterize the surface and Global mapping of the surface SAR imaging (S/C) subsurface structures and the Global topography Altimetry/stereo imaging/interferometric topography determine their correlation (Goals (S/C) 1 and 2) Proﬁles of subsurface structures and thickness of the Radar sounding (S/C) with 10 km penetration surface organic material layer depth and 30 m vertical resolution Correlate the surface composition Global mapping of the surface SAR imaging (S/C) with 30 to 50 m resolution and their distribution with surface Global mapping of the surface composition Infrared spectral imaging (S/C) within the methane structures (Goal 2) band windows

Atmosphere Characterize the atmospheric Determine the dynamics and thermal state of the Infrared spectra imaging of the atmosphere (S/C) circulation and the composition of atmosphere including the clouds Lidar (S/C) the atmosphere from orbit and Determine the composition Infrared spectra imaging of the atmosphere (S/C) during probe descent (Goal 3) Mass spectrometry with direct sampling from orbit (S/C) and during decent (L/P) Lidar (S/C) Characterize the lake/atmosphere Determine temperature, pressure, composition, Mass spectrometry (L/P) interactions in situ (Goal 3) evaporation rate and physical properties that Physical properties package (L/P) characterize lake and atmosphere interactions

Lakes/seas Characterize lakes/seas and their Lake and sea composition, including low/high mass Mass spectrometry/chemical analyzer (L/P) composition/chemistry hydrocarbons, noble gases, and carbon isotopes Atmosphere physical properties package (astrobiological potential) (temperature sensor, barometer, anemometer)/ (Goals 4, 1, 2, and 3) meteorological package/thermo-gravimetry (L/P) Exchange processes at the sea-air interface to help Atmosphere physical properties package constrain the methane cycle (temperature sensor, barometer, anemometer)/ meteorological package (L/P) Presence and nature of waves and currents Physical properties package (L/P) Surface imaging (250 μrad/pixel) Properties of sea liquids including turbidity and Lake physical properties package (turbidity and dielectric constant dielectric constant measurements) (L/P) Sea depths to constrain basin shape and sea volume Sonar (L/P) Shoreline characteristics, including evidence for past Surface Imaging (250 μrad/pixel) changes in sea level Descent camera imaging/stereogrammetry (L/P)

Explorer (TiME) (Stofan et al., 2010, 2013) and balloon, Titan Aerial the lower atmosphere of Titan's equatorial regions for 3–6 months. Explorer (TAE) (Lunine et al., 2011) probes within NASA and ESA While both TiME and TAE discarded the orbiter, another NASA respectively. To reduce scale and cost, both proposals did not Discovery candidate, Journey to Enceladus and Titan (JET), jet- include an orbiter and both probes would use Direct-to-Earth tisoned the probes and proposed examination of Enceladus' plume (DTE) communication to relay data back to Earth. TiME was a and Titan's atmosphere and surface using only an orbiter with an floating lake probe that would drift across Ligeia Mare for 3–6 instrument suite consisting of two instruments and a radio science months investigating lake composition and processes as well as experiment (Sotin et al., 2011). JPL prepared a study on Titan lake local meteorology and atmosphere. TiME was a finalist for NASA's probe missions for the NASA Planetary Science Decadal Survey and Discovery mission. TAE was a proposal in response to ESA's call for investigated several potential mission architectures and nautical a medium (M) M3 class mission for its Cosmic Vision (2015–2025) probe types for the exploration of Titan's lakes and seas including program and included a pressurized balloon that would explore a relay orbiter with a nautical probe with floating and submersible

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i 12 G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ components, a floating probe with DTE communication capabil- gravity field measurements and maximizes the imaging coverage ities and submersible and floating probes both with relay com- of the surface. A 1500 km altitude orbit has negligible drag effect munication capabilities (JPL, 2010). on the S/C; however lower altitude orbits are also possible Several other possible missions for in situ examination of Titan increasing the orbit maintenance. After orbit insertion around were proposed in the literature including an airplane (AVIATR) Titan, mapping of Titan with remote sensing experiments will probe that called for aerial reconnaissance of Titan's atmosphere begin, lasting from several months to a few years. The lake probe (Barnes et al., 2010) and a propelled lake probe known as Titan will relay data to the orbiter, which will serve as the communica- Lake In-situ Sampling Propelled Explorer TALISE (Urdampilleta tions link between the probe and Earth. Direct-to-Earth (DTE) et al., 2012). communication from the lake lander (feasible with Earth view from northern polar seas starting from 2036 to 2050 s and later) allows for augmented communication and tracking (Coustenis 6. Proposed mission concept et al., 2009b). Scenarios for the lake probe are (1) the lake probe will have propulsion capabilities and mobility, (2) the lake probe The proposed mission concept consists of two elements, the will not have propulsion capabilities, rather it will float around the spacecraft (S/C), and a lake probe (lander). The S/C will carry a lake driven by winds or waves (although they are expected to be scientific payload consisting of remote sensing experiments and an rather small) and possible tides. Fig. 5 shows simulations of five Entry, Descent and Landing (EDL) module containing a lake lander splashdowns in Ligeia Mare and subsequent drift tracks using equipped with an instrument suite capable of carrying out in situ winds predicted by the GCM (Lorenz et al., 2012). The wind measurements of Titan's polar lakes. During the descent the probe direction varies over the course of a Titan day, and so the will make in situ measurements of the atmosphere. Once a trajectories have loops and curves. For predicted winds in the successful splashdown has been achieved, the lake lander will be take measurements, including the sampling of both the liquid of the sea and the low-atmosphere. A valuable augmentation may be the capability to sample solid material on the shoreline or on the shallow sea-bed. The possible targets of the lake probe are Ligeia Mare (781N, 2501W) and Kraken Mare (681N, 3101W). Fig. 4 shows the design of the lake probe and the internal view with the location of the scientific instruments by the MasterSAT10 Team at University La Sapienza (Italy). The duration of the cruise from Earth to Saturn is estimated to be on the order of 10 years or less, depending on the mass of the spacecraft and the gravitational pull options. Following orbit insertion at Saturn, the S/C will conduct a tour of the satellite system, with the possibility of several flybys at Enceladus, while flybys at Titan would be used both as a scientific opportunity and as a way to modify the trajectory in order to finally enter orbit around Titan. A different mission scenario proposes that the orbit around Titan could be achieved through an aero-capture maneu- ver, followed by other maneuvers to refine the spacecraft's final 1500 km circular high-inclination orbit. Release and entry of the lake probe, and relay of its data to Earth will take place either during (1) Titan flybys phase, or (2) Titan orbital phase. The Fig. 5. Simulations of five splashdowns in Ligeia Mare and subsequent drift tracks architecture of the mission will be defined by the scientists and using winds predicted by the GCM (see Lorenz et al. (2012)). The wind direction the agencies in dedicated studies. We considered as a possible varies over the course of a Titan day, and so the trajectories have loops and curves. fi For predicted winds in the season shown (2023) the vehicle makes landfall on the nominal nal orbit of the S/C (orbiter) around Titan a 1500 km western shore of Ligeia after several weeks, except in one instance where it reaches altitude circular near-polar orbit. A polar orbit optimizes the an island.

Fig. 4. Design of the lake probe and the internal view of the scientiﬁc instruments (Courtesy of the MasterSAT10 Team at University La Sapienza, Italy).

Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i G. Mitri et al. / Planetary and Space Science ∎ (∎∎∎∎) ∎∎∎–∎∎∎ 13 season shown (2023) the vehicle would make landfall on the Atreya, S.K., Adams, E.Y., Niemann, H.B., Demick-Montelara, J.E., Owen, T.C., western shore of Ligeia after several weeks, except in one instance Fulchignoni, M., Ferri, F., Wilson, E.H., 2006. Titan's methane cycle. Planet. Space Sci. 54, 1177–1187. where it reaches an island. Barnes, J.W., Brown, R.H., Radebaugh, J., Buratti, B.J., Sotin, C., Le Mouelic, S., The power source both for the S/C and the lake probe is critical Rodriguez, S., Turtle, E.P., Perry, J., Clark, R., Baines, K.H., Nicholson, P.D., 2006. for this mission on Titan and the Saturn system. Preliminary Cassini observations of flow-like features in western Tui Regio, Titan. Geophys. Res. Lett. 33, L16204. http://dx.doi.org/10.1029/2006GL026843. analysis indicates that, to meet power requirements through solar Barnes, J.W., Brown, R.H., Soderblom, L., Buratti, B.J., Sotin, C., Rodriguez, S., power, the orbiter would need large solar arrays (total surface Le Mouèlic, S., Baines, K.H., Clark, R., Nicholson, P., 2007. Global-scale 130–200 m2). Given the opacity of Titan's atmosphere and the low surface spectral variations on Titan seen from Cassini/VIMS. Icarus 186, – solar elevation from the polar seas, the use of a solar powered 242 258. Barnes, J.W., Brown, R.H., Soderblom, L., Sotin, C., Le Mouèlic, S., Rodriguez, S., generator for the lake-probe is not feasible. The use of batteries for Jaumann, R., Beyer, R.A., Buratti, B.J., Pitman, K., Baines, K.H., Clark, R., the lake probe will only provide power to the lander on the order Nicholson, P., 2008. Spectroscopy, morphometry, and photoclinometry of fi – of hours. 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Please cite this article as: Mitri, G., et al., The exploration of Titan with an orbiter and a lake probe. Planetary and Space Science (2014), http://dx.doi.org/10.1016/j.pss.2014.07.009i