Cassini image of a landslide on Iapetus
Landslides can wreak enormous destruction, especially when they travel farther than expected. When an avalanche occurs, dirt both falls vertically and spreads horizontally, with the horizontal distance usually no more than twice the vertical drop. But in a sturzstrom
, some unknown factor decreases the coefficient of friction
, allowing the earth to slide much farther; it acts more like a glacier or a lava flow than a regular avalanche. Theories about that friction-reducing factor abound---trapped air, water, or mud, pressure, rubbed and heated rock becoming more slippery, rock nanoparticles, sound waves, changes in local gravity---but its true nature is still unknown. By examining sturzstroms that occur on distant planets and moons---whose forces of gravity, atmospheres, fluids, and soil differ from those on Earth---researchers hope to unravel the factors that contribute to a landslide’s length. This information could help us predict landslides’ shapes and alleviate the damage they cause. Avalanches happen on bodies throughout the solar system, including our Moon, Mars, and Saturn’s moon Iapetus
. Now, thanks to the Cassini
probe’s high-resolution images of Saturn and its satellites, researchers can map Iapetus’s landslides in detail. A new study
examined the traces of avalanches on this moon’s surface, comparing their vertical drop to their horizontal sweep, and found that Iapetus has an unusual number of sturzstroms. What is making Iapetian landslides relatively friction-free? Because Iapetus has no air and is cold enough to keep any water frozen, researchers eliminated trapped air or water from the possible culprits. They instead suggested that friction itself contributes to reducing friction: as rock and ice rub together, this contact heats up the ice and makes it slippery, lowering friction and enabling a sturzstrom. But this hypothesis doesn't explain sturzstrom on Earth, where the soil does not always contain ice. Don't let that failure rub you the wrong way though---the finding will still help researchers study landslides and ice tectonics on other icy planets and moons, and it is related to one possible theory
for Earth's sturzstroms: that an avalanche’s energy heats dirt and rock to transform their physical properties, their coefficient of friction.