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.

SB_Image1

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.

Specifically, the South polar region of Titan is characterized by four large, SAR-dark depositional basins (Image 1). The area covered by the southern basins is similar to the area of the northern, liquid-filled seas (South: 6.80 x 105 km2, North: 7.30 x 105 km2; Birch et al. [2017b]). Each of the basins are located within topographic lows, and are floored by a SAR-dark plain that is indicative of a relatively smooth surface. We interpret this surface to be a depositional plain (or seafloor) that formed underneath a liquid surface. Surrounding each basin are highly dissected, SAR-bright terrains that we interpret as past shorelines. Topographically, these shorelines are consistent with an equipotential surface, and morphologically they are the termini of numerous large channel networks that drain towards the basins. With other morphologic and topographic data described in Birch et al. [2017b], these observations all suggest that the four basins we identify were indeed past locations of large seas, equivalent to those we observe at the north today.

While morphologic and topographic data support the four basins we identify having been formerly filled seas, compositional data offers a challenge to our interpretations. Specifically, there are few 5 micron-bright deposits, interpreted as evaporite, identified within the basins [Barnes et al., 2009; MacKenzie et al., 2014]. If the southern paleoseas were once filled, the prevalence of evaporite deposits at the North raises questions as to why the two poles would have such different sedimentary processes. If our hypothesis that the South polar basins are paleoseas is correct, then some mechanism, perhaps erosion or burial, must be at work to mask any evaporite signature from the top few microns of the surface. Alternatively, the southern paleoseas may have had a different composition than the northern seas that wasn’t conducive to the formation of large evaporite deposits. As the likely time scale that determines the presence or absence of liquids in a given hemisphere is ~100,000 years [Lora & Mitchell, 2015; Aharonson et al., 2009], it can be expected that significant erosion/deposition of materials has occurred and any large 5 micron-bright deposits are hidden from our view today. Interestingly, this then requires the floors of putative equatorial paleoseas Tui and Hotei Regio to have been wetted more recently than the southern basins.

Despite these minor challenges, our study is a necessary first step toward understanding the distribution of liquid in Titan’s past. Future spacecraft missions to Titan may test our interpretations, as a definitive understanding of the evolution and distribution of volatiles on Titan is essential to unravelling the volatile budget and liquid/sediment transport timescales in Titan’s hydrological cycle.

Further Reading:

Aharonson, O., Hayes, A. G., Lunine, J. I., Lorenz, R. D., Allison, M. D., and Elachi, C. (2009), An asymmetric distribution of lakes on Titan as a possible consequence of orbital forcing, Nature Geoscience, 2, 851–854. doi:10.1038/ngeo698.

Barnes, J. W., Brown, R. H., Soderblom, J. M., Soderblom, L. A., Jaumann, R., Jackson, B., Le Mouélic, S., Sotin, C., Buratti, B. J., Pitman, K. M., Baines, K. H., Clark, R. N., Nicholson, P. D., Turtle, E. P., and Perry, J. (2009), Shoreline features of Titan’s Ontario Lacus from Cassini/VIMS observations, Icarus, 201, 217–225. doi:10.1016/j.icarus.2008.12.028.

Birch, S. P. D., Hayes, A. G., Dietrich, W. E., Howard, A. D., Bristow, C. S., Malaska, M. J., Moore, J. M., Mastrogiuseppe, M., Hofgartner, J. D., Williams, D. A., White, O. L., Soderblom, J. M., Barnes, J. W., Turtle, E. P., Lunine, J. I., Wood, C. A., Neish, C. D., Kirk, R. L., Stofan, E. R., Lorenz, R. D., and Lopes, R. M. C. (2017a), Geomorphologic mapping of titan’s polar terrains: Constraining surface processes and landscape evolution, Icarus, 282, 214–236. doi:10.1016/j.icarus.2016.08.003.

Birch, S. P. D., Hayes, A. G., Corlies, P. Stofan, E. R., Hofgartner, J. D., Lopes, R. M.C., Lorenz, R. D., Lunine, J. I., MacKenzie, S. M., Malaska, M. J., Wood, C. A., and the Cassini RADAR Team (2017b), Morphological evidence that Titan’s southern hemisphere basins are paleoseas, Icarus, in press. doi:10.1016/j.icarus.2017.12.016.

Hayes, A. G., Aharonson, O., Lunine, J. I., Kirk, R. L., Zebker, H. A., Wye, L. C., Lorenz, R. D., Turtle, E. P., Paillou, P., Mitri, G., Wall, S. D., Stofan, E. R., Mitchell, K. L., Elachi, C., and Cassini Radar Team (2011), Transient surface liquid in Titan’s polar regions from Cassini, Icarus, 211, 655–671, doi:10.1016/j.icarus.2010.08.017.

Hayes, A. G. (2016), The Lakes and Seas of Titan, Annual Review of Earth and Planetary Sciences, 44, 57–83, doi:10.1146/annurev-earth-060115-012247.

Lora, J. M., and Mitchell, J. L. (2015), Titan’s asymmetric lake distribution mediated by methane transport due to atmospheric eddies, Geophysical Research Letters, 42, 6213–6220. doi:10.1002/2015GL064912.

MacKenzie, S. M., Barnes, J. W., Sotin, C., Soderblom, J. M., Le Mouélic, S., Rodriguez, S., Baines, K. H., Buratti, B. J., Clark, R. N., Nicholson, P. D., and McCord, T. B. (2014), Evidence of Titan’s climate history from evaporite distribution, Icarus, 243, 191–207. doi:10.1016/j.icarus.2014.08.022.

Stofan, E. R., Elachi, C., Lunine, J. I., Lorenz, R. D., Stiles, B., Mitchell, K. L., Ostro, S., Soderblom, L., Wood, C., Zebker, H., Wall, S., Janssen, M., Kirk, R., Lopes, R., Paganelli, F., Radebaugh, J., Wye, L., Anderson, Y., Allison, M., Boehmer, R., Callahan, P., Encrenaz, P., Flamini, E., Francescetti, G., Gim, Y., Hamilton, G., Hensley, S., Johnson, W. T. K., Kelleher, K., Muhleman, D., Paillou, P., Picardi, G., Posa, F., Roth, L., Seu, R., Shaffer, S., Vetrella, S., and West, R. (2007), The lakes of Titan, Nature, 445, 61–64. doi:10.1038/nature05438.

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