Titan: Earth-like on the Outside, Ocean World on the Inside
Shannon M. MacKenzie, Samuel P.D. Birch, Sarah Horst, Christophe Sotin, Erika Barth, Juan M. Lora, Melissa G. Trainer, Paul Corlies, Michael J. Malaska, Ella Sciamma-O'Brien, Alexander E. Thelen, Elizabeth P. Turtle, Jani Radebaugh, Jennifer Hanley, Anezina Solomonidou, Claire Newman, Leonardo Regoli, Sebastien Rodriguez, Benoit Seignovert, Alexander G. Hayes, Baptiste Journaux, Jordan Steckloff, Delphine Nna-Mvondo, Thomas Cornet, Maureen Palmer, Rosaly M.C. Lopes, Sandrine Vinatier, Ralph Lorenz, Conor Nixon, Ellen Czaplinski, Jason W. Barnes, Ed Sittler, Andrew Coates
DDraft version February 18, 2021
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Titan: Earth-like on the Outside, Ocean World on the Inside
Shannon M. MacKenzie, Samuel P.D. Birch, Sarah H¨orst, Christophe Sotin, Erika Barth, Juan M. Lora, Melissa G. Trainer, Paul Corlies, Michael J. Malaska, Ella Sciamma-O’Brien, Alexander E. Thelen, Elizabeth Turtle, Jani Radebaugh, Jennifer Hanley, Anezina Solomonidou, Claire Newman, Leonardo Regoli, S´ebastien Rodriguez, Benˆoit Seignovert, Alexander G. Hayes, Baptiste Journaux, Jordan Steckloff, Delphine Nna-Mvondo, Thomas Cornet, Maureen Palmer, Rosaly M.C. Lopes, Sandrine Vinatier, Ralph Lorenz, Conor Nixon, Ellen Czaplinski, Jason W. Barnes, Ed Sittler, andAndrew Coates Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA Massachusetts Institute of Technology Johns Hopkins University Jet Propulsion Laboratory, California Institute of Technology Southwest Research Institute, Boulder, Colorado, USA Yale University NASA Goddard Space Flight Center NASA Ames Space Science and Astrobiology Division, Astrophysics Branch, USA Department of Geological Sciences, Brigham Young University, S-389 ESC Provo, UT 84602, United States Lowell Observatory California Institute of Technology, Pasadena, CA, USA Aeolis Research, 333 N. Dobson Road, Unit 5, Chandler, AZ 85224, USA Universit´e de Paris, Institut de Physique du Globe de Paris, CNRS Laboratoire de Plan´etologie et G´eodynamique, Universit´e de Nantes, Nantes, France Cornell University, Ithaca NY, USA University of Washington Planetary Science Institute University of Maryland Baltimore County, Center for Space Sciences and Technology, Baltimore, Maryland, USA European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villanueva de la Canada, Madrid, Spain Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721, USA Jet Propulsion Laboratory, California Institute of Technology. LESIA, Observatoire de Paris, Universit´e PSL, CNRS, Sorbonne Universit´e, Universit´e de Paris, 5 place Jules Janssen, 92195Meudon, France University of Arkansas Department of Physics, University of Idaho, Moscow, Idaho, USA Mullard Space Science Laboratory, University College London (Received 10 February 2020; Revised; Accepted)
Submitted to PSJABSTRACTThanks to the
Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager en-counters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offeringunparalleled opportunities to explore prebiotic chemistry. And while
Cassini-Huygens revolutionizedour understanding of each of the three “layers” of Titan—the atmosphere, the surface, and the inte-rior—we are only beginning to hypothesize how these realms interact. In this paper, we summarizethe current state of Titan knowledge and discuss how future exploration of Titan would address someof the next decade’s most compelling planetary science questions. We also demonstrate why exploring
Corresponding author: Shannon M. [email protected] a r X i v : . [ a s t r o - ph . E P ] F e b MacKenzie et al.
Titan, both with and beyond the
Dragonfly
New Frontiers mission, is a necessary and complementarycomponent of an Ocean Worlds Program that seeks to understand whether habitable environmentsexist elsewhere in our solar system.
Keywords: editorials INTRODUCTIONAt the turn of the millennium, advancements in ground-based and space-based telescopes enabled the first glimpsesof Titan’s surface (e.g. Smith et al. 1996; Meier et al. 2000; Coustenis et al. 2001, 2003; Griffith et al. 2003). Theheterogeneous surface albedo was inconsistent with a global ocean, immediately prompting discussion over the fateof ethane, anticipated to be one of the most abundant products of the atmospheric photochemistry (Lunine et al.1983). When
Cassini-Huygens arrived in the Saturn system in 2004, a slew of fundamental advances were enabled bythe combined in situ and remote sensing observations over the next 13 years. Newly identified features and processesprompted new questions, many of which will remain unanswered until a return mission to Titan (e.g. Nixon et al.2018; Rodriguez in review).In light of these revelations–that Titan is a an organic-rich, ocean world where Earth-like geological processes reworkthe landscape and the complex atmospheric products that fall upon it–Titan was considered a target of high importancegoing into the 2012-2023 Decadal Survey (National Research Council 2011). Similar to the Titan Explorer study of2007 (Lockwood et al. 2008; Lorenz et al. 2008a), the Titan Saturn System Mission concept study (TSSM) employeda comprehensive, three-pronged approach: an orbiter, a lander (targeting the lander to the northern lakes rather thanthe equatorial dunes), and a montgolfiere (Coustenis et al. 2009a; Reh 2009). This particular mission architecturewas ultimately not put forward as the highest priority, instead deferred to the next decade “primarily because of thegreater technical readiness of [the Europa flagship mission]”. The Vision and Voyages report further noted that “[aTitan-returning mission’s] high scientific priority, however, is especially noteworthy” and thus recommended continueddevelopment of the technologies needed to support such a mission.As the 2012-2023 decade unfolded, however, new technologies, scientific revelations, and congressional inertia mo-tivated the addition of Titan and Enceladus to the New Frontiers 4 competition. With the selection of
Dragonfly ,some–but not all–of the high priority science identified by the TSSM will be addressed. Where, then, does Titanscience stand now and what questions will be beyond the scope of
Dragonfly ? How is the exploration of Titan’s atmo-spheric, surface, and subsurface processes relevant to Ocean Worlds and other planets? We discuss the answers to thesequestions as the community enters the purview of a new decadal survey, stemming from considerations submitted as awhite paper by these same authors to the 2023-2032 National Academies Planetary Science and Astrobiology DecadalSurvey. TITAN IS AN ORGANIC WORLDTitan hosts the most Earth-like atmosphere in the solar system. Similarities include the atmospheric structure(Fulchignoni et al. 2005) (Figure 1), a nitrogen-dominated composition (95% N , 4% CH , and 1% trace species atthe surface), and a surface pressure of 1.5 bar. The photolytic destruction of atmospheric methane initiates a chainof photochemical reactions responsible for the plethora of organic species that make up the haze observed by Cassini-Huygens and ground-based facilities (Marten et al. 2002; Gurwell 2004; Ali et al. 2013; Cordiner et al. 2014, 2015,2018; Cordier & Carrasco 2019; Molter et al. 2016; Desai et al. 2017; Lai et al. 2017; Palmer et al. 2017; Teanby et al.2018; Lombardo et al. 2019; Thelen et al. 2019b,a, 2020; Nixon et al. 2020). While unlike present-day Earth, Titan’shaze production may be similar to processes on Early Earth (Trainer et al. 2006; Trainer 2013).2.1.
Pressing Questions and Future InvestigationsCassini identified the compositions of neutral and positive ions up to 99 Da but could only detect the presence ofnegative ions with a mass-to-charge ratio so large as to be similar to that of terrestrial proteins (Coates et al. 2007;Waite et al. 2007; Wellbrock et al. 2013; Woodson et al. 2015). These molecules grow larger during polar winter andwith decreasing altitude (Wellbrock et al. 2019), but their ultimate fate remains unknown. Thus, our current list ofknown compounds represents only the tip of the organic factory iceberg. Ongoing modeling efforts and laboratoryexperiments continue to investigate what reactions might be at work (Rannou et al. 2004; Waite et al. 2007; Vuittonet al. 2007, 2019; Krasnopolsky 2009, 2014; Nixon et al. 2012; Waite et al. 2013; Larson et al. 2014; Dobrijevic et al. itan: Earth-like on the Outside, Ocean World on the Inside Figure 1. (top) Comparison of the temperature profiles and structure of the atmospheres of Titan and Earth. Verticalbars at left show the regions probed by Cassini instruments (Ion and Neutral Mass Spectrometer, INMS; CAssini PlasmaSpectrometer, CAPS; Radio and Plasma Wave Science, RPWS; UltraViolet Imaging Spectrometer, UVIS; Imaging ScienceSubsystem, ISS; Visual and Infrared Mapping Spectrometer, VIMS; Composite InfraRed Spectrometer, CIRS), the HuygensGas Chromatograph Mass Spectrometer (GCMS), and the Atacama Large Millimeter/submillimeter Array (ALMA) telescope.(bottom) Similar to the cycling of water on Earth, Titan’s troposphere hosts the hydrological cycle of methane between thesurface and on atmosphere. Irreversible loss of methane in the thermosphere suggests that a methane replenishment mechanismis necessary.
MacKenzie et al.
Cassini-Huygens . How seasonal trends incondensate distributions extend to lower altitudes (Coates et al. 2007; Desai et al. 2017) and whether these affectsedimentation onto the surface remains unknown. Models indicate that microphysics plays an important role in cloudand haze formation (Barth 2017); increased understanding of the distribution, optical properties, and compositionof hazes and clouds from observational (at Titan and ground-based) (Jennings et al. 2012a,b, 2015; Vinatier et al.2012; Seignovert et al. 2017, 2021; Le Mou´elic et al. 2018; West et al. 2018; Anderson et al. 2018b) and laboratorydata (Anderson et al. 2018a; Nna-Mvondo et al. 2019) would constrain physical models (e.g. Loison et al. 2020).Several approaches to constrain the age of Titan’s atmosphere overlap at ∼ Role in an Ocean Worlds Program
Atmospheric organic species are the ultimate source of the organic sediments that dominate Titan’s surface, sotheir formation and evolution in the atmosphere have important implications for surface geology and possible sub-surface nutrient availability. Furthermore, investigating the processes that create complex species in Titan’s atmo-sphere—without, presumably, biological catalysts like those responsible for large molecules here on Earth—offersfundamental insight into the chemistry that may precede or facilitate the rise of biochemistry on Early Earth (Traineret al. 2006; Trainer 2013) and beyond. The study of Titan’s atmospheric chemistry therefore offers crucial context forthe habitability potential of other ocean worlds where the essential elements may be less abundant.2.3.
Relevance to other planets
Questions surrounding the dynamics and longevity of Titan’s atmosphere link to questions about the gas and icegiants (Robinson et al. 2014; Toledo et al. 2019) and—given the coupling between the atmosphere and surface—aboutEarth, Venus, Mars, and Pluto (Mitchell et al. 2014; Mandt et al. 2015; Brain et al. 2016; Guendelman & Kaspi 2018;Read & Lebonnois 2018; Crismani et al. 2019; K¨ohn et al. 2019; Faulk et al. 2020; Kite et al. 2020) (Figure 2). Withoutits own magnetic field, Titan’s interactions with the solar wind and Saturn’s magnetosphere offer the opportunity toexplore whether magnetic fields are necessary for habitability. Moreover, Titan’s atmosphere serves as a powerfulbackyard analog for hazy exoplanets—from understanding the formation and evolution of atmospheric aerosols to howwe might best detect and observe them—as we have ground truth from both remote and in situ sensing (de Kok &Stam 2012; Forget & Leconte 2014; Tokano 2015; Arney et al. 2016; Checlair et al. 2016; Mu {\ ˜n } oz et al. 2017; Heet al. 2017; H¨orst et al. 2018; Levi & Cohen 2019; Lora et al. 2018; Alvarez Navarro et al. 2019; Mart´ınez-Rodr´ıguezet al. 2019; Miguel 2019). TITAN IS AN ACTIVE HYDROLOGICAL AND SEDIMENTARY WORLDTitan also has a methane-based hydrologic cycle akin to Earth’s (Mitchell & Lora 2016; Hayes et al. 2018). Theseasonal timing and magnitude of surface-atmosphere fluxes of methane, including observed clouds and rainstorms itan: Earth-like on the Outside, Ocean World on the Inside Figure 2.
Titan’s atmospheric density compared to that of terrestrial worlds both within and beyond (e.g. Morley et al. 2017)our solar system, highlighting the potential for comparative planetology between Titan and other worlds with atmospheres. (Turtle et al. 2009, 2011b,a, 2018; Brown et al. 2010; Barnes et al. 2013; Lemmon et al. 2019; Dhingra et al. 2019),are probably linked to existing surface and subsurface liquid reservoirs (Mitchell 2008; Mitchell & Lora 2016; Tokano2009; Tokano & Lorenz 2019; Tokano 2020; Lora & Mitchell 2015; Lora et al. 2015, 2019; Newman et al. 2016; Faulket al. 2017, 2020). The hydrological cycle shapes the surface, producing landforms that bear a striking resemblanceto those found on Earth (Hayes 2016) (Figure 3). Lakes and seas of liquid methane and ethane (Brown et al. 2008;Mastrogiuseppe et al. 2014, 2016, 2018) up to hundreds of meters deep (Mastrogiuseppe et al. 2014, 2016, 2018;Stofan et al. 2007) are found across Titan’s polar regions (Turtle et al. 2011b; Sotin et al. 2012; Barnes et al. 2013;Dhingra et al. 2019; Barnes et al. 2015; Hofgartner et al. 2014, 2016; Cornet et al. 2015; MacKenzie et al. 2019b;Solomonidou et al. 2020a). River channels and rounded cobbles imaged by Huygens (Karkoschka & Schr¨oder 2016)and the radar-bright channels (Barnes et al. 2007; Lorenz et al. 2008b; Burr et al. 2009, 2013; Le Gall et al. 2010;Cartwright et al. 2011; Black et al. 2012; Langhans et al. 2013) and fans (Birch et al. 2016; Radebaugh et al. 2018;Cartwright & Burr 2017) observed by
Cassini (Wasiak et al. 2013; Poggiali et al. 2016) demonstrate that Titan’shydrologic cycle is intimately connected with the sedimentary cycle: complex organic compounds synthesized andadvected in and by the atmosphere are further transported and modified across the surface by the only known activeextraterrestrial hydrologic cycle. Perhaps the best studied sediments on Titan are the organic sands that occupy 17%of the moon’s surface (Soderblom et al. 2007; Barnes et al. 2008; Bonnefoy et al. 2016; Le Gall et al. 2011, 2014;Rodriguez et al. 2014; Brossier et al. 2018). Linear dunes (100s of kilometers long and ∼
100 m in height) demonstratethe importance of aeolian processes and underlying topography in the redistribution of Titan’s organics (Rodriguezet al. 2014; Radebaugh et al. 2008, 2010; Lorenz & Radebaugh 2009; Malaska et al. 2016b; Paillou et al. 2016; Telferet al. 2019). Titan’s vast mid-latitude plains ( ∼
65% of the surface) are also hypothesized to consist of organic materials(Malaska et al. 2016a; Lopes et al. 2016, 2020; Solomonidou et al. 2018; MacKenzie et al. 2019a), but their compositionand origin remains unknown. 3.1.
Pressing Questions and Future Investigations
Despite advances in our understanding of the landscapes of Titan, the composition of the surface remains ill-constrained.
Cassini observations, limited in spectral and spatial resolution, suggest two general categories of materials:organic-rich and water-ice rich (Barnes et al. 2008; Rodriguez et al. 2014; Brossier et al. 2018; Solomonidou et al. 2018;Soderblom et al. 2009; Griffith et al. 2019). Continued laboratory and theoretical work into the possible compositionsand physical properties of Titan’s solid (M´endez Harper et al. 2017; Cable et al. 2018, 2019, 2020; Maynard-Caselyet al. 2018; Yu et al. 2018, 2020) and liquid (e.g. Farnsworth et al. 2019; Hanley et al. 2020; Engle et al. 2020; Steckloffet al. 2020; Vu et al. 2020) surface materials are crucial for informing interpretations of
Cassini-Huygens data andsupporting future exploration of the surface like the
Dragonfly mission.Dramatic advances in our understanding of Titan’s seasonally-evolving weather and climate (Mitchell et al. 2006;Mitchell & Lora 2016; Hayes et al. 2018) are similarly accompanied by new questions and key unknowns. The principalmechanisms controlling the timing and distribution of humidity ( ´Ad´amkovics et al. 2016; ´Ad´amkovics & de Pater 2017),convection, methane cloud formation, and precipitation remain incompletely understood, as do the sources and sinksof atmospheric methane and the roles of atmospheric variability (Griffith et al. 2008; Mitchell et al. 2009; Roe 2012;Mitchell & Lora 2016; Hayes et al. 2018) and transient phenomena like dust storms (Rodriguez et al. 2018) in theclimate. Likewise, the impact of heterogeneous surface-atmosphere coupling—for example, how Titan’s lakes affect thenorth polar environment (Rafkin & Soto 2020)—and the magnitude and importance of regional climate variability arestill largely unexplored. Further observations of Titan’s weather phenomena, coupled with improvements in physical
MacKenzie et al.
Figure 3.
Select examples of terrains shaped by Titan’s hydrological and sedimentological processes as viewed by CassiniRADAR and terrestrial analogs observed by SENTINEL 1: (a) shorelines of Kraken Mare from T28; (b) Chesapeake Bay(38.884 ◦ N, 76.398 ◦ W) ; (c) river channels terminating in fans from Ta; (d) Death Valley channels and fans (36.688 ◦ N,117.177 ◦ W);(e) organic sands organized into dunes from T49; (f) Namibian longitudinal dunes (24.285 ◦ S,15.437 ◦ E). itan: Earth-like on the Outside, Ocean World on the Inside < Cassini data covers only 9% of Titan at scales too coarse for detailed geophysical andhydrological analysis of hydrologic catchments, mountain wave effects, or orographic clouds and precipitation (Corlieset al. 2017).
Huygens data offer higher resolution but only over a few square kilometers (Daudon et al. 2020). Inconjunction with maps of surface composition at high spatial and spectral resolution, global imaging and topographicdata would address fundamental questions surrounding the hydrological, sedimentological, and meteorological cyclesof Titan, augmenting
Cassini data and complementing
Dragonfly ’s planned local in situ investigations.3.2.
Role in an Ocean Worlds Program
Titan represents the organic-rich endmember of the Ocean World spectrum (Figure 4). Understanding the surfaceand atmospheric processes that create, modify, and transport these materials on Titan, and the timescales and volumeson which they act, would elucidate the role these processes play in planetary habitability and their significance.3.3.
Relevance to other planets
Titan’s surface and climate system serves as a natural laboratory for studying the fundamentals of a planetary-scalehydrologic cycle, offering the unique opportunity to observe how this cycle controls the physical and chemical evolutionof the landscape in an environment akin to but less complex than Earth’s. For example, sea level rise is likely ongoingand has dramatically shaped the coasts of Titan’s large seas (Aharonson et al. 2009; Hayes et al. 2011, 2017; Loraet al. 2014; MacKenzie et al. 2014, 2019b; Birch et al. 2018; Tokano & Lorenz 2019; Tokano 2020) and is likely ongoingalthough the rates remain loosely constrained; study of Titan’s coasts and ongoing erosional/depositional processescould be directly compared to the rapid changes on Earth and inform the study of paleo coastlines on Mars. TITAN IS AN OCEAN WORLDA subsurface water ocean lies beneath Titan’s organic-covered ice crust (Nimmo & Pappalardo 2016), evidence forwhich includes gravitational tides (Iess et al. 2012; Mitri et al. 2014) and larger-than-expected obliquity (Baland et al.2011, 2014). 4.1.
Pressing Questions and Future Investigations
The thickness of Titan’s crust is loosely constrained to 50-200 km (Choukroun & Sotin 2012; Nimmo & Pappalardo2016; Hemingway et al. 2013; Lefevre et al. 2014) and the extent and duration of convection within the ice crust(Hemingway et al. 2013; Lefevre et al. 2014; Noguchi & Okuchi 2020) is still debated. Estimates of the oceanic depthspan 500-700 km (Iess et al. 2012; Castillo-Rogez & Lunine 2010; Gao & Stevenson 2013; Chen et al. 2014) and thestate of differentiation in the core is unknown (Nimmo & Pappalardo 2016; Baland et al. 2014; Gao & Stevenson 2013;O’Rourke & Stevenson 2014). The presence of salts or ammonia may explain the ocean’s high density (Mitri et al.2014; Leitner & Lunine 2019), but magnesium sulfate is also a potential solution (Vance et al. 2018). Primordial icybodies provided noble gases and organic matter during Titan’s accretion, making the interior an even vaster sourceof organics than the atmosphere, with some models predicting 1000 × the current atmospheric methane abundance(Tobie et al. 2012). These considerations, coupled with detection of radiogenic Ar by
Huygens (Niemann et al.2005) suggest that outgassing from the interior may be responsible for the atmosphere. New isotopic measurementsof noble gases and methane are necessary to resolve key questions concerning the ocean composition, the evolution ofthe interior and atmosphere, and the formation of Titan (Glein 2015, 2017; Marounina et al. 2015, 2018; Miller et al.2019; Journaux et al. 2020a).At pressures >
500 MPa, a layer of high-pressure ice may separate Titan’s core from the ocean (Vance et al. 2018),but if the heat flux is high enough and/or salinity high enough, the ocean may be in direct contact with the silicate core(Journaux et al. 2020b). Initially, the presence of high-pressure ice prompted the oceans of the largest icy satellites tobe deemed inhospitable, assuming that separation by ice precluded exchange between the ocean and core. However,advances in our knowledge of how ices behave at high pressure show that convection can move material through the
MacKenzie et al.
Figure 4.
Titan on the water-rock interaction spectrum of Ocean Worlds, as anticipated from models of interior structure(insets, based on the work of Vance et al. 2018)). ice layer (Choblet et al. 2017; Kalousov´a et al. 2018), including salts and volatiles like Ar (Journaux et al. 2017;Kalousov´a & Sotin 2018). More laboratory and theoretical investigations into the properties of high pressure ices andhydrates are needed before we fully understand their implications on Ocean World habitability.4.2.
Role in an Ocean Worlds Program
Determining whether Titan’s ocean is in contact with the rocky core would provide a key constraint to the formationand longevity of large Ocean Worlds both within and beyond our solar system (Journaux et al. 2013). Studying the veryorigins of Titan’s organic cycle—from the primordial to hydrothermally altered material—informs our understandingof the role of volatile-rich ices in the early solar system. IS TITAN A HABITABLE WORLD? itan: Earth-like on the Outside, Ocean World on the Inside
Pressing Questions and Future Investigations
Titan’s deep crustal ice and subsurface ocean could be one of the largest habitable realms in the solar system, witha volume of liquid water 18x that of the Earth’s oceans and CHNOPS (potentially available from primordial and/orthermally processed materials; Miller et al. 2019). Tectonic activity and cryovolcanism may facilitate the delivery ofsurface organics through the crust. Whether any or all of these processes are at work and on what timescales theyoperate on Titan remain open questions, with implications for other ocean worlds where habitability may rely evenmore heavily upon the exchange of surface and subsurface material. For example, temperature and pressure conditionsat the putative depth of Titan’s stagnant lid/convective ice transition are very similar to those encountered withinterrestrial deep glacial ice, which hosts a diversity of microbial life (Miteva et al. 2009) in the intergrain channelsbetween solid ice grains (Price 2007; Barletta & Roe 2012). In these intergrain regions, microbial metabolism is slowenough that the environment may be habitable for 10,000 years—only a few orders of magnitude lower than Titan’shypothesized convective cycle.A frigid ambient temperature of ∼
90 K (Jennings et al. 2009, 2011, 2016, 2019; Cottini et al. 2012) makes Titan’ssurface largely inhospitable for Earth-like life using water as the biochemical solvent. However, there are ephemeralscenarios in which liquid water is present at Titan’s surface: lavas erupting from cryovolcanoes and impact-generatedmelt. While some geomorphological evidence supports the existence of cryovolcanism (Lopes et al. 2020, 2007, 2013),its mechanics (Mitri et al. 2008; Moore & Pappalardo 2011) are not well understood, in part due to the lack ofconstraints on the extent, makeup, and activity of the crust as well as the ocean composition. However, impact cratersare found across Titan’s surface (Lorenz et al. 2007; Le Mou´elic et al. 2018; Soderblom et al. 2010; Neish & Lorenz 2012;Neish et al. 2013; Neish & Lorenz 2014; Neish et al. 2016; Werynski et al. 2019; Hedgepeth et al. 2020; Solomonidouet al. 2020b). During the impact, crustal material and surface organics mix; the resulting pockets of liquid watereventually freeze on timescales loosely constrained to up to 10,000s of years (Artemieva & Lunine 2003, 2005; O’Brienet al. 2005; Neish et al. 2006; Davies et al. 2010, 2016). Mixing tholins with liquid water in the laboratory producesamino acids on a timescale of days (Neish et al. 2008, 2009, 2010, 2018). Titan’s transient liquid water environmentsare thus extraterrestrial laboratories for exploring how far prebiotic chemistry can progress under time and energyconstraints that are difficult to realistically reproduce experimentally (Neish et al. 2018). The
Dragonfly mission willtake advantage of this opportunity with surface composition measurements near a large impact crater.Without an understanding of the chemical processes necessary for the emergence of life, it is impossible to saywith certainty how long it takes for life to arise (Orgel 1998). This timescale is a critical unknown in our conceptof habitability: is there a minimum time necessary for all the key ingredients to be collocated? The answer to thisquestion has immediate implications for strategizing the search for life elsewhere (both where to search and whether totarget extant or extinct life), especially since the lifetime of the liquid oceans on both confirmed and candidate oceanworlds remains an active area of research (Nimmo & Pappalardo 2016; Neveu & Rhoden 2019). Any constraints onhabitability timescales from Titan’s transient liquid water environments would provide key context for exploration ofpotentially habitable environments and the search for life.Finally, Titan’s lakes and seas of liquid hydrocarbons offer a unique opportunity to investigate whether the solventnecessary for biochemistry must be water. Theoretical considerations suggest alternative chemistries are possible(Benner et al. 2004; Lv et al. 2017) and the abundance of solid and liquid organic molecules available on the surfaceand lack of UV radiation make the surface of Titan an advantageous place for exploring the possibility of a truesecond genesis (Lunine & Lorenz 2009; Lunine 2010; McKay 2016). Theoretical investigations are exploring boththe possibilities for lipid membrane-like structures in low temperature environments and whether cell membranes areeven necessary (Palmer et al. 2017; Stevenson et al. 2015; Rahm et al. 2016; Sandstr¨om & Rahm 2020). Laboratoryand theoretical models are revolutionizing our understanding of the possible conditions within Titan’s lakes and seas(Cordier & Carrasco 2019; Luspay-Kuti et al. 2015; Cordier et al. 2012, 2016, 2017; Cordier & Liger-Belair 2018; Hodysset al. 2013; Corrales et al. 2017; Malaska et al. 2017; Hartwig et al. 2018; Czaplinski et al. 2019, 2020; Farnsworth et al.2019). Employing these new findings to constrain the habitability potential of Titan’s liquid hydrocarbons requires0
MacKenzie et al. both determining the composition of Titan sediments—as the
Dragonfly mission’s plans to do by exploring at a portionof one of Titan’s low-latitude dune fields—and monitoring the composition, physical conditions, and seasonal evolutionof Titan’s polar lakes and seas with future missions. FUTURE INVESTIGATIONS AT TITAN
Dragonfly , the next New Frontiers (NF) mission, is a relocatable lander, to explore the prebiotic chemistry of Titan’ssurface (Turtle et al. 2017; Lorenz et al. 2018a). (For a detailed description of
Dragonfly ’s science goals and objectives,see Barnes submitted). Arriving in the 2030s,
Dragonfly will resolve a critical unknown: the chemical compositionof Titan’s solid sediments. By using a mass spectrometer to measure compositions of the organic-rich sands of theequatorial dune fields, water-ice rich clasts from the relatively unaltered interdunes, and previously melted impactmelt ejecta from an impact crater,
Dragonfly will begin to answer the question of how far prebiotic chemistry canprogress in environments that provide long-term access to key ingredients for life, thereby providing crucial context forastrobiological investigations across the solar system.
Dragonfly will also determine elemental abundances in the nearsubsurface beneath the lander with a gamma ray neutron spectrometer, thus informing the availability and distributionof elements key to habitability.Sample provenance both at the scale of
Dragonfly ’s immediate environs and the local region is essential to interpretingthe chemical findings in context.
Dragonfly is thus equipped with a suite of cameras to conduct imaging campaignsat local, nested scales. Meteorological and geophysical instruments will determine aeolian transport rates and monitorlocal weather conditions, as well as probing the thermal and electrical properties of the surface. Geophones anda seismometer round out the contextual measurements by probing the dynamics and properties of the ice crust,potentially constraining the depth to the ocean (St¨ahler et al. 2018).
Dragonfly ’s payload is thus poised to revolutionize not only our understanding of Titan’s chemistry and geology butaddress more broadly how far prebiotic chemistry can progress and what chemical and geological processes make aplanet or moon habitable. But, just as
Curiosity addresses different fundamental science than the
Mars Reconnais-sance Orbiter , the NF-scope and architectural choices that make
Dragonfly best suited for its local in situ investigationnecessarily preclude addressing many other outstanding questions at Titan, especially those requiring a global per-spective.Thus, as demonstrated by exploration of Mars, a sequence of opportunities is needed to build upon and sufficientlyleverage the detailed exploration of Titan begun by
Cassini-Huygens and to be continued by
Dragonfly in the comingdecades. In particular, exploring the polar lakes and seas, their influence on Titan’s global hydrologic cycle, andtheir potential habitability, will remain out of even Dragonfly’s impressive range. Such measurements would alsobe complemented by orbital imaging at higher spatial and temporal resolutions than what
Cassini or ground-basedobservations could provide. A higher order gravity field would reveal eroded craters and thus constrain the prevalenceof transient liquid water environments. More specifically,
Dragonfly ’s seismic investigation of the interior would besignificantly enhanced by a global topographic dataset and higher fidelity mapping of the gravity field.Further study of the dynamics of Titan’s climate and the seasonal evolution of hazes and weather phenomena (e.g.clouds and haboobs, Smith et al. 2016; West et al. 2016; Le Mou´elic et al. 2018; Rodriguez et al. 2018; St¨ahler et al.2018; Vinatier et al. 2018; Lemmon et al. 2019) requires continued long-term monitoring with ground- and space-basedassets as Titan’s northern summer unfolds. A global imaging dataset would facilitate understanding the beginning-to-end life cycle of the materials sampled by
Dragonfly . Furthermore, as new species are identified in Titan’s atmosphere,such as with ALMA (Figure 5), the needs of Titan exploration evolve. For example, as some of these species are onlydetected above 300 km and thus require orbital monitoring since low vapor pressures in the troposphere would makedetection difficult for
Dragonfly .At least two examples for how to manifest these complementary investigations in the next decade were described inwhite papers to the 2023-2032 National Academies Planetary Science and Astrobiology Decadal Survey, representingNew Frontiers and Flagship-scope efforts. But these are far from a comprehensive representation of possible architec-tures for returning to Titan. A return to the Saturn system could orbit Titan (Sotin et al. 2017) for global mappingand geophysics or leverage the proximity of two prime Ocean World targets to jointly explore both Enceladus andTitan, via orbiting Saturn with plume flythroughs and frequent Titan flybys (Coustenis et al. 2009b; Sotin et al. 2011)or shuttling between Titan and Enceladus (Russell & Strange 2009; Sulaiman in review). Titan’s thick atmospherecan be leveraged for long-duration flight (Lorenz 2008; Barnes et al. 2012; Ross et al. 2016) at altitudes high enoughto maximize areal coverage and minimize atmospheric interference on compositional surface mapping (e.g. Corlies itan: Earth-like on the Outside, Ocean World on the Inside Nitrogen Compounds
Oxygen Compounds
Hydrogen and Hydrocarbons
Methane AcetyleneEthylene Diacetylene BenzenePropadiene PropylenePropane Ethane CyclopropenylidenePropyne Nitrogen HydrogenIsocyanideAcrylonitrile Propionitrile CyanopropyneAcetonitrile Cyanogen CyanoacetyleneCarbon Monoxide Water Carbon DioxideHydrogen
Molecules Detected in Titan’s Atmosphere by Remote Sensing
Hydrogen Cyanide
Figure 5.
Molecules in Titan’s atmosphere that have been uniquely identified via remote sensing. (Molecule image credit: BenMills/Wikimedia Commons) et al. 2021). Ride-along small satellites can be exploited for gravity science (Tortora et al. 2018). On the surface, thediversity of interesting terrains inspired the study of a fleet of shape-changing robots (Tagliabue et al. 2020), a fleetof mini-drones, and a drone capable of also floating on the surface of the seas (Rodriguez in review). A mission tofloat on Titan’s seas has been proposed (Stofan et al. 2013) and submerged instrumentation and/or vessels have beenstudied (e.g. Lorenz et al. 2016, 2018b). These in situ elements would benefit and/or require an orbiter for data relay.The diversity of mission concepts (and combinations thereof) that have been proposed and studied reflect the diversityof science questions left to answer at Titan and, importantly, demonstrate that compelling architectures span the fullspectrum of NASA and ESA mission classes. TITAN IS AN UNPARALLELED DESTINATIONTitan offers the opportunity to study a myriad of fundamental planetary science questions. The processes thatgovern its atmosphere, surface, and interior and interactions between these three environments make Titan an analogfor destinations across the solar system and beyond. In the next decade,
Dragonfly will continue the legacy of
Cassini-Huygens and radically transform our understanding of Titan’s chemistry, geology, and astrobiological potential. Butif the last decade has taught us anything, it’s that this moon’s complexity tends to defy our imagination. There isstill much left to learn before we fully understand Saturn’s largest moon, requiring mission opportunities in additionto
Dragonfly in the next decade. ACKNOWLEDGMENTSWe thank the broader Titan community for supporting the submission of a white paper of similar content to the2023-2032 Decadal Survey. We also thank Caleb Heidel (JHU APL) for graphical contributions to Figure 1. S.M.M.acknowledges support from JHU APL and Cassini Data Analysis and Participating Scientst Program (CDAP) grant
MacKenzie et al.
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