Sturzstroms on Saturn’s Moon Iapetus.

Post by Kelsi Singer. Ph.D Candidate, Earth and Planetary Sciences, Washington University, USA.

A typical landslide runs out less than two times its drop height whereas a long-runout landslide can extend 20-30 times the height it dropped from. Long-runout landslides (sturzstroms) are found across the Solar System.  They have been observed primarily on Earth (Image 1) and Mars, but also on Venus, and Jupiter’s moons Io and Callisto.

Image 1: An example of a long-runout landslide on Earth is the Blackhawk landslide in the Lucerne Valley, California. This landslide travelled ~8 km. Image Source USGS

Iapetus is the third largest moon of Saturn. The low density indicates that it is mostly composed of ice, with only a small (~20%) amount of rocky materials. Recently, a large number of long-runout landslides (~30) have been identified (Singer et al., 2012). Three morphological indicators aided the identification of these landslide deposits.  First, the landslides themselves often had a distinct surface texture compared to the surrounding terrain; either hummocky (called blocky type landslides, see Image 2) or relatively smooth and uncratered (called lobate, see Image 3).  Second, landslide frontal or lateral margins often appear as a distinct border, and in some cases these margins are quite steep.  Third, the landslides are often associated with an adjacent alcove on the crater wall or structural ridge from which they fell.  These landslides occur in smaller craters, large impact basin rims (rising higher than 10 km in some cases), and from the unique equatorial ridge (rising up to 20 km; see Image 4).

Landslides on Iapetus are the largest and most numerous observed on any icy body, and they rival the longest runout landslides seen elsewhere in the Solar System (up to 80 km).  So, why are there so many landslides on Iapetus?  One important factor is the extreme topography. It is the largest for its size, of any major body in the Solar System. The antiquity of the surface also plays a role. A long history of impacts, without resurfacing from other geological process, would lead to a largely unconsolidated surface. The surface therefore has marginally stable slopes that can be triggered to fail over time – most likely triggered by impacts elsewhere on the body.

Image 2: Global view of Iapetus’ dark, leading hemisphere and a close up of a large, blocky type, landslide in the crater Malun. Malun crater formed right on the edge of the large Turgis basin, which likely triggered the fall of material from the tall (~8 km) Turgis rimwalls. This landslide extends 55 km at its greatest length. The equatorial ridge is also visible in the global view, giving Iapetus a walnut-like appearance. Global dark side: NASA/JPL/Space Science Institute . Malun close up: NASA/JPL/Space Science Institute

Image 3: Global view of Iapetus’ bright, trailing hemisphere and a close up of a large, lobate type, landslide in the Engelier basin (~250 m/px). The landslide extends up to 80 km away from the 10 km high Engelier rimwall. There are several landslides along the walls of Engelier, likely accounting for the crenulated appearance of the basin (rather than a perfectly circular rim). The landslide shown exhibits multiple, overlapping lobes, and there is a hint of longitudinal furrows (sets of ridges parallel to the lateral margins). Global bright side: NASA/JPL/Space Science Institute, Zoomed high res: Cassini Product ID N1568145272

 Typical height:length ratios of landslides on Iapetus lie between 0.1 and 0.3. On the lower end, this is analogous to terrestrial submarine landslides and mudflows; the upper end is comparable to small subaerial rock avalanches on Earth or large landslides on Mars. The coefficient of friction is lower than expected for ice (which is approximately 0.55-0.7). These data indicated that the frictional properties of the Iapetus landslides were reduced. Many theories have been proposed for a reduction of friction in large runout landslides (e.g., acoustic fluidization, mobilization on an air cushion). The mechanism proposed for Iapetus is flash heating along the base of the landslides. This process produces a concentrated amount of heating; not enough to melt ice, but sufficient to increase lubrication and make the ice more slippery.

Earth and a small icy satellite such as Iapetus may seem very dissimilar, however many of the same geomorphic processes are observed. A better understanding of the long landslides on Iapetus may help us to understand the causes of similar catastrophic events on our own planet.

Image 4: Iapetus’ unique and ancient equatorial ridge shows diverse morphologies, sometimes flat-topped, other times sharp and steep-sided, and in some places there are individual mountainous peaks. This portion of the ridge (Toledo Montes) shows where landslides have modified the flat-topped ridge (at ~225 m/px). Arrows indicate landslide margins and dotted lines show alcoves that are possibly sites of more ancient landslides. No matter how the ridge originally formed (a debated topic), its appearance has been considerably altered by a long history of mass wasting. Cassini Product ID: W1568125439

Further reading:

Icy Satellites:

Dombard, A.J., Cheng, A.F., McKinnon, W.B. & Kay, J.P. (2012) Delayed formation of the equatorial ridge on Iapetus from a sub-satellite created in a giant impact. J. Geophys. Res. 117, 03002.

Moore, J. M. et al. (1999) Mass movement and landform degradation on the icy Galilean satellites: Results of the Galileo Nominal Mission. Icarus 140, 294–312.

Singer, K.N., McKinnon, W.B., Schenk, P.M., and Moore, J.M. (2012) Massive ice avalanches on Iapetus caused by friction reduction during flash heating. Nature Geoscience, 5, 8, pp. 574 – 578.

 Mars:

McEwen, A. S. (1989) Mobility of large rock avalanches: Evidence from Valles Marineris, Mars. Geology 17, 1111–1114.

Quantin, C., Allemand, P. & Delacourt, C. (2004) Morphology and geometry of Valles Marineris landslides. Planet. Space Sci. 52, 1011–1022.

Friction reduction on Earth from flash heating (landslides and faults):

De Blasio, F. V. & Elverhøi, A. (2008) A model for frictional melt production beneath large rock avalanches. J. Geophys. Res. 113, F02014.

Di Toro, G. et al. (2011) Fault lubrication during earthquakes. Nature 471, 494–498.

Goldsby, D.L., & Tullis, T.E. (2011) Flash heating leads to low frictional strength of crustal rocks at earthquake slip ratesScience 334, 216–218.

 General:

Rosenberg, R. (2005) Why is ice slippery? Physics Today 58, 50–55.

Melosh, H. J. (2011) Planetary Surface Processes. Cambridge Univ. Press.

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