Titan’s labyrinth terrain

Post contributed by Michael J. Malaska, PhD, Scientist, Jet Propulsion Laboratory / California Institute of Technology, USA.

Saturn’s moon Titan is where organic chemistry and surface geomorphology intersect to create an enigmatic landscape with many features in common with Earth, but that are made of completely different materials. Much of Titan’s surface is made up of organic sedimentary materials; recent mapping shows that plains and dunes cover over 80 percent of the globe. The Cassini spacecraft’s Synthetic Aperture Radar (SAR) was able to penetrate Titan’s thick haze and reveal areas of highly dissected plateaux on the surface that are called labyrinth terrain. Image 1 shows an SAR image of an example of this type of terrain, the Sikun Labyrinth. Detailed examination of Titan’s labyrinth terrain can tell us a lot about Titan’s geological history and surface evolution.

Image 1. Top: Image of the Sikun Labyrinth in the south polar terrain of Titan. The blue arrow and number at top left indicates direction of radar illumination and incidence angle for this scene. Bottom: diagram showing how radar illumination interacts with terrain of valleys and plateaux. Image credit: Mike Malaska.

SAR images are incredibly useful for examining the surface of Titan, but they provide different information from those of visible or infrared images. For one thing, the radar beam acts like a flashlight lighting up crinkled aluminum foil from the side – surfaces that face the radar beam reflect back the radar energy and look brighter, while surfaces facing away from the radar beam reflect away the radar beam and look darker. In Image 1, we can see valley slopes facing away from the radar beam (radar indicated by blue arrow) that are dark, and valley slopes facing the radar beam that are brighter. SAR also gives information on surface roughness, too. The Cassini radar had a wavelength of 2.2 cm: objects smaller than this wavelength don’t reflect much radar energy back to the spacecraft and thus look dark, while objects larger than this reflect radar energy back to the spacecraft antenna and look bright. In Image 1 we can see valley bottoms that look darker, perhaps mantled by smaller grained materials, while adjacent plateaux tops look brighter, perhaps owing to coarser surface texture. We can trace the valley floors together and see that they are interconnected and networked like fluvial valleys here on Earth. Additional processing of SAR data from the radar instrument was also able to extract some limited topographic data. Some of the labyrinth plateaux are elevated over 500 m relative to the surrounding terrain, which on Titan is a significant topographic excursion. Another observing mode with the Cassini RADAR instrument was able to measure the microwave emissivity of the labyrinth terrains and surrounding plains – this showed that labyrinth terrains and surrounding areas are all consistent with high emissivity organic materials at least to a depth of a few meters.  All this data is consistent with labyrinths being thick plateaux of organic materials that have been fluvially eroded. When we estimate the total volume of Titan’s organics, accounting for the thickness and amount of material removed via erosion, it turns out that somewhere between 15-42% of Titan’s solid organic inventory is contained in the labyrinth terrains.

All of this has really important implications for Titan. It tells us that Titan’s atmospheric organic factory has been operating for a geologically long period of time – at least long enough to build up a thick plateau of organics and then later dissect them.

Image 2 shows an artist impression of the greater Sikun Labyrinth region which contains many other labyrinths as well as empty lake basins. Further evidence of fluvial activity in this area is the large meandering channel of Celadon Flumina, which is roughly the size of the Mississippi River in width and meander pattern.

Image 2. Artistic representation of the greater Sikun Labyrinth region. Many labyrinth terrains are in this scene. The mid-left foreground shows Mississippi-sized Celadon flumina (dark meandering channel debouching into the dark basin), with the Palma Labyrinth in middle foreground, Naraj Labyrinth in the right-midground, the Sikun Labyrinth in far midground left, followed by the Tupile Labyrinth behind that, and finally, the Ecaz Labyrinth at far background of the scene. View is tracking longways north towards south. Width of the scene is approximately 190 km across at the midpoint. Colors and topography as interpreted by Mike Malaska. Image credit: NASA/JPL/ESA/SSI and Mike Malaska/Bjorn Jonsson/Doug Ellison. Image link: https://solarsystem.nasa.gov/resources/14888/sikun-labyrinthus-an-artistic-view/

But how did the labyrinths get eroded? One clue is that some of the labyrinth terrains appear to contain closed valleys (Image 3), at least at the resolution limit of the Cassini radar instrument (~300 m). Closed valleys suggest that material originally inside the valley had to be moved somewhere – but how exactly did it get moved out? On Earth, closed valleys are usually diagnostic for karstic dissolution and subsurface transport. Karstic dissolution occurs when liquids dissolve soluble geologically important materials and these materials are then transported away through either surface or subsurface flow. As this occurs, the landscape begins to change as these materials are removed. Interestingly, laboratory experiments and theoretical calculations also provide support that Titan’s methane rains could dissolve some of Titan’s surface organics in a similar fashion to dissolution here on Earth, even though the two worlds have different surface liquids and solids. Geomorphological observation, laboratory experiments, and numerical modelling of Titan’s labyrinths all seem to point towards dissolution geology (karst) occurring on this moon.

Image 3. A: diagram showing how radar interacts with open and closed valleys. B: Image of a section of the Ecaz labyrinth. C: Detail of Ecaz labyrinth showing an example closed valley – this shows the SAR pixels. D. Image of (B) indicating closed valleys in the Ecaz labyrinth. Blue arrow indicates the radar incidence angle and illumination direction in this scene. Image credit: Mike Malaska.

Titan has a completely different set of parameters compared to those here on Earth, but the similarity in surface features suggest the same fundamental geomorphologic processes. By studying the labyrinth terrains in more detail, we might learn more about Titan’s past organic history as well as better understanding karstic processes both on Titan and on Earth.

Acknowledgement:

The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

Further reading:

Labyrinth terrain on Titan:

Malaska, M.J., Radebaugh, J., Lopes, R.M.C., Mitchell, K.L., Verlander, T., Schoenfeld, A.M., Florence, M.M., Le Gall, A., Solomonidou, A., Hayes, A.G., Birch, S.P.D., Janssen, M.A., Shurmeier, L., Cornet, T., Ahrens, C., Farr, T.G., Cassini RADAR Team. Labyrinth Terrain on Titan. Icarus, in press. https://doi.org/10.1016/j.icarus.2020.113764

Geologic map of Titan:

Lopes, R.M.L., Malaska, M.J., Schoenfeld, A.M., Solomonidou, A., Birch, S.P.D., Florence, M., Hayes, A.G., Williams, D.A., Radebaugh, J., Verlander T., Turtle, E.P., , Le Gall, A., Wall, S., 2020. A global geomorphological map of Saturn’s moon Titan. Nature Astronomy 4, 228–233. https://doi.org/10.1038/s41550-019-0917-6

Theoretical studies of Titan karst:

Cornet, T., Cordier, D., Le Bahers, T., Bourgeois, O., Fleurant, C., Le Mouélic, S., Altobelli, N., 2015. Dissolution on Titan and on Earth: Towards the age of Titan’s karstic landscapes. Journal of Geophysical Research Planets 120, 1044-1074. https://doi.org/10.1002/2014JE004738

Laboratory studies of Titan karst:

Malaska, M.J. and Hodyss, R., 2014. Dissolution of benzene, naphthalene, and biphenyl in a simulated Titan lake. Icarus 242, 74-81. https://doi.org/10.1016/j.icarus.2014.07.022

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