Saturn's moon Iapetus has many unique features that stand out
even among the odd Solar System satellites. Its shape is very
walnut-like: flattened at the poles, bulging strongly at the
equator, with an equatorial mountain ridge nearly 20 kilometers
high running about three-quarters of its circumference. Unlike
any other known object, the face that points in the direction of
its motion is dark in color, while the trailing face is bright
white. Some of these mysteries have plausible solutions, but
others remain unsolved. A new observation described by Kelsi N.
Singer, William B. McKinnon, Paul M. Schenk, and Jeffery M.
Moore adds to Iapetus' marvelous weirdness. Analyzing images
from the Cassini space probe, they found huge ice avalanches on
the moon's surface. These avalanches are known as long-runout
because they extend much farther than a typical landslide.
Because Iapetus has no atmosphere or groundwater—implicated in
some long-runout landslide models—the researchers concluded
local melting of ice reduces t