In this Chapter, I focus on the interplay of the two main geological types of processes that develop the surface globe, which are the exogenous and endogenous processes. Both terms have been introduced basically for terrestrial geological processes but find equivalants on other planetary bodies as well. On Earth, as exogenic processes are considered the energy (heating) from the Sun, the force of gravity, the activity of organisms, weathering, the blowing wind, running water, underground - water, waves and currents in surface water bodies, glaciers and more. Critical products of such activities are the aggradation and
degradation21; the latter is mainly caused by weathering, mass wasting and erosion (e.g. Graniczny, 2006). Hence, precipitation, fluvial, aeolian and drain phenomena are included in the exogenic active processes of Titan. A short description of the general term of endogenic processes can be found in 4.1.4.
General
Since Titan harbors a thick nitrogen-dominated atmospheric envelope rich in organic substances, a dynamic coupling between the surface and the atmosphere is expected. Indeed, Titan’s active cycle of methane appears to link the atmosphere with the surface and the interior (Fig. 4.7). As mentioned in the introduction, methane is the second most abundant compound in the atmosphere of Titan (Flasar et al. 2005; Waite et al., 2005; Niemann et al., 2010). The existence of a large number of hydrocarbons and nitriles found in Titan's atmosphere (Hanel et al., 1981; Coustenis et al., 1989), originates through methane and nitrogen photolysis essentially and recombination with nitrogen (Strobel, 1974).
Fig. 4.7 - The methane cycle on Titan (Image credit: Atreya et al. 2006).
Short-lived radicals22 of CHn are the products of the photochemistry that occurs above the
level of 700 km in the atmosphere and the reaction of CH3 radicals produce ethane, which is a
significant molecule during the process that methane follows (Choukroun & Sotin, 2012). Moreover, the methane photolysis produces various complex hydrocarbons, which form
21 Major categories of gradation: natural levelling of land as a result of the building up or wearing down of pre-
existing formations
22A radical in chemistry is an atom, molecule, or ion that has unpaired valence electrons or an open electron
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Titan’s haze layer. Some of the aerosols deposit on the surface and form lake-like features (precipitation-fluvial) with major significance in the process of the methane cycle and the habitability of Titan. When the deposition occurs on the equator, the climate turns the liquid hydrocarbons into hydrocarbon sand dunes (aeolian) (Lunine and Lorenz, 2009). In addition, photochemical reactions and hydrogen escape dissociate methane in the upper atmosphere (Strobel, 1974; Yung et al. 1984; Wilson and Atreya, 2004; Lavvas et al. 2008). Lack of any source that could replenish this loss means that methane should dissapear from Titan in the course of 10-30 Myr (Atreya, 2010).
Hence, this active process includes chemical reactions and climate and geological activity. This latter part, that appears to control major factors on Titan’s evolution and current status, is a main part of my PhD researches.
Numerous models have proposed organic precipitation on Titan (Toon et al. 1988; Lorenz, 1993; Graves et al. 2008), either in the form of violent torrential storms (Hueso and Sanchez-Lavega, 2006), and smooth drizzle in the lower atmosphere (Tokano et al. 2006) or in the form of methane rainfalls from occasional short-lived clouds that have been modeled or observed (Griffith et al. 1998; Graves et al. 2008; Porco et al. 2005; Rannou et al. 2006). Methane rains can carve channels and fill lakes, while mass transport by winds causes erosion and creates extensive dune fields (Lancaster, 2006; Lorenz et al. 2006a; Radebaugh et al. 2008; Tokano, 2008; Lorenz and Radebaugh, 2009; Hayes et al. 2013). The fact that Titan is undergoing seasonal weather changes helps it maintain an active-dynamic climate, which forms complex surficial edifices.
Consequently, climate related processes, such as fluvial and aeolian, have created structures similar to those on Earth such as lakes, seas, riverbeds, sand dunes, shorelines and dendritic drainage networks. Moreover, seasonal climate variability, as illustrated among other, by the reduction of the southern lake shorelines (Turtle et al. 2008; Wye et al. 2009; Wall et al. 2010), is most probably present on Titan. I present hereafter the major types of non-tectonic in origin features on Titan, as presented in Solomonidou et al. (2010) and Solomonidou et al. (2013a) in addition to unpublished material.
4.1.3.1 Fluvial features
Lakes
As mentioned earlier in this Chapter, image and numerical data from Cassini-Huygens exhibited evidence for the presence of stable surface liquid extents, called “lakes”, or “seas”
(e.g. Stofan et al. 2007; Aharonson et al. 2009; Hayes et al. 2010) instead of a global surface ocean or even large basins homogeneously distributed. This is one exceptional characteristic for an outer Solar system body since up-to-date, Titan, is the only object other than Earth for which clear evidence of stable bodies of surface liquid has been reported. The other important aspect is that the lakes are mostly concentrated in the North, with some exceptions. Currently, the Cassini/RADAR data confirmed the observations of hydrocarbon lakes on Titan’s surface and their presence is now well established (Fig. 4.7) (Lopes et al. 2007). These lakes, observed on Titan, resemble terrestrial lakes (Cornet et al. 2012). Moreover, Titan, despite its small size in comparison with that of Earth’s, exhibits massive lakes that compares to that of largest terrestrial lake (Lake Superior, USA).
More than ten currently identified Titan lakes have a mean diameter greater than 200 km. Most of the other lakes are quite smaller with diameters less than 100 km. Kraken Mare, Ligeia Mare (which reside in an endorheic basin dominated by seas –Wasiak et al. 2013) and Punga Mare located at the northern part of Titan have adequate sizes to be characterized as seas (according the IAU Committee on Planetary Nomenclature) (Fig. 4.8). Ontario Lacus is the largest lake located in the southern hemisphere. It has been suggested that a depression drains and refills from the undergroung, exposing liquid regions ringed by materials like saturated sand (Cornet et al. 2012).
Figure 4.8 - The large seas of Titan greater than 200 km located in the satellite’s North Pole from Cassini/ISS data (Image credit: NASA/JPL/SSI). For scale purposes, Kraken Mare extends for almost 400,000 km2.
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A major classification of Titan lakes has been made based on the RADAR obsevations: (a) the dark – liquid filled lakes that thoroughly absorb the incident RADAR beam, (b) the granular – partially filled lakes as recorded from SAR microwave penetrations and (c) the bright – empty lakes (Stofan et al. 2007) that appear brighter in SAR compared to their exteriors and have a depth of 200-300 m (Hayes et al. 2008) (Fig. 4.9).
Fig. 4.9 - Lakes on Titan’s surface as recorded from the Radar mapper during the Cassini T16 flyby on July 22, 2006 (Image adapted from Stofan et al. 2007).
In the course of the Cassini mission, examples of current or past fluvial activity have been found in the past lake-level changes of Ontario Lacus and changes in surface albedo of Arrakis Planitia, possibly due to evaporation (Barnes et al. 2009; Hayes et al. 2011; Turtle et al. 2009; 2011b).
Moreover, in spite of the detection of several lakes concentrated in the northern hemisphere (Stofan et al. 2007), the total amount of liquid methane on the surface remains too small (Sotin et al. 2012) for the required replenishment of methane in the atmosphere and in the absence of adequate hydrocarbon liquid exposed sources, several studies have suggested the existence of undersurface methanofers in various forms (Stevenson, 1992; Lunine et al. 1999).
Drainage networks
The Cassini SAR data presented evidence for the presence of fluvial networks on Titan’s surface, interpreted as products of liquid alkane flow (Burr et al. 2009). The fluvial processes on Titan’s surface, present a major geodynamic activity that contributes to the satellite’s morphology.
Before SAR, the DISR observed branched lineations were interpreted as fluvial valley networks with inset streams formed by flowing methane (Tomasko et al. 2005; Perron et al. 2006). Data from RADAR and VIMS also confirmed the presence of network lineations interpreted as drainage networks (Elachi et al. 2005; Porco et al. 2005; Barnes et al. 2007b; Lorenz et al. 2008). Indeed, according to Burr et al. (2013) fluvial networks are present on SAR images covering ~40% of Titan from the RADAR up through T71 and on visible light images of the HLS collected by the DISR (see section 4.1.1).
Figure 4.3 shows a complex network of narrow drainage channels from Huygens/DISR, while Fig. 4.10 shows dendritic networks on Titan as seen with SAR and from morphological maps (Lorenz et al. 2008). The system in Fig. 4.10 is located at the western end of Xanadu close to an area that I discuss in detail here and from which data I analyze in Chapters 6 and 7, named Tui Regio. Possible changes in the fluvial deposits on Titan have been suggested by Burr et al. (2013).
Fig. 4.10 - This fluvial network extends over 400km while it presents a dendritic morphology. The dendritic network drainage is the pattern formed by the streams, rivers and lakes in a particular ‘watershed’. In this case incisions were not observed but a well-developed branching structure on both channels (red and green circle) is present (Image adapted from: Lorenz et al. 2008).
4.1.3.2 Aeolian features
Sand dunes
From several studies, it has been inferred that the surface age is about half a billion years old (Artemieva and Lunine, 2003; Lorenz et al. 2007), not too different from the average ages of Earth’s and Venus’. In addition to the erosional and burial phenomena that may have eliminated most of the impact structures, tectonic disruption of the crust may also obliterate
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craters. The consideration of Titan's young surficial age indicates the possible existence of active regions among the satellite. Contrary to impact craters surficial structures that are commonly seen on Titan are the dunes (Fig. 4.1d; 4.11). A sand dune is a semi-permanent accumulation of loose sand that forms in areas where the wind tends to blow in one direction, at velocities high enough to move sand, across a land surface that permits sand to amass in a regular and consistent form. Cassini/RADAR images have shown many dune formations at equatorial latitudes, mainly from 0°N to 30°N and in some isolated regions up to 55°N (Radebaugh et al. 2008). Due to their extent the equatorial dunes have been interpreted as sand seas (Lorenz et al. 2006a) and they are mainly concentrated in zonal east-west direction. The presence of these vast dune fields indicates the wind blow direction which is towards east, but they wrap around the topographically high features they meet like mountains and craters (Radebaugh et al. 2008).
Titan dunes are 1-2 km in width and are separated almost 1-4 km from one another, extending up to 150 km height and more than 100 km in length (Elachi et al. 2006). Cassini/ISS and VIMS observations have also captured many of the dunes fields (Porco et al. 2005; Soderblom et al. 2007b; Barnes et al. 2008).
The sand composition is possibly a mixture of organic materials and ice grains (Radebaugh et al. 2009). The dunes are generally smooth surfaces that diverge around topographic obstacles resembling terrestrial dunes (Radebaugh et al. 2009). Moderately variable winds that either follow one mean direction or alternate between two different directions have formed the observed longitudinal dunes (Lorenz et al. 2006) (Fig. 4.11).
Fig. 4.11 - Sand dunes from the Belet sand sea on Titan formed by aeolian processes from SAR data (Image credit: NASA/JPL).
Recent studies indicate the dunes as active features due to the presence of distinguishable interdunes in both RADAR and VIMS dune observations (Barnes et al. 2008). For the latter, as it happens on Earth the interdunes possibly represent the substrate surface layer that the displacement of dune material should blanket with time. Consequently, the presence of both dunes and interdunes on Titan probably indicate current surface activity in these areas (Barnes et al. 2008; Savage, 2011).