Post by Dr. Alexis Rodriguez. Planetary Scientist, Planetary Science Institute, Tucson, AZ, USA.
The Martian northern lowlands are thought to currently be extensively covered by an ice-rich deposit, interpreted by some researchers to be the residue of an ancient ocean that existed ~3.4 Ga (Kreslavsky and Head., 2002). However, evidence for this ocean has remained a subject of intense dispute and scientific scrutiny since it was first proposed (Parker et al., 1989, 1993) several decades. The controversy has largely stemmed in the fact that the proposed Martian paleo-shoreline features exhibit significant elevation ranges (Head et al., 1999), a lack of wave-cut paleoshoreline features (Malin and Edgett, 1999), and the presence of lobate margins (Tanaka et al., 1997, 2005).
The discovery of potential tsunami deposits as discussed in a recent article by Rodriguez et al. (2016) brings in a new perspective in which coastal margins shaped by enormous tsunami waves likely characterized early Mars oceans. Tsunami waves are high energy run-up flows with upper reaches, which, unlike shoreline margins, do not conform to a constant elevation, and consequently their role in shaping the coastal terrains of early Mars can explain much of the long-debated absence of topographic equipotentiality; the absence of widespread wave-cut paleoshoreline features, which would have been extensively buried or resurfaced by the waves; and the presence of lobate fronts, which are characteristic of tsunami run-up flows.
The features that Rodriguez et al. (2016) interpreted as tsunami deposits consist of lobate deposits reaching several hundred kilometers in lengths over a few hundred meters of relief gains (Fig. 1).
Their diagnostic criteria combine a complex set of spatial relationships and geomorphic observations, including the lobes’: (1) highland-facing shapes [consistent with emplacement by run-up flows; Figs. 1-3]; (2) lengths typically oriented perpendicular to regional HLB slopes [consistent with the flows sourcing from within the northern plains; Figs. 1, 2]; (3) boulder and ice-rich make-up [consistent with high energy flows capable of transporting coarse bedload and water; Fig. 3]; and (4) regional dissection by channels that drain into the northern plains [consistent with a backwash phase as the displaced water rushed back into the ocean].
Wind streaks comprise another type of geologic feature generated by gravity-defying uphill motion of sediments. However, these are made up of sand and silt sediments, exhibit surface bedforms and generally have greater length-width ratios than the proposed tsunami deposits (Rodriguez et al., 2016). Glaciers can overflow uphill floors, but their bulk mass is still displaced downslope by gravity, propagate through valleys, and exhibit numerous characteristic features such as terminal and marginal moraines. The investigated lobes do not source from valleys and are not flanked by moraines-like ridges (Rodriguez et al., 2016). Furthermore, flow separation patterns (e.g., Fig. 3) documented by Rodriguez et al. (2016) are not consistent with thick glacial overflows, and instead, they reflect upslope thinner flow motion affected by low-relief topography.
*In May 2013, the Saskatchewan Water Security Agency filmed an ice surge in the Codette Reservoir near Nipawin, Saskatchewan, Canada. The surge comprises a spectacular terrestrial analog of rarely observed catastrophic slurry ice-rich flows leading to the emplacement of enormous lobate fronts, which are strikingly similar to those shown in Figure 3. Video link: https://www.youtube.com/watch?v=OgMBQFf64JM
Head, J. W. et al. Possible ancient oceans on Mars: Evidence from Mars Orbiter Laser Altimeter data. Science 286, 2134-2137 (1999).
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Malin, M. C. & Edgett, K. S. Oceans or seas in the Martian northern lowlands: High resolution imaging tests of proposed coastlines. Geophys. Res. Lett. 26, 3049-3052 (1999).
Parker, T. J., et al. Coastal geomorphology of the Martian northern plains. J. Geophys. Res. 98, 11061-11078 (1993).
Parker, T. J., Saunders, R. S. & Schneeberger, D. M. Transitional morphology in west Deuteronilus Mensae, Mars: Implications for modification of the lowland/upland boundary. Icarus 82, 111-145 (1989).
Rodriguez, J. A. P., et al., Tsunami waves extensively resurfaced the shorelines of a receding, early Martian ocean. Scientific Reports 6:25106 (2016), DOI: 10.1038/srep25106.
Tanaka, K. L., Skinner, J. A. & Hare, T. M. Geologic map of the northern plains of Mars, (2005) (Date of access: (24/11/2015). U.S. Geological Survey Scientific Investigations Map 2888, scale 1:15,000,000 (1 mm = 15 km) at 90° N and 1:7,500,000 at 0° N, http://pubs.usgs.gov/sim/2005/2888/
Tanaka, K. L. Sedimentary history and mass flow structures of Chryse and Acidalia Planitiae, Mars. J. Geophys. Res. 102, 4131-4149 (1997).
Acknowledgements: I am particularly grateful to Ken L. Tanaka for his invaluable scientific contribution to discovery of the potential tsunami deposits on Mars.