Editor's note: This entry originally appeared on the Planetary Geomorphology Image of the Month, a monthly blog feature maintained by the International Association of Geomorphologists Planetary Geomorphology Working Group. I occasionally repost these features because they are written by the geologists who are discovering and explaining cool landforms across the solar system. I have edited it to make it more accessible to non-geomorphologists. -- Emily Lakdawalla
Typical landslides run out about once or twice as far as the distance that they fall. But some landslides, called "long-runout" landslides or "sturzstroms," can extend 20 to 30 times the height they dropped from. Sturzstroms are found across the Solar System. They have been observed primarily on Earth and Mars, but also on Venus, and Jupiter's moons Io and Callisto. Here is an Earth sturzstrom:
Kerry Sieh, USGS
The Blackhawk Landslide, San Bernardino mountains, California
The largest slide in the Transverse Range province is the Blackhawk, on the north slope of the San Bernardino Mountains, California. This prehistoric slide is one of the largest known in North America. It was studied in detail by R. L. Shreve in the 1950's and 60's, who introduced the hypothesis that long-runout landslides may move on a cushion of compressed air. However, landslides on Mars and icy satellites occur even without any air to ride on, and other hypotheses have been considered since Shreve's original work.
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, I worked with a group of scientists to identify about 30 long-runout landslides, and published a paper about them in Nature Geoscience. These landslides occur in smaller craters, large impact basin rims (rising higher than 10 km in some cases), and from the unique equatorial ridge.
We identified the landslides based on three characteristics. First, the landslides themselves often had a distinct surface texture compared to the surrounding terrain, either hummocky (called blocky-type landslides) or relatively smooth and uncratered (called lobate). Here are examples of both types:
NASA / JPL / SSI
Landslide in Iapetus' Malun crater
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. Large white arrows indicate the direction of incoming sunlight.
NASA / JPL / SSI
Large, lobate landslide in Iapetus' Engelier basin
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).
A second indicator that these are landslides is that their frontal or lateral edges often appear as a distinct border, and in some cases they are quite steep.
Third, the landslides are often associated with an adjacent alcove on the crater wall or structural ridge from which they fell. You can see those alcoves in this image of Iapetus' equatorial ridge:
NASA / JPL / SSI / annotated by Kelsi Singer
Landslide modification of Iapetus' ridge
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.
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. No other body that is similar in size to Iapetus has such a wide range of elevations. The antiquity of the surface also plays a role. A long history of impacts, without resurfacing from other geological processes, would leave an uppermost surface that was pulverized and not well consolidated. The surface therefore has slopes that are only barely stable and which can be triggered to fail over time. Sturzstroms are most likely triggered by impacts elsewhere on Iapetus.
Typical height-to-length ratios of landslides on Iapetus lie between 0.1 and 0.3. Terrestrial submarine landslides and mudflows have low coefficients of friction and height-to-length ratios of around 0.1. Small subaerial rock avalanches on Earth and large landslides on Mars have higher coefficients of friction and have height-to-length ratios closer to 0.3. The frictional properties of the Iapetus landslides were smaller than one would predict for an icy body.
Many theories have been proposed to explain a reduction of friction in large-runout landslides (such as acoustic fluidization or 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. But the same geomorphic processes operate on both worlds. A better understanding of the long landslides on Iapetus may help us to understand the causes of similar catastrophic events on our own planet.