What is happening in Titan’s equatorial belt?

Post by Jeremy BrossierDeutsches Zentrum für Luft- und Raumfahrt (DLR), Institute of Planetary Research, Berlin, Germany.

During the last thirteen years (2004 – 2017), the Cassini-Huygens mission allowed a real revolution in the exploration of Titan, the largest moon of Saturn. This mission has revealed that Titan is – in many aspects – very similar to Earth. Titan is a frozen version of Earth, where methane behaves as water, and water ice may be as hard as rock. Despite its strange characteristics, Titan undergoes a rich variety of surface processes that are likewise analogous to those on our planet. Titan being entirely shrouded by a dense atmosphere made of dinitrogen, methane and solid organic particles (i.e. tholins), direct observation of its surface is only possible through radar data, as well as infrared data within specific wavelengths intervals. SAR images from the radar, allowed identifying various landscapes on the moon (see Image 1), and evaluating their global distribution, notably for the lakes and dunes. Lakes are mostly confined around the poles, while the dunes dominate the equatorial belt. Thus, the shape of Titan’s surface seems quite well understood thank to SAR images, however, it is crucial to determine not only the morphology, but also the nature of the material composing or coating the various landscapes to better understand the geology of this intriguing moon.


Image 1: A few examples of Titan’s landscapes seen through SAR images, including (A) mountain chains embayed by plains, (B) undifferentiated plains, (C) impact crater, (D) dunes, (E) river channels, (F) small lakes, and (G) a close up of the second largest sea, namely Ligeia Mare. SAR images were acquired during Titan flybys (A, B) T43 in May 2008, (C) T77 in June 2011, (D) T21 in Dec. 2006, (E) T44 in May 2008, (F) T19 in Oct. 2006, and finally (G) T28 in April 2007. Note that Titan flybys are tagged with the abbreviated target name “T” and the flyby number.



Morphological Evidence that Titan’s Southern Hemisphere Basins are Paleoseas

Post by Samuel Birch, Cornell University, Ithaca, USA.

Titan is the only body in the solar system, besides the Earth, known to currently have standing bodies of liquids on its surface [Stofan et al., 2007]. Presently, liquids are restricted to the polar regions (>50o) with liquid bodies in the North encompassing 35 times more area as compared to the South [Hayes et al., 2011; Birch et al., 2017a]. Apsidal precession of Titan’s obliquity over ~100,000 year cycles, analogous to the Earth’s Croll-Milankovitch cycles, likely forces liquids from pole-to-pole, and has been invoked as a physically plausible mechanism to account for the dichotomy [Aharonson et al., 2009]. General circulation models support such a mechanism, as Titan’s current orbital configuration produces more intense, high-latitude, baroclinic eddies over the southern hemisphere, preferentially depositing more liquid at the northern pole [Lora & Mitchell, 2015]. These models, therefore, imply that the presence of northern liquids is transient over geologic timescales. Large basins able to accommodate ~70,000 km3 of liquid methane and ethane [Hayes, 2016] are required when orbital and climatic conditions become favorable for the accumulation of southern seas. Our study [Birch et al. 2017b] identifies four large basins, all of which show morphological evidence for having been formerly filled by liquids.


Image 1: Polar stereographic projection of SAR image data of the South polar region extending out to 60o latitude. SAR image data includes all flybys up to and including T98. A mosaic of ISS data underlays the SAR mosaic. The perimeters of the four basins that we identified are highlighted in yellow.


Is the Xanadu region on Titan an impact basin?

Post by Dr. Mirjam Langhans, GFZ German Research Centre for Geosciences, Helmholtz Centre Potsdam, Germany.

The surface of Titan, Saturn’s largest moon, is subject of great geologic interest, particularly since the arrival of Cassini/Huygens mission in the Saturnian System. Titan’s largest distinct and highly reflective surface feature, named Xanadu, is located close to the equator. The image depicts Xanadu in full extension with a rich diversity of geologic landforms, such as fluvial valleys, mountain ridges and impact craters. Despite the high volume of image data in this region, the geologic history behind Xanadu remains enigmatic to this day.

Geomorphologic map of Xanadu. Data: Cassini SAR data, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). background: Cassini-ISS, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). Inner and outer boundary of the Xanadu Circular Feature (XCF) are highlighted at Western Xanadu (black lines, according to Brown et al. (2011)). Green dots: impact craters listed in Wood et al. (2010) and Neish & Lorenz (2012), red dots: potential impact craters. Fluvial channels are delineated in blue. Dark green: lineations seen in mountain ranges, from Radebaugh et al. (2011). Light green: lineations in mountain ranges (Langhans et al. 2013).

Geomorphologic map of Xanadu. Data: Cassini SAR data, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). background: Cassini-ISS, source: (http://pds-imaging.jpl.nasa.gov/portal/cassini_mission.html). Inner and outer boundary of the Xanadu Circular Feature (XCF) are highlighted at Western Xanadu (black lines, according to Brown et al. (2011)). Green dots: impact craters listed in Wood et al. (2010) and Neish & Lorenz (2012), red dots: potential impact craters. Fluvial channels are delineated in blue. Dark green: lineations seen in mountain ranges, from Radebaugh et al. (2011). Light green: lineations in mountain ranges (Langhans et al. 2013).


Cryo-volcanic “Mount Doom” on Titan

Post by Rosaly LopesRandy Kirk,  and Mary Bourke,

Jet Propulsion Laboratory, California Institute of Technology, California, USA.
US Geological Survey, Astrogeology Science Center, Flagstaff, Arizona, USA.
Geography, Trinity College, Dublin, Ireland.

Data from the Cassini mission have revealed that Titan is a planetary body where the interior, the surface, and atmospheric processes interact to create and modify landforms (Loppes et al, 2010). In terms of recent surface processes, Titan is one of the most earth-like bodies in our solar system. Landforms include the largest area of aeolian dunefields in our solar system (e.g., Radebaugh et al., 2008), lakes (e.g., Stofan et al., 2006), fluvial channels (e.g., Langhans et al., 2012), mountains (e.g., Radebaugh et al., 2007), and features that have been interpreted as volcanic (e.g., Lopes et al., 2007).

Image 1: The  RADAR (SAR) images in black and white over a false-color mosaic of VIMS data.  The globe at upper left shows the location of the map on Titan (arrow). The white lines show the approximate boundaries of the perspective view in Image 2.

Image 1: The RADAR (SAR) images in black and white over a false-color mosaic of VIMS data. The globe at upper left shows the location of the map on Titan (arrow). The white lines show the approximate boundaries of the perspective view in Image 2.


Surface dissolution on Titan and Earth: Ontario Lacus and the Etosha Pan (Namibia) .

Post by Thomas Cornet, Olivier Bourgeois, Stephane Le Mouelic et al.,

Laboratoire de Planétologie et Géodynamique de Nantes, , Université de Nantes, UMR 6112, CNRS, Nantes, France.

Titan, Saturn’s major moon, possesses hydrocarbon lakes and seas in the polar regions [Stofan et al., 2007, Hayes et al., 2008]. Among these, Ontario Lacus (72°S, 180°E, Image 1) is the largest in the south (235 km-long, 75 km-wide). So far it is interpreted as a liquid-covered lake in Titan’s southern hemisphere because of its dark appearance in Cassini image data [Barnes et al., 2009; Turtle et al., 2009; Hayes et al., 2010; Wall et al. 2010], the identification of liquid ethane in its interior [Brown et al., 2008] and the smoothness of its surface [Wye et al., 2009].

Image 1

Image 1: Ontario Lacus (Titan) and the Etosha Pan (Namibia) as surface dissolution morphologies under arid climates. Credits: Envisat ASAR, data provided by the European Space Agency ©ESA 2009, ESA ®; Cassini RADAR, data provided by JPL/NASA. Link to high resolution image


Latitudinal-dependent Surface Runoff on Titan

Post by Dr Mirjam Langhans, Istituto di Astrofisica Spaziale e Fisica Cosmica – INAF, Roma, Italy

Saturn‘s largest moon Titan is one of only a few bodies in the Solar System with an active volatile cycle. Besides Earth, only ancient Mars is supposed to have hosted a water cycle. Titan‘s volatile cycle is based on methane (CH4), occurring in liquid and gaseous state given Titan‘s environmental conditions (e.g. Flasar 1983, Lorenz & Lunine 2005). Despite the different volatiles involved, similar atmospheric processes occur on Titan and Earth, such as the formation of clouds and precipitation .

Following the action of the methane cycle, surface runoff and the incision of linear valleys take place. As a result, fluvial landscapes evolved on Titan, analog to those on Earth (e.g. Tomasko et al. 2005; Perron et al. 2006, Lorenz et al. 2008, Langhans et al. 2012).

Image 1

Image 1: Cassini-Radar-SAR image shows a dendritic valley network at high northern latitudes of Titan, ending in Kraken Mare, captured by radar-SAR (Radar-SAR T28, April 10, 2007). The image is centered at 280°W, 78°N.


Evaporites on Titan

Post by Jason W. Barnes, Assistant Professor of Physics, University of Idaho

Evaporites form on planetary surfaces when dissolved chemical solids precipitate out of saturated solution as their liquid solvent evaporates. Until recently theywere known to exist on only two planets: Earth and Mars. On Earth there are a variety of evaporite constituents including carbonates (CaCO3), sulfates (CaSO4), and halides (NaCl), progressing in order of increasing solubility.  NASA’s rover Opportunity discovered evaporitic deposits on Mars that are primarily composed of sulfates — different from Earth’s due to a highly acidic formation environment.

A third planetary instance of evaporite has now been discovered in an exotic location:  Saturn’s moon Titan.  Being so far from the Sun, Titan has a low surface temperature of 90°K (-183°C), just warmer than liquid nitrogen.  Hence all of Titan’s water is permanently frozen.  However methane on Titan plays the same role that water does on Earth and Mars. Titan has methane clouds, methane rain, methane rivers, and methane lakes and seas (Image of the Month, March 2010).

Therefore the evaporites on Titan have an unusual nature relative to those on rocky planets.  Instead of water being the solvent, on Titan the solvent is methane.  And instead of salts being the solute, on Titan organic molecules derived from ultraviolet photolysis of methane dissolve in rain, surface, and ground liquid.  Those organics precipitate out of lakes when the liquid methane solute evaporates, becoming evaporite.

Image 1

Image 1: Cassini VIMS/RADAR hybrid image of filled and dry lakes south of Titan’s methane sea Ligeia Mare. The brightness of the image is determined by synthetic aperture radar which indicates roughness, and the colors by Cassini’s Visual and Infrared Mapping Spectrometer indicate composition. Some of the small lakes in the image are filled (cyan arrows). Other lakes show lacustrine morphology, but no evidence for liquids. Some of those dry lakes have the same composition as the surrounding terrain, but others show evaporites in bright orange.


Rectilinear Fluvial Networks on Titan

Post by Devon Burr1, Sarah Drummond1 and Robert Jacobsen2.

1Earth and Planetary Sciences Department and Planetary Geosciences Institute, University of Tennessee Knoxville, USA
2Geology Department, Colorado College, USA

Titan, like Earth, has a solid surface enveloped by a substantial atmosphere. Both atmospheres contain a few mass percent of volatiles – hydrocarbons on Titan, water on Earth – that are close to their triple points. These conditions are conducive to precipitation and runoff, resulting in fluvial processes. At Titan, data from the Cassini-Huygens mission indicate the occurrence of methane rainfall and precipitation runoff [Lunine et al., 2008]. In addition, the Descent Imager and Spectral Radiometer (DISR) on the Huygens probe observed branched lineations interpreted as fluvial valley networks with inset streams formed by flowing methane [Tomasko et al., 2005; Perron et al., 2006].

Image 1: Network patterns (Howard, 1967). The implications of some of these patterns are provided in Table 1.


Lakes on Titan

Posted by Dr. Mary Bourke .

Surface conditions on Titan are near the triple point of methane, suggesting a methane-based hydrologic cycle which may incorporate solid, liquid, and gaseous phases. Albedo patterns on Titan’s surface evident in early Earth-based observations were interpreted as dark hydrocarbon liquids in topographic lows between exposures of bright water-ice bedrock (Lorenz and Lunine, 2005; Smith et al., 1996).

Initial data from the Cassini-Huygens mission detected more than 75 radar dark patches in the northern portion of a 6,000 km long swath of the surface (Image 1). These features measured from 3 km to in excess of 70km across. The backscatter of some of the dark patches had much lower reflectivity than previously imaged areas on Titan, including the radar-dark sand dunes observed near Titan’s equator (Sept. 2007 PGWG featured image).

mage 1: Radar imaging data from a Cassini flyby. The intensity in this false-coloured image is proportional to how much radar brightness is returned. The lakes, darker than the surrounding terrain, are emphasized by tinting regions of low backscatter in blue. Radar-brighter regions are shown in tan. The strip of radar imagery is foreshortened to simulate an oblique view of the highest latitude region, seen from a point to its west. This radar image was acquired by the Cassini radar instrument in synthetic aperture mode on July 22, 2006. The image is centered near 80° north, 35° west and is about 140 kilometers (84 miles) across. Smallest details in this image are about 500 meters (1,640 feet) across. Credit: NASA/JPL


Cryovolcanic features on Titan

Post by Dr Catherine Neish

Cryovolcanism (or ‘cold’ volcanism) describes the eruption of substances that are generally considered to be volatiles on the surface of Earth (eg. water, water-ammonia mixtures, etc.). Cryovolcanism is functionally similar to the volcanism we see on Earth, except that cryolavas (‘cold’ lavas, such as water) erupt at much lower temperatures than rock lavas. As with all forms of volcanism, two conditions must be met for cryovolcanic flows to be present on the surface of an icy moon: liquids must be present in the interior, and those liquids must then migrate to the surface. The latter requirement is more difficult to achieve for cryolavas than rock lavas, given that solid ice is less dense than water. The addition of some amount of ammonia can reduce the density difference – a liquid ammonia-water mixture of peritectic composition (33 wt. % ammonia, 946 kg m3) is near neutral buoyancy in ice (917 kg m3). Though these pockets would not easily become buoyant on their own (given the difference in density of ~20-30 kg m3), they are sufficiently close to the neutral buoyancy point that large-scale tectonic stress patterns (tides, non-synchronous rotation, satellite volume changes, solid state convection, or subsurface pressure gradients associated with topography) could enable the lavas to erupt effusively onto the surface.

Ganesa Macula, Titan

Image 1: A portion of the RADAR swath taken during the Cassini spacecraft’s TA (Titan-A) encounter on October 26, 2004 (Elachi et al. 2005). This image shows several possible cryovolcanic features, including overlapping flow features (right) and the large circular feature Ganesa Macula (left). Radar illumination is from the bottom.


Longitudinal dunes on Saturn’s moon Titan

Posted by  Dr. Jani Radebaugh, Department of Geological Sciences, Brigham Young University, Utah, USA

(Re-posted from IAG Image of the Month, September, 2007)

The Cassini spacecraft is in orbit around Saturn, and occasionally flies close to one of its many icy moons. Because of specially designed instruments on Cassini, the surface of Saturn’s largest moon, Titan, enshrouded in a thick, hydrocarbon haze-rich atmosphere, has been observed for the first time by this spacecraft.

Dunes on Titan

Cassini RADAR SAR image is north up, with resolution ~300 m. RADAR illumination direction and inclination angle is indicated by the open arrow. Image courtesy of the NASA Cassini Project.


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